Finaldesignproject.docx

ID: 1114132

Design Project 1

University of Mauritius Faculty of Engineering Department of Chemical and Environmental Engineering 11/28/2014 Soomaree Keshav 1114132

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DESIGN OF A SEWAGE TREATMENT PLANT

Student Group: 3A

Members Soomaree Keshav St Paul M.M Eldora Ramanah R. Devi Ramdewar P.Kumar Ramdhonee A.K.R Mishra Coordinator: Mr A Mudhoo

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Table of Contents Table of Figures .......................................................................................................................................... 7 Acknowledgment ....................................................................................................................................... 8 List of abbreviations .................................................................................................................................. 9 CHAPTER 1

: INTRODUCTION ................................................................................................... 10

1.1

Aim of the design ..................................................................................................................... 10

1.2

Environmental and social impact .......................................................................................... 10

1.3

The trend of treated water, demand and supply ................................................................. 11

1.4

Existing and Future market for treated water...................................................................... 12

1.5

Brief comparison and competitors......................................................................................... 12

1.6

Development of new process techniques ............................................................................. 12

1.6.1

Preliminary/Primary Treatment ................................................................................... 12

1.6.2

Secondary Treatment ....................................................................................................... 13

1.6.3

Tertiary Treatment ........................................................................................................... 13

1.7

The problem statement .............................................................................................................. 14

CHAPTER 2 2.1

: LITERATURE REVIEW .......................................................................................... 15

Literature review for primary Treatment ............................................................................. 15

2.1.1

Oil-water separator Design ............................................................................................. 15

2.1.2

Equalisation tank .............................................................................................................. 15

2.2

Literature review for secondary Treatment ......................................................................... 16

2.2.1

Primary Clarifier .............................................................................................................. 16

2.2.2

Aerobic versus Anaerobic treatments ........................................................................... 17

2.2.3

The Aerobic Membrane Bioreactor ................................................................................ 20

2.3

Literature review for tertiary treatment................................................................................ 24

2.3.1

Filtration ............................................................................................................................ 25

2.3.2

Disinfection ....................................................................................................................... 28

2.4

Literature review on sludge thickening, digestion and gas handling .............................. 29

2.4.1

Thickening of sludge ....................................................................................................... 29

2.4.2

Sludge disposal................................................................................................................. 30

2.4.3

Sludge dewatering ........................................................................................................... 30

CHAPTER 3:

PROCESS CONSIDERATION .................................................................................... 32 Page 3 of 134

ID: 1114132 3.1

Process consideration for preliminary treatment ................................................................ 32

3.1.1

Screenings.......................................................................................................................... 32

3.1.2

Balancing/equalizing tank ............................................................................................. 33

3.2

Process consideration for secondary treatment ................................................................... 34

3.2.1

Circular primary settling tank ........................................................................................ 34

3.2.2

Secondary treatment methods........................................................................................ 34

3.2.3

Aeration equipments ....................................................................................................... 36

3.3

Process consideration for tertiary treatment ........................................................................ 36

3.3.1

Filtration processes .......................................................................................................... 36

3.4

Disinfection ............................................................................................................................... 37

3.5

Process consideration for sludge thickening and digestion ............................................... 37

3.5.1

Thickening primary and waste activated sludge ........................................................ 37

3.5.2

Stabilization....................................................................................................................... 37

3.6

Process Consideration for process in anaerobic digestion ................................................. 38

3.7

Process consideration for sludge conditioning, dewatering and disposal ...................... 38

3.7.1

Type of sludge conditioning ........................................................................................... 38

3.7.2

Type of chemical conditioning aids............................................................................... 39

3.7.3

Dewatering ........................................................................................................................ 39

3.8

Sludge disposal......................................................................................................................... 39

CHAPTER 5: 5.1

PREIMINARY DESIGN [SIZING] ............................................................................. 43

Sizing of the preliminary treatment unit .............................................................................. 43

5.1.1

The Oil-Water Separator ................................................................................................. 43

5.1.2

Bar Screen .......................................................................................................................... 43

5.1.3

Fine screen ......................................................................................................................... 44

5.1.4

Equalization Tank ............................................................................................................ 44

5.2

Sizing of the secondary treatment units ............................................................................... 45

5.2.1

Primary Clarifier .............................................................................................................. 45

5.2.2

The Membrane Bioreactor............................................................................................... 45

5.3

Sizing of the tertiary treatment units .................................................................................... 46

5.3.1

Sand Filter ......................................................................................................................... 46

5.3.2

Chlorination ...................................................................................................................... 47 Page 4 of 134

ID: 1114132 5.3.3

Thickener ........................................................................................................................... 47

5.3.4

Sludge Digester ................................................................................................................ 48

5.3.5

Dewatering Tank .............................................................................................................. 49

CHAPTER 6:

ENERGY BALANCE ................................................................................................... 50

CHAPTER 7:

PRELIMINARY HAZOP ........................................................................................... 51

7.1

The oil-water separator ........................................................................................................... 51

7.2

Screening ................................................................................................................................... 52

7.3

Equalization Tank .................................................................................................................... 54

7.4

Primary Clarifier ...................................................................................................................... 54

7.5

The Membrane Bioreactor....................................................................................................... 55

7.6

Sand Filter ................................................................................................................................. 56

7.7

Thickener ................................................................................................................................... 56

7.8

Digester...................................................................................................................................... 57

CHAPTER 8: 8.1

WASTE TREATMENT ................................................................................................ 59

PRELIMINARY TREATMENT ......................................................................................................... 59

8.1.1

Oil-Water Separator ......................................................................................................... 59

8.1.2

Screen bars and grit chamber ......................................................................................... 59

8.1.3

Primary sedimentation tanks ......................................................................................... 59

8.2

SECONDARY TREATMENT ................................................................................................. 59

8.2.1 8.3

The MBR Tank .................................................................................................................. 59 TERTIARY TREATMENT ....................................................................................................... 59

8.3.1 8.4

Sand filter .......................................................................................................................... 59

SLUDGE TREATMENT .......................................................................................................... 59

8.4.1

GRAVITY THICKENER .................................................................................................. 59

8.4.2

FINAL DISPOSAL OF SLUDGE CAKE........................................................................ 60

8.5

BIOGAS ........................................................................................................................................ 61

CHAPTER 9:

CONTROL STRATEGY ............................................................................................... 61

9.1

Control of Liquid level ............................................................................................................ 61

9.2

Control of Flow......................................................................................................................... 62

9.3

Control of temperature ............................................................................................................ 64

9.4

Control of pH ............................................................................................................................ 64 Page 5 of 134

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Control of Pressure .................................................................................................................. 65

CHAPTER 10:

PRELIMINARY COSTING ..................................................................................... 65

10.1

Cost of Equipment ................................................................................................................... 66

10.2

Calculating of working capital ............................................................................................... 66

10.3

Calculating payback period .................................................................................................... 67

CHAPTER 11:

CONCLUSIONS ....................................................................................................... 67

References ................................................................................................................................................. 68 Appendices 1: Mass Balance................................................................................................................... 75 Mass Balance for Preliminary treatment............................................................................................... 75 Mass Balance for Secondary treatment ................................................................................................. 79 Mass Balance for Tertiary treatment ..................................................................................................... 89 Mass Balance for Sludge Treatment ...................................................................................................... 91 Appendix 2: Energy Balances ............................................................................................................... 94 Energy balance for Preliminary treatment ........................................................................................... 94 Energy balance for Secondary treatment .............................................................................................. 95 Energy balance for Sludge treatment .................................................................................................... 97 Appendix 3: SIZING ............................................................................................................................ 100 Sizing of the Bars Screen ..................................................................................................................... 100 Sizing of Oil water separator ................................................................................................................ 103 Sizing of equalisation tank .................................................................................................................... 111 Sizing of the primary clarifiers ............................................................................................................. 112 Sizing of the MBR................................................................................................................................... 113 Sizing the Sand filter .............................................................................................................................. 117 Sizing of chlorination unit .................................................................................................................... 119 Sizing of thickener.................................................................................................................................. 120 Sizing of Sludge digester....................................................................................................................... 123 Sizing of dewatering unit ...................................................................................................................... 126 Freeze Design ............................................................................................................................................ 128 Minutes of meeting ................................................................................................................................ 129 09/08/2014 ............................................................................................................................................... 129 20/09/2014 ............................................................................................................................................... 131 Page 6 of 134

ID: 1114132 04/10/2014 ............................................................................................................................................... 132 11/10/2014 ............................................................................................................................................... 132 25/10/2014 ............................................................................................................................................... 133 08/11/2014 ............................................................................................................................................... 133

Table of Figures Table 1.1: Characteristic of waste water Table 2.1:

A brief comparison between aerobic and anaerobic systems

Table 2.2:

Characteristic comparison between aerobic and anaerobic systems

Table 2.3:

Advantages and disadvantages of MBR

Table 2.4:

Comparison of the main MBR systems

Figure 1:

Aeration demand for the biodegradation of the organic matters as a function of target MLSS

Figure 2:

Relative cost decrease of kubata membrane and MBR systems

Table 2.5:

Sand filter maintenance

Table 3.1:

Manually versus mechanically cleaned screen bar

Table 3.2:

Design criteria for the screening

Table 3.3:

Comparative analysis of rectangular and circular clarifiers

Table 4.1:

Summary of mass balance for the sewage flow over the system

Table 4.2:

Mass balance for biogas

Table 9.1:

The control of liquid level for the whole plant

Table 9.2:

The control of flow rate for the whole plant

Table 9.3:

The control of temperature for the whole plant Page 7 of 134

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Table 9.4:

The control of pH for the plant

Table 9.5:

The control actions for pressure in the plant

Table 10.1:

Equipment costs

Acknowledgment I wish to thank Mr. Ackmaz Mudhoo, our design coordinator, for guiding us and giving us all the possible help that he could. I am thankful to him since he was always present when we needed him and was here to direct us to the right way. I am also grateful to Mr. Arvinda Ragen, our program coordinator, who ensured that we did not lack anything with regards to the project and he did his best to give us all the facilities we needed. A special thank goes to all my group members; St Paul, Ramanah, Ramdewar and Ramdhonee, who actively participated in the design project to make it successful. I would also like to thank Dr. Dinesh Soorup, the head of department, who responded positively to all the problems faced by us.

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List of abbreviations OWS:

Oil Water Separator

B.S:

Bar Screen

C1:

Primary Clarifier 1

C2:

Primary Clarifier 2

MBR:

Membrane Bioreactor

S.D:

Sludge Digester

BOD:

Biological Oxygen Demand

COD:

Chemical Oxygen Demand

TSS:

Total suspended solids

EQ tank:

Equalisation tank

Veq:

Equalisation Volume

Abs:

Absolute

Q:

Influent Flowrate

SE:

BOD in Effluent

S0:

BOD in influent,

X:

MLSS concentration

SRT:

Sludge Retention Time

F/M:

food to microorganism ratio

Xo :

Amount of TSS entering

Xe:

Amount of TSS leaving

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CHAPTER 1 :

INTRODUCTION

1.1 Aim of the design Without treatment the water from toilets, baths, sinks and washing machines from domestic and residential premises contaminated with metals, oils and other pollutants in rainwater run-off from urban areas draining to sewers would have significant adverse impacts on the water environment. The aim of this project is to design an economically-realistic wastewater treatment plant to process raw sewage from urban residential areas, the treated water can be re-used for any other activities except for consumption. The design includes all unit operations from inlet works to the disposal of the final effluent and also the handling and safe disposal of sludge and consideration potential economic use of the final sludge. This design also makes a comprehensive assessment of the possibility to capture and process the metabolically generated biogas for eventual electricity conversion. 1.2 Environmental and social impact Since independence, Mauritius has experienced environmental problems owing to economic development and demographic expansion. The household and industrial wastewater is currently discharged generally without any proper treatment into the lagoons and oceans around the island, threatening the tourism industry (which is a major source of earnings in foreign currency) and the livelihoods of artisanal fishermen with oceanic pollution. The standard of living in terms of per capita income has increased, but the quality of life was adversely affected by increased public health and sanitation problems including intestinal and eye diseases.1 The demand for wastewater treatment in Port Louis was expected to increase from 25000 m3 per day (1997), through 48 thousand m3 per day (2005), to 61000 m3 per day (2007). This exceeds the existing capacity of wastewater treatment plants (17000 m3 per day) in Fort Victoria and Pointe aux Sable, both of which was in a deteriorated condition and requires immediate replacement with new treatment systems. Meanwhile, the Government of Mauritius requested the Japanese Government to help establish a new wastewater treatment system in the Montagne Jacquot area, which was located six km south-west of the city. Wastewater was to be sent to this facility through compression pipes from new pumping stations, covering 1,340 ha (or a population of 118 thousand, as of 1997) in residential and commercial districts.2

http://www.defimedia.info/news-sunday/nos-news/item/8181-news-inbrief.html?tmpl=component&print=1 1

2

http://statsmauritius.gov.mu (2007)

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The Environmental Protection Agency was established in 1993 to license, regulate and control activities for the purposes of environmental protection. In Section 60 of the Environmental Protection Agency Act, 1992, it is stated that "the Agency may, and shall if so directed by the Minister, specify and publish criteria and procedures, which in the opinion of the Agency are reasonable and desirable for the purposes of environmental protection, in relation to the management, maintenance, supervision, operation or use of all or specified classes of plant, sewers or drainage pipes vested in or controlled or used by a sanitary authority for the disposal of any sewage or other effluent to any waters ". Criteria and procedures in relation to the treatment and disposal of wastewater are being published by the Agency in a number of manuals under the general heading: 'Wastewater Treatment Manuals'. Where criteria and procedures are published by the Agency, a sanitary authority shall, in the performance of its functions, have regard to them. 3 1.3 The trend of treated water, demand and supply Many water treatment technology trends have been observed for some time in Europe, where supranational directives forced the enhancement of national standards for water quality and wastewater discharges. With a population roughly equivalent to that of the U.S. compacted into approximately one-fourth as much space and a historic location of infrastructure in central settings, Europe has already faced the challenge of upgrading treatment plants despite limited room for expansion (Limbachyia, et al., 2001). Growth in population is leading to an appreciation of the water value within wastewater streams and increasingly municipalities are recognizing the need to recover this water globally.

For example, Florida has set a target to reuse 75 percent of

wastewater by 2025. A major sustainability initiative introduced in the UK is to reduce overall potable water consumption from 180 litres per day, a 56 percent reduction, by 2016. It is estimated that 25 percent of the potable water entering homes is used to flush toilets. Given the future risks of water shortages and the cost of providing purified water for drinking, any unnecessary waste of potable water on activities such as

3

http://www.gov.mu

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flushing toilets, washing clothes, washing the car and watering the garden is not sustainable (Laakkoken et al., 2010)4. 1.4 Existing and Future market for treated water Many municipal wastewater treatment systems are large, centralized works that often waste the water resource rather than reuse it and consume significant amounts of energy to move wastewater from the source to the treatment plant. This has led to a growing recognition that infrastructure investment should be on the principle of sustainable, decentralized water reuse technologies where water is captured, treated and reused locally. In both developed and developing countries, sludge disposal is an issue growing in line with the increase in the volume of wastewater treated. New technologies such as moving bed bioreactors and submerged aerated filters are starting to replace the traditional activated sludge systems due to their costeffectiveness, smaller footprints, use of recycled plastics for media, and lower power consumption. The ability to upgrade existing systems with minimal new construction provides for a low-impact solution. For residential wastewater systems, which require minimal maintenance, do not require mechanical blowers and feature very low power consumption. These are becoming a popular solution. 1.5 Brief comparison and competitors In the present days there is concretely no market for treated water in Mauritius resulting in almost no competitors. However, it is expected to have an increased competition in this particular market for the next coming decades which will bring more evolution to the local Sewage treatment plants. 1.6

Development of new process techniques

1.6.1 Preliminary/Primary Treatment  Salsnes Filter The Salsnes filter uses a removable fine mesh screen attached to an inclined moving belt of wire cloth to sieve solids from wastewater simultaneously filtering the water and dewatering the solids. The belt rotates to an “air knife” for self-cleaning with compressed air to remove the solids to a sludge compartment. In one installation, the Salsnes filter has proven to reduce influent BOD and TSS by 40% and 65% respectively (McElroy, 2012)5. Performance depends on the size distribution of influent solids and 4 5

LAAKKONEN, LAURILA, KANSANEN and SCHULMAN, [ca.2010]. History of wastewater treatment McElroy, R. et al., “Restoring Lost WWTP Capacity through Innovative Technologies”, WEFTEC 2012

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the size of the mesh selected for the filter screen which typically ranges from 100 to 500 microns (Sutton et al. 2008)6 although a 1000 micron mesh screen was installed at the Daphne Utilities WWTF. The screen surface hydraulic loading rate is an important factor affecting screen performance. A pressure transmitter varies belt speed to maintain liquid level at near the overflow elevation to assure effective flow distribution. The belt is backwashed to remove fats, oils, and grease. Filters are available in sizes with capacities up to 2200 gpm for free standing units and 3500 gpm for units installed in a concrete channel. Multiple units may be installed in parallel to achieve the desired capacity. A dewatering screw press is available to transport the solids, and when used can produce sludge at up to 27% solids (Sutton 2008). 1.6.2 Secondary Treatment  Vacuum Rotation Membrane System This membrane system employs flat-sheet, ultrafiltration-membrane segments configured into disks rotating on a horizontal shaft. The hydrophilic membrane has a pore size of approximately 38 nm. Sequential cleaning of the rotating membranes is achieved with scouring air introduced next to the shaft at about half the water depth, providing high-intensity scouring of 1/6 to 1/8 of the disk near the 12 o’clock position. The membranes rotate through the scouring section several times per minute. Operating results show that neither back-pulsing nor regular cleaning is required. Average flux is typically 8-12 gal/ft2/day with a suction head of less than 10 feet. (Shear forces introduced by the rotational movement together with the high-intensity air scour remove solids buildup on the membranes to decrease membrane fouling. Chemical cleaning once or twice a year has shown to be sufficient for operating VRM plants.7 1.6.3 Tertiary Treatment  Microwave Ultraviolet (UV) Disinfection UV disinfection transfers electromagnetic energy from a mercury arc lamp to wastewater. Electromagnetic radiation, between the ranges of 100 to 400 nm (UV range), penetrates bacterial cells, and works as a bactericide. Typical mercury vapor UV lamps contain electrodes that facilitate the generation of UV radiation by striking an electric arc.8 These electrodes are delicate and their deterioration is the primary source of failure in UV disinfection systems. Microwave UV disinfection technology eliminates the need Sutton, P. et al. “Rotating Belt Screens: An Attractive Alternative for Primary Treatment of Municipal Wastewater” WEFTEC 2008. 7 Schuler, S. “Operating Experience with Rotating Membrane Bioreactors”, Water World, March 2009. 8 Meera, V., et al., “Microwave UV Comes to Texas,” WEFTEC Proceedings, 2010 6

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for electrodes by using microwave-powered, electrodeless, mercury UV lamps. In this technology, microwave energy is generated by magnetrons and directed through wave guides into quartz lamp sleeves containing argon gas.9,10 The directed microwave energy excites the argon atoms, which in turn excite the mercury atoms to produce radiation as they return from excited states to lower energy states, as is the case with other mercury UV lamps. The intensity of the radiation increases when the applied microwave power is increased. Microwave UV disinfection systems are available in modular, open-channel, and closed-vessel designs.11

1.7

The problem statement

The Sewage Treatment plant (STP) is expected to receive raw sewage of the characteristics shown in table 1.1. Table 1.1: Characteristic of water Parameter

Value at inlet

Sewage flow (m3/day)

70 000

TSS (mg L-1)

285

pH

7.40

Total NH3–N (mg L-1)

25

Temperature (0C)

29

Total NO3—N (mg L-1)

20

BOD5 (mg L-1)

