Membrane Bioreactors

1. INTRODUCTION The technologies most commonly used for performing secondary treatment of municipal wastewater rely on microorganisms suspended in the wastewater to treat it. Although these technologies work well in many situations, they have several drawbacks, including the difficulty of growing the right types of microorganisms and the physical requirement of a large site. The use of microfiltration membrane bioreactors (MBRs), a technology that has become increasingly used in the past 10 years, overcomes many of the limitations of conventional systems. These systems have the advantage of combining a sus-pended growth biological reactor with solids removal via filtration. The membranes can be designed for and operated in small spaces and with high removal efficiency of contaminants such as nitrogen, phosphorus, bacteria, bio-chemical oxygen demand, and total suspended solids. The membrane filtration system in effect can replace the secondary clarifier and sand filters in a typical activated sludge treatment system. Membrane filtration allows a higher biomass concentration to be maintained, thereby allowing smaller bioreactors to be used. Membrane bioreactors are able to provide the benefits of biological treatment with a physical barrier separation. Compared to conventional treatment processes, membranes are able to provide better quality effluent with a smaller, automated treatment process.

2. APPLICABILITY For new installations, the use of MBR systems allows for higher wastewater flow or improved treatment performance in a smaller space than a conventional design, i.e., a facility using secondary clarifiers and sand filters. Historically, membranes have been used for smallerflow systems due to the high capital cost of the equipment and high operation and maintenance (O&M) costs. Today however, they are receiving increased use in larger systems. MBR systems are also well suited for some industrial and commercial applications. The high-quality effluent produced by MBRs makes them particularly applicable to reuse applications and for surface water discharge applications requiring extensive nutrient (nitrogen and phosphorus) removal.

3. ADVANTAGES  The retention of all suspended matter and most soluble compounds within the bioreactor leads to excellent effluent quality capable of meeting stringent discharge requirements and opening the door to direct water reuse.  The possibility of retaining all bacteria and viruses results in a sterile effluent, eliminating extensive disinfection that would be required otherwise and eliminate the corresponding hazards related to disinfection by products.  It results in more compact systems than conventional processes significantly reducing plant footprint making it desirable for water recycling applications.  The process is more compact than a Conventional Activated Sludge process (CAS), skipping three (3) individual processes of the conventional scheme. The feed wastewater only needs to be screened (1-3 mm) just prior to removal of larger solids that could damage the membranes.  In addition it is easier to operate and maintain.  It has a higher Nitrogen Removal rate than any other treatment process.  Finally, it has a comparatively low sludge yield; thereby reducing the IOM cost of sludge handling.

4. MEMBRANE

During MBR wastewater treatment, solid-liquid separation is achieved by Microfiltration (MF) or Ultrafiltration (UF) membranes. A membrane is simply a twodimensional material used to separate components of fluids usually on the basis of their relative size or electrical charge. The capability of a membrane to allow transport of only specific compounds is called semi-permeability (sometimes also permselective). This is a physical process, where separated components remain chemically unchanged. Components that pass through membrane pores are called permeate, while rejected ones form concentrate or retentate. Fig 1:Structure of membrane unit

4.1 MEMBRANE TYPES 4.1.1 Plate and Frame – The plate and frame membranes consist of two flat sheets of membrane material, usually an organic polymer, stretched across a thin frame. The space between the membrane sheets is placed under vacuum in order to provide the driving force for filtration. Several plates are arranged in a cassette to allow for increased surface area and convenient modular design. The membrane cassette is immersed in the mixed liquor and the separation flow is from outside-in. For example, Kubota membranes have air induced liquid cross-flow along the plates. This creates turbulence and hinders cake formation and subsequent fouling. The organic polymer, polyethylene for example, has the required flexibility to move slightly in the cross-flow to allow three-dimensional dynamic forces to reduce cake formation. The cross-flow of air also acts to dissolve oxygen to and mix the contents of the reactor.

4.1.2 Hollow fibre – Hollow fibre membranes consist of long strands, or fibres, of hollow extruded membrane. They are most often of organic polymer construction and are applied much the same as plate and frame membranes. The fibres are mounted to a supporting structure that serves as a manifold for permeate transport as well as an air delivery system. Similar to the plate and frame modules, air induced liquid cross flow prevents excessive cake formation and increases the lifespan of the membrane.

Fig 2:Hallow fiber membrane

Fig 3:Flat Plate membrane

4.1.3 Tubular – As the name implies, tubular membranes are hollow tubes with the membrane placed on the surface of the tube. Below the membrane surface is a supporting structure with high porosity. In most cases, tubular membranes are made of inorganic material such as ceramic and have a metal oxide membrane surface to provide a small nominal pore size. Tubular membranes have a different separation driving force than the previous two. Rather than vacuum pressure, the material to be separated flows along the membrane at high velocity under pressure. The velocity provides a transverse force to drive the water through the membrane while leaving the larger diameter particles behind. A tubular membrane could be used in the outside-in arrangement with the feed water flowing along the centre of the tube and the permeate passing to the outside walls, or the inside-out arrangement where the influent travels along the centre of the tube and travels axially outward. Fig 4: Tubular membrane

