Biological Wastewater Treatment

UNIT II WASTEWATER BIOTREATMENT

Biological treatment of waste water Methods  Aerobic : removal of organic pollutants in wastewater by bacteria that require oxygen to work End products: Water and carbon dioxide and biomass  Anaerobic :bacteria digests biosolids in the absence of oxygen End products: methane and carbon dioxide gas and biomass

Aerobic Biological Treatment  Steps  Primary  secondary  Tertiary treatments

Aerobic Biological Treatment Technologies 1) 2) 3) 4)

Conventional Activated Sludge Process (CASP) System Cyclic Activated Sludge System (CASS) Membrane Bioreactor (MBR) Biofilm reactors in aerobic biological wastewater treatment: - Trickling Filters - Integrated Fixed Film Activated Sludge (IFAS) System - Submerged Aerobic Fixed Film Reactor - Fluidized Bed Bioreactor - Hybrid Biofilm/suspended Growth processes - Rotating biological contactors

Conventional Activated Sludge Process (CASP) System  most common and oldest biotreatment process  used to treat municipal and industrial wastewater  wastewater after primary treatment i.e. suspended impurities removal is treated in an activated sludge process based biological treatment system comprising aeration tank followed by secondary clarifier.  The aeration tank is a completely mixed or a plug flow (in some cases) bioreactor where specific concentration of biomass (wide variety of microorganisms) (measured as mixed liquor suspended solids (MLSS) is maintained along with sufficient dissolved oxygen (DO) concentration (typically 2 mg/l) biodegradation of soluble organic pollutants (measured as Chemical oxygen demand (COD) and Biological oxygen demand (BOD)

 The aeration tank is provided with fine bubble diffused aeration pipework at the bottom to transfer required oxygen to the biomass and also ensure completely mixed reactor.  The aerated mixed liquor from the aeration tank overflows to the secondary clarifier unit to separate out the biomass and allow clarified, treated water to the downstream filtration system for finer removal of suspended solids.  The separated biomass is returned to the aeration Tank by means of return activated sludge (RAS) pump.

Activated sludge plant

Cyclic Activated Sludge System (CASS)  Cyclic Activated Sludge System (CASS) - one of the most popular sequencing batch reactor (SBR) processes employed to treat municipal wastewater and wastewater from a variety of industries including refineries and petrochemical plants.  This technology offers several operational and performance advantages over the conventional activated sludge process.  The CASS SBR process performs all the functions of a conventional activated sludge plant (biological removal of pollutants, solids/liquid separation and treated effluent removal) by using a single variable volume basin in an alternating mode of operation, thereby dispensing with the need for final clarifiers and high return activated sludge pumping capacity.

 The Cyclic Activated Sludge System (CASS), incorporates a high level of process sophistication in a configuration which is cost and space effective and offers a methodology that has operational simplicity, flexibility and reliability that is not available in conventionally configured activated sludge systems.

 The reactor basin is divided by baffle walls into three sections Zone 1: Selector, Zone 2: Secondary Aeration, Zone 3: Main Aeration).  Sludge biomass is intermittently recycled from Zone 3 to the Zone 1 to remove the readily degradable soluble substrate and favor the growth of the floc-forming microorganisms.  System design is such that the sludge return rate causes an approximate daily cycling of biomass in the main aeration zone through the selector zone.  No special mixing equipment or formal anoxic mixing sequences are required to meet the effluent discharge objectives.

 CASS utilizes a simple repeated time-based sequence which incorporates: 1 & 2) Aeration (for biological reactions) 2) Settle (for solids-liquid separation) 3) Decant (to remove treated effluent)

Characteristics of aerobic activated sludge:  Overall, activated sludge must contain a microorganisms capable of producing all enzyme systems required for the biodegradation of both soluble and insoluble pollutants.  Activated sludge contains prokaryotes (bacteria) and Eukaryotes (Protozoa and fungi)  The primary consumers of organic wastes are the heterotrophic (An organism that cannot synthesize its own food and is dependent on complex organic substances for nutrition) bacteria  The majority of the bacterial genera in activated sludge are Gram-negative  Pseudomonas, Arthrobacter, Comamonas, Lophomonas, Zoogloea, Sphaerotilus, Azotobacter, Chromobacterium, Achromobacter,Flavobacterm, Bacillus, and Nocardia.  Certain genera Sphaerotilus or Nocardia-causes poor settling  Many types of protozoa : 50,000 cells/ml

Design of activated sludge process • The main parameters that have to be taken into account when designing activated sludge systems are: • Influent : Characteristics must be known before treatment - Flow rate of the inflow of waste water - Substrate concentration in inflow of waste water (For e.g. BOD) - Active biomass concentration in inflow of wastewater • • • •

Kinetic data and stoichiometric coefficients Sludge recycle rate Sludge wasting rate Oxygen uptake rate

hydraulic loading rate, HLR kg BOD/m3/day = raw BOD (kg/m3) x flow rate (m3/day) aeration volume (m3) hydraulic retention time (HRT, h) = reactor volume (m3) flow (m3/h) sludge loading rate SLR, kg BOD/kg MLSS/day = raw BOD (kg/m3) x flow rate (m3/day) MLSS (kg/m3) x aeration tank volume (m3)

Membrane Bioreactor (MBR)  Latest technology for biological degradation of soluble organic impurities.  Extensively used for treatment of domestic sewage  but for industrial waste treatment applications, its use has been somewhat limited or selective.  MBR is an improvement of CASP, where the secondary clarifier is replaced by a membrane unit for the separation of treated water from the mixed solution in the bioreactor  System comprises of activated sludge process with the biomass separation carried out by membrane process  Separation of biomass using membrane provides filtered quality final effluent – can reuse

Biofilm reactors in aerobic biological wastewater treatment  2 categories: (1) Fixed-medium systems : biofilm media are static in the reactors and the biological reactions take place in the biofilm developed on the static media, and (2) Moving-medium systems : biofilm media are kept continually moving by means of mechanical, hydraulic, or air forces. ()Biofilms can be used in various types of reactors Trickling filters and biological towers Rotating biological contactors Fed Batch Reactors (FBRs) Granular media filters Fluidized bed biofilm reactors Hybrid biofilm/suspended growth processes