307

Total Phosphorus (mg L-1)

10

COD (mg L-1)

984

Total coliforms count (MNP 100mL-1)

420000

Black and Veatch Corporation, “White’s Handbook of Chlorination and Alternative Disinfectants,” 5thed., Wiley, 2010 9

11

Newton, J., “Disinfection Utilizing an Innovative Microwave UV System,” WEFTEC Proceedings, 2009

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The raw sewage will enter the STP from a complex network of pipes and it is assumed that the site requirements needed for the construction of the plant is available. Also, some amount of oil is assumed to be entering in the effluent. CHAPTER 2 : 2.1

LITERATURE REVIEW

Literature review for primary Treatment

2.1.1 Oil-water separator Design The oil-water separation is based on the difference in density. Oil being less dense than water will float on the surface of water under quiescent conditions. Flow of water should be sufficiently low enough to provide the quiescent condition. In essence, an oil trap is a chamber designed to provide flow conditions sufficiently quiescent so that globules of oil rise to the water surface and coalesce into a separate oil phase, to be removed by mechanical means. Oil-water separation theory is based on the rise rate of the oil globule (vertical velocity: Vt) and its relationship to the surfaceloading rate of the separator. The AIP Oil-Water Separator is highly efficient and can remove up to 99.9% of oil droplets 20 microns or larger. It has a compact design, requires smaller footprint and space. It has also almost zero operation and maintenance cost with zero power consumption. Furthermore, no chemical cleaning of the media is required as the media can be cleaned by just high pressure water jet. 2.1.2 Equalisation tank The sewage from the oil-water separator and bar screen chamber comes to the equalization tank which is the first collection tank in an STP. Its main function is to collect the incoming raw sewage that comes at widely fluctuating rates, and pass it on to the rest of the STP at a steady flow rate. During the peak hours, the equalization tank stores sewage coming at a high rate and lets it out during the non-peak time when there is little/no incoming sewage.(Ananth S. Kodavasal, 2011) Thus, the tank dampens fluctuations and provides a constant outflow rate, which can improve performance of subsequent steps in the STP. This can help design the rest of the plant with smaller equipment (less capital investment) because of this improvement in performance. Sewer line is gravity-fed, and is likely to be at considerable depth below the ground level. Hence, it is effective not to place the tank too deep; otherwise it will demand very deep excavations and expensive construction. It also renders the maintenance and cleaning processes very hazardous and costly. The equalization tank is used only for Page 15 of 134

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buffering the daily fluctuations in the sewage flow quantity and must be of sufficient capacity to hold the peak time inflow volumes, else the tank will overflow. Peak time volumes are site-specific and variable. In the case of residential complexes, there is a distinct morning major peak when all residents are using their kitchens, bathrooms and toilets, followed by a minor peak in the late evening hours. In addition, the sewage generation may be heavier during the weekends. In a typical residential complex, an equalization tank with a capacity to hold 4-6 hours of average hourly flow should be adequate. (Ananth S. Kodavasal, 2011) 2.2 Literature review for secondary Treatment Secondary treatment can be defined as the utilization of microorganisms to treat the primary effluent, where a considerable amount of BOD and COD as well as nutrients such as nitrogen compounds are removed. Secondary treatment consists of reactor tanks; that is, activated sludge, and final clarifiers 2.2.1 Primary Clarifier Primary clarifiers or primary sedimentation tank are a physical treatment process used to remove settleable solids and solids in suspension by the principle of specific gravity. Selection of sedimentation tanks depends on plant size, the nature of the wastewater to be treated, and effluent parameters. Primary sedimentation is currently used as a preliminary step ahead of biological treatment and is designed to operate continuously. The three different types of sedimentation tanks are rectangular, circular and square clarifiers. In the rectangular type, wastewater flows from one end of the tank to the other. In the circular and square types, the wastewater typically enters at the center and flows towards the outside edge with the settled solids scraped or otherwise transported to the center. The main function of primary sedimentation is firstly to reduce the load on the biological treatment units and to increase sludge solids concentration in sludge thickening. For efficient design and operation, primary clarifiers should remove 50 to 65 percent of the suspended solids and 25 to 40 percent of the BOD. The clarification tanks are designed to provide shorter detention time and a higher rate of surface loading. Efficient grit removal is very important at plants since it prevents abrasion and wear of mechanical equipment, deposition of grit in pipes and also accumulation in aerators and anaerobic digesters (Marcos von Sperling et aL, 2005).

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2.2.2 Aerobic versus Anaerobic treatments Table 2.1: A Brief comparison between Aerobic and anaerobic Systems

Parameter

Aerobic Treatment

Aerobic Treatment

• Microbial reactions take • Microbial reactions take place in place in the the Process Principle

presence oxygen

of

molecular/

free

• Reactions products carbon dioxide,

• Reactions products are carbon dioxide, water and excess biomass Wastewater with medium organic

Applications

low

impurities (COD < 1000 and for

absence of molecular/ free oxygen are

methane and excess biomass

to Wastewater with medium to high organic ppm) impurities (COD > 1000 ppm) and easily

wastewater that are difficult to biodegradable biodegrade e.g. food and

wastewater

e.g. municipal sewage, refinery beverage wastewater rich in wastewater starch/sugar/

Reaction Kinetic

Net Sludge Yield

Post Treatment

etc.

alcohol

Relatively fast

Relatively slow Relatively low (generally one fifth to one

Relatively high

Typically direct filtration/ disinfection

tenth of aerobic treatment processes) discharge

or

Invariably followed aerobic treatment

by

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Foot-Print

Capital Investment

Relatively large

Relatively small and compact

Relatively high

Relatively low with pay back

Continuously stirred tank reactor/digester, Upflow Activated Sludge e.g. Extended Anaerobic sludge Blanket Aeration, Oxidation Ditch, MBR, Fixed Film Processes e.g. Trickling (UASB), Ultra High Rate Filter/Biotower. Fluidized Bed

Example Technologies

Reactors.

Table 2.2: Characteristic comparison between aerobic and anaerobic systems Feature Organic efficiency

Aerobic removal High

Anaerobic High

Effluent quality

Excellent

Moderate to poor

Organic loading rate

Moderate

High

Sludge production

High

Low

Nutrient requirement

High

Low

Alkalinity requirement

Low

High for certain industrial waste

Energy requirement

High

Low to moderate

Temperature sensitivity

Low

High

Start up time

2–4 weeks

2-4 month

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Odor

Less opportunity for odors

Bioenergy and nutrient No recovery Mode of treatment

Potential odor problems Yes

Total (depending on feedstock Essentially pretreatment characteristics)

2.2.2.1 Advantages of Aerobic Digesters:  Fewer operational problems. 

Less daily maintenance.



Lower BOD Concentrations in supernatant liquor.



Lower capital costs.

2.2.2.2 Disadvantages of Aerobic Digesters:  Higher energy requirement (lot of aeration and mixing required). 

No methane produced.

 Digested sludge has lower solid content, thus volume of sludge to be dewatered is much larger. 2.2.2.3 Advantages of Anaerobic Digesters:  Methane recovery of microbial biomass produced in aerobic growth (biogas), can be used as alternate fuel source.  Reduces production of landfill gas which when broken down aerobically releases methane into atmosphere. 

Sludge occupies less volume, easier to dry.



Lower operating costs.

2.2.2.4 Disadvantages of Anaerobic Digesters:  Accumulation of heavy metals and contaminants in sludge 

Narrow temperature control.



Longer start up time.

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2.2.3 The Aerobic Membrane Bioreactor Over the last two decades the technology of membrane bioreactors (MBRs) has reached a significant market share in wastewater treatment and it is expected to grow at a compound annual growth rate (CAGR) of 13.2%, higher than that of other advanced technologies and other membrane processes, increasing its market value from $ 337 million in 2010 to 627 million in 2015 (BCC, 2011). Aerobic MBRs represent an important technical option for wastewater reuse, being very compact and efficient systems for separating suspended and colloidal matter, which are able to achieve the highest effluent quality standards for disinfection and clarification. The main limitation for their widespread application is their high energy demand – between 0.45 and 0.65 kWh/m3 for the highest optimum operation from a demonstration plant, according to recent studies (Garcés et al., 2007; Tao et al., 2009). Table 2.3: Advantages and Disadvantages of the MBR Advantages of MBR 1. There is complete biomass retention in the aerobic reactor, which decouples the sludge retention time (SRT) from the hydraulic retention time (HRT), allowing biomass concentrations to increase in the reaction basin, thus facilitating relatively smaller reactors or/and higher organic loading rates (ORL) 2. In addition, the process is more compact than a conventional activated sludge process (CAS), removing 3 individual processes of the conventional scheme and the feed wastewater only needs to be screened (1-3 mm) just prior to removal of larger solids that could damage the membranes

Disadvantages of MBR 1. The implantation is limited by its high costs, both capital and operating expenditure (CAPEX and OPEX), mainly due to membrane installation and replacement and high energy demand. This high energy demand in comparison with a CAS, is closely associated with strategies for avoiding/mitigating membrane fouling (70% of the total energy demand for MBR) (Verrech et al., 2008; Verrech et al., 2010) 2. Fouling is the restriction, occlusion or blocking of membrane pores or cake building by solids accumulation on the membrane surface during operation which leads to membrane permeability loss.

2.2.3.1 Pre-treatment Membranes are very sensitive to damage with coarse solids such as plastics, leaves, rags and fine particles like hair from wastewater. In fact, a lack of good pretreatment/screening has been recognized as a key technical problem of MBR operation (Santos and Judd, 2010a). For this reason fine screening is always required for protecting the membranes. Typically, screens with openings range between 1 mm (HF Page 20 of 134

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modules) to 3 mm (FS modules) are common in most facilities. However, data reported by Frechen et al. (2007) for 19 MBR. 2.2.3.2 Membrane fouling control and cleaning It is generally accepted that the optimal operation of an MBR depends on understanding membrane fouling (Judd, 2007). Abatement of fouling leads to elevated energy demands and has become the main contribution to OPEX (Verrech et al., 2008). In addition, uncertainty associated with this phenomenon has led to conservative plant designs where the supplied energy is so far to be optimized. Traditional strategies for fouling mitigation such as air sparging, physical cleaning techniques (i.e. back flushing and relaxation) and chemical maintenance cleaning have been incorporated in most MBR designs as a standard operating strategy to limit fouling. Air sparging, expressed as specific aeration demand SADm, takes a typical value for full-scale facilities between 0.30 Nm3/h m2 (FS configuration) to 0.57 Nm3/h m2 (HF configuration). Relaxation and back flushing (only for HF) are commonly applied for 30–130 seconds every 10–25 min of filtration (Judd, 2010). Frequent maintenance cleanings (every 2–7 d) are also applied to maintain membrane permeability. However, these pre-set fixed values of key parameters, based on general background or the recommendations of membrane suppliers, lead to under-optimized systems and results in loss of permeate and high energy demand. Recently, several authors have proposed a feedback control system for finding optimal operating conditions. For example, Smith et al. (2006) have successfully validated a control system for back flush initiation by permeability monitoring. This system automatically adjusts the back flushing frequency as a function of the membrane fouling, which results in a reduction of up to 40% in the back flushing water required. Ferrero et al. (2011) have used a control system at semi-industrial pilot scale trials based on monitoring membrane permeability, which achieved an energy saving between 7 to 21% with respect to minimum aeration recommended by membrane suppliers. 2.2.3.3 Sludge retention time (SRT) and biomass concentration SRT contributes to a distinct treatment performance and membrane filtration, and therefore, to system economics. Specifically, these parameters act on biomass concentration (MLSS), generation of soluble microbial products (SMP) and oxygen transfer efficiency. Increasing the SRT increases the sludge solids concentration and therefore, reduces bioreactor volume required. Furthermore, because of the low growth rates of some microorganisms (specifically nitrifying bacteria), a longer SRT will achieve a better treatment performance, as well as generating less sludge. In addition, it has been reported that high values of SRT can increase membrane permeability by Page 21 of 134

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decreasing SMP production (Trussel et al., 2006). Conversely, high solids concentration results in a higher viscosity of the microbial suspension (Rosenberger et al., 2002b), as a consequence, higher concentrations decrease air sparging efficiency and oxygen transfer rate to the microorganisms, resulting in a higher energy demand as well as increasing membrane fouling and the risk of membrane clogging. Given all of these factors, for economical reasons, most full-scale facilities are designed for MLSS range of 8-12 g/l and SRT range of 10-20 d (Asano et al., 2006; Judd, 2010). 2.2.3.4 Application and Cost Analysis of a Membrane Bioreactor Table 2.4: Comparison of the main MBR systems (Source: Yang et al., 2006)

Pore size (μm) Material Module size (m2)

1538 FS Vertical immersion 0.4 Chlorinated PE 0.8

Mitsubishi-Rayon (Japan) 374 HF Horizontal immersion 0.1/0.4 PE 105

Cleaning method

Relax

Relax

Cleaning frequency(min/min)

1/60

2/12

0.5/15

Recovery method

Chlorine backwash

Chlorine backwash

Chemical soak

Kubota (Japan) Number of installations Membrane Configuration

Zenon (Canada) 374 HF Vertical immersion 0.04 PVDF 31.6 Backpulse relax

and

Energy usage for membrane aeration is a significant operating cost for any membrane bioreactor facility (Yoon et al., 2004) calculated the total variable operational cost of MBR by summing the decreasing sludge-treatment cost and increasing aeration cost(See fig 2). Since minimized sludge production implies maximized aeration cost, and vice versa, they considered the existence of an optimum point between these two extreme cases, where the total operational cost is minimized. They concluded that for reasonable ranges of HRT and MLSS sludge treatment cost overwhelms aeration cost, so the most adequate strategy for MBR cost reduction would be maintenance of low sludge production conditions.

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Fig 1 Aeration demand for biodegradation of organic matters as a function of target MLSS and HRT. Flow rate and COD of influent were 1000m3 day–1 and 400 mg/L, respectively (Source: from Yoon SH et al., 2004)

Despite its relative youth, MBR technology has developed over a decade to a mature product available for all sizes of application, in domestic, municipal, or industrial sector. Further improvement of the process will increase its cost-effectiveness and MBR technology is expected to play a key role for wastewater treatment in the next years, in Europe as well as worldwide. To date, European countries with the highest number of full-scale MBR plants are England, France, Germany, Belgium, and the Netherlands. MBR markets are expected to open in other countries as well: in dry southern states like Spain, Greece, and Italy, due to their water shortages, and in Central and Eastern European countries (such as Hungary, Poland, Bulgaria, etc.) that will be obligated to develop their wastewater treatment technologies and adapt them to the standards and environmental legislation of the European Union.

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Fig 2 Relative cost decrease of Kubota membranes and MBR systems (Source: Kennedy et al., 2005)

2.3 Literature review for tertiary treatment Tertiary treatment consists of a balancing tank which controls the flow of wastewater going to the filtration unit where solids that have not been removed in the FST are removed. Before the final disposal of wastewater, it should be treated for pathogen removal thus the need for disinfection. The main reason for tertiary treatment is to meet the requirements of the standards of the environment for irrigation. Classifying advanced wastewater treatment is to differentiate on the basis of desired treatment goals. Advanced wastewater treatment is used for: Additional organic and suspended solids removal, Removal of nitrogenous oxygen demand (NOD), Nutrient removal and Removal of toxic materials (Spellman et al. 2012).

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2.3.1 Filtration Filtration is a physical and usually the final step in the solids removal process in a plant with no advanced treatment. Filters like screens are used to remove about 99.5 percent of solids which comprise mainly floc and associated minerals but of a more particulate size, some BOD, nutrients, heavy metals and some coliforms in water. . There are four mechanisms which form part of the filtration process namely straining, adsorption, biological action, and absorption. Each filter is equipped with a sequential automatic wash system known as filter backwash. A sensor will detect the level of water after filtration. After which a backwash pump takes part of the filtered water and recirculates it through the media and porous plate. This washes the filter media and solids are aspired by a pump which goes to a backwash tank. 2.3.1.1 Types of filter media There are three types of filter media: granular media which includes sand, anthracite, granular activated carbon, garnet, ilmenite and gravel chosen for their particular grain size and specific density, micro screens which are two dimensional and are mainly metal screens, wire cloth, metal fiber, natural fiber or fabric, synthetic fiber or fabric, paper, plastic, fiberglass chosen for their specific opening size and other medias such as diatomaceous earth, synthetic fuzzy balls and resin beads. 2.3.1.2 Types of filters The main types of filters are gravity and pressure filters.

In gravity filters, the

wastewater to be treated flows through the filter from top to bottom. The wastewater is evenly distributed to the filters through an inlet distribution system. The water is allowed to pass through a porous plate and a filter media like sand whereby solid particles are trapped. . Gravity filters are divided into rapid and slow type. Rapid gravity filters are 20 to 30 times more rapid than slow filters so that the former require less space.

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2.3.1.3 The Sand filter Sand filters works in an aerobic condition. As the wastewater percolates slowly through the filter media, natural physical processes such as straining and sedimentation, biological processes, and chemical processes such as chemical adsorption of pollutants onto media surfaces plays a finite role in the removal of some chemical constituents (e.g., phosphorus) combine to provide the treatment. In the first 6 to 12 inches of the filter surface, most of the treatment occurs. Biomat is a thick layer which forms part of the filter ecosystem. This layer is formed near the surface of the filter. This layer contains bacteria which consume particles in the wastewater. In turn, protozoa feed on the bacteria and help prevent the biomat from becoming so dense that it clogs the filter. This balance between the various life forms and the physical and chemical processes that take place in the sand filter results is extremely efficient in wastewater treatment requiring minimal operation and maintenance. Eventually, the biomat becomes clogged, and the top layer of sand needs to be raked or removed as part of regular filter maintenance. Sand filters cost less to construct in rural areas than centralized treatment systems. 



They are energy-efficient

They have relatively low maintenance requirements but should be serviced by trained technicians. 



They can provide high quality effluent.



Sand filters may enable development in difficult sites.



They can remedy an existing malfunctioning system.



They can be a good option for homes in environmentally sensitive areas.

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Table 2.5: Sand Filter Maintenance Item

Requirement

Pre-treatment

Depends on process (septic tank, aerobic unit, etc.)

Pumps controls

Check every 6 months.

and

Timer sequence

Check and adjust every 6 months.

Appurtenances

Check every 6 months.

Raking

Check every 6 months. If drainage time between doses has increased significantly, rake top 3 in. (for surface filters only).

Replacement

Skim media when heavy incrustations occur. Add new media when depth falls below 24 in. Rest when ponded continuously. Replace top 2-3 in. media when surface ponds more than 12 in. deep. Rest while alternate unit in operation (60 days).

Other

Weed as required, Maintain distribution device as required, Protect against ice sheeting on the surface of the filter, Check high water alarm (for single pass sand filters only).

The filter design and local costs for labour and materials are dependent on exact costs for sand filter construction, operation, and maintenance. Costs for pre-treatment and additional treatment and disposal also need to be factored in when calculating the entire system costs. The filters can be constructed or assembled onsite using local labour and materials as construction of the sand filter units themselves usually is economical. Land and media costs are two most significant factors that affect the cost of sand filter treatment. In areas where media is expensive or needs to be hauled a long distance, costs are much higher. Page 27 of 134

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Operating costs include electricity used by the pump, and the cost for inspections and maintenance. 2.3.2 Disinfection Disinfection is normally the last step before final disposal. This practice removes and reduces about 99.5 percent of microorganisms and waterborne pathogenic organisms that would otherwise be transmitted to human beings. Protection of public water supplies, fish and aquatic life, irrigation and agricultural waters is ensured by disinfection. The most common types of disinfection are UV radiation, halogens compounds like chlorine, chlorine dioxide, bromine, sodium hypochlorite and use of ozone. 2.3.2.1 UV radiation This method is a highly effective means of disinfection but has no residual capacity which means re-growth of microorganisms is possible. The efficiency of UV radiation depends on the quality and temperature of water, turbidity, flow rate and also UV transmittance.