5. MEMBRANE CHARACTERISTICS Membrane treatment is an advanced treatment process that has become increasing popular over the past ten years. Membrane processes have been understood but underutilized since the 1960’s due to high capital costs. Recent developments in membrane manufacturing have enabled the production of better quality membranes at a reduced price. Compared with increasing conventional water treatment costs, membrane treatment is now considered economically feasible. 5.1 Filtration processes There are six commercially used membrane separation processes; Microfiltration (MF), Ultrafiltration (UF), Nanofiltration (NF), Reverse Osmosis (RO), Dialysis, and Electrodialysis (ED). Membrane processes can be classified based on membrane separation size and mechanism, membrane material and configuration, or separation driving forces used. Membrane processes utilize set terminology to discuss membrane performance. The rate of fluid transfer across the membrane is referred to as the flux, and has units of kg/m²h.The pressure experienced across the membrane is referred to as the trans-membrane pressure (TMP). The fluid that passes through the membrane is the permeate, while the flow retained by the membrane is the retentate. Separations based on membrane pore size include MF (0.1- 0.2 μm),UF (0.002 – 0.1 μm), and NF (0.0001 – 0.001 μm). The ranges are not strictly defined and some overlap exists. These three types of filtration rely on a sieving action to remove particulate matter. All four varieties of membranes rely on hydrostatic pressure differences to drive the separation process. Microfiltration or Ultrafiltration is the most commonly used membrane size in wastewater and

MBR treatment. Microfiltration is able to remove protozoa, bacteria and turbidity, while UF has the added benefit of virus removal). 5.2 Membrane materials Membranes are made from either organic polymers or ceramic materials. Polymers offer the advantage of low cost production but may contain natural variations in pore size, and are prone to fouling and degradation. Ceramic membranes offer excellent quality and durability but are economically unfeasible for large scale operations, although they may be well suited for industrial applications (Scott and Smith, 1996). All of the commercial MBR manufacturers use polymeric MF membranes. Table lists the most common types of polymer materials used to construct membranes. Polymeric membranes are manufactured in several forms, the most common types for MBR are hollow fiber and plate and frame. TABLE 1: Polymer Membrane Materials and Characteristics Material

Advantages

Disadvantages

Polypropylene

Low cost

No chlorine tolerance

High pH range tolerance

Expensive cleaning

Polyvinylidene fluoride

High chlorine tolerance

Chemicals required Cannot sustain pH>10

Polyether Sulphone and

Simple cleaning chemicals Chlorine tolerance

Brittle material requires

Polysulphone

Reasonable cost

support or flow inside to

Polyacrylonitrile

Low cost, typically used for

outside Less chemically resistant than

Cellulose Acetate

UF membranes Low cost

PVdF Narrow pH range Biologically active

5.3 Membrane integrity Monitoring membrane integrity is necessary for all processes. Membrane breakage or Degradation can lead to the loss of physical separation and possible contamination of the effluent. Membrane integrity can be monitored by particle counters or pressure decay testing (PDT). PDT is the preferred method due to its reliability and increased accuracy. In PDT the

membrane module is pressurized to a high pressure and monitored for leaks, the PDT is sensitive enough to detect the breakage of a single fibre. 5.4 Membrane fouling Membrane fouling is the largest concern in the design of membrane and MBR systems. Membrane fouling can be due to particulate build-up, chemical contamination or precipitation. Particulate fouling occurs as matter in the wastewater collects on the surface of the membrane. As the layer builds up the membrane pores can be blocked reducing the flux through the membrane and increasing the TMP. Particulate matter can foul membranes by either plugging or narrowing the pores or through the formation of a cake layer on the surface. Membrane fouling can be controlled through the use of periodic maintenance back-flushing and chemical cleans in place (CIP). Back-flushing is completed by reversing the flow of air or water through the membrane to unclog the pores. If the membrane is heavily fouled a chemical clean may be necessary. Sodium hydroxide and surfactant solution is the most common chemical used for cleaning, but other chemicals such as citric acid, chlorine, hydrogen peroxide, or aluminium bifluoride may be used depending on manufacturer’s guidelines. Long term fouling due to the precipitation of manganese of silica has been observed in some instances, but can generally be reversed with cleaning. In membrane bioreactors several additional steps are taken to reduce fouling due to the high suspended solids in the retentate. Coarse bubble aeration is introduced at the bottom of hollow fiber membranes and travels vertically along their length. This has a two-fold purpose of aerating the wastewater and vibrating the membrane fibers to remove particulate matter, increasing the time between cleanings. The membranes are operated near critical flux to minimize fouling and in a periodic fashion, with a back-flush every 5 to 15 minutes.

6. MBR SYSTEM CONFIGURATIONS MBR systems can be classified into two major categories according to the location of the membrane component.

6.1 Submerged or Immersed MBR In the submerged MBR process, the membrane is submerged directly in the aeration tank. By applying low vacuum or by using the static head of the mixed liquor, effluent is driven through the membrane leaving the solids behind.

6.2 External / Sidestream MBR In the external MBR, the mixed liquor is pumped from the aeration tank to the membrane at flow rates that are 20-30 times the product water flow to provide adequate shear for controlling solids accumulation at the membrane surface. The high cost of pumping makes EMBR system impractical for full scale municipal wastewater treatment plants.

Fig 5:Immersed membrane bioreactor

Fig 6:sidestream membrane bioreactor

TABLE 2 – Comparison of External and Internal Membrane Based MBR System Configuration. Comparative Factor Membrane area Requirement Space or Footprint Requirements

External MBR Systems

Internal MBR Systems

Characterized by higher flux and Lower flux but higher membrane therefore lower membrane area packing density (i.e., membrane area per unit volume). requirement. Higher flux membranes with bioreactor operating at higher VSS concentration and skidded assembly construction, results in, compact system.

Higher membrane packing density and operation at bioreactor VSS concentration of 10 g/l or greater translates to compact system.

Bioreactor and Membrane Component Bioreactor can be designed and Design and operation of bioreactor Design and