Trickling Filters

Flow Diagram for Trickling Filters Recycle Final clarifier Final effluent

Influent

Primary clarifier

Trickling filter

Waste sludge

Trickling Filters • Not a true filtering or sieving process • Material only provides surface on which bacteria to grow • Can use plastic media – lighter - can get deeper beds (up to 12 m) – reduced space requirement – larger surface area for growth – greater void ratios (better air flow) – less prone to plugging by accumulating

Trickling Filters

Filter Material

Typical Trickling Filter

Random Packing

Structured Media

Bio-towers

Trickling Filter •

• • • • • • •

Tank is filled with solid media – Rocks – Plastic Bacteria grow on surface of media Wastewater is trickled over media, at top of tank As water trickles through media, bacteria degrade BOD Filter is open to atmosphere, air flows naturally through media Treated water leaves bottom of tank, flows into secondary clarifier Bacterial cells settle, removed from clarifier as sludge Some water is recycled to the filter, to maintain moist conditions

Trickling Filter System

Trickling Filter Process

Types of Trickling Filters •

•

•

Standard or low rate – single stage rock media units – loading rates of 1-4 m3 wastewater/m2 filter crosssectional area-day – large area required High rate – single stage or two-stage rock media units – loading rates of 10-40 m3 wastewater/m2 filter crosssectional area-day – re-circulation ratio 1-3 Super rate – synthetic plastic media units- Plastic media depths of 5-10 m • modules or random packed • specific surface areas 2-5 times greater than rock • much lighter than rocks • can be stacked higher than rocks – Loading rates of 40-200 m3 wastewater/m2 filter

Design Criteria for Trickling Filters T a b le 1 0 .5 T y p ic a l D e s ig n C r it e r ia f o r T r ic k li n g F il t e r s

I te m

L o w -r a te filte r

H ig h - r a te f ilte r

S u p e r - ra te f ilte r

H y d r a u l ic lo a d in g ( m 3 /m 2 - d )

1 - 4

10 - 40

40 - 200

O r g a n ic lo a d in g ( k g B O D 5 /m 3 - d )

0 .0 8 - 0 .3 2

0 .3 2 - 1 .0

0 .8 - 6 .0

D e p th (m )

1 .5 - 3 .0

1 .0 - 2 .0

4 .5 - 1 2 .0

R e c ir c u la tio n r a tio

0

1 - 3

1 - 4

F ilte r m e d ia

R o c k , s la g , e tc .

R o c k , s la g , s y n th e tic s

F ilte r f lie s

M any

F e w , la rv a e a re w ash e d a w a y

Few or none

S lo u g h in g

I n te r m itte n t

C o n t in u o u s

C o n t in u o u s

D o s in g in te rv a ls

< 5 m in

< 15 s

C o n t in u o u s

E f flu e n t

U s u a l ly f u l ly n it r if ie d

N it r i f i e d a t l o w lo a d in g s

N it r i f i e d a t l o w lo a d in g s

Submerged Aerobic Fixed Film Reactor  cost-effective method of waste water treatment  sewage sanitation that is primarily used in residential and commercial complexes  This equipment primarily works on the three stages that are Primary Settlement, Secondary Treatment and Final Settlement / Clarification  Improved treatment quality  particularly for small to medium sized treatment plants where available land is limited, and where full time operational manning would be uneconomical  A well built Submerged Aerated Filter plant has no moving parts within its main process zones, any serviceable items will be positioned to access easily without disrupting the ongoing sewage treatment

Fluidized Bed Bioreactor Fluidized bed bioreactors (FBR) have been receiving considerable interest in wastewater Treatment. A fluidized bed bioreactor consists of microorganism coated particles suspended in wastewater which is sufficiently aerated to keep the gas, liquid and the solid particles thoroughly mixed. capable of achieving treatment in low retention time because of the high biomass concentrations that can be achieved . successfully applied to an aerobic biological treatment of industrial and domestic wastewaters.

Rotating Biological Contactor Rotating Biological Contactors, commonly called RBC’s, are used in wastewater treatment plants (WWTPs). The primary function of these bioreactors at WWTPs is the reduction organic matter. Very effective for low strength organic wastes

Rotating Biological Contactors

RBC Schematic Film of Microorganisms

Rotation

Wastewater

Primary Settling Sludge Treatme nt

Secondary Settling

Sludge Treatment

• Design of Rotating Biological Contactors  Organic loading rate ( g BOD per m3 disc area per day)  Hydraulic retention time (d)  Diameter of discs (m)  Number of discs  Depth of submergence of rotating disc (m)  Rate of rotation (rpm)

Hybrid Biofilm/suspended Growth processes  Activated sludge process can be enhanced in terms of capacity and reliability through the introduction of biofilm media that create hybrid biofilm/suspended growth system  Existing activated sludge-upgraded by adding biofilm surface area inside the aeration basin  The goal is to increase the total mass of active bacteria in the system and to increase the SRT  Suspended and biofilm bacteria have different SRTs  Biofilm SRT-longer- which makes it especially important for accumulating slow growing species such as nitrifiers  Eg: Activated biofilter • In the settler MLVSS--- maintained around 2,500 mg/L • BOD loading--- 9 Kg BOD5/m3/day

Anaerobic waste-water treatment Anaerobic treatment is often performed for solids and high-strength industrial waste-waters (upto 1,00,000 COD).  both facultative and obligate anaerobic microorganisms, in the absence of oxygen are being involved in the anaerobic waste water treatment.  They biodegrade the organic pollutant to generate methane, carbon dioxide and biomass.  The microbiology of these anaerobic digestion processes is complex.  Their efficient and stable operation requires a microbial population containing at least 3 different interacting microbial groups. 1) Fermentative/hydrolytic bacteria (group 1) 2) Acetogenic bacteria (group 2) 3) Methanogenic bacteria (group 3) 

) There are 3 stages: 1)Hydrolysis 2) Acetogenesis 3) Methanogenesis

 The first stage of the process, hydrolysis involves the hydrolysis of complex organic wastes (proteins, fatty acids and polysaccharides) by the hydrolytic bacteria.

The hydrolysis of these wastes results in the production of simplistic, soluble organic compounds (amino acids, volatile acids and alcohols).  The second stage of the process, acetogenesis, involves the conversion of the volatile acids and alcohols to substrates such as - acetic acid or acetate (CH3COOH) and - hydrogen gas by acetogenic bacteria further that can be used by methane-forming bacteria.  The third and final stage of the process, methanogenesis, involves the production of methane and carbon dioxide from the products of acetogenesis process by methanogenic bacteria.