UV disinfection is now becoming an economically competitive

alternative to chlorination and ozonation and does not generate toxic or genotoxic by products. It is a physical process that instantaneously neutralizes microorganisms as they pass by ultraviolet lamps submerged in the effluent. 2.3.2.2 Ozone Air or oxygen-generated ozone is a highly effective disinfectant.

It is

normally generated on-site by electrical discharge and is thus energy intensive. Ozone has the same effects as UV radiation means it has no residual effects. 2.3.2.3 Chlorination Chlorine is the most widely used disinfectant in water and wastewater treatment. It is used to destroy pathogens, control nuisance microorganisms, and for oxidation.. It is, however, a highly toxic substance and recently concerns have been raised over handling practices and possible residual effects of chlorination.

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2.4 Literature review on sludge thickening, digestion and gas handling Sewage sludge consists of organic and inorganic solids present in the raw wastewater that were removed in the primary settling tank, as well as the organic solids generated during the activated sludge process, removed in the final sedimentation tank. Typically in the form of a liquid or semisolid, the sludge contains 0.25 to 12 percent solids by weight, depending on the treatment chosen for the wastewater (Metcalf and Eddy, 2003). Sludge handling, treatment and disposal are quite complex as a result of the unpleasant constituents present, varying with the type of wastewater and the process involved. 2.4.1 Thickening of sludge Thickening is a process of increasing the solids content of sludge by removing a portion of its liquid content, decreasing its volume, and also to increase the efficiency and decrease the costs of subsequent sludge-processing steps (Ghebremichael, 2004. The thickened sludge remains in the fluid state. Thickening of WAS is important owing to its high volume and low solids concentration. A number of technologies are available to achieve thickening namely: gravity thickener, gravity belt, rotary drum thickener. Described below, dissolved air flotation and centrifugation (Ghebremichael and Hultman, 2004). 2.4.1.1 Gravity thickener The mechanism of a gravity thickener is similar in design to primary clarifier; sludge is fed to a centre feed well and is allowed to settle and compact before being withdrawn from the bottom of the basin. The sludge scraping mechanism is often provided with vertical pickets, which gently agitate the sludge, contributing to its densification by releasing trapped gas and waters. The thickened sludge is pumped to digesters or dewatering equipment, while the supernatant is returned to the head works of the treatment plant, or to the primary settling tank (Metcalf & Eddy, 2003) 2.4.1.2 Flotation Dissolved air flotation is used for thickening of sludge that originates from suspended growth biological treatment processes. It involves the introduction of air into a sludge solution that is being held at an elevated pressure. When the solution is depressurized,

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the dissolved air is released as finely divided bubbles which attach to the suspended solids. The floating solids are collected by a skimming mechanism similar to a scum skimming system. (Metcalf & Eddy, 2003) 2.4.1.3 Centrifugation Centrifuges are used to thicken and dewater waste activated sludges. They involve the settling of sludge particles under the influence of centrifugal forces. Two basic types of centrifuges are the solid bowl and the imperforate basket. 2.4.1.4 Gravity belt Gravity belt thickeners consist of a gravity belt that moves over rollers driven by a variable-speed drive unit. It is used for the thickening of raw and digested sludges after conditioning by the addition of polymer. The conditioned sludge is fed into a box, which distributes it evenly across the width of the moving belt. (P.A.Vesilind, 2003) 2.4.2 Sludge disposal 2.4.2.1 For the production of electricity It is the process of burning dewatered sludge with a moisture content of fifty percent at existing incineration stations as a fuel for the production of electricity. This way of disposal can be used if more profit is made with incineration rather than soil conditioner due to reduce waste volume as ash and electricity production. 2.4.2.2 Sludge as fertilizer It is a process of converting sludge into NPK granulated fertilizer in which the water content is first reduced to fifty percent through a filtration process followed by an exothermic reaction to evaporate the remaining water. Normal dewatered sludge can achieve a moisture content of fifty percent but for fertilizing purposes, more water removal is required. 2.4.3 Sludge dewatering This is a mechanical unit operation where there is a volume reduction greater than that attained by the thickening process; that is, from about 4 – 20 percent to that of one-fifth, resulting in a non-fluid material known as sludge cake. The different technologies Page 30 of 134

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available are: 1) vacuum filtration, 2) centrifuge, 3) belt filter press, 4) filter press, 5) drying beds and 6) lagoons. i.Vacuum filtration In this method, vacuum is applied downstream to the filter media, causing the liquid phase to move through the porous media, thus the separation of solid particles from water. The vacuum filter consists of a horizontal cylindrical drum that rotates which is partially submerged in a mat of conditioned sludge and there is a porous media that is covered on the surface of the drum.

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CHAPTER 3: PROCESS CONSIDERATION The wastewater treatment plant designs require an efficient working capacity with the lower energy consumption at the lower possible cost, thus the need to carefully choose the different treatment methods and equipments with regard to the benefits they have to offer, their inconveniences but also their costs in terms of capital investment and maintenance. The choices made are not necessarily the best but practical under certain circumstances. Once the construction of the plant is completed changes in the design or any other alteration in the choice of equipments or methods cannot be entertained, so the selection of treatment process and equipments must take into account the amount of wastewater that will be generated in the design period. Below is the different treatment methods and equipments used involved for each part of the treatment plant. 3.1

Process consideration for preliminary treatment

3.1.1 Screenings Table 3.1: Manually versus mechanically cleaned screen bar. Manually cleaned screen bar

Mechanically cleaned screen bar

Inexpensive

Expensive

Labor intensive

Low labor/ automatic

Inefficient

Efficient

Usually found in small plants

Used in both small and large plants

Difficult to collect screenings

Screenings easily gathered

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Require

frequent

raking

to

avoid Less prone to clogging

clogging Low maintenance costs

High maintenance costs

The mechanically cleaned screen bar is chosen because even though it is expensive, it is the one that involves less labor as well as is more efficient in terms of removal of screenings. It has been used at the head works of most medium to large wastewater treatment plants for the past fifty or more years. The fact that the sewage plant would be operated continuously, automated units would be favorable for us. Table 3.2: Design criteria Item

Manually cleaned

Mechanically cleaned

Bar size: Width (mm)

5 - 15

5 - 15

Depth (mm)

25 - 80

25 - 80

Bar spacing (mm)

20 - 50

5 - 80

Angle of inclination

45o – 60o

18o – 90o

Approach velocity (m/s)

0.3 – 0.6

0.6 – 1.0

The medium screening mechanical screen bar that has been chosen has bar spacing of 25mm. Two medium screening will be used; of which one will be continuously in use and the remaining one is kept as spare in case of breakdown. Also, the medium screen bars will be followed by fine mechanically cleaned screen bars of bar spacing 6mm. Two fine screening will be used; one of them to be continuously in use while the remaining one is kept as spare. 3.1.2 Balancing/equalizing tank Balancing tanks can of two types: in-line equalizing and off-line equalizing tanks, in which the in-line tank is chosen since off-line tanks require an overflow structure, hence Page 33 of 134

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the need for more space and an increase in construction costs. It provides sufficient storage volume to permit a non-uniform flow of waste water to be collected, mixed and pumped forward to a treatment system at a uniform rate. For the design, the flow entering the balancing tank is by gravity. 3.2

Process consideration for secondary treatment

3.2.1 Circular primary settling tank Two circular or upward-flow tanks, connected in parallel, are being chosen for the purpose of this design. Wastewater enters at the center and rises vertically to be drawn off by flowing over a peripheral weir situated at the surface. Mechanical scrapers collect the sludge, concentrating it towards the center, from where it is removed for further treatment. Circular tank diameters range from 25 to 150 feet. In a circular tank, the feed often flows through the center column and then outwards through ports in the column. The water then flows in a radial direction to a peripheral weir where a baffle is not required (Metcalf and Eddy, 2003). Table 3.3: Comparative analysis of rectangular and circular clarifier Shape

Rectangular

Circular

Retention time/h

4-8

2-3

Dead spaces

Yes

No

Short circuiting

No

Yes

Energy consumption

Low

High

Sludge collection

Complicated

Simple

High

Low

Maintenance and operation costs

3.2.2 Secondary treatment methods After well analyzing all the data obtained (see literature review), we decided to use an aerobic treatment system mainly because of the following reasons: 

Minimum odor when properly loaded and maintained,

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 Large biochemical oxygen demand (BOD) removals providing a good quality effluent,  High rate treatment allowing smaller scale systems, e.g., less land required;  The final discharge may contain dissolved oxygen which reduces the immediate oxygen demand on a receiving water,  The aerobic environment eliminates many pathogens present in agricultural wastes. Compared to anaerobic treatment systems, the most striking advantages of high-rate aerobic wastewater treatment are: 1. A higher level of treatment which may make a smaller drain field possible, 2. May work when the soil or ground water level will not support a standard septic system, 3. Help reduce environmental impacts, 4. Helps to protect valuable water resources, 5. Higher efficiency regarding removal of nutrient/COD/BOD, 6. Lower capital cost and rapid recovery of the cost due to high efficiency and good quality of product, 7. It provides much more sludge than anaerobic systems which is important to our design because it includes sludge treatment with biogas removal, By comparison between anaerobic and aerobic systems, anaerobic digestion has more benefits for consuming less energy to achieve good BOD and COD removal, for the production of energy as methane which can be used for heating purposes and less sludge production. However, anaerobic digestion works best for wastewater with COD above 4000 mg/L, which is not the case with the design COD which is below 1000mg/L, thus aerobic system is chosen. Furthermore, solids that form anaerobically do not flocculate well, also its effluent needs further treatment which is not economical and such systems are susceptible to small changes in temperature and pH, thus the need to control these parameters. There is nutrients removal like nitrogen and phosphorus and the possibility of yielding methane from waste activated sludge in aerobic systems making it a better choice than anaerobic systems (Metcalf and Eddy, 2003). However, the activated sludge process differs in two ways: i.Aerobic tank with mechanical aerators ii.Aerobic tank with diffusers (Diffused air systems)

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3.2.3 Aeration equipments Diffusers are preferred over mechanical aerators since they have low maintenance cost and high oxygen efficiency, thus more efficient in terms of energy consumption. They also have the inconvenience of being clog, but still this can be overcome by a sudden increase in air pressure. Fine bubble diffusers, specially ceramic diffusers due to its resistivity to chemical and biological fouling, are chosen for its high oxygen transfer and aeration efficiencies; therefore satisfying the demand of oxygen. 3.3

Process consideration for tertiary treatment

3.3.1 Filtration processes TSS and TDS of secondary treated sewage (from secondary clarifier) will still be at a high concentration so that further processes will be required. Filtration is one of the most cost effective processes for removing TSS since the energy requirement is low and bears no complex reactions. TSS removal methods such as reverse osmosis chemical precipitation, electrodialysis, distillation and micro/ultrafiltration are overlooked due to their high energy requirement and investment as well as operating costs. Media filtration and surface filtration are hence compared. There are various types of filter media used such as sand, coal, dual media for example sand and coal, or mixed media (coal, sand and garnet). 3.3.1.1 Sand Filtration Slow sand filters proved to be uneconomical when compared to rapid sand filters and micro screen where the efficiency reached were about 60 % for suspended solids (TSS) and about 40 % for BOD. Operation of a rapid sand filter consists of regular backwashing. The period between backwashes depends on the quality of the water being filtered. The purpose of backwashing is to remove the suspended material that has been deposited in the filter bed during the filtration cycle. Periodic repacking of the filter bed may be required at infrequent intervals to ensure efficient operation

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(Negulescu, 2011). Rapid sand filter are preferred compared to slow filter as back washing occurs rapidly and it has a high filtration rate of about 150 to 200 million gallons of water per acre per day. 3.4

Disinfection

The most frequent disinfectants are chlorine, ozone and UV rays (See literature review). UV rays, being highly effective for pathogen sterilization, are also very expensive. Ozone is highly toxic but not readily available. Considering the effects and inconveniences of UV rays and ozone, Chlorination is chosen, even though it is somehow toxic as it increases the total dissolved solids in the effluent, it is the less expensive one (Hung et al. 2012). It must be noted that due to the selection of the membrane bioreactor in our system, which has a very high initial cost, we need to minimize the cost for our other units. 3.5

Process consideration for sludge thickening and digestion

3.5.1 Thickening primary and waste activated sludge Gravity thickener is the most common type of thickener which can act as a thickener and achieve blending, important to produce a uniform mixture for subsequent processes at the same; thereby eliminating the need for a separate blend tank. This type of thickener is chosen since the solids concentration of 4 – 6 % achieved by a gravity thickener that is required by the high rate digester. It requires the minimum power consumption compared to dissolved air flotation and centrifugal thickener. It also has the least operation skill requirement and operating costs. Conditioning chemicals and polymer are typically not needed as is the case for gravity belt thickener and rotary drum thickener. The pH can also be adjusted in this type of thickener which can provide space for storage (Metcalf and Eddy, 2003). 3.5.2 Stabilization Anaerobic digestion is the process chosen for the stabilization of sludge since it is a closed system which eliminates odors. Even though it involves a high capital costs, the recovery of methane and thus the production of energy offset the high operational costs Page 37 of 134

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of the energy intensive aerobic process. This energy can be used for the plant’s own use decreasing the dependency on external sources. The digested sludge can then be used as fertilizers or a fuel source. Although a large detention time is required and therefore a large reactor, the positive outcome is the destruction of pathogens and at the same time reducing about 50 to 65 % of the total solids mass. Methane captured can be piped to boilers to generate electricity and produce hot water which can be used for heating purposes. 3.6 Process Consideration for process in anaerobic digestion High rate digestion is deemed suitable for the process as it requires less space and short detention time of 10 – 30 days compared to low rate digestion. Mesophilic digestion is chosen since the operating temperature for these organisms is 30 – 38 0C; thus reducing less energy requirement compared to the thermophilic digestion where fluctuations in temperature can cause process instability and a risk of higher odor potential. Thermophilic and 2 stage digestion process produce a poorer quality supernatant which contains dissolved materials; requiring further special treatment. As for the 2-stage digestion process, it is still under pilot studies and will be more energy intensive; requiring 2 mixing devices for both tanks. 3.7

Process consideration for sludge conditioning, dewatering and disposal

3.7.1 Type of sludge conditioning Chemical conditioning is preferred over physical methods where the most common method is thermal conditioning. Thermal treatment of sludge has a high capital and operation costs owing to its mechanical complexity and corrosion resistant material and its requirements in terms of skilled personal. Production of odorous off-gases, which are not environmental and societal-friendly, can be an inconvenience as it needs to be collected and treated before release. Chemical conditioning, on the other hand, is economical, produces increased yields and has a greater flexibility.

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3.7.2 Type of chemical conditioning aids Despite, the high costs of polyelectrolyte, an organic polymers; it is chosen over inorganic chemical aids for their easy handling and less space requirements for the feed system.

Besides, inorganic chemicals can severely corrode dewatering equipment,

increasing the costs of operational and maintenance in the storing, handling. They also produce an extra kilogram of sludge for every kilogram of inorganic aid used, increasing the disposal costs. Organic polymers, in contrast, has the advantage of reducing conditioning costs and are much safer for use and has a dosage feed rate of 3.632 kilogram per ton of dry solids; thereby compensating for its high cost, around $ 19.75 per ton dry polymer. 3.7.3 Dewatering The aim of the dewatering equipment is to achieve fifty percent by dry weight solids content and centrifuge equipment can produce the required percent. In addition, its capital cost is low when compared to other methods, there is minimization of odor as it is an enclosed unit and requires little supervision; thus continuous solid bowl centrifuge is chosen since it is more suited for high solid content and used for medium and large plants compared to imperforated basket centrifuge. Filter press has many inconveniences such as mechanical complexity; high chemical and labor costs and limitation on filter cloth life while vacuum filtration has low operational costs but higher initial costs and land requirements. 3.8 Sludge disposal The proposed method for disposing of dewatered sludge is as a fuel source in power stations since the electricity demand in on the rise in Mauritius and it will help to move towards a green island and reduce our dependency on exported fuel sources. Sludge as compost is a good option but requires large land area and causes odor problems. Digested sludge usually is too low in nutrients such that its use as fertilizers has some drawbacks. The other methods such as pyrolysis and gasification are complex disposal options on pilot basis, requiring highly trained and experienced operators. As for dumping in landfills, it is not very environment friendly and there is the risk of odor problems with the leachate formed.

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Chapter 4:MASS BALANCE Table 4.1: Summary of mass balance for sewage flow over the system Bar Influent

screens

to

to

Stream

Bar screens

Oil-water separator

Oil-water separator

Eq. tank to Primary

MBR

to

Primary

Clarifiers

sand

clarifiers

to MBR

filter

Eq. tank

to

Sand filter to disinfectio n unit

70,000

70,000

70,000.0

70,000.0

69,789

69,778

69,673.5

68,880

68,880

68,880.0

69,091

38,000

1221

1221

21,490

21,490

12,105

21,491

9,671

2800

1148

TSS (kg/d) 19,950

1596.7

10,376.6

4,988

4,988

1364.6

478

1,750

1,750

1,750

1,750

1,750

140.0

140

1,400

1,400

1,400

1,400

1,400

623.1

623

700

700

700

700

700

24.5

24.5

420,000

420,000

420,000

420,000.0

420,000

420,000

420,000

114,170

114,170

114,170

98,020

56,509

6,174 3634.5 Page 40 of 134

Q (m3/d) COD (kg/d) BOD (kg/d)

NH3-N (kg/d) NO3—N (kg/d) P (kg/d) Coliforms (MPN 100mL–1) Total (kg/d)

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Table 4.1: Summary of mass balance for sewage flow over the system [continued from table 4.1] Disinfection unit

Thickener to Sludge Digester

Sludge Digester to Dewatering Unit

Sludge output

to Irrigation authority

to thickener

Primary clarifier to Thickener

Q (m3/d)

69,777.70

1,831

211

532.3

505.7

69,777.70

COD (kg/d)

1,221.40

9,945

31,091

41,036

4,082.5

1,221.40

BOD (kg/d)

1,120.00

20,852

11,820

23,286

2,328.6

1,120.00

TSS (kg/d)

122.80

13,140

14,963

27,810

9,733.5

122.80

NH3-N (kg/d)

140.00

490

490

490

135.45

140.00

NO3—N (kg/d)

623.12

903

903

903

490

623.12

P (kg/d)

24.50

655

655

655

589.5

24.50

Coliforms (MPN 100mL–1)

420

-

-

-

-

420

Total (kg/d)

3,252

45,985

59,922

94180

16770

3,251.82

Streams

MBR

Note: Refer to appendix 1 for detailed calculations

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MASS BALANCE FOR BIOGAS Table 4.2: Mass balance for biogas Parameters

Flow rate (m3/d)

Total biogas

7,566.7

CH4

5069.7

CO2

2421.33

H2O

52.8065

H2S

0.1604

Note: Refer to appendix 1 for more detail about the biogas balances

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CHAPTER 5: PREIMINARY DESIGN [SIZING] 5.1

Sizing of the preliminary treatment unit

5.1.1 The Oil-Water Separator Vertical Velocity

0.18083 cm/s

Horizontal flow velocity

1.5 cm/s

minimum vertical cross-sectional area

54 m2

number of channels

2

Width of channel

8m

Depth of channel

3.38 m

Length of channel

3.33 m

5.1.2 Bar Screen Depth of chamber, d

1.50 m

Total width of opening at the rack, w

0.6m

Clear bar spacing

50 mm

Number of bars

12

Width of bar

10 mm

Thickness of bar

50 mm

Width, W of the chamber

0.72 m

Height of rack( allowing 0.6 m of freeboard)

2.0 m

Angle of inclination of the bars to the horizontal, θ

80˚

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5.1.3 Fine screen Depth of chamber, d

2m

Total width of opening at the rack, w 0.45 m

Clear bar spacing

9.5 mm

Number of bars

48

Width of bar

10 mm

Thickness of bar

50 mm

Width, W of the chamber

0.93 m

Height of rack( allowing 0.6 m of 2.63 m freeboard) Angle of inclination of the bars to 80˚ the horizontal, θ Material of construction

Mainly of stainless steel for protection against corrosion and for higher lifetime

5.1.4 Equalization Tank Number of Equalization tank

3

Flow rate per tank, Q (m3/h)

967.87 m3/h

Retention time (hours)

3 hours

Area (m2)

556.42 m2

Volume (m3)

3,338.52 m3

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Length (m)

33.36 m

Breadth (m)

16.68 m

Freeboard (m)

0.5 m

Inlet velocity (m/h)

1.73 m/h

Material of construction

Concrete

5.2

Sizing of the secondary treatment units

5.2.1 Primary Clarifier Number of tank

2 clarifiers in parallel

Flow rate per tank, Q (m3/h)

1458.3

Retention time (hours)

2.5

Area (m2)

1000

Volume (m3)

3646

Tank Diameter (m)

36

Height (m)

3.65

Material of construction

Concrete

5.2.2 The Membrane Bioreactor Shape

Rectangular

Total volume

14934 m3

Dimensions (L x B x H)

42.5m × 88 m × 4.4 m

Number Of channels

20 channels

Material

reinforced concrete

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Water temperature

28 0C brought to 20 0C

Liquid depth

7m Fine bubble ceramic diffuser

Aeration system

Oxygen demand 3 1565.40 𝑚 𝑜𝑓 𝑂2⁄𝑑𝑎𝑦

Aeration period

=

5.12 ℎ𝑜𝑢𝑟𝑠

Diffuser submergence

7m

Oxygen transfer efficiency (OTE)

35%

Aeration configuration

Covering the floor completely

Air supply

Using a centrifugal blower feeding 7454.24 𝑚3 𝑜𝑓 𝑎𝑖𝑟⁄𝑑𝑎𝑦

SRT

10 days

BOD vol. loading F/M

5.3

0.938

𝐾𝑔 𝑚3 . 𝑑𝑎𝑦

0.3

Sizing of the tertiary treatment units

5.3.1 Sand Filter Number of sand filter

1

Flow rate/ sand filter, Q (m3/h)

69,674 m3/day

Area (m2)

87.5 m2

Breadth (m)

9.35 m

Length (m)

9.35 m

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Volume (m3)

305.98 m3

Under drain depth (m)

0.8 m

Filtration duration (h)

20 hours (per day) Reinforced concrete is used

Material of construction

for the tank with the filter media being sand.