Overview of anaerobic digestion processes

 Several reactor designs exist for anaerobic treatment.  simple conventional mixed sludge reactors

utilized primarily to treat solid waste materials such as primary and secondary sludges,  anaerobic filter or upflow anaerobic sludge blanket (UASB) digester. Conventional methods for the disposal of sludges and other solid wastes:  Methods routinely used for the disposal of final sludges and other solid wastes are as follows. 1) Landfilling 2) Incineration 1) Landfilling: used for agricultural, industrial and urban wastes. ) materials can leach into adjacent water courses when unsuitable sites are used. ) expensive due to a lack of suitable landfill sites.

2) Incineration: routinely used for solids.  For sludges, the system operates with limited energy input due to their high calorific value, leading to self combustion.  However, there may be problems with the disposal of residue ash, as it often contains high concentrations of heavy metals.

 The types of bacteria within an anaerobic digester : - saccharolytic bacteria - proteolytic bacteria fermentative bacteria - lipolytic bacteria - methane-forming bacteria  3 important bacterial groups in anaerobic digesters with respect to the substrates utilized by each group.  These groups include - the acetate-forming (acetogenic) bacteria (AFB), - the sulfate-reducing bacteria (SRB), - the methane-forming bacteria (MFB).

ACETATE-FORMING BACTERIA (AFB)  Acetate-forming (acetogenic) bacteria grow in a symbiotic relationship with methane-forming bacteria. Acetate serves as a substrate for methane-forming bacteria.  Eg: when ethanol (CH3CH2OH) is converted to acetate, carbon dioxide is used and acetate and hydrogen are produced.

CH3CH2OH + CO2

CH3COOH + 2H2

 When AFB produce acetate, hydrogen also is produced. If the hydrogen accumulates and significant hydrogen pressure occurs, the pressure results in termination of activity of acetateforming bacteria and lost of acetate production. However, methane-forming bacteria utilize hydrogen in the production of methane so that a significant hydrogen pressure does not occur. CO2 + 4H2 CH4 + 2H2O

 AFBs are obligate hydrogen producers and survive only at very low concentrations of hydrogen in the environment.  They can only survive if their metabolic waste—hydrogen— is continuously removed.  This is achieved in their symbiotic relationship with hydrogen-utilizing bacteria or methane-forming bacteria.  Acetogenic bacteria reproduce very slowly.  Generation time for these organisms is usually greater than 3 days.

SULFATE-REDUCING BACTERIA (SRB)  SRB also are found in anaerobic digesters along with AFB and MFB.  If sulfates are present, SRB such as Desulfovibrio desulfuricans multiply.  Their multiplication or reproduction often requires the use of hydrogen and acetate—the same substrates used by MFB

 When sulfate is used to degrade an organic compound, sulfate is reduced to H2S.  Hydrogen is needed to reduce sulfate to H2S.  The need for hydrogen results in competition for hydrogen between two bacterial groups, SRB and MFB.  When SRB and MFB compete for hydrogen and acetate SRB obtain hydrogen and acetate more easily than MFB under low-acetate concentrations.  At substrate-to-sulfate ratios <2, SRB out-compete MFB for acetate.  At substrate-to-sulfate ratios between 2 and 3, competition is very intense between the two bacterial groups.  At substrate-to-sulfate ratios >3, MFB are favored.

METHANE-FORMING BACTERIA (MFB)  MFB are known by several names - Methanogens -Methanogenis bacteria -Methane forming bacteria - Methane producing bacteria  morphologically diverse group of organisms that have many shapes, growth patterns, and sizes.  The range in diameter sizes of individual cells is 0.1–15 µm.  Filaments can be up to 200 µm in length.  Motile and non motile bacteria as well as sporeforming and non-spore-forming bacteria can be found.

 MFB are some of the oldest bacteria and are grouped in the domain Archaebacteria (from arachae, meaning “ancient”).  The domain thrives in heat.  MFB are - oxygen-sensitive, - fastidious anaerobes - free-living terrestrial and aquatic organisms.  MFB found in habitats that are rich in degradable organic compounds.  In these habitats, oxygen is rapidly removed through microbial activity.  MFB also have an unusually high sulfur content: Approximately 2.5% of the total dry weight of the cell is sulfur.

 MFB has several unique characteristics 1) a “nonrigid” cell wall and unique cell membrane lipid, The cell wall lacks muramic acid, and the cell membrane does not contains an ether lipid as its major constituent. 2) substrate degradation that produces methane as a waste 3) specialized coenzymes. - coenzyme M and - the nickel containing coenzymes F420 and F430. )Coenzyme M is used to reduce CO2 to CH4. )The nickel-containing coenzymes are important hydrogen carriers in methane-forming bacteria.

Anaerobic Reactor Technologies  a number of novel anaerobic reactor configurations have been developed.  2 types - Conventional - High rate Anaerobic digesters (Ads)

Conventional

High-rate AD

Eg: Anaerobic batch reactor (Anaerobic lagoons)

(Many types of reactors)

Stratified

Homogeneous due to mixing

Intermittent feeding and withdrawal

Continuous or intermittant feeding and withdrawal

HRT based on liquid input is 30-60 days

HRT based on liquid input is 15 days or less

VS loading: 500-1600 kg/m3.day

VS loading: 1600-1800 kg/m3.day

High-rate anaerobic reactors        

Completely mixed anaerobic digester (AD) Anaerobic contact process (ACP) Anaerobic sequencing batch reactor (ASBR) Anaerobic packed bed or anaerobic filter Anaerobic fluidized and expanded bed reactors Upflow anaerobic sludge blanket (UASB) reactor Anaerobic baffled reactor (ABR) 2-Phase AD Process

Completely mixed AD  Wastewater and anaerobic bacteria are mixed together and allowed to react.  When the organic pollutant is reduced to desired level, treated wastewater is then removed

 Can be operated in either batch or continuous mode  At least 10 days of SRT and HRT are required because of slow growing methanogens.  This necessitates a reactor with a very large volume  Large volume requirements wash-out of microorganisms in effluent pose serious problems and make ADs unsuitable for use with most industrial wastewaters

Anaerobic contact process (ACP)  Link between high biomass concentration, greater efficiency and smaller reactor size is the idea of ACP.  Settling of anaerobic sludge in a settling tank and its return back to the reactor allows further contact between biomass and raw waste.  due to sludge recycling, the SRT is no longer coupled to the HRT.  As a result, considerable improvements in treatment efficiency can be achieved.  Disadvantage : poor sludge settlement arose from gas formation by anaerobic bacteria in settling tank.