5.3.2 Chlorination Number of tank

4

Flow rate of NaOCl /channel, Q

0.00564 m3/s

Volume of 1 tank

362.9 m3

Volume of 4 tanks

1,451.5 m3

Depth

1.8 m

Breadth

2.2m

Cross sectional area

3.96 m2

Length

91.6 m

Length of each channel

30.5 m

5.3.3 Thickener Number of thickeners

2

Volume (m3)

956.12 m3

Surface area (m2)

152.59m2

Diameter (m)

13.94 m

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Side water depth (m)

5.8 m

Depth at central hopper (m)

7.19 m

Thickening period (days)

3.48 days

Total flow (m3/d)

548.94 m3/d Reinforced concrete for tank while inner

Material for construction

equipments

are

of

stainless

steel,

resistant to corrosion

5.3.4 Sludge Digester Number of digesters

2

Volume (m3)

2019.6 m3

Area (m2)

176.71m2

Diameter (m)

15 m

Side wall height (m)

13.22 m

Central hopper depth (m)

14.72 m

Amount of VSS in thickened sludge (Kg /d)

Material for construction

13511.96 Kg/d Reinforced concrete is used for the digesters while any other equipment is made up of stainless steel.

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5.3.5 Dewatering Tank FIRST TANK Number of tank

1

Surface area

1.85 m2

Volume designed

1.92 m x 0.96 m x 1m + 0.5m (freeboard)

Total solids flowrate

763 kg/hr

total amount of polymer used Material for construction

2.771 kg/hr Fiber glass is used. Inox stainless steel is chosen the stirrer.

MATURATION TANK Number of tank

1

Volume designed

1.85m3

Surface area

1.85 m2

Flow

rate

of

polymer

solution Material for construction

17.08 m3 /hr Fiber glass is used. Inox stainless steel is chosen the stirrer

Note: Detailed calculation can be obtained from appendix 3.

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CHAPTER 6:

ENERGY BALANCE

Infrastructure

Preliminary treatment

Primary treatment

Units using power

Total (kWh)/day

 Screen Bar (Fine + -72.0 Medium) 

Oil-water separator

-23.6

4 Scrappers in PST + 4 sludge pump + -5604.48

% of power total

the

(%)

0.25

14.37

8 dosing pump

Secondary treatment

Tertiary treatment

MBR tanks + clarifiers

-8241.11

12 centrifugal pumps + 1 -435.2 centrifugal blower Pumps

-25.6

Total

1.12

4.57

Thickening + 2 pumps+ 2 -2693.96 scrappers

6.90

2 pumps

17.89

-6978.16

Sludge handling & Digestion + 2 pumps + 2 -7758 conditioning draft tube

Building

21.13

19.89

2 Stirrers (polymer and -1725.3 ageing tank) + 2 pumps

4.42

Dewatering + 2 pumps

-6.356

0.02

Electricity

-2340

6

-35808

100

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CHP

+17185

48

Net electricity to be taken from CEB

18623

52

Energy is the measure of the use of electricity over time. It is measured in kilowatthours (kWh). One kWh is a kW used for 1 hour. The minus sign (-) indicates electricity consumption whereas the plus sign (+) refers to electricity production. Energy balances can be calculated theoretically, based on the running time and power consumption of the equipment. The calculations are shown in Appendix 2. CHAPTER 7: PRELIMINARY HAZOP The common hazards in a wastewater treatment plant include physical injuries, confined spaces, electric shock, explosive gas mixtures and noise. Most of the accidents and incidents occurring on the site are due to unsafe work practice or incorrect procedures and inadequate supervision. The HAZOP process is used to identify potential hazards and operational problems in terms of plant design and human error. 7.1 The oil-water separator Hazard Deviatio Parameters s n

Person al Safety

Toxic substances present in wastewater

High; Good State/ Bad state

Possible causes 1. Contact with the Wastewate r;

Consequenc es 1. Chemicals absorbed through skin when in contact with wastewater.

2. Needle stick injury when removing screenings from a bar screen

2. Disease can also enter the body through cuts and abrasion

Safeguard

Action

Provide hand gloves and stay away from the wastewate r

Assistance should be provided and immediate actions should be taken in case of Skin Contact

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Cleaning and Maintenan ce

7.2

Clean/ Unclean; Good State/ Bad state

Screen blockage or clogging

1. Efficiency Two- stage of screens screen in decreases; order to facilitate safe cleaning; to reduce the possibility of blockage; 2. Wastewate to allow r unable to proper pass through maintenan causing ce overflow;

1. Provide a sensor and an alarm to monitor the risks of blockage;

regular check up and maintenan ce should be done

Screening

Hazards

Obnoxious odors and vermin

Excessive screen clogging

Causes

Consequences Can result in health problems such as nausea, nerve irritations in respiratory tracts, Inappropriate mouth or eyes and may cause difficulties, or extended breathing storage of sneezing, swelling of nasal membranes, tearing of the eyes screenings etc.

Abnormal quantity of debris in the wastewater.

Preventive measures Provide correct storage, increase frequency of removal and disposal

Slows down or completely Spot the source of stops the uniform wastewater excessive debris and flow and this may stop the stop it. treatment process completely.

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Provide a coarser rack

Low velocity through the rack

Retune the timer cycle or fit a level override. Automatic rake action not frequents enough.

Excessive Low grit velocities in accumulation the channel Jammed raking mechanism will not reset

Obstruction still on the screen

Remove flow irregularities, reslope the floor, rake the channel and flush frequently. Eliminate obstruction

the

Moment setting too fine

Seek advice from manufacturer concerning setting arrangement

Screen not Broken chain, being raked cable or limit but motor is switch working

Inspect the rakes, switches and chains, replace them if needed.

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Rake not Defective functioningcontrol no detectable mechanism reasons

Check control circuits and motors and replace them is necessary.

7.3 Equalization Tank Hazard Deviatio Parameters s n

Risks of overflo Solid w due accumulati to on Blockag e

7.4

high

Possible causes Solids building up can cause septicity, growth of filaments, organic acids, gassing and sometime s even H2S generatio n.

Consequenc es This can mean safety issues, excess electricity costs, excess polymer or chemical consumption and excess solids handling costs.

Safeguar d Protective goggle, face shields, impervio us gloves and chemical boots should be worn.

Action Local exhaust ventilation is installed to remove airborne contaminant s.

Primary Clarifier

Parameters

Sludge Removal

Deviation

High/ Low

Possible causes Without a support at the bottom, the rake assembly swings around.

Consequenc es 1. the blades get damaged when it strikes the bottom violently; 2. the blades do not sweep the floor uniformly

Safeguard

Action

Provide a bottom steady bush which supports the central rotary shaft of the rake.

Regular maintenance should be done

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7.5 The Membrane Bioreactor Hazard Deviatio Parameters s n Safety Constructio High n and maintenanc e

Possible causes Safety rules and regulation s not properly abided

Consequence s Falling into the ponds causing injuries;

Safeguar d Hand railing provided at all places where there is potential of falling around all tanks and other places where falling height is greater than 1.5m

Action Emergenc y measures should be provided; provide with protective clothing and other personal protective equipmen t and chemical resistant clothing to avoid exposure of skin

Disease caused by infectious agents like protozoa, virus upon skin contact;

Use of life buoys and Safety jackets to get the person out of water; chronic Bridges poisoning by must inhalation

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7.6

Sand Filter Paramete Hazards rs

Risk of causing UV health radiation problem

Deviatio n

Possible causes Exposur e to UV radiation

High/lo w

Thickener Parameter Deviatio Hazards s n Major Gases High spills of such as liquor or methane cake and HsS sludge:

Consequence Safeguard s Serious health Wear problems personal protective equipmen t and Chemical resistant Clothing

Action 1. Limit the UV radiation;

2. Equipmen t must be designed with safe handling in mind

7.7

Risks of slippery surfaces

Possible causes Odor Spreadin g

Consequence s 1. Discomfort and psychological problems related to bad smells of the waste 2. Contain the spills within an earth bund and absorb the contained liquid using additional sand / earth

Safeguar d Safety mask are provided

Action 1. Limit the access to such places

2. Check valves and pipe works for leaks

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Spillage / Pollutio n

3. Earth contaminated with liquid sludge should be treated as such 4. For cake spills, dispose of cake to an open skip for removal offsite

Low height objects:

3. Respect the maintenanc e and servicing schedules 4. Safe systems of work and practice should be adopted 5. Clean-up personnel must shower and disinfect themselves before leaving the site

Injury on the head

7.8

Digester

Hazards

Parameters

Safety problems

Flammable gases

Deviati Possible on causes High Formati on of Flamma ble gases

Consequen ces Risks of fire or explosion

Safeguar d 1. Avoid being exposed to this unit for a long period of time;

Action 1. Obey all safety instructions concerning entry into confined spaces e.g. check atmosphere for oxygen or poisonous gases,

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2. Wear personal protective equipmen ts and Chemical resistant Clothing

2. use respiratory protection equipment if needed,

3. have a coworker stand guard in case of need for help Risks of Friction hearing damage from the operation of pumps Risks of cleaning clogging and maintenanc e (formation of scum layer in the settler)

Risks of Blockage of the nozzle at the inlet

cleaning and maintenanc e

Low/H Due to Damage to Seek Wearing of ear igh vibratio the ears medical protection n and help if equipments motors exposed too long Low/H Due to leads to igh presence high of fats effluent suspended solids

Remove scum layer from the container and regularly do the maintena nce Compl Due to uneven Regular ete/ hydrolys distribution maintena Incomp is of fats of waste nce lete over the should be reactor done system

Install a skimmer in the settling compartment

Arrange for a system to clean the reactor easily

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CHAPTER 8: WASTE TREATMENT As the treatment of wastewater is ongoing, there is generation of waste products that needs to be handled and disposed of in the most efficient, economical way to prevent any environmental harm. When waste is created, it gives priority to preparing it for reuse, then recycling, then recovery, and last of all disposal (Da Zhu,P U Asnani,2008).

8.1

PRELIMINARY TREATMENT

8.1.1 Oil-Water Separator The oil removed from the oil-water separator can be sent to an oil refinery industry. 8.1.2 Screen bars and grit chamber Screen bars and grit particles such as sand, clay are removed and are sent to the landfill since it is considered to be the most efficient and environmental friendly method. 8.1.3 Primary sedimentation tanks Primary sludge has to be treated and handled properly as it contains pathogens and can cause odor and health problems. It is sent to the sludge handling unit for further treatment such as anaerobic digestion. 8.2

SECONDARY TREATMENT

8.2.1 The MBR Tank Sludge from the MBR has more or less the same characteristics as primary sludge but the particles are fine and cause more odors. A large portion of it is recycled in the aeration basin. 8.3

TERTIARY TREATMENT

8.3.1 Sand filter Solids particles are trapped in the media during the filtration in the sand filter. These are further treated and safely disposed as a waste solid. These can be used in fertilizers or other by products. 8.4

SLUDGE TREATMENT

8.4.1 GRAVITY THICKENER  Supernatant from thickener:

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As the solid content of the sludge is increased, a small fraction of the liquid is removed. This liquid is overflowed from the thickener. It is returned to the inlet of the primary wastewater treatment plant for further treatment.



Scum on the surface of the thickener:

It is found on the surface of the thickener: caused by prolonged retention time in the thickener. It is pumped to digester immediately after removal for treatment since scum is full of pathogens that can be deactivated during digestion process. 8.4.2 FINAL DISPOSAL OF SLUDGE CAKE 8.4.2.1 Land filling

Landfills can be used for all types of sludge since they are designed to prevent the contamination of ground water and to prevent the migration of the wastes from the landfill. It also has a cover for avoiding penetration of wastewater by rainwater. The sump collects leachate from the landfill and is pumped to a wastewater treatment plant. 8.4.2.2 Composting

It is the biological decomposition of organic constituents in the water. Compost is made by mixing sludge with a bulking agent to ensure that the mixture can be aerated for an accelerated aerobic degradation process; and thus drying it. The resultant product can be applied on land. (Metcalf & Eddy, 2003) 8.4.2.3 Pyrolysis and gasification of sludge



Pyrolysis

It is a thermal treatment process in which the sludge (or biomass) is heated under pressure to a temperature of350–500 °C in the absence of oxygen. In this process, the sludge is converted into char, ash, pyrolysis oils, water vapor, and combustible gases. Part of the solid and/or gaseous products of the pyrolysis process are incinerated and used as heating energy in the pyrolysis process.



Gasification

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This involves the breakdown of dried sludge (or biomass) in an ash and in combustible gases at temperatures usually about 1000 °C in an atmosphere with a reduced amount of oxygen. Pyrolysis and gasification of sewage sludge have some potential advantages compared to incineration. One advantage is that the conversion of the combustible gases of both systems into electrical power can be achieved more efficiently (Metcalf & Eddy, 2003). 8.4.2.4 Sludge incineration:

All organics are incinerated with the resulting heat recovered by preheating fluidizing air and/or generating steam. Ash/fines are recovered using a flue gas cyclone with water quench/ slurrying conveying system at its bottom and a two stage flue gas scrubber. The main types of incinerators are: multiple hearth and fluidized bed. (Metcalf & Eddy, 2003)

8.5

BIOGAS

Biogas is a gas that is formed by anaerobic microorganisms in the anaerobic digester. These microbes feed off carbohydrates and fats, producing methane and carbon dioxides as metabolic waste products. This gas can be harnessed by man as a source of sustainable energy. . During the upgrading process the calorific value of the biogas is increase from 6.5 kW to 9.7 kW.

CHAPTER 9: CONTROL STRATEGY As per the Process instrumentation diagram, several pump control, chemical storage tank and process level controls or alarms are implemented to properly control the flow rates. 9.1 Control of Liquid level Table 9.1: The control of liquid level for the whole plant Sensing device

Unit

Float: The liquid level is In detected and converted receiving onto an electric signal to

Control actions When water is at its maximum level, the valve at receiving chamber will close, causing the opening of the storage tank Page 61 of 134

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produce a control signal chamber for immediate actions. Screening PSTs

Polymer solution in maturation tank

Ultrasonic level measuring devices: A pulse of ultrasonic wave is generated which bounce off the liquid surface. The echo and echo’s travel time is detected and calculated respectively, which is then converted to a level measurement.

(another receiving chamber). When water already to its level an alarm will be heard to avoid overflow or there will be an automatic closing of valve and the next valve will be open to fill the other tanks. When there is a change in the level of solution, the position the stem of the level control valve is altered, controlling the amount of polymer fed into the maturation tank from the dissolving tank.

Equalizing tank

When the maximum level has been reached, the valve will be closed, causing water to be stored in preceding In rapid units. gravity sand filters

In anaerobic digester

As the maximum water surface has been reached, the sludge inflow to digester and outflow of gravity thickener close. This will cause the primary and WAS to be stored in the spare primary sedimentation tank.

an optical sludge level In the whereby the sludge level blanket should detector is used which Gravity be measured, taking into consideration determines the thickener its corrosivity and sticking process, concentration of sludge through the intensity of light 9.2 Control of Flow Table 9.2: The control of flow rate for the whole plant Page 62 of 134

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Flow nozzle meter: works by differential In pipes pressure. Electromagnetic Flow meter: It works by the principles of electromagnetic induction. When the fluid flows through the metering tube, a magnetic field is applied, resulting in a potential difference which is proportional to flow velocity perpendicular to the flux lines

Diluted polymer from dissolving tank to maturation tank Water for polymer dilution in the dissolving tank Digested sludge towards centrifuge Centrate towards the primary units As the flow increases or decreases, a signal is sent such Dry polymer that the inlet valve adjusts itself from hopper to to the preset flow rate. dissolving tank

Ultrasonic flow meter: A pulse of Rapid gravity ultrasonic wave is generated on the liquid filters surface. The resonance and resonance time Chlorination is sensed and measured. This time is then converted to flow measurement. Magnetic flow meter: As the sludge passes waste activated through the meter, generating a magnetic sludge in field, the voltage produced is measured thickener and converted into velocity; thus flow rate. To anaerobic digester RAS (Return Page 63 of 134

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Activated Sludge) Orifice meter: It consists of a straight Digester length of pipe inside which an orifice gas flow affects the flow by inferring the rate of flow by measuring the pressure difference.

If the flow rate is too high or low, the flow is adjusted to the pre-set value.

9.3 Control of temperature The process of digestion occurs through the action of microorganisms which break down the organic matter into the resulting products of carbon dioxide and methane. However, these microorganisms have an optimum growth temperature. Large fluctuations in temperature can cease or slow down considerable the production of the large, stable population needed for digestion process. To measure the temperature, a thermocouple is used. Table 9.3: The control of temperature for the whole plant Thermocouple: It operates on the principle that current flows in a circuit made of two different metals when the two electrical junctions between the metals are at different temperatures.

MBR Aeration tank

In low or high temp the mean cell residence time can be raised or decreased accordingly.

Chlorination

Any change in temp will cause the flow/amount of chlorine to be added to change

Anaerobic digestion

If the temp of the digester goes down, part of the digester contents is recycled to the heaters.

9.4 Control of pH The control of pH is an important parameter to achieve the norms for irrigation and to be within the range, chemicals are added to control pH.

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Table 9.4: The control of pH for the whole plant pH meter

MBR Aeration tank

Glass electrode: The electrode In produces a voltage related to anaerobic hydrogen ion activity and to pH. The digester pH is determined by measuring the voltage against a reference electrode.

If the pH is not that required, buffering agents such as NaHCO3 will be added. Buffering agents is added into the mixing stream of the digester.

9.5 Control of Pressure Pressure should be controlled in the gas holder to prevent bursting and this is achieved by using a pressure relief valve whereby as soon as the desired pressure is reached, the valve is automatically closed. Also, Pressure monitoring should be done in the anaerobic digester, too high or too low pressure will cause an explosion or even collapse; thus the flow of gas must be increased or gas stored in the digester respectively. Table 9.5: The control actions for pressure in the plant Mechanical pressure gauges: Anaerobic It works on the principle of digester physical displacement caused by changes in pressure, shown by a link to indicator or pointer on a scale.

Too high or low pressure will cause an explosion / collapse; thus the flow of gas must be increased or gas is stored in the digester respectively.

CHAPTER 10: PRELIMINARY COSTING A large sum of money is required to purchase and install the necessary machinery and equipment before the full operation of industrial plant. Land and service facilities must be obtained and the plant must be constructed with all the piping system, controls and services, together with the money for the payment of expenses involved in the plant operation known as working capital.

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10.1 Cost of Equipment Table 10.1: Equipment costs Equipment

1

2 3

QTY

Coarse screens (Inclined bar screen with rack and pinion 2 system- Infilco Degremont bar screen) Fine bar screen (Fine straight bar 2 screen GFD type) Rectangular in-line equalization 1 tank equipped with mixer

Cost ( Rs) / unit

Total Cost / Rs

253,150

506,300

305,000

610,000

300,212.17

300,212

4

Circular clarifier

2

23,082,825.07

46,165,650

5

MBR

1

3,536,920

3,536,920

1

226,441.24

226,441

1

40,371,838.51

40,371,839

1

6,955,415.34

6,955,415

6 7 8

Rapid Sand Filtration equipped with an inlet chamber Thickener Square

Based

UASB

sludge

digester

9

Centrifuge

1

2,003,400

2,003,400

10

Pumps

9

61,500

553,500

11

Oil separator

1

15,762,107.85

15,762,108

Total Purchase Equipment Cost

116,991,785

10.2 Calculating of working capital Assumptions: 

1 kg of sludge cake will be sold at Rs 3.00



1 m3 of treated water will be sold at Rs 1.

 all biogas produced will be used in the plant itself for providing energy for pumps and other equipments(Calculation to be done in appendix) Page 66 of 134

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1) For sludge cake, total sludge cake = 14,492.71 kg/d (from mass balance) Total revenue = 14,482.91 kg/d× 365×2 = Rs 10,572,524.30 2) For treated water, total amount of treated water = 70000m3/d Total revenue 70000 × 365 × 0.70= Rs 17,885,000 Hence, total income = Rs 28,457,524 3) 10 % income tax = 10 % × Rs 32,837,498.75 = Rs 2,845,752.4 4) Net income = Rs 28,457,524 - Rs 2845752.4 = Rs 25,611,771.6 10.3

Calculating payback period

Payback period = Total capital investment/ total income = Rs 181,821,085.30/ Rs 25,611,771.6 = 7.0 Hence the payback period is estimated to be around 7 years. CHAPTER 11:

CONCLUSIONS

The objective of the project was to design a wastewater treatment plant which can treat domestic wastewater with the inlet parameters as stated in the design statement in the introduction. The wastewater received at the head works undergoes oil-water separation, screening, grit removal as preliminary treatment, primary sedimentation, activated sludge process for the destruction of many of the inlet parameters such as BOD, COD and also nitrification and de-nitrification and the most important treatment in the plant being the disinfection unit where 99.8 percent of the total coliforms count were deactivated. All these treatment units were necessary for the treated wastewater to meet the standards for effluent regulations for irrigation. Mauritius, being a water-stressed country can Page 67 of 134

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benefit on the re-use of wastewater such that it does not have to utilize fresh water for irrigation and this water can be used for other purposes. Almost all the waste within the treatment unit was analyzed, with sludge production being the major waste. The sludge was treated by anaerobic digestion after being thickened. The choice for this treatment was the recovery of energy to be used for the plant, thus reducing the electricity bill. An energy analysis was performed for the consumption of electricity for the different equipments in the treatment plant with 22% taken up by the activated sludge process, while on the other side; energy is being generated by the biogas. The percentage of energy generated is 48, that is; requiring only 52 percent for the operation of the plant. The equipments used in the treatment plant have to be purchased and the tanks, chambers and other buildings constructed making up the capital investment together with the installation, piping and other costs. The income for the plant comes from the sales of wastewater for irrigation and sludge cake to power station and thus the range for payback period is about 7 years. Hence we can conclude that the wastewater treatment plant can provide an environmentally friendly, cost effective production of the product and most probably socially acceptable.

References  Albertson,O.E ,1991,Dewatering Municipal Wastewater sludges, Noyes Data Corporation Publications 

Ananth S. Kodavasal, 15 August 2011. The STP Guide – Design, Operation and

Maintenance.(Pg 33, 34) Available at: http://kspcb.kar.nic.in/STP-Guide-web(Lo).pdf  APHA (1992). Standard Methods for the examination of Water and Wastewater, 18th ed. AmericanPublic Health Association/Water Environment Federation, Washington, DC, USA.

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 Icon. (2008). the 2009-2014 world outlook for membrane bioreactor (MBR) systems for wastewater treatment. Icon Group Publications.  Judd, S. (2008). The status of membrane bioreactor technology. Trends in Biotechnology, Vol. 26, No. 2, pp. 109-116.  Judd, S. (2010). The MBR Book. Principles and Applications of Membrane Bioreactors in Water and Wastewater Treatment, Elvesier, ISBN: 978-0-08-096682-3, 2nd Ed, London. Page 70 of 134

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Sudarshan Prasad Mahajan, 2009, Air Pollution Control, TERI Press, India p. 139  Syed. R, Motley.E.M, Guang Zhu, Chiang.P & Yerby, Water Works Engineering Planning, Design and Operation, First Edition. Prentice Hall PTR  Tao, G.; Kekre, K.; Oo, M-H.; Viswanath, B.; Lew, C-H.; Kan, L-M. & Seah, H. (2009). Large scale membrane bioreactor plant design (retrofit) and optimisation. Proceedings of the 4th IWA Membrane Technology Conference, Beijing, China, Sept 1-3.  Tchobanoglous G., Burton F.L., Stensel H.D., Wastewater Engineering Treatment and Reuse, 4th Edition, Metclf & Eddy Inc, China: McGraw-Hill Companies, Inc  Tottie O., 2008. Evaluation of sludge management in Wuhan, China, Department of Microbiology, Swedish University of Agricultural Sciences, Uppsala, ISSN 1401-5765  Trusell, R.; Merlo, R.; Hermanowicz, S. & Jenkins, D. (2006). The effect of organic loading on process performance and membrane fouling in a submerged membrane bioreactor treating municipal wastewater. Water Research, Vol. 40, pp. 2675-2683.  Turovskiy, I.S., Mathai, P.K., 2006. Wastewater Sludge Processing. New Jersey: John Wiley & Sons, Inc.  United Nations, 2003. Waste-water Treatment Technologies: A General Review. New York: United Nations, E/ESCWA/SDPD/2003/6  Van Loosdrecht, M.C.M. & Hence, M. (1999). Maintenance, endogenous respiration, lysis, decay and predation. Water Science and Technology. Vol. 39, No.1, pp. 107-117.  Vanrolleghem P.A., van Daele, M. & Dochain, D. (1995). Practical identifiability of a biokinetic model of activated sludge respiration. Water Research, Vol. 29, pp. 25612570.  Verrecht, B.; Judd, S.; Guglielmi, G.; Mulder, J. W. & Brepols, C. (2008). An aeration energy model for an immersed membrane bioreactor. Water Research, Vol. 42, pp. 4761-4770.  Verrecht, B.; Maere, T.; Nopens, I.; Brepols, C. & Judd, S. (2010). The cost of a large-scale hollow fibre MBR. Water Research, Vol. 44, No. 18, pp. 5274-5283.  Vol. 163B, pp. 224-231.  Wastewater Treatment Plants: Planning, Design and Operation, 2nd edition by Syed Qasim, Chemical Coagulation and precipitation, p.347  Wastewater Treatment plants; planning,design and operations, 2nd edition, Syed Qasim, p.322-323 Page 73 of 134

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Appendices 1: Mass Balance Mass Balance for Preliminary treatment a.

Coarse screen

From literature review: 5-25% BOD removal efficiency (assuming 15 % efficiency) 15-30% TSS removal efficiency (assuming 20% efficiency)

BOD 307 mg/l

BOD 260.95mg/l Coarse screen TSS 228mg/l

TSS 285 mg/l

COD 984mg/l

COD 984mg/l

Nitrogen 45mg/l

Nitrogen 45mg/l Phosphorus 10mg/l

Phosphorus 10mg/l BOD 46.5 mg/l TSS 57.0 mg/l

By product 15% of 307 mg/l ----------------- 46.5mg/l 20 % of 285mg/l ----------------- 57.0 mg/l Performing mass balance for BOD: Mass inlet = mass of by product + mass of outlet 307 mg/l = 46.5 mg/l + Q Q = 260.95mg/l Page 75 of 134

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The same procedure is done for TSS Flow rate calculation Conversion of mg/l to kg/s For BOD 307 mg/l 307 mg/l-----------------307x10-6 kg/l 307x10-6 kg/l----------------- 307x10-3 kg/m3 1 sec ----------------- 0.8103 m3 1m3 ----------------- 1/0.8103 sec 1m3 ----------------- 1.234 sec Therefore 307x10-3 kg/m3----------------- 307x10-3/(1.234x1000) kg/s 307mg/l ----------------- 0.2488 kg/s Kg/s

Kg/m3

BOD

0.2438

0.307

COD

0.7974

0.9841

TSS

0.2130

0.2850

NITROGEN

0.0398

0.0450

PHOSPHORUS

0.0081

0.01

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Total

1.6311

Approximate density of wastewater in: 1.6311 + 1,000 = 1,001.6311 kg/m3 By product flowrate (kg/s) = 0.0373 kg/s + 0.0462 kg/s = 0.0835kg/s Flowrate of by-product (m3/s) = by-product flowrate (kg/s) / density of wastewater in (kg/m3) Flowrate of by-product (m3/s) = 0.0835/1001.63 = 0.00008336 m3/s Therefore flowrate of wastewater at outlet = 0.8102 m3/s - 0.00008336 m3/s = 0.810 m3/s b.

Fine screen

From literature review: 20-45% BOD removal efficiency (assuming 30 % efficiency) 25-50% TSS removal efficiency (assuming 35% efficiency)

BOD 260.95mg/l

Coarse screen

BOD 182.7 mg/l

TSS 228mg/l

TSS 148.2 mg/l

COD 984mg/l

COD 984 mg/l

Nitrogen 45mg/l

Nitrogen 45 mg/l

Phosphorus 10mg/l

Phosphorus 10 mg/l BOD 78.3 mg/l TSS 79.8 mg/l

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By product 30% of 307 mg/l BOD ----------------- 78.3 mg/l 35% of 285mg/l TSS----------------- 79.8 mg/l Flow rate calculation

Kg/s

Kg/m3

BOD

0.2115

0.2610

COD

0.7974

0.9841

TSS

0.1848

0.2280

NITROGEN

0.0398

0.0450

PHOSPHORUS

0.0081

0.01

Total

1.528

Approximate density of wastewater in: 1.528 + 1,000 = 1,001.528 kg/m3 By product flowrate (kg/s) = 0.06345 kg/s + 0.06468 kg/s = 0.1280 kg/s Flowrate of by-product (m3/s) = by-product flowrate (kg/s) / density of wastewater in (kg/m3) Flowrate of by-product (m3/s) = 0.1280/1001.528 = 0.0001278 m3/s Therefore flowrate of wastewater at outlet = 0.810 m3/s - 0.0001278 m3/s = 0.8099 m3/s

Performing mass balance for BOD: Mass inlet = mass of by product + mass of outlet 260.95 mg/l = 78.3 mg/l + Q Page 78 of 134

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Q = 182.7 mg/l The same procedure is done for TSS c.

Equalization Tank

Mass Balance for Secondary treatment a. Primary Clarifiers Total flow rate= 2916.7 m3/day Number of clarifier tank = 2 Flow in one tank = (2916.7/2) = 1458.3 m3/day I.Assumptions12 : Average removal efficiencies of primary clarifier, TSS Removal efficiency= 75% BOD Removal efficiency = 55% COD Removal efficiency= 45 %

II.TSS Mass Balance in 1 tank Concentration = 285 mg/L Concentration = 0.285 kg/m3 Mass flow rate in = 0.285 × 1458.3 = 415.625 kg/h Mass out in underflow (in sludge) = 0.75 × 415.625 = 311.71875 = 311.7 kg/h Mass out in overflow = (415.625 - 311.71875) =103.90625 = 104 kg/h Total mass in effluent to MBR = 207.8125 = 207.8 kg/h 12

Wastewater Treatment Plants: Planning, Design and Operation, 2 nd edition by Syed Qasim, Chemical Coagulation and precipitation, p.347

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III.BOD balance 1 tank Concentration of BOD = 307 mg/L Concentration of BOD = 0.307 kg/m3 Mass flow rate in = 447.7 kg/h Mass out in underflow (in sludge) = 0.55 × 447.7 = 246.2 kg/h Mass out in overflow = 447.7 – 246.2 = 201.5 kg/h COD Balance in 1 tank Concentration = 987 mg/L Concentration = 0.987 kg/m3 Mass flow rate in = 1439.4 kg/h Mass out in underflow (in sludge) = 0.45 × 1439.4 = 647.7 kg/h Mass out in overflow = 1439.4 – 647.7 = 791.7 kg/h IV.VOLUME OF SLUDGE DISCHARGED TO THICKENER Mass flow rate of sludge from one clarifier = 311.7 kg/h Knowing that typical solids concentrations in raw primary sludge from settling municipal wastewater are 6%-8%13, an average concentration of 7% will be assumed to be the solids concentration. Mass flow rate of sludge equals to the mass of dry solids. Therefore let mass of dry solids be Sdry = 311.7 kg/h and wet solids be Swet Accounting for 7% solids by weight, Swet =

𝑆𝑑𝑟𝑦 0.07

311.7

= 0.07 = 4453.125 kg/h

13

Information retrieved from a word document available online at http://home.engineering.iastate.edu/~leeuwen/CE%20523/Supplementary%20Notes/Sludge%20Disposal.doc

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The primary sludge density ranges from 1.0 to 1.03 g/cm3 [14] which makes an average of 1.015 g/cm3 or 1015 kg/m3 Therefore, assuming a sludge density of ρ = 1015 kg/m3, this corresponds to a flow of Qsludge from one clarifier =

𝑆𝑤𝑒𝑡 ρ

=

4453.125 1015

= 4.38 m3/ h

Therefore total flow rate from both clarifiers, Qs = 4.38 × 2 = 8.77 m3/ h Hence, Flow rate to MBR = 2916.67 – 8.77 = 2907.9 m3/h

14

Wastewater sludge processing, Izrail S. Turovskiy et al, Physical and Biological properties, p.47

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b. Mass Balance for the MBR

I.Mass Balance on the MBR Influent = Waste Sludge + Effluent 

Q = Influent Flowrate



SE = BOD in Effluent, 𝑚𝑔⁄𝐿



S0 = BOD in influent, 𝑚𝑔⁄𝐿

Hence BOD efficiency can be calculated as follows: Given: BOD in domestic wastewater entering Primary Settling Tank = 307 𝑚𝑔⁄𝐿 Assuming BOD removal efficiency in primary clarifier = 40 % Primary effluent BOD = 307 𝑚𝑔⁄𝐿 × (1 − 0.40) = 184 𝑚𝑔⁄𝐿 Page 82 of 134

ID: 1114132 Therefore; As assumed permissible limits, that is 40 mg/L = 2800 kg/day is in the effluent BOD Removal Efficiency in MBR =

184 𝑚𝑔⁄𝐿 −40𝑚𝑔⁄𝐿 184𝑚𝑔⁄𝐿

× 100 = 78%

Hence; 𝑂𝑣𝑒𝑟𝑎𝑙𝑙 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = (

307 𝑚𝑔⁄𝐿 − 40 𝑚𝑔⁄𝐿 ) × 100 = 87% 307 𝑚𝑔⁄𝐿

The recycle ratio can be expressed as follows: 𝑅𝑒𝑐𝑦𝑐𝑙𝑒 𝑅𝑎𝑡𝑖𝑜 =

𝑅𝑒𝑐𝑦𝑐𝑙𝑒 𝐹𝑙𝑜𝑤𝑟𝑎𝑡𝑒, 𝑚3 ⁄𝑑𝑎𝑦 𝐼𝑛𝑓𝑙𝑢𝑒𝑛𝑡 𝐹𝑙𝑜𝑤𝑟𝑎𝑡𝑒, 𝑚3 ⁄𝑑𝑎𝑦

Typical recycle ratio for conventional activated sludge process = 0.25 – 0.50 Average Recycle Ratio = 0.375 Therefore; 𝑅𝑒𝑐𝑦𝑐𝑙𝑒 𝐹𝑙𝑜𝑤𝑟𝑎𝑡𝑒, 𝑄𝑅 = 𝑅𝑒𝑐𝑦𝑐𝑙𝑒 𝑟𝑎𝑡𝑖𝑜 × 𝐼𝑛𝑓𝑙𝑢𝑒𝑛𝑡 𝑓𝑙𝑜𝑤𝑟𝑎𝑡𝑒, 𝑄0 = 0.375 × 70000 𝑚3 ⁄𝑑𝑎𝑦 𝑅𝑒𝑐𝑦𝑐𝑙𝑒 𝐹𝑙𝑜𝑤𝑟𝑎𝑡𝑒, 𝑄𝑅 = 26250 𝑚3 ⁄𝑑𝑎𝑦 Also by the methodology of Shu Dar lin (2005), 𝑄𝑅 can be expresses as follows: 𝑄𝑅 =

𝑄0 (𝑥 − 𝑥0 ) 𝑥𝑤 − 𝑥

Whereby: X = MLSS concentration, 2500 𝑚𝑔⁄𝐿 X0 = Primary Effluent TSS, 102.6 𝑚𝑔⁄𝐿 𝑄𝑅 = 𝑟𝑒𝑐𝑦𝑐𝑙𝑒 𝑓𝑙𝑜𝑤𝑟𝑎𝑡𝑒, 26250 𝑚3 ⁄𝑑𝑎𝑦 26250 𝑚3 ⁄𝑑𝑎𝑦 =

70000 𝑚3 ⁄𝑑𝑎𝑦 (2500 𝑔⁄𝑚3 − 102.6 𝑔⁄𝑚3 ) 𝑥𝑤 − 2500 𝑔⁄𝑚3 𝑥𝑤 = 3893.07 𝑔⁄𝑚3 Page 83 of 134

ID: 1114132 Also; 𝑅𝑒𝑐𝑦𝑐𝑙𝑒 𝑅𝑎𝑡𝑖𝑜 =

𝑄𝑅 𝑋 = 𝑄0 𝑋𝑅 − 𝑋

Whereby: 𝑋𝑅 is the concentration in recycle sludge and as assumed R is 0.375. 𝑅𝑒𝑐𝑦𝑐𝑙𝑒 𝑅𝑎𝑡𝑖𝑜 =

2500 𝑔⁄𝑚3 𝑋𝑅 − 2500 𝑔⁄𝑚3

Inlet Flowrate:XO = Concentration of TSS in influent Primary Settling Tank Outlet Flowrate:Xe = Concentration of Biomass in effluent Return Sludge: QR = return flow pumping rate, 𝑚3 ⁄𝑑𝑎𝑦 Xu = return microorganism concentration Waste Activated Sludge: QW = Waste flow from recycle waste line, 𝑚3 ⁄𝑑𝑎𝑦 And µ = growth of solids (𝑑𝑎𝑦 −1 )