 Although simple in concept, individual units make ACP more complex than other high rate ADs.

Anaerobic Sequencing Batch Reactor (ASBR)  Anaerobic sequencing batch reactor (ASBR) process is a batch-fed, batch-decanted, suspended growth system and is operated in a cyclic sequence of four stages: - feed, - react, - settle - decant.

 Since a significant time is spent in settling the biomass from the treated wastewater, reactor volume requirement is higher than for continuous flow processes.  Not require - additional biomass settling stage or solids recycle.  No feed short-circuiting is another advantage of ASBRs over continuous flow systems.  Operational cycle-times for the ASBR can be as short as 6 hours if biomass granulation is achieved.

Anaerobic Filter (Packed Bed)  Anaerobic filter is a fixed-film biological wastewater treatment process in which a fixed matrix (support medium) provides an attachment surface that supports the anaerobic microorganisms in the form of a biofilm.  Treatment occurs as wastewater flows upwards through this bed and dissolved pollutants are absorbed by biofilm.

 Anaerobic filters were the first anaerobic systems that eliminated the need for solids separation and recycle while providing a high SRT/HRT ratio  Various types of support material can be used - plastics, - granular activated carbon (GAC), - sand, - reticulated foam polymers, - granite, - quartz and stone.  These materials have exceptionally high surface area to volume ratios (400 m2/m3) and low void volumes.  Its resistance to shock loads and inhibitions make anaerobic filter suitable for the treatment of both dilute and high strength wastewaters. Limitations: deterioration of the bed structure through a gradual accumulation of non-biodegradable solids.

 This leads eventually to channelling and short-circuiting of flow, and anaerobic filters are therefore unsuitable for wastewaters with high solids contents.  Additionally, there is a relatively high cost associated with the packing materials. Anaerobic Filter Packings

Fluidized Bed Reactor (FBR)  In order to achieve fluidization of the biomass particles, units must be operated in an upflow mode.  Rate of liquid flow and the resulting degree of bed expansion determines whether the reactor is termed a fluidized bed or expanded bed system.  Expanded bed reactors have a bed expansion of 10% to 20% compared to 30% to 90% in fluidized beds.  In FBR, biomass is attached to surface of small particles (anthracite, high density plastic beads, sand etc.) which are kept in suspension by upward velocity of liquid flow.

 Effluent is recycled to dilute incoming waste and to provide sufficient flow-rate to keep particles in suspension.  Large surface area of support particles and high degree of mixing that results from high vertical flows enable a high biomass conc. to develop and efficient substrate uptake.  Biomass concentration: 15-40 g/l  The greatest risk with FBR is the loss of biomass particles from the reactor following sudden changes in particle density, flow rate or gas production.  In practice, considerable difficulties were experienced in controlling the particle size and density of flocs due to variable amounts of biomass growth on particles.  Therefore FBRs are considered to be difficult to operate.

Upflow Anaerobic Sludge Blanket (UASB) Reactor  The problem associated with anaerobic filters and FBRs has led to development of unpacked reactors that still incorporate an immobilized form of particulate biomass.  most widely used high-rate anaerobic system for domestic & industrial wastewater treatment.  UASB reactor is based on that anaerobic sludge exhibits inherently good settling properties, provided the sludge is not exposed to heavy mechanical agitation.  Adequate mixing is provided by an even flow-distribution combined with a sufficiently high upflow velocity, and by agitation that results from gas production.  Biomass is retained as a blanket or granular matrix, and is kept in suspension by controlling the upflow velocity.

Wastewater flows upwards through a sludge blanket located in lower part of reactor, while upper part contains a three phase (solid, liquid, gas) separation system. Three-phase separation device is the most characteristic feature of UASB reactor. It facilitates the collection of biogas and also provides internal recycling of sludge by disengaging adherent biogas bubbles from rising sludge particles.

 Superior settling characteristics of granular sludge allows higher sludge concentrations  Granular sludge development is now observed in UASB reactors treating many different types of wastewater.

Granulation  Granulation is a process in which a non-discrete flocculent biomass begins to form discrete well-defined granules.  These vary in dimension and appearance depending on the wastewater and reactor conditions, but generally have a flattened spherical geometry with a diameter of 1-3 mm.  Mechanism of biomass granulation has been widely studied the objective being that the rate and extent of granule formation could be manipulated, particularly in wastewaters that show little intrinsic propensity to granulate (e.g. fat and oil containing effluents). Anaerobic sludge granules from a UASB reactor treating effluent

Expanded Granular Sludge Bed (EGSB) Reactor

Hybrid (UASB+Packed Bed) Reactor

Anaerobic Baffled Reactor (ABR)  Anaerobic baffled reactor (ABR) consists of a number of UASB reactors connected in series.  In ABR, wastewater passes over and under the staggered vertical baffles as it flows from inlet to outlet.  Advantages: Unique baffled design enables ABR to reduce biomass washout, hence retain a high active biomass content.  It can recover quickly from hydraulic and organic shock loads.  No special gas or sludge separation equipment is required  Owing to its compartmentalized configuration, it may function as a two-phase anaerobic treatment system with separation of acidogenic and methanogenic biomass.

 ABR has a simple design and requires no special gas or sludge separation equipment. It can be used for almost all soluble organic wastewater from low to high strength.  Considering its simple structure and operation, it could be considered a potential reactor system for treating municipal wastewater in tropical and sub-tropical areas of developing countries

Anaerobic Membrane Bioreactor (AMB)  AMB is still in development stage. Advantages:  Higher biomass concentrations in AMB reduce the size of reactor and increase organic loadings.  Almost complete capturing of solids (much longer SRT) results in maximum removal of VFAs and degradable soluble organics and provide a higherquality effluent.

 Maximum capture of effluent SS improves greatly the effluent quality.  Longer SRT and low effluent SS concentration allow AMB to compete with aerobic treatment processes.  High liquid velocities across the membrane and gas agitation systems might be used to minimize fouling. Disadvantages  Organic fouling problems are typically caused by accumulation of colloidal material and bacteria on the membrane surface.  High pumping flow rates across the membrane may lead to the loss of viable bacteria due to cell lysis.  Developments in membrane design and fouling control measures could make AMB a viable technology in future.

2-Phase AD Process  Two-phase AD implies a process configuration employing separate reactors for acidification and methanogenesis.  These are connected in series, allowing each phase of digestion process to be optimized independently since the microorganisms concerned have - Different nutritional requirements - Physiological characteristics - pH optima - Growth and nutrient uptake kinetics and - Tolerances to environmental stress factors

Advantages of Two-Phase AD  Improvement in process control  Disposal of excess fast growing acidogenic sludge without any loss of slow growing methanogens  Degradation of toxic materials in the first phase (protects the sensitive methanogens)  Precise pH-control in each reactor  Higher CH4 content in biogas from methanogenic phase  Increased loading rate possible for methanogenic stage Disadvantages of Two-Phase AD  High sludge accumulation in the first phase  Lack of process experience and so more difficult to operate  Difficulty in maintaining a balanced segregation of the phases.

Advantages of Two-Phase AD  Improvement in process control  Disposal of excess fast growing acidogenic sludge without any loss of slow growing methanogens  Degradation of toxic materials in the first phase (protects the sensitive methanogens)  Precise pH-control in each reactor  Higher CH4 content in biogas from methanogenic phase  Increased loading rate possible for methanogenic stage Disadvantages of Two-Phase AD  High sludge accumulation in the first phase  Lack of process experience and so more difficult to operate  Difficulty in maintaining a balanced segregation of the phases.

Nitrogen and phosphorous removal

Biological Nitrogen Removal • Uptake into biological cell mass • Nitrification (conversion to Nitrate) • Denitrification (conversion to N2 gas)

NITRIFICATION

Nitrogens are introduced into waste water in the forms of , • Urine • Food processing waste • Chemical cleaning agents • Many other industrial waste

Negative Impacts of Nitrogen on the Environment Nitrate in drinking water can cause Blue Baby Syndrome

Increased levels of nitrate in water supplies can increase the acidity of the water and make toxic metals such as mercury soluble Growth more of nuisance algae can cause decreased water quality and cause fish kills 

DEFINITION Nitrification is a microbial process of removing the nitrogen from environment with the help of nitrifying bacteria which are called nitrifiers. • Nitrosomonas (ammonia to nitrite) • Nitrobacter (nitrite to nitrate)

Majority of ammonia (gas) is converted to ammonium (ion). The equilibrium between gas and ionic forms are maintained by pH of water. More acidic solution leads to ionic form(6-9) More basic solution leads to gas form (12)

Molecular mechanism

First step nitrification: (nitrosifying bacteria) ammonia is converted in nitrite with the help of autotrophs which can produce ammonia mono oxygenase (hydroxyl amine) as well as hydroxyl amine oxydo reductase (nitrite) NH3 + O2 → NO2 + 3H+ + 2e− NH2OH +  H2O → NO2 + 5H+ + 4e−

Second step nitrification : (nitrifying bacteria) Nitrite which is produced in the first step can be converted in to nitrate with the help of nitrite oxydoreductase. NO2 +  H2O → NO3 + 2H+ + 2e−

NITROGEN CYCLE Total Kjeldahl Nitrogen (TKN)

Ni tr

D

en

it

ri

fi

ifi ca ti o n

ca

ti

on

NITRIFYING BACTERIA • • • •

Autotrophs Chemolithotrophs Obligate aerobes ANAMMOX micro organisms

Basic Design

THE INFLUENCE OF ENVIRONMENTAL FACTORS

• • • • • •

Alkalinity (50-150 mg/l in the form of CaC03) Temperature pH (6-9) Toxic substances Oxygen concentration Concentration of the substances

Carbon Source (Methanol )

Nitrification

Denitrification

Influent

Effluent Media Fill

Media Fill

Alkalinity

Aeration

Mechanical Mixing

Nitrification Tank Design •Nitrification rate of 1 gNH4+-N/m2d (Kaldnes) •Decided on 30% Fill •Ammonia-N Load of 1250 lb/d •Reactor Volume of 130,000 ft3 •Depth of 12 ft •Length to Width 2:1 used •HRT 3.9 hrs Media Design and Characteristics -7 mm Long and 10 mm Diameter -Density of 0.96 g/cm3 -Surface Area of 500 m2/m3 -Typical Fill of 30-50%

• • • •

Aeration Requirements Course Bubble Diffusers used to Suspend Media and Oxygen Requirements Oxygen Transfer Rate of 1%/ft depth Results in 11% Oxygen Transfer Rate (Kaldnes) Based on earlier stoichiometry 4.6 gO2/gNH4+-N (Nitrification) Ammonia Loading used to   C determine SOTR  AOTR     F (   C  C )  110 kgO2/hr  Standard Oxygen Transfer Rate SOTR Air _ flow  Determined SOTE (60 min/ h)(( 0.270kgO / m air ) s , 20

s ,T , H

•

3

2

•

Air Flow Rate then Determined

Alkalinity Requirements Stoichiometry NH4++2HCO3-+2O2→NO3-+2CO2+3H2O 7.14 g Alkalinity as CaCO3/g N is required Design Considerations •Assumed Influent Alkalinity of 120 mg/L as CaCO3 •Desired Effluent Alkalinity of 80 mg/L as CaCO3 •Alkalinity Required used for Nitrification is 164 mg/L as CaCO3 •Alkalinity Addition Needed is 124 mg/L as CaCO3 •Total Alkalinity Required 6,200 lbs/d

Full nitrification: ammonium less than 2mg/l nitrite less than 0.5 mg/l • To reduce 1 pound of BOD 1.2 pounds of oxygen is required • To reduce 1pound of ammonium to nitrite 4.6 pounds of oxygen is required.

Nitrification processes • • • • •

One sludge activated process Two sludge activated process Biofilm nitrification processs Hybrid process ANNAMOX process

One sludge activated process • Both heterotrophic and nitrifying bacteria coexist in a single mixed liquor that simultaneously oxidizes ammonium and organic BOD. • It has one reactor and one settler for all types of biomass. • One sludge nitrification otherwise called as one stage nitrification, because it has only one reactor stage.