II.BOD Balance around MBR 𝐼𝑛𝑝𝑢𝑡 − 𝑜𝑢𝑡𝑝𝑢𝑡 + 𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛 = 𝑎𝑐𝑐𝑢𝑚𝑢𝑙𝑎𝑡𝑖𝑜𝑛 (𝑄0 𝑆0 + 𝑄𝑅 𝑆𝐸 ) − (𝑄0 + 𝑄𝑅 )𝑆𝐸 −

𝜇𝑋𝑉 𝑑𝑆 = 𝑉 𝑌 𝑑𝑡

𝑑𝑆 𝑉=0 𝑑𝑡 Y = cell yield in kg MLSS/ kg BOD removed At steady state 𝜇𝑋𝑉 = 𝑄0 𝑆0 − 𝑄0 𝑆𝐸 𝑌 𝜇 𝑄0 (𝑆0 − 𝑆𝐸 ) = 𝑓⁄𝑚 𝑟𝑎𝑡𝑖𝑜 = 𝑌 𝑉𝑥 𝑆0 = 184 𝑚𝑔⁄𝐿 Page 84 of 134

ID: 1114132

Assuming no chemical changes occur: 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝐵𝑂𝐷𝐸 = 40 𝑚𝑔⁄𝐿 Therefore: 𝐵𝑂𝐷𝑅 = 𝐵𝑂𝐷𝑊𝐴𝑆 = 40 𝑚𝑔⁄𝐿 III.Flow Balance Influent Flow Q0 = 70,000 m3/day Initial BOD = BOD0 = 184 g/m3 As calculated: BOD Efficiency = 78 % BOD in Effluent = 40 g/m3 IV.Solid Balance on membrane 𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑇𝑆𝑆 𝑖𝑛 𝑅𝐴𝑆 = 𝑇𝑆𝑆𝑅 = 𝑋𝑈 𝑔⁄𝑚3 × 𝑄𝑅 𝑚3 ⁄𝑑𝑎𝑦 𝑇𝑆𝑆𝑅 = 10.65 𝑘𝑔⁄𝑚3 × 26250 𝑚3 ⁄𝑑𝑎𝑦 = 279,562.5 𝐾𝑔⁄𝑑𝑎𝑦 𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑇𝑆𝑆 𝑖𝑛 𝑊𝐴𝑆 = 𝑋𝑢 𝑘𝑔⁄𝑚3 × 𝑄𝑊 𝑚3 ⁄𝑑𝑎𝑦 = 2367.07 𝐾𝑔⁄𝑑𝑎𝑦 Assume TSS Removal Efficiency = 81% Therefore: 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑂𝑓 𝑇𝑆𝑆 𝑖𝑛 𝐸𝑓𝑓𝑙𝑢𝑒𝑛𝑡 = 0.19 × 102.6 𝑚𝑔⁄𝐿 = 19.494 𝑚𝑔⁄𝐿 = 1364.6 𝐾𝑔⁄𝑑𝑎𝑦 V.Balance on VSS on membrane The removal efficiency of VSS in an MBR system is 90%, therefore; 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑉𝑆𝑆 𝑖𝑛 𝐸𝑓𝑓𝑙𝑢𝑒𝑛𝑡 = 10% × 82.08 𝑔⁄𝑚3 = 8.208 𝑔⁄𝑚3= 574.6𝐾𝑔⁄𝑑𝑎𝑦 Whereby VSS = 80% TSS = 82.08𝑔⁄𝑚3 =5745.6𝐾𝑔⁄𝑑𝑎𝑦 Assuming: 𝑉𝑆𝑆𝑊𝐴𝑆 = 𝑉𝑆𝑆𝑅 = 5745.6 𝐾𝑔⁄𝑑𝑎𝑦 − 574.6 𝐾𝑔⁄𝑑𝑎𝑦 = 5171.04 𝐾𝑔⁄𝑑𝑎𝑦 Page 85 of 134

ID: 1114132

VI.NH3-N Balance on the membrane skid Overall Balance: 𝑁𝐻3 − 𝑁𝑖𝑛 = 𝑁𝐻3 − 𝑁𝑜𝑢𝑡 𝑁𝐻3 − 𝑁𝑂𝑈𝑇 = 𝑁𝐻3 − 𝑁𝐸 + 𝑁𝐻3 − 𝑁𝑊𝐴𝑆 By the methodology of N.F.Gray, 2005, the removal efficiency of 𝑁𝐻3 − 𝑁 in an ASP system is 90% 𝑁𝐻3 − 𝑁𝑖𝑛 = 1400 𝐾𝑔⁄𝑑𝑎𝑦 𝑁𝐻3 − 𝑁 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 𝑖𝑛 𝑎𝑒𝑟𝑎𝑡𝑒𝑑 𝑡𝑎𝑛𝑘 = 90% × 1400 𝐾𝑔⁄𝑑𝑎𝑦 = 1260 𝐾𝑔⁄𝑑𝑎𝑦 𝑁𝐻3 − 𝑁𝐸 = 𝑁𝐻3 − 𝑁𝑊𝐴𝑆 = 𝑁𝐻3 − 𝑁𝑅 = 1400 𝐾𝑔⁄𝑑𝑎𝑦 − 1260 𝐾𝑔⁄𝑑𝑎𝑦 = 140 𝐾𝑔⁄𝑑𝑎𝑦 VII.NO3-N Balance on membrane skid Overall Balance: 𝑁𝑂3 − 𝑁𝑖𝑛 = 𝑁𝑂3 − 𝑁𝑜𝑢𝑡 Since nitrification occurs, let 𝑁𝑂3 − 𝑁𝑥 be the amount of nitrates obtained by the nitrification of ammonia. Therefore: 𝑁𝑂3 − 𝑁𝑖𝑛 = 𝑁𝑂3 − 𝑁1 + 𝑁𝑂3 − 𝑁𝑥 Assuming 𝑁𝑂3 − 𝑁𝑥 = 5031.2 Kg⁄day 𝑁𝑂3 − 𝑁𝑜𝑢𝑡 = 𝑁𝑂3 − 𝑁𝐸 + 𝑁𝑂3 − 𝑁𝑊𝐴𝑆 𝑁𝑂3 − 𝑁𝑥 + 𝑁𝑂3 − 𝑁𝑖𝑛 = 𝑁𝑂3 − 𝑁𝐸 + 𝑁𝑂3 − 𝑁𝑊𝐴𝑆 According to N.F.Gray (2005), Removal Efficiency=90% Since 𝑁𝑂3 − 𝑁𝑖𝑛 = 1200 Kg⁄day 𝑎𝑛𝑑 𝑎𝑠 𝑎𝑠𝑠𝑢𝑚𝑒𝑑 𝑁𝑂3 − 𝑁𝑥 = 5031.2 Kg⁄day 𝑇𝑜𝑡𝑎𝑙 𝑁𝑂3 − 𝑁𝑖𝑛 = 6231.2 Kg⁄day 𝑁𝑂3 − 𝑁 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 𝑖𝑛 𝑎𝑒𝑟𝑎𝑡𝑒𝑑 𝑡𝑎𝑛𝑘 = 0.90 × 6231.2 Kg⁄day = 5608.08 Kg⁄day 𝑁𝑂3 − 𝑁𝐸 = 6231.2 Kg⁄day − 5608.08 Kg⁄day = 623.12 Kg⁄day 𝑁𝑂3 − 𝑁𝑊𝐴𝑆 = 623.12 Kg⁄day 𝑁𝑂3 − 𝑁𝑅 = 623.12 Kg⁄day Page 86 of 134

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VIII.Phosphorus Balance on the membrane skid Overall Balance: [𝑇𝑜𝑡𝑎𝑙 𝑃ℎ𝑜𝑠𝑝ℎ𝑜𝑟𝑢𝑠 𝐸𝑛𝑡𝑒𝑟𝑖𝑛𝑔 𝑡ℎ𝑒 𝑠𝑦𝑠𝑡𝑒𝑚] = [𝑃ℎ𝑜𝑠𝑝ℎ𝑜𝑟𝑢𝑠 𝑙𝑒𝑎𝑣𝑖𝑛𝑔 𝑡ℎ𝑒 𝑠𝑦𝑠𝑡𝑒𝑚] + [𝑇𝑜𝑡𝑎𝑙 𝑃ℎ𝑜𝑠𝑝ℎ𝑜𝑟𝑢𝑠 𝑝𝑟𝑒𝑠𝑒𝑛𝑡 𝑖𝑛 𝑊𝐴𝑆] 𝑃𝑖𝑛 = 70 Kg⁄day = 1 g⁄m3 Assuming Efficiency of Phosphorus Removal is 65% 𝑃ℎ𝑜𝑠𝑝ℎ𝑜𝑟𝑢𝑠 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 𝑖𝑛 𝑎𝑒𝑟𝑎𝑡𝑒𝑑 𝑡𝑎𝑛𝑘 = 65% × 70 Kg⁄day = 45.5 Kg⁄day = 0.65 g⁄m3 𝑃𝐸 = 70 Kg⁄day − 45.5 Kg⁄day = 24.5 Kg⁄day 𝑃𝑊𝐴𝑆 = 24.5 Kg⁄day 𝑃𝑅 = 24.5 Kg⁄day

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Summary of balances: Streams

Influent to screening

Oil water Equalization Primary Membrane Bioreactor seperator tk clarifiers Skid

Q (m3/d)

70,000.00

70,000.00

70,000.00

69,984.28 69,777.7

69,777.7

COD (kg/d)

65,436.00

68,880.00

65,436.00

48,363.00 1221.4

1221.4

BOD (kg/d)

20,415.50

21,490.00

20,415.50

12,894.00 2800.0

2800.0

TSS (kg/d)

17,955.00

19,950.00

17,955.00

3,990.00

3,990.00

1364.6

NH3-N (kg/d)

1,750.00

1,750.00

1,750.00

1,750.00

1,750.00

140.0

NO3—N (kg/d)

1,400.00

1,400.00

1,400.00

1,400.00

1,400.00

623.1

P (kg/d)

700.00

700.00

700.00

700.00

700.00

24.5

Coliforms (MPN 420,000.00 420,000.00 420,000.00 –1 100mL )

420,000.0 420,000.0

420,000.0

Total (kg/d)

69,984.28 11861.4

6,173.6

107,656.50 114,170.00 107,656.50

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Mass Balance for Tertiary treatment a.

Sand Filter

Filtration across the sand filter normally achieves reductions of 65% in TSS and 59% in BOD.

Qo

SAND FILTER

Qe

So

Se

Xo

Xe

Qo: Flow rate coming from second clarifier Qe: Flow rate of effluent coming from sand filter So: Amount of BOD entering sand filter Se: Amount of BOD leaving sand filter Xo: Amount of TSS entering sand filter Xe: Amount of TSS leaving sand filter Given, Qo & Qe = 69,673.5 m3/day (flow rate is same for both inlet and outlet) So = 2800.0 kg/day, Xo = 1364.6 kg/day Se = So – (59/100 x So) = 2800.0 – (59/100 x 2800.0) = 1148.0 kg/day Xe = Xo – (65/100 x Xo) = 1364.6 – (65/100 x 1364.6) = 477.61 kg/day Page 89 of 134

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Calculating whether amount of BOD & TSS are within permissible limits In a flow rate of 69,673.5 m3/day we get 1148.0 kg/day of BOD Therefore in, 69,673.5 m3

1148.0 kg of BOD

69,673.5 x 103 L

1148.0 x 106 mg of BOD

1L

(1148.0 x 106) ÷ (69673.5 x 103)

1L

16.48 mg of BOD

BOD = 16.48 mg/L

69,673.5 m3

477.61 kg of TSS

69,673.5 x 103 L

477.61 x 106 mg of TSS

1L

(477.61 x 106) ÷ (69673.5 x 103)

1L

6.84 mg of TSS

TSS = 6.84 mg/L Both BOD and TSS are within permissible limit. b.

Chlorination Parameters r Q in

Q out Disinfection

Parameters in

Parameters out

Page 90 of 134

ID: 1114132

Assumptions: 

Chlorination is used for the removal of coliforms only.



Q in = Q out remains constant through UV disinfection.

Efficiency of chlorination unit = 99.9% Therefore coliforms out = 420 MPN 100 mL–1

Mass Balance for Sludge Treatment a. Thickener S thickened sludge

S influentt

Seffluent Gravity thickener

Parameters in

S influent

Parameters out

= Seffluent+Sthickened slugde

Efficiency of gravity thickener is taken to be 90% from wastewater engineering (Metcalf & Eddy, 2003) S effluent=0.90*S influent = 0.90 x (326.45+ 222.26 + 1282.37) =1831 m3/d. S thickened sludge= Sinfluent - S effluent = 183.11 m3/d. Parameters thickened sludge = Removal efficiency of parameters × Parameters influent COD thickened sludge= 0.9 x 3454.4 Kg/d COD thickened sludge = 3109 Kg/d

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b.

Sludge Digester

Qin

Qout

CODin

CODout

BODin

Sludge digester

BODout

NH3in

NH3out

PO4-in

PO4-out

TSSin

TSSin

Flowrate, Qin = 532.3 m3/day The sludge digester has an efficiency of 95% Qout = 0.95 x 532.3 = 505.7 m3/day % removal in sludge digester: TSS: 65%, COD: 90%, BOD: 90%, NH3: 85% and PO4-: 10%  TSSout = TSSin – (0.65 x TSSin) = 27,809.9 – (0.65 x 27,809.9) = 9733.5 kg/day  CODout = CODin - (0.9 x CODin) = 40825 – (0.9 x 40825) = 4082.5 kg/day  BODout = BODin - (0.9 x BODin) = 23826 – (0.9 X 23286) = 2328.6 kg/day  NH3out = NH3in - (0.9 x NH3in) = 903 – (0.85 x 903) Page 92 of 134

ID: 1114132

= 135.45 kg/day  PO4-out = PO4-in - (0.9 x PO4-in) = 655 – (0.1 x 655) =589.5 kg/day

c.

Dewatering unit Scentrate Parameterscentrat e

S in Centrifuge Parameters in

S cake Parameters cake

S in = Scake+ Scentrate Scentrare= Removal efficiency = 0.90 (D.C Bacley, 1997) x Sin Scentrate = 0.9 x 583.77 = 525.39 Kg/d S cake= Sin - Scentrate S centrate= 58.38Kg/d Parameters in = Parameters centrate+ Parameters cake Parameterscentrate= Removal efficiency of parameters × Parameters in Parameterscake = Parameters in- Parameters centrate

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Appendix 2: Energy Balances Energy balance for Preliminary treatment a) Screen bars Assuming both screens uses same amount of power, the energy consumption can be computed as follows: Wattage of screen bar = 1.5 kW E = 2(1.5 x 24) = 72 kWh/d Pump energy consumption, PE =

ρgHQ Ƞ

(Frank R. Spellman, 2003)

PE is the input power required (W) ρ is the fluid density (kg/m3) g is the standard acceleration of gravity H is the energy Head added to the flow (m) Q is the flow rate (m3/s) Ƞ is the efficiency of the pump plant as a decimal ρ = 2650 kg/m3 for grit removal g = 9.81 m/s2 Assume H = 2m, Q = 0.001 m3/s and Ƞ = 90 % Therefore, total PE = 2(2650 x 9.81 x 1 x 0.001) / 0.9 = 57.77 W 57.77 x 24 = 1.386 kWh/d Total energy consumed by screen bars = 4.374 + 1.386 = 5.76 kWh/d

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Energy balance for Secondary treatment a) Circular settling tank Wattage

for

Energy

for

each

Energy

for

all

Wattage Energy

electric motor

for

per

motors

for

per

each

each

motor

dosing

day

=

day

=

2.18

(2.18

x

24)

(52.32

x

4)

dosing pump

=

pump =

(2.5

= =

= x

24)

kWh

52.32

kWh/d

209.28

kWh/d

2.5 =

60

kWh kWh/d

Energy for all dosing pumps =(60 x 8) = 480 kWh/d Wattage Energy

for for

each

each pump

pump =

(51.2

= x

24)

51.2 =

kWh

1228.8

kWh/d

Energy for all pump = (1228.8 x 4) = 4915.2 kWh/d b)

Distribution Chamber

It is assumed that the flow from the distribution chamber to the primary circular settling tanks is through gravity. Therefore there is no consumption of energy in this section. Secondary treatment a)

Power requirement for the Membrane Bioreactor

In order to meet the oxygen demand in the aeration tanks, diffusers are placed and these diffusers require energy. Energy Balance on Aeration Tank Normally the newer technology include centrifugal blowers or positive displacement blowers, since positive displacement blowers is limited to only 425 m3/min volumetric flow rate and centrifugal blowers can be used for volumetric flow rate beyond 80 m3/min, a centrifugal pump is preferred in this case. By the methodology of Frank R. Spellman (2013), the power requirement for aeration is calculated as follows: Average power required for aeration = Pav QxRxT = x [(Pdis /P) 0.283– 1] 3600 x 29.7 x 0.283 x η Page 95 of 134

ID: 1114132

Whereby: P is the atmospheric pressure Q is the air flow rate T is the inlet temperature in Kelvin (280C = 301.15 K) η is the efficiency of the blower which is normally within the range 70%-80% R is the gas constant, 8.314kJmol-1K-1 Pdis = discharge pressure of blower which varies between 1.7 and 2.4 bar for fine bubble diffusers. Therefore: Pav =

1,447.22 𝑚3 ⁄𝑚𝑖𝑛 x 8.314 x 301.15 K 2.05 𝑏𝑎𝑟 0.283 x [( ) – 1] 3600 x 29.7 x 0.283 x0.75 1.01325 𝑏𝑎𝑟 Pav = 35.24 x 24 = 845.76 kWh/d

b)

Energy Balance on membrane biofilter

Using the equation:𝑇 = 𝑤𝑅 2 Whereby: T is the torque in Kg.m W is the work kg/m R is the radius of the clarifier, m Assuming W = 20 lb/ft , W = 20 x 1.49 = 29.8 kg/m Since Diameter of clarifier = 26m Radius of clarifier, R = 13 m Hence; kg

𝑇 = 29.8 m × 13.02 𝑚2 = 5036.2 𝐾𝑔. 𝑚 = 49,405.12 𝑁𝑚 𝑃𝑜𝑤𝑒𝑟 = 𝑇𝑜𝑟𝑞𝑢𝑒 × 𝐴𝑛𝑔𝑢𝑙𝑎𝑟𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 = 𝑇 ×

𝐿𝑖𝑛𝑒𝑎𝑟𝑆𝑝𝑒𝑒𝑑 𝑟𝑎𝑑𝑖𝑢𝑠

Page 96 of 134

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From N.F Gray (2005), the speed of rakes varies between 3-6m/min and taking an average speed; Speed of rake = 4.5m/min = 0.075m/s 𝐴𝑛𝑔𝑢𝑙𝑎𝑟𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 =

Speed of rake 0.075 𝑚⁄𝑠 = = 0.00577 𝑟𝑎𝑑⁄𝑠 𝑟𝑎𝑑𝑖𝑢𝑠 13.0𝑚

Hence: 𝑃𝑜𝑤𝑒𝑟 = 49,405.12 𝑁𝑚 × 0.00577 𝑟𝑎𝑑⁄𝑠 = 285.07 𝑥 24 = 6841.68 𝑊ℎ/𝑑 Since 6 clarifiers are used, overall power requirements= 6841.68 x 6 = 41.05 kWh/d

c)

Energy balance on the amount of pumps

As illustrated in the process flow diagram, 13 pumps are being used, therefore, the energy required by the pumps are calculated as follows: Number of pumps= 12 centrifugal pumps which operate 24 hours + 1 centrifugal blower which operate only 5.12 hours /day The wattage of the centrifugal pump = 25.1 kW Energy required = (12 × 25.1 x 24hr) kW + (25.1 ×5hr) = 7354.3 kWh/day

Energy balance for Sludge treatment The input power calculation formula is

Ph = q ρ g h / (3.6 x106)

Where: Ph = power (kW) q = flow capacity (m3/h) ρ = density of fluid (kg/m3) g = gravity (9.81 m/s2) h = differential head (m) Assuming a head of 2 m

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Pump for

Q m3/d

Q m3/hr

kW/d

Number pumps

of Total kW/d

Blending tank

1831.08

76.3

9.98

1

9.98

Thickener

1831.08

76.3

9.98

2

19.96

Digester

183.1

7.6

0.998

2

1.996

Polymer

409.9

17.08

2.236

2

4.472

Centrifuge

583.77

24.3

3.178

2

6.356

(U.S. Department of Energy’s Office of Industrial Technologies, 2001 and F. Spellman, 2003) In the high rate sludge digestion, the sludge is mixed by recirculating the gas formed in a draft tube or by pumping. Power consumption for stirrers is generally 20-100kW/m3. (R.L. King, R.A Hiller and G.B Tatterson, 2004) i.scrapper Energy consumption 55.5kWh for scrapper Energy consumed by 1 scrapper = 55.5x 24 =1332 kW/d ii.Thickener Energy consumed by 2 scrappers +2 pumps = (1332x 2) + (19.96) = 2683.96 kW/d iii.Digester Average energy consumed by draft tube = 80 kWh/ m3 Volume of tank = 2019.6 m3 Energy consumed by 1 draft tube = 161568 W x 24 hour = 3.878 MW/d Energy consumed by 2 draft tube +2 pumps = (3.878x 2 x 1000) + (1.996) = 7758 kW/d iv.Polymer Energy consumed by 1 stirrer= 16 kW/ m3 Volume of tank = 1.85 m3 Page 98 of 134

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Energy consumed by 1 stirrer = 29.6 kW x 24 hour = 710.4 kW/d Energy consumed by 2 stirrers +2 pumps = (710.4 x 2) + (4.472) = 1725.3 kW/d 1.