Two sludge activated process • Two stage is an attempt to reduce the competition between heterotrophs and nitrifiers by oxidizing most of the organic BOD in a first stage, while ammonium is oxidized in second stage. • Here two stages are available. Both stage produce their own biomass which are recycled with respective stages. • The first sludge is essentially free of nitrifiers, while the second stage has full of nitrifiers, since all the nitrification and only a small amount of BOD oxidation occurs there.

Biofilm nitrification • The nitrifying bacteria are being attached to a solid surface in the form of a biofilm. • Biofilm structures are heterogeneous • physical factors influencing biofilm structure are hydraulic erosion, detachment,mass transfer, shape of the substrate. • Chemical factors- physico-chemical environment, substrate concentration, type of substrate • During nitrification the nitrifying bacteria grow continuously and hence there is a growth of the thickness of the biofilm. If this is not balanced by a corresponding detachment of the biofilm for trickling filters, the biofilm will be too thick resulting in clogging of the trickling filters. It is therefore desirable to have a stable state with equilibrium between growth and detachment.

Hybrid Biofilm/suspended Growth processes • Activated sludge process can be enhanced in terms of capacity and reliability through the introduction of biofilm media that create hybrid biofilm/suspended growth system • Existing activated sludge-upgraded by adding biofilm surface area inside the aeration basin • The goal is to increase the total mass of active bacteria in the system and to increase the SRT • Suspended and biofilm bacteria have different SRTs • Biofilm SRT-longer- which makes it especially important for accumulating slow growing species such as nitrifiers • Eg: Activated biofilter • In the settler MLVSS--- maintained around 2,500 mg/L • BOD loading--- 9 Kg BOD5/m3/day

ANAMMOX process • ANAMMOX, an abbreviation for ANaerobic AMMonium OXidation, is a globally important microbial process of the nitrogen cycle. • If only half of the ammonia nitrogen in a wastewater flow were oxidized just to nitrite, then the remaining ammonia nitrogen and the nitrite nitrogen that has been formed could be converted to nitrogen gas by anammox bacteria in an anaerobic (absence of oxygen) environment. • The discovery of ANAMMOX is one of the most startling ones in environmental biotechnolgy. • ANAMMOX bacterium uses ammonium as its donor and nitrite as its electron acceptor. The energy reaction is, • NH4+ + NO2- = N2 + 2H2O • The yield and specific growth rate reported for ANAMMOX are low.

DENITRIFICATION PROCESS

HIGHLIGHTS

Physiology of Denitrifying Bacteria Tertiary Denitrification One- sludge denitrification

INTRODUCTION •

Denitrification is a microbially facilitated process of nitrate reduction

•

Performed by a large group of heterotrophic facultative anaerobic bacteria

•

Ultimately produce molecular nitrogen (N2) through intermediate gaseous nitrogen oxide products.

•

This respiratory process reduces oxidized forms of nitrogen in response to the oxidation of an electron donor such as organic matter.

•

The preferred nitrogen electron acceptors in order of most to least thermodynamically favourable include nitrate (NO3−), nitrite (NO2−), nitric oxide (NO), nitrous oxide (N2O) finally resulting in the production of dinitrogen (N2) completing the nitrogen cycle.

•

Since denitrification can lower leaching of NO3 to groundwater, it can be strategically used to treat sewage or animal residues of high nitrogen content.

•

Denitrification allows for the production of N2O, which is a greenhouse

a

series

of

SCHEMATIC OF DENITRIFICATION PROCESS

CONDITIONS  Anaerobic Conditions  Heterotrophic and Autotrophic Bacteria  Nitrite or Nitrate is electron acceptor  Carbon Source Required  Neutral pH (pH 7)

DENITRIFYING BACTERIA •

A group of bacteria that reduce nitrates or nitrites to nitrogen containing gases.

•

Important as it allows nitrogen to be recycled back into the atmosphere.

•

Denitrifying heterotrophic bacteria only exist in environments where oxygen is limited.

•

Denitrifying bacteria use reactive substrates to perform denitrification.

•

Denitrification is one of the few ways to permanently remove reactive nitrogen from ecosystems. Since excess reactive nitrogen in water contributes to serious water quality and human health problems like toxic algal blooms and bowel cancer, denitrification in streams can be considered a valuable ecosystem service. 

•

Potential examples include Thiobacillus denitrificans, denitrificans, Paracoccus denitrificans and Pseudomonas.

•

Genes known in microorganisms which denitrify include nir (nitrite reductase) and nos (nitrous oxide reductase).

nitrogen

and

organic

carbon as

Micrococcus

PHYSIOLOGY

 Denitrification process proceeds in a stepwise manner:

 Very low conc. of the electron donor or too high DO can lead to accumulation of denitrification intermediates. (NO2-, NO and N2O). i) a low conc. of the electron donor limits the supply of electrons to drive the reductive half-reactions.

)

) ii) a high DO tends to repress the nitrite and nitrous oxide reductases before the nitrate reductase is suppressed. pH values outside the optimal range of 7 to 8 can lead to accumulation of intermediates. - In low acidity waters, pH control can become an issue,



Thiobacillus denitrificans  Characteristics : • Short, rodshaped, Gram-negative • Grows as a facultatively anaerobic chemolithotroph • Coupling the oxidation of inorganic sulfur compounds to the reduction of oxidized nitrogen compounds • The optimum conditions for denitrification are pH 6.85 at 32.8°C, and optimal growth conditions are pH 6.90 at 29.5°C. • Widely found in soil, mud, fresh-water and marine sediments, sewage and industrial waste treatment ponds • T. denitrificans can remediate natural groundwater and engineered water treatment systems by removing excess nitrate

 Application : • In T. denitrificans, sulfide is oxidized by a series of reactions catalyzed by a sulfide/sulfite oxidoreductase, similar to that found in some SRB. • T. denitrificans finds use in environmental remediation, particularly for nitrate removal. • Excess nitrogen can cause problems such as eutrophication and methemoglobinemia (blue baby syndrome), caused primarily by discharge of waste waters and overuse of fertilizers. • Thiobacillus denitrificans is an autrophic denitrifier, which has the competitive advantages of not needing an external carbon source to be added to the process, and not producing as much sludge. • Sulfur-based systems that T. denitrificans thrive in have been considered as a viable solution to remove nitrate from groundwater and surface water that have been contaminated, as well as remediating waste waters with high nitrate levels such as nitrified leachate. Thiobacillus denitrificans is able to reduce not only nitrate, but nitrite as well.

Paracoccus denitrificans  Characteristics : • A coccoid shaped gram negative bacteria. Includes a double membrane with a cell wall. • They live in the soil in either aerobic or anaerobic environments and also have the ability to live in many different kinds of media including C1 and sulfur. • They can either use organic energy sources, such as methanol or methylamine, or act as Chemolithotrophs. • A feature of this bacteria is its ability to single-handedly convert nitrate to dinitrogen in a process called denitrification.

 Application : • Finds application in the construction of a bioreactor, a tubular gel containing two bacteria, for the removal of nitrogen from wastewater. • Paracoccus denitrificans has the unusual ability of reducing nitrite to nitrogen gas. • In this bioreactor, Paracoccus denitrificans is paired up with Nitrosomonas europaea, which reduces ammonia to nitrite. This system simplifies the process of removing nitrogen from wastewater.

TYPES OF DENITRIFICATION PROCESS

Tertiary denitrification : exogenous electron donor is needed and added. One sludge denitrification : donor already present in the waste-water.

TERTIARY TREATMENT Nutrient Removal, Solids Removal, and Disinfection

Why is tertiary treatment needed ? •

To remove total suspended solids and organic matter those are present in effluents after secondary treatment.

•

To remove specific organic and inorganic constituents from industrial effluent to make it suitable for reuse.

•

To make treated wastewater suitable for land application purpose or directly discharge it into the water bodies like rivers, lakes, etc.

•

To remove residual nutrients beyond what can be accomplished by earlier treatment methods.

•

To remove pathogens from the secondary treated effluents.

•

To reduce total dissolved solids (TDS) from the secondary treated effluent to meet reuse quality standards.

•

To provide additional treatment when soils or receiving waters cannot.

Biological Denitrification • A modification oxygen) – Same bacteria aerobically

of that

aerobic consume

pathways carbon

(no

material

• Denitrifying bacteria obtain energy from the conversion of NO3- to N2 gas, but require a carbon Organic source matter Cell mass

NO3- + CH3OH + H2CO3  C5H7O2N + N2 + H2O + HCO3-

Denitrification (cont.) • Separate denitrification reactor or • Combined Carbon Oxidationnitrification-denitrification reactor – A series of alternating aerobic and anoxic stages – Reduces the amount of air needed – No need for supplemental carbon source

Combined Nitrification/Denitrification

(note alternating regions of aerobic and anoxic)

Tertiary Denitrification using Activated Sludge • SRT (5 d) >>HRT

SRT 

Mass Sludge In System Mass Sludge Leaving System Per Day

• High cell concentration increases reaction rate

Biofilm Process • Submerged fixed beds of rocks, sand, limestone, or plastic media. • Fluidized beds of sand, activated carbon, and pellets of ion-exchange resin. • Circulating beds of a range of lightweight particles. • Membrane bioreactors (membrane supplies H 2 and is the attachment surface) . • HRT can be less than 10 minutes.

LIMITATIONS IN TERTIARY DENITRIFICATION SYSTEM

• Recalcitrant Forms of Nitrogen • Adequate Nutrients • Partially Denitrified Versus Full Nitrogen Load as Nitrate • Final barrier of treatment prior to disinfection • Ammonia bleed through from upstream process (partial nitrification).

ONE-SLUDGE DENITRIFICATION

One-sludge denitrification  The water contains an electron donor that can drive denitrification. ) One-sludge denitrification uses the BOD in the influent of a wastewater to drive denitrification  Two goals in one-sludge denitrification : 1) Providing aerobic conditions that allow full nitrification 2) Providing anoxic conditions and Reserving BOD (organic electron donor) for denitrification. ) In one sludge denitrification, the TKN must be oxidized to NO3-- N without oxidizing all the BOD before denitrification takes place. -Total Kjeldahl nitrogen (TKN) = organic BOD and

Basic One-Sludge Strategies Three basic strategies for reserving organic electron donor while nitrification takes place :

1)Biomass storage and decay 2) Classical predenitrification 3) Simultaneous denitrification

nitrification

with

Biomass storage and decay The synthesis of biomass stores electron equivalents that originally came from the BOD and can be released through endogenous respiration to drive denitrification.

Biomass storage and decay  It is called Wuhrmann biomass decayer (Swiss engineer K. Wuhrmann, 1964)  Biomass storage and decay has limited applicability by itself and is not often employed as a stand-alone process due to two shortcomings: 1) Endogenous respirations has slow kinetics o high conc. of MLVSS (operating problems with settler and recycle) and a long HRT in a anoxic tank o high capital costs are necessary 2) The decay of biomass always releases NH4+-N from the anoxic step although at concentrations lower than in the influent.

Classical Predenitrification - The first tank (anoxic) ; the influent BOD (electron donor) is directly utilized for denitrification. - The second tank (aerobic); the influent TKN is nitrified to NO3- and any remained BOD is oxidized. - The nitrate formed in the aerobic tank is recycled to an anoxic tank

Classical Predenitrification Advantages: i) direct use of influent BOD for denitrification - reduces aeration costs for the removal of BOD. ii) faster kinetics than with biomass storage and decay iii) no release of NH4+-N in the effluent Disadvantage: i) large mixed-liquor recycle rate -) increases costs of piping and pumping.

Simultaneous nitrification with denitrification Three factors allow all reactions to occur simultaneously. 1) Various nitrogen reductases using N as e- acceptor are repressed only when the D.O. conc. is well above 1 mg/L. 2) However, inhibition of the nitrogen reductase is not severe when the D.O. conc. is less than 1 mg/L. 3) D.O. conc. is depressed inside the aggregates that normally form in treatment systems; thus, denitrification can occur inside the floc (or biofilm), as long as the electron donor (BOD) penetrates inside. - When D.O. concentration is poised at a suitably low level (< ~ 1 mg/L), anoxic denitrification can occur in parallel to the aerobic reactions of nitrification and aerobic BOD oxidation.

Simultaneous nitrification with denitrification Advantages: •

Simultaneous nitrification with denitrification offers all the advantages of predenitrification, but overcomes the main disadvantage, the high recycle rate.