Energy production from biogas

For biogas containing 67% of methane: 1 m3 of biogas

6.7 kWh (FNR, 2009)

Assumption: 

Density of biogas is 1.18 kg/m3



Efficiency of CHP plant is 40%

Amount of biogas going in the CHP = 7566.7 kg/d Flowrate

of

biogas

1 m3 of biogas 6412.46 m3 of biogas

=

𝑚𝑎𝑠𝑠𝑓𝑙𝑜𝑤𝑟𝑎𝑡𝑒 𝑑𝑒𝑛𝑠𝑖𝑡𝑦

=

7566.7 1.18

=

6412.46

m3/d

6.7 kWh (6412.46 x 6.7) = 42963.48 kWh

Efficiency of CHP plant is Energy obtained from CHP per day = (42963.48 x 0.4) = 17185.39 kWh/d

40%

Page 99 of 134

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Appendix 3: SIZING Sizing of the Bars Screen Sizing coarse screening Head-loss range: 0.15-0.76 m Depth of channel: 1.5-5.0 m Width of channel: 0.6-2.0 m Bar spacing: 12-80 mm

Bar screen spacing: Assuming velocity through aperture= 0.9m/s Area = peak flow/velocity through aperture Area= 0.81/0.9= 0.9m2 Calculating total width of opening (W): W= area (A)/ depth (d) Assuming depth= 1.5m W=A/d = 0.9/1.5= 0.6m Calculating number of opening (n): Choose a 50mm clear opening n= W/opening size = 0.6/0.05 = 12 To use 12 bars with 10mm width and 50mm thick Calculating width of chamber (w) w = 0.6 + (0.01 x 12) w = 0.72 m Height of rack (h): Page 100 of 134

ID: 1114132

h = 1.5m/ sin 80o (assuming inclination angle 80o) h= 1.52m (assuming 0.6m free board) h = 2m Efficiency coefficient = clear opening/width of chamber Eff. Coe. = 0.6/0.72 = 0.833

Head loos of rack (H): Selecting rectangular bar with semi-circular upstream face (β=1.83) 𝑤

𝑣2

H = β. ( 𝑏 )4/3. (2𝑔). Sin Q H= 1.83 x 1 x (0.92/2 x 9.81). sin 80o H= 0.0744m H = 74.4mm

Fine screen: Spaced bar: 1.5-6.4mm Head loss H = 1/2g x (V/c)2 H = ½ x (Q/ A.c)2 A: area of effective opening c: discharge coefficient Q: discharge through the screen Typical value c= 0.6 Area, A = Peak velocity/ velocity through screen aperture A = 0.81/0.9 = 0.9m2

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Width of rack (W) W= A/d = 0.9/2 (d= 2.0m for minimum excavation) W= 0.45m Calculating number of opening: Choosing 9.5mm clear opening n= W/ opening space n= 0.45/ 0.0095m n=47.36 Therefore 50 bars with 10mm width and 50mm thick Width of chamber (W): W = (0.01 x 50) + 0.45m W = 0.95m Height of rack: 2m/ sin 80o = 2.03m h = 2.63m (with 0.6m freeboard) Head loss (H): H = 1/2g x (V/c)2 H = (0.5 x 9.81) x (0.9/0.6)2 H = 0.115m (115mm)

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Sizing of Oil water separator

In an ideal separator, any oil globule with Vt equal or greater than surface loading rate will reach the separator surface and be removed.

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Design variables Qm = Flow of oily water into the oil-water separator, m3/s d = depth of water in channel, m L = length of channel, m B = width of one channel, m n = number of channel AH = total surface area, m2 Ac = total cross section, m2 VH = horizontal flow velocity, m/s Vt = rise rate of oil globule, m/s The design The design is based on the rise rate of oil globule:

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Assume oil globule diameter of 0.015 cm:

Data Given: 

Sewage Flow = 70000 m3/day = 0.8102 m3/s



Temperature = 29 °C

Assumptions taken: 

Specific gravity of water = 0.99



Specific gravity of oil = 0.92 Page 105 of 134

ID: 1114132



Viscosity of wastewater = 0.0062 poise = 0.3871 cm2/s



Oil globule size = 0.015 cm

Design constraints •

VH ≤ 1.5 cm/s or VH = 15 Vt, whichever the smaller

•

1.0 m ≤ d ≤ 2.5 m

•

0.3 ≤ d/B ≤ 0.5

•

1.8 m ≤ B ≤ 6.0 m

•

n = 2 (minimum 2 channels)

•

L/B ≥ 5

Calculating Vt using: Vt = 0.0123[(Sw - So)/μ] Vt = 0.0123[(990 – 920)/ (0.3871)] Vt = 2.224 cm/s Vertical Velocity = 2.224 cm/s Horizontal flow velocity is taken as 1.5 cm/s Also,

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Hence, Ac = (0.8102 x 100)/1.5 Ac = 54 m2 Therefore the minimum vertical cross-sectional area is 54 m2

Similarly,

By assuming ‘B’ to be 8m and n = 2 Page 107 of 134

ID: 1114132

d = 54/ (8 x 2) d =3.38 m Depth of channel = 3.38 m Depth/width ratio = 3.38/8 = 0.4 Calculating L using:

Page 108 of 134

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Using the following graph:

F is found to be 1.46

Page 109 of 134

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Hence, L = F x (VH/Vt) x d L = 1.46 x (1.5/2.224) x 3.38 L = 1.46 x 0.67446 x 3.38 L = 3.33 m Length of channel = 3.33 m Design inputs: Sewage Flow (in m3/s)

0.8102

Temperature (in °C)

29

Specific gravity of water

0.99

Specific gravity of oil

0.92

Viscosity of wastewater (in poise)

0.0062

Oil globule size (in cm)

0.015

Results: Vertical Velocity (in cm/s)

0.18083

Horizontal flow velocity ( in cm/s)

1.5

minimum vertical cross-sectional area (in m2)

54

number of channels

2

Width of channel (in m)

8

Depth of channel (in m)

3.38

Length of channel (in m)

3.33

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Sizing of equalisation tank Daily volumetric flow rate from Primary Settling Tank = 69,673.55 m3/d (from mass balance). Flow rate per hour =

69,673.55 24

= 2,903.06 m3/h

Assumptions: 

The number of equalization used is 3.



Detention time is 2-5 hours (Yung-Tse Hung et al., 2012). A typical detention time of 3 h is chosen as the detention time needs to be long enough so as to effectively balance fluctuating flows and to assist self-neutralization.



A safety factor of 15% is considered to make sure that there is no overflow of wastewater (Water Environment Federation, 2008).



A freeboard of 0.5 m is considered (Karia. G.L et al., 2006).



A depth of 5.5 m is considered with a 0.5 m of freeboard with a total of 6 m.

i.

Volume = Flow rate (m3/h) × detention time (h) = 2,903.06 × 3 = 8,709.18 m3

ii.

Total volume = volume of tank + 15 % volume of tank = 8,709.18 + (0.15 × 8,709.18) = 10,015.56 m3 10,015.56

iii.

Volume/ tank =

iv.

Surface area/ tank =

3

= 3,338.52 m3

3,338.52 6

= 556.42 m2

Assuming Length to Breadth ratio is 2:1 2× Breadth2 = 556.42 m2 v.

Breadth = 16.68 m and Length = 2× 16.68 m = 33.36 m

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vi.

2,903.06 m3

(

Flow rate per tank

)

3 h Inlet velocity = Surface area of tank = 556.42 = 1.73 m/h (m2)

Sizing of the primary clarifiers Primary Clarifiers Design parameters: Flow rate per day = 70 000 m3 Flow rate per hour = 2917 m3 ≈ 2920 m3 Detention time = 2 h Assumptions: From Qasim (livre avec amit) p. 329, a surface loading rate of 35 m3/m2.day and a weir loading rate of 250 m3/m.day are assumed for a daily flow rate of 113,500 m3 , therefore, the same design parameters will be assumed for a flow of 70, 000 m3. Surface loading rate per hour = Weir loading rate per hour =

35 (m3/m2.day) 24 (ℎ)

250 (m3/m.day) 24(ℎ)

= 1.46 m3/m2.h

= 10.42 m3/m.h

Design Calculation: Flow in unit tank per hour =

2920 2

= 1460 m3

Area required for one tank (m2) =

Flow rate Surface loading rate

=

1460 1.46

= 1001.14 ≈ 1002 m2

Volume of unit clarifier tank (m3) = Flow rate × retention time = 1460 × 2 = 2920 m3 Knowing that, area of circle = 𝜋r2, diameter of tank can be found using cross sectional area of circular clarifier. 𝜋

𝐷2 4

= 1001.14

D = 35.7 ≈ 36 m Hence, Diameter of one clarifier tank (m) = 36 m Page 112 of 134

ID: 1114132

Given, volume of cylinder = πr2h, height of cylindrical part of clarifier can be found using π

D2 4

h = 𝑉 = 2920 m3

D= 35.7 m Making h subject of formula, h = 2.92 ≈ 3 m From Clarifier Design 2nd Edition, p.468, it is said that for tanks having diameter approximately above 25 m, a slope of 18 m is usually taken. Therefore the bottom slope of the primary clarifier will be 18 m. Sizing of the MBR 1. Sludge age or sludge retention time As per methodology, the SRT for a membrane bioreactor tank is typically 5 to15 days so as to achieve efficient achieve BOD & nitrogen removal. Therefore, the average SRT is assumed: 𝑆𝑅𝑇 = 2.

5 + 15 = 10 𝑑𝑎𝑦𝑠 2

Feed to microorganism ratio

As the biomass is actively removing the organic substrate in the wastewater, it follows that the BOD loading should be related to the volume of the biomass in the aeration tank (i.e. Sludge Loading) Assume F/M ratio for conventional process = 0.2-0.4 𝐴𝑣𝑒𝑟𝑎𝑔𝑒

𝐹 0.2 + 0.4 𝑟𝑎𝑡𝑖𝑜 = = 0.3 𝑚 2

The lower the f/m ratio, the lower the rate of metabolism and the greater the BOD removal and sludge settleability. However, as removal efficiency increases so does the overall oxygen demand of the system and so the overall cost of BOD removal. F/m ratio is also the rate of BOD or COD applied per unit volume of mixed liquor. From Metcalf and Eddy, 2003; F/M ratio can be expressed as:

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𝐹 𝑄0 (𝑆0 − 𝑆𝐸 ) = 𝑀 𝑉𝑥 𝑊ℎ𝑒𝑟𝑒𝑏𝑦: F/M = food to microorganism ratio, 𝐾𝑔𝐵𝑂𝐷⁄𝐾𝑑𝑀𝐿𝑆𝑆, 𝑑𝑎𝑦 𝑄0 = Inlet Flowrate, 𝑚3 ⁄𝑑𝑎𝑦 𝑆0 = Inlet 𝐵𝑂𝐷, 𝑚𝑔⁄𝐿 𝑆𝐸 = 𝑂𝑢𝑡𝑙𝑒𝑡𝐵𝑂𝐷, 𝑚𝑔⁄𝐿 X = Reactor solids, 𝑚𝑔⁄𝐿 V = Volume of Aeration tank, 𝑚3

3.

Total aeration volume and dimensions of the MBR tank

From the equation of food to microorganism ratio 𝐹 𝑄0 (𝑆0 − 𝑆𝐸 ) = 𝑀 𝑉𝑥 𝑊ℎ𝑒𝑟𝑒𝑏𝑦: F/M =0.3 𝐾𝑔𝐵𝑂𝐷⁄𝐾𝑑𝑀𝐿𝑆𝑆, 𝑑𝑎𝑦 𝑄0 = 70,000 𝑚3 ⁄𝑑𝑎𝑦 𝑆0 = 200𝑚𝑔⁄𝐿 = 14000 kg/day 𝑆𝐸 = Taking into consideration permissible limits, that is 40 mg/L = 2800 kg/day X = Reactor solids, 𝑚𝑔⁄𝐿 Assuming that the bioreactor have a concentration varying in the range 2000 𝑚𝑔⁄𝐿3000𝑚𝑔⁄𝐿 Hence, an average MLSS concentration is calculated which equals to 2500 𝑚𝑔⁄𝐿 V = Volume of Aeration tank, 𝑚3

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(𝐾𝑔𝐵𝑂𝐷) 70,000 𝑚3 ⁄𝑑𝑎𝑦 (200 𝑚𝑔⁄𝐿 − 40 𝑚𝑔⁄𝐿) 0.3 = (𝐾𝑑𝑀𝐿𝑆𝑆, 𝑑𝑎𝑦) 𝑉 × 2500 𝑚𝑔⁄𝐿

𝑉 = 14934 𝑚3 From methodology, range of depth should vary from 4m to 7 m Assuming depth 4 m and width 4.4 m typical length to width ratio for MBR Therefore: Width = 4.4 m Length = 44 m 14934 𝑚3 𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑡𝑎𝑛𝑘 = = 3733.33 𝑚2 4𝑚

3733.33 𝑚2 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐶ℎ𝑎𝑛𝑛𝑒𝑙𝑠 = = 19.28 = 20 𝐶ℎ𝑎𝑛𝑛𝑒𝑙𝑠 4.4 𝑚 × 44𝑚 Therefore: Actual Length =

𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎 𝐴𝑐𝑡𝑢𝑎𝑙 𝑤𝑖𝑑𝑡ℎ

=

3733.33 4.4×20

= 42.42 𝑚

Dimensions of one channel = 42.5 m × 4.4m ×4.5 m Total surface area of the Aeration tank = 42.5m × (4.4m × 20) = 3740 𝑚2 Dimensions of the tank = 42.5m × 88 m

4.

Aeration Period or Hydraulic Retention time 𝑉 𝑚3 14934 𝐻𝑅𝑇 = = = 0.213 𝑑𝑎𝑦 = 5.12 ℎ𝑜𝑢𝑟𝑠 3 𝑄 𝑚 ⁄𝑑𝑎𝑦 70000

5.

Volumetric BOD loadings

The volumetric BOD loading is defined as the ratio of BOD (Kg/day to the Volume (m3). Page 115 of 134

ID: 1114132

𝑉𝑜𝑙𝑢𝑚𝑒𝑡𝑟𝑖𝑐 𝐵𝑂𝐷 𝐿𝑜𝑎𝑑𝑖𝑛𝑔 = 6.

𝐵𝑂𝐷, 𝐾𝑔/𝑑𝑎𝑦 13,968.5𝐾𝑔/𝑑𝑎𝑦 𝐾𝑔 = = 0.938 3 3 3 𝑉𝑜𝑙𝑢𝑚𝑒, 𝑚 14934 𝑚 𝑚 . 𝑑𝑎𝑦

Oxygen Requirements

Air requirement = 0.8 Kg/ Kg BOD removed Therefore: 𝐴𝑖𝑟 𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑚𝑒𝑛𝑡 = 0.8 Kg × 11200 Kg/ day = 8960 Kg / day Also since density of air = 1.202 Kg/𝑚3 Air Requirement =

8960 Kg / day = 7454.24 𝑚3 𝑜𝑓 𝑎𝑖𝑟⁄𝑑𝑎𝑦 1.202 Kg/𝑚3

Assuming 21% of oxygen in air; Oxygen Requirement = 7454.24 𝑚3 𝑜𝑓 𝑎𝑖𝑟⁄𝑑𝑎𝑦 × 0.21 = 1565.40 𝑚3 𝑜𝑓 𝑂2 ⁄𝑑𝑎𝑦 Air supplied per 𝑚3 of wastewater treated 𝐴𝑖𝑟𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 =

7454.24 𝑚3 𝑜𝑓 𝑎𝑖𝑟⁄𝑑𝑎𝑦 = 0.106 𝑚3 𝑜𝑓𝑎𝑖𝑟⁄𝑚3 𝑤𝑎𝑠𝑡𝑒𝑤𝑎𝑡𝑒𝑟 70000 𝑚3 ⁄𝑑𝑎𝑦

Oxygen supplied per 𝑚3 of wastewater treated

1565.40 𝑚3 𝑜𝑓 𝑂2⁄𝑑𝑎𝑦 𝑂𝑥𝑦𝑔𝑒𝑛𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 = = 0.022 𝑚3 𝑜𝑓𝑂2⁄𝑚3 𝑤𝑎𝑠𝑡𝑒𝑤𝑎𝑡𝑒𝑟 70000 𝑚3 ⁄𝑑𝑎𝑦 Membrane biological reactor Shape

Rectangular

Total volume

14934 m3

Dimensions (L x B x H)

42.5m × 88 m × 4.4 m

Number Of channels

20 channels

Material

reinforced concrete

Page 116 of 134

ID: 1114132

Water temperature

280C brought to 200C

Liquid depth

7m Fine bubble ceramic diffuser

Aeration system

Oxygen demand 3 1565.40 𝑚 𝑜𝑓 𝑂2⁄𝑑𝑎𝑦

Aeration period

=

5.12 ℎ𝑜𝑢𝑟𝑠

Diffuser submergence

7m

Oxygen transfer efficiency (OTE)

35%

Aeration configuration

Covering the floor completely

Air supply

Using a centrifugal blower feeding 7454.24 𝑚3 𝑜𝑓 𝑎𝑖𝑟⁄𝑑𝑎𝑦

SRT

10 days

BOD vol. loading F/M

0.938

𝐾𝑔 𝑚3 . 𝑑𝑎𝑦

0.3

Sizing the Sand filter Sizing of sand filter The filter should be able to treat all the water that is decanted from secondary clarifier tank. The following calculations and assumptions show the filter capacity required for our sewage treatment plant:

Page 117 of 134

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

Assumptions

Parameters

Value

Remarks

Inlet flowrate

70,000 m3/day

-

Number of sand filter

4

-

Effective size of the sand

0.45 mm

-

average porosity for sand

0.435

-

specific gravity for silica 2.6 sand

-

Sphericity for sand

0.75

-

Duration of filtration

20 hours (per day)

Allow 4 hours for rest, backwash, etc.

Filtration rate

10 m/h

-

Depth of sand layer

0.8 m

-

Height of filter

3.5 m

-



Calculations

Flow rate in each filter bed = Area =

m3 ) d

Flowrate ( h d

70000 4

= 17500 m3/d

17500

m h

(20 ×Filtration rate )

= 20×10 = 87.5 m2

Assuming square base, Length = √Area = √87.5 m2= 9.35 m Breadth = Length = 9.35 m Volume = Surface Area of 1 sand filter × bed depth = (9.35 m × 9.35 m) × 3.5 m = 305.98 m3

Page 118 of 134

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Available head loss in each filter unit is assumed to be 2.5 m Sizing of chlorination unit Chlorination system: Assumption (Dar Lin, S., 2007 & Metcalf & Eddy,2002) 

Considering 4 contacts tanks, the flow in the chlorination system divided in 4.

 Recommended dosage of chlorine/mg/l from activated sludge effluent without any filter: 7mg/l 

Concentration of available chorine in NaOCl =10% correction factor=0.45



Considering a minimum contact time of 30 minutes



Depth to width ratio<1



Length to width ratio ranges between 40-70



Considering a basin with 3 passes and an free overboard of 0.6m



Stock of NaOCl is kept for 15 days and considering a decay rate of 0.03% per day

From literature review Dosage: 7mg/l (7ppm) Daily requirement = (dosage of chlorine (kg/m3) x flowrate (m3/day) Daily requirement = 7/1000 x 69,673.5 = 487.7kg/day Chlorine concentration in solution (kg/m3) = 10% =100g/l = 100kg/m3 Amount of chlorine in solution (kg/s) = 487.7/ ( 86400 x 10) = 0.000564 kg/s Amount of solution required (m3/day) = 487.7/100 = 4.87m3/day Flowrate of NaOCl (m3/s) = 4.72/86400 = 0.00564 m3/s

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Determining the volume of one contact tank (m3) Volume of contact tank = flowrate in contact tank (m3/s) x contact time (s) Volume of contact tank = 0.806/4 x 30 x 60 = 362.9 m3 4 contact tank = 1,451.5 m3 Applying correction factor= number of day×decay/day=15day×0.03decay/day= 0.45 Storage tank volume for NaOCl soln/m3 = ((volume of NaOCl/m3/day× number of day ×10)/(10-Correction factor)) =((4.87 m3/day ×15×10)/(10-0.45)) =76.5 m3 Length, Width and Height of tank = 3m×4m×7m

From assumptions above Depth of the tank/m =1.8m Width of the tank/m=2.2m Therefore cross sectional area = 1.8x 2.2 = 3.96 m2 Length of tank = volume of tank / cross sectional area Length of tank = 362.9 m3/ 3.96 m2 = 91.6m Length of each pass (m) = length of tank inside basin / number of passes in basin = 91.6/3 = 30.5m

Sizing of thickener Solids from primary clarifier = 10773 kg/d Solids from final clarifier = 7538.07 kg/d 1)

Total solids = (10773+7538.07) = 18311.07 Kg/d

Flow from primary clarifier = 326.45 m3/d

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Flow from final clarifier = 222.26 m3/d 2)

Total flow = (326.45 +222.26) = 548.71 m3/d

Assumptions from Metcalf & Eddy, 2003: (a)

Specific gravity of combined sludge = 1.02

(b)

Hydraulic loadings for combined sludge = 6 - 12 m3/m2.d

(c)

Solids capture efficiency = 90 %

(d) According to table 14-19 in Wastewater Engineering Book, for a solids concentration of 3.27 %, a solids loading of 40 – 80 Kg/m2.d (e)

Bottom of thickener is sloped at 20 cm/m (1:5)

(f)

6% solids concentration in thickened sludge

(g)

The typical value of diameter obtained is 10-24 m.

3)

Percentage

solids

in

sludge

=

(

𝑠𝑜𝑙𝑖𝑑𝑠 𝐹𝑙𝑜𝑤 𝑥 𝑠.𝑔 𝑥 1000

)

x

100

18311.07

= (548.71 𝑥 1.02 𝑥1000 ) x 100 = 3.27 % 𝑠𝑜𝑙𝑖𝑑𝑠

4)

Total area =𝑠𝑜𝑙𝑖𝑑𝑠 𝑙𝑜𝑎𝑑𝑖𝑛𝑔= (18311.07Kg /d) / (60 kg/ m3.d) = 305.18 m3

5)

Diameter of thickener

2 thickeners are provided; area of 1 thickener =

305.18 2

= 152.59 m2

Required diameter of 1 thickener = √ {(4 x 152.59 m2) / ∏} = 13.94 m 6)

Checking hydraulic loading

Hydraulic loading =

𝑓𝑙𝑜𝑤𝑟𝑎𝑡𝑒 𝑎𝑟𝑒𝑎

548.71

= 305.18= 1.80 m3/m2.d

For combined primary and waste activated sludge, hydraulic loading should be in the range of 6 – 12m3/m2.d. 1.80 m3/ m2.d is less than the minimum value; therefore provision for dilution water should be provided. Amount of dilution water = (6 m3/m2.d x 305.18 m2) –548.71m3 = 1282.37m3 Page 121 of 134

ID: 1114132

7)

Side – water depth

Generally, clear water and settling zone is in the range of 1.2 m – 1.8 m and the thickening zone is 3.0 m (Metcalf & Eddy, 2003). For the purpose of design, clear water zone = 1.1 m; settling zone = 1.7 m; thickening zone = 3.0 m Total side – water depth = (1.1 + 1.7 + 3.0) m = 5.8 m + freeboard of 0.6 m 8)

Depth of central hopper 20

Depth of central hopper = 100 x

13.94 2

= 1.394 m

Total water depth at central hopper = (5.8 + 1.394) m = 7.194 m 9)

Thickening period

Volume of each thickener = {(∏ / 4) x 13.942 m2 x 5.8 m} + {(∏ / 12) x 13.942 m2 x 1.394 m} = 956.12 m3 Thickening period =

2 𝑥 956.12 548.71

= 3.48 days

For the total amount of thickened sludge, TSS in dilution water has been assumed to be negligible in thickened sludge withdrawal: Total TSS in sludge = 18311.07Kg/d; 90 % of TSS removal in thickened sludge Amount of thickened sludge = 0.9 x 18311.07Kg /d = 16478 Kg/d Volume of 6% solids concentration = (16478 kg /d) / (0.06 x1.02 x 1000 kg / m3) = 269.28 m3 10)

Quality of thickener overflows

Overflow solids in supernatant = (0.1 x 18311.07Kg/d) = 1831.107Kg/d Overflow of recovered water = (inflow + dilution water) – thickened sludge outflow = (548.94+1282.37) – 305.18 m3 = 1526.13 m3 / d 1831.107 𝑥 106

Concentration of solids in recovered water = 1526.13 𝑥 1000=1200 mg/ L

Page 122 of 134

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Sizing of Sludge digester Assumption: (a)

Percentage VSS in primary sludge = 70 %

(b) From equation 14 – 14 in Wastewater Engineering, solids retention time at 40 0C = 10 days as a minimum requirement; 15 days is chosen for higher destruction of VSS (liptak equation) (c)

2 digesters are provided

(d) According to Metcalf & Eddy, Diameter should be 6-38 m and height should be 7.5 -15 m (e)

Conical tank floor is at a slope of 1:5

(f)

Gas produced is 1 m3/Kg per VSS destroyed

(g)

67 % of digester gas is methane gas (water environment research,2004)

(h) Specific gravity Takikawa,2009))

of

digested

sludge

=

1.01(Metcalf&

eddy

,2003,

S.

Solids from primary clarifier = 10773 Kg/d; vss Solids from final clarifier = 7538.07Kg/d Percentage VSS for combined primary and WAS =(

10773 𝑥 0.7+7538.07 18311.07

) x 100 % = 82%

Amount of VSS in thickened sludge = 0.82 x 16478 Kg / d = 13511.96Kg / d 1)

Digester volume = (269.28 m3 / d) x 15 d = 4039.2 m3

2) Checking VSS loading rate = (13511.96 kg VSS / d) / (4039.2m3) = 3.35 kg VSS / m3.d From table 14-28, VSS loading rate should be between 3.3-3.8 Kg VSS / m3.d for 15 days Volume of 1 digester =

4039.2 2

= 2019.6 m3

Since diameter is taken to be 15m; radius = 15/2 = 7.5 m Area = ∏ x (7.5m) 2 = 176.71 m2 2019.6

Active depth = 176.1 =11.4 m Page 123 of 134

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3)

Additional depth as follows

Grit deposit = 0.608 m; scum blanket = 0.608m; space below cover at maximum level = 0.608m 4)

Side wall height & central hopper depth:

Total sidewall height = 11.4 m + (0.608 x 3) m = 13.22 m 1 15

Depth of central hopper = 5x 2 = 1.5 m Total water depth at central hopper = (13.22 + 1.5) m = 14.72m 5)

Percentage & Amount of VSS destroyed

By equation: Vd = 13.7 x ln (SRTdes) + 18.9 = 13.7 x ln(15 ) +18.9 = 56% = 0.56 Amount of VSS destroyed = (0.56 x 13511.96 Kg/d) = 7566.7 Kg/d 6)

Amount of methane gas produced due to destruction of VSS

Gas produced = (7566.7 Kg / d) x (1 m3 / d) = 7566.7 m3 /d Methane gas produced = 0.67 x 7566.7m3 m3 / d = 5069.7 m3 / d 7)

Quality of digested sludge

Fixed solids in feed sludge = (16478–13511.96)Kg/d = 2966.04 Kg/d VSS remaining after digestion = (13511.96–7566.7) Kg/d = 5945.26Kg/d Total solids in digested sludge = (2966.04 +5945.26) Kg/d = 8911.3Kg/d Single stage digesters operate without supernatant withdrawal. Therefore, the volume of m3/d fed is the same as the volume withdrawn. Volume of digested sludge removed = 159.2 m3/d 8911.3 𝑥 100

Solids concentration in digested sludge =269.28 𝑥 1.01 𝑥 1000= 3.28 % 1.

Calculations over sludge heating

Assumptions: a)

The minimum air temperature = 19.80C

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b)

The average temperature for earth around wall = 22.4 0C

c)

The temperature of earth below floor = 25.2 0C

d)

The temperature of raw sludge feed = 250C

e)

Heat transfer coefficient of:

I) Insulated wall exposed to air = 0.6 – 0.8 W/m2.0C ii) Wall exposed to dry earth = 0.57 – 0.58 W/m2.0C iii) Moist earth below floor = 0.7 W/m2.0C iv) Insulated roof = 0.16 – 0.18 W/m2.0C f)

The specific heat of sludge = 4200 J/Kg. 0C

g)

Half of the digester is found below the earth (Metcalf& Eddy,2003)

Sludge solids feed to each digester = 16478Kg / d = 8239Kg / d 1)

Heat required for 1 digester

Heat required for 1 digester, Q1 = feed sludge weight * specific heat of sludge * (operating temp. of digester – temp. of incoming sludge) = (8239kg /d) x (4200 J /kg. 0C) * (40 – 250C) =519.06 MJ/d Each digester will have its own heat exchanger; thus the need for 2 heat exchangers with capacity of 519.06 MJ/d each. 2)

Computing area of each component

a)

Wall area = 2∏rh = 2 x ∏ x (5 m) x (13.98m) = 439.19 m2

b)

Floor area = ∏rl = ∏ x (5m) x √[(5m)2 + (1m)2] = 80.10m2

c)

Roof area = ∏r2 = ∏ x (5m)2 = 78.54m2

Assumption: (a) Heat loss to surroundings is minimized due to insulation and will be analyzed in detailed design (b)

Constant properties of materials Page 125 of 134

ID: 1114132

(c)

Fouling factors and tube resistance will be taken care of in detailed design

(d)

Mass flow rate of water (the heating liquid) will be twice that of sludge

(e)

Temperature of water is available at 60 0C from the CHP

(f)

Sludge has the same properties as water

(g) Diameter of inner and outer tubes are 75 mm and 150 mm respectively to heat thickener sludge (Metcalf& Eddy, 2003) Sizing of dewatering unit a. Tank Sizing Total solids = (10773+7538.07) = 18311.07Kg/d= 763 kg/hr From literature review, the amount of polymer used = 8 lb of polymer per ton of dry solids = (8 x 0.454) = 3.632 kg of polymer per ton of dry solids Thus, total amount of polymer used = [18311.07 kg/d / (1000kg)] x 3.632kg = 39.27 kg/ day = 2.771 kg/hr The following assumptions are made: o

A detention time of 6.5 minutes is considered for the first tank.

o

The depth of balancing tank is taken to be 1 m.

o The polymer is diluted by water to 0.1 to 0.2 % concentration. A concentration of 0.15 % is chosen for this tank design (MELTCALF & EDDY, 2003) 1)

Volume of Tank

The volume of tank is calculated as follows: A concentration of 0.15 % is chosen for this tank design. 1.5 g of polymer is present in 1 L of water Thus, 2771 g of polymer (total amount of polymer per hour) is present in (1/1.5) x 2771= 1847.4L of water Page 126 of 134

ID: 1114132

Thus the volume of water (and polymer) in the first tank = [1847.4L/ (1000 L / m3)] =1.85m3 2)

Surface Area of Tank

The surface area of the tank is calculated as follows: Surface area of tank = Volume of tank / Depth of tank= 1.85 m3 / 1 m= 1.85 m2 1 tank is used. Length: Breadth = 2:1 L x B = 2B * B =2B2 Thus; B = 0.96 m and L = 1.92 m Volume designed = 1.92 m x 0.96 m x 1m + 0.5m (freeboard) 3)

Flow rate of polymer solution to maturation tank

Flow rate of polymer solution = volume of tank/detention time = 1.85m3 / (6.5 min / 60 min) = 17.08 m3 /hr

b.

Maturation tank

The following assumptions were made: a.

Detention time in maturation tank must be in the range of 1hour.

b.

The depth of balancing tank is taken to be 1 m .

c. The tank mixer should be low speed with a low shear impeller designed for a maximum tip speed of less than152.4 m /min 1)

Sizing of Tank

The volume of the dissolving tank is the same as the volume of maturation tank since all the contents of the dissolving tank goes into the maturation tank; thus the volume and surface area are the same. Volume = 1.85 m3

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Surface area = 1.85 m2, since depth of tank is 1 m.

Freeze Design This primary design was an overview of a typical sewage treatment plan. However, many basic assumptions were taken, where detailed information are not taken into consideration. The detailed design introduces a more specific plan of the plant where each and every single steps of calculations, sketches, instrumentation and control, detailed mass and energy balances are further discussed. The detailed design will include:        

A much further description of the equipment and processes used, Heat and mass transfer around specific equipment allocated, Energy balance around each equipments, Detailed mechanical design sketches of each equipment in concerned, Detailed information about the material of construction, More detailed instrumentation and control measures, Safety considerations, A feasible economic consideration of the whole plan.

Hence, all the units discussed in this primary design would be further elaborated in much more detail in the detailed design coming next.

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Minutes of meeting

09/08/2014 Chairperson: Mr. Mudhoo Secretary: Keshav Soomaree MEETING DETAIL: Discussion on the weekly meetings: 

Each meeting on the will consists of a secretary and a chairperson.

 The roll of secretary or chairman will be assigned to every member of the group in a specific order at each meeting intervals: Secretary

Chairperson

Keshav

Mr.Mudhoo

Teesha

Keshav

Amit

Teesha

Pravish

Amit

Mr.Mudhoo

Pravish



The secretary need to take notes of the important topics talked in the meeting.



Notes taken must be converted to PDF and handled to the supervisor.

Discussion on the Project: Design of a sewage treatment plant: 

Overview on the problem statement.



Understanding what need to be done.

Note: Members of the group agreed that they will be able to follow the guideline given. 

A backup of the project must be safely kept.

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 Each member of the group needs to keep record of the different individual work of all the group members in separate folders. 

There will be at least ten unit operations both majors and minors.

Discussion on the project introduction: 

Brief intro on sewage system



Impact on environment



Why must it be treated



The different harmful constituents of the waste



What are the benefits associated



Cost implications(Locally)



Actual sewage treatment plant in Mauritius

The introduction must be concluded by an intro on the design project: “The design project will consist of designing: ....” Discussion on the literature review and researches to be done:  Each member must do their own research and give their ideas on the next meeting scheduled.  The overall PFD of the system must be ready and submitted to the supervisor in five weeks time, that is, on the 30/08/2014.  Different types of equipments or material to be used must be analysed and one of them must be selected with appropriate justification. 

Chapter 1, 2 & 3 must be ready by the fifth week (30/08/14).

 Each member of the group must have a printed copy of the work done on every chapter covered. Discussion about the Process Consideration:  Group members should bring innovative ideas or improvements relevant to the design project.

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 Any module done in the previous academic years must be pin pointed in every relevant section worked. Discussion on the distribution of work:  The name of each member must be noted on a separate sheet specifying which work they performed.  A note need to be affixed at the end of the project confirming: “Each member of the group has fairly contributed in every part of the design works”. 

The note must be signed by all members and approved by the supervisor.

End of meeting

20/09/2014 Chairperson: Pavish Ramdewar Secretary: Keshav Soomaree Members present: Keshav Soomaree, Pravish Ramdewar, Eldora St Paul, Amit Ramdhonee, Teesha Ramanah. Discussion we made on: 

The researches made on PFD



Selection of an appropriate PDF for our plant

We agreed to work on the followings for the equipment we were assigned: 

Introduction



Literature review



Process consideration



Mass balance



Energy balance



Sizing

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

Control strategies



Preliminary Hazop



Waste treatment



Costing

End of meeting

04/10/2014 Chairperson: Eldora St Paul Secretary: Pravish Ramdewar Members present: Pravish, Eldora, Keshav, Teesha 

Discussion on the problems of mass balances,



Discussion on how to manage our energy balances,



Tackled the problems faced on sizing of the equipments,



We agreed to complete our mass balances and share the work by the week after.

Note: All problems tackled and discussion made were shared with Amit

End of meeting 11/10/2014 Chairperson: Keshav Soomaree Secretary: Amit Ramdhonee Members present: Amit, Keshav, Pravish  Discussion was done on the biogas balances and energy strategies of the out but biogas. Page 132 of 134

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 We decided to fully use the electricity, being obtained from the biogas, for the self consumption of the plant (Pumps Blower, mechanical stirrer, etc...).  We discussed how to manage our recycle streams especially for the secondary treatment. 

The issue of sludge digester was also raised.



We analysed the screening units and the oil-water separator in detail.

End of meeting

25/10/2014 Chairperson: Pravish Ramdwar Secretary: Teesha Ramanah Members Present: Teesha, Pravish, Amit, Eldora, Keshav 

Significant changes in our units,



Replacing the anaerobic system to aerobic one,



All biogas to be removed would be obtained only from the sludge digester,

 The unit of carbon adsorption is removed from the plant: No need of it; costly and difficult to design, 

The addition of two primary clarifiers + one secondary clarifier,



Clarifiers are changed from rectangular to circular one.

Re distribution of the work Note: The new work format of our treatment plant was approved by Mr. Mudhoo.

End of meeting 08/11/2014 Chairperson: Amit Ramdewar Page 133 of 134

ID: 1114132

Secretary: Eldora St Paul Members Present: Eldora, Amit, Keshav, Pravish 

Discussion of the compilation of our work,



Each member brought their work

We discussed the followings: 

Sizing of the equipments + finalisation of the Material balances,



Control strategies



PID



Primary costing(seen in detail)

A specific format was chosen for the compiled work.

End of meeting

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