•

Maintaining a low D.O. conc. throughout one reactor creates an infinitely high recycle ratio and allows essentially 100 percent N removal.

Drawback : •

The combinations of SRT, HRT, and D.O. conc. that guarantee reliability are not known yet.

riations on the Basic One-Sludge process Barnard process

Benefits of One Sludge Denitrification - No exogenous electron donor needs to be added. • chemical costs are reduced over tertiary denitrification. - Some of the influent BOD is oxidized with nitrate as the electron acceptor (not O2). • aeration costs are reduced compared to alternative systems that oxidize all BOD and nitrify the reduced - nitrogen forms in the influent with O2 . - Full or nearly full N removal is achieved • protecting receiving waters at risk eutrophication.

from

cultural

Phosphorous removal

 Major Use: Fertilizer (Triple Superphosphate, Monoammonium Phosphates)  Other Uses: • surfactants • cleaners • metal treating • lubricants • fire retardants • tooth paste

 Environmental Impacts  Problem: Fertilizer Runoff and Product Waste Result: Eutrophication-Water body receives excess nutrients that stimulate primary production (algae, nuisance plants) Negative Effects • Health (Algae releases neurotoxins) • Depletes Dissolved Oxygen • Fish Kills

 Phosphorous can be removed from wastewater prior to biological treatment, after biological treatment  Chemical precipitation: PO43- with Ca2+, Al3+ or Fe3+

Methods for the removal as part of the microbial treatment process: • Precipitation by metal-salts addition to a microbiological process • Normal phosphorous uptake into biomass • Enhanced biological phosphorous uptake into biomass

Form of phosphates

Chemical Phosphorus Removal

Ortho Phosphate (Soluble)

plus

Metal Salts (Soluble)

form

Insoluble Phosphorus Compounds

Chemical Removal M+3 PO4-3 + ( M+3 = Metal in Solution )

MPO4

PRECIPITATION Metals used are: Aluminum, Al Iron, Fe

Precipitation by metal-salts addition to a microbiological process Al3++ PO43-

AlPO4

Undergo competing acid/base complex reaction results formation of protonated species - pH-important role There are two important disadvantages associated to chemical treatment method: (1) a certain overdosing of metal salts is necessary to obtain the required low effluent phosphorus values, resulting in high costs of chemicals and a significant increase of excess sludge production (2) the accumulation of ions (increased salt content) may seriously restrict the reuse possibilities of the effluent

Chemical Phosphorus Removal

Total Phosphorus Organic Phosphorus Condensed (Poly) Phosphates Ortho Phosphates

Metal Salt Addition Ortho Phosphates

Normal phosphorous uptake into biomass: Stoichiometric fromula for the biomass can be modified to include this amount of P. C5H7O2NP0.1 has a formula weight of 116 g/mol, of which P is 2.67%  Sludge wasted from the process removes P in proportion to the mass rate of sludge VSS wasted (QwXvw)  A steady state mass balance on total P is  P0 and P =influent and effluent total P concentrations, Q=influent flow rate

Biological P Removal-Enhanced biological phosphorous removal Anaerobic Conditions (EBPR) Heterotrophic Bacteria Break Down Organics Fermentation Volatile Fatty Acids (VFAs) Acetate (Acetic Acid)

Anaerobic

Fermentation Acetate Production Selection of PAO - Phosphate Accumulating Organisms Acinetobacter/P AO P Released to Produce Energy

Mechanism for uptake of phosphorous by biological method:  phosphate accumulating organisms (PAOs) store polyphosphate as an energy reserve in intracellular granules  Under anaerobic conditions, in the presence of fermentation products, PAOs release orthophosphate, utilizing the energy to accumulate simple organics and store them as polyhydroxyalkanoates (PHAs) such as poly-β-hydroxybutyrate (PHB)  Under aerobic conditions, the PAOs grow on the stored organic material, using some of the energy to take up

Phosphorus Removing 3Mechanism PO Facultative Substrabacteria te

Acetate plus fermentati on products

Ener gy

PH B

Anaero bic

Aerob ic

Ener gy

BOD + O 2

3-

PO 4

4

PH B PolyP

(Phosphoru s Acinetobacter spp. removing bacteria, Polyslow P grower)

+

CO 2 + H2O

New biomass

Biological P Removal Important Considerations

Adequate Influent BOD ( Enough O2 demand to achieve anaerobic conditions)

BOD:P 20:1

Adequate Anaerobic Detention Time 1-3 hrs (Not so long as to reduce sulfate to sulfide-septicity)

Adequate Aerobic Detention Time 4-5 hrs. (Enough time for BOD removal & Low Effluent Suspended Solids Nitrification) Below 20 mg/L (SS result in P in effluent)

Sludge Handling

(Supernatant P can overload P removal system)

Biological P Removal Benefits No Chemical Feed (Usually, Sometimes) Lower Cost Safety No Tramp Metals No Chemical Sludge Produced

nhibits Growth of Filamentous Organisms (Cycling between Anaerobic & Aerobic)

Biological P Removal Most often Used Processes A/O Phostrip A2/O Concentric Ring Oxidation Ditch Sequencing Batch Reactor

A/O Process (Anaerobic/Oxic)

Head End of Aeration Tank Baffled and Mechanically Mixed Primary Effluent and RAS Produce Anaerobic Conditions Phosphorus Released “Luxury Uptake” of Phosphorus in Aerated End

A/O Process

A2/O

(Anaerobic/Oxic)

(Anaerobic/Anoxic/Oxic)

Phostrip Some Return Sludge Diverted to Anaerobic Stripper Phosphorus Released Elutriated (Washed) to a Precipitation Tank Precipitated With Lime – Sludge Removed

Concentric Ring Oxidation Ditch Three Aeration Tanks in Concentric Rings

Anaerobic Aerobic Aerobic

Concentric Ring Oxidation Ditch

Three Aeration Tanks in Concentric Rings

Wasting Aerobic the Bio-solids Removes Phosphorus

Sequencing Batch Reactor

Sequencing Batch Reactor

Batch Treatment in Sequence of Steps Anaerobic P Release

Static Fill

Settle

Mixed Fill

Decant

React Fill React

Aerobic P Taken Up by Biomass.

Waste Idle

P Remove d in Sludge

Ortho-P

D.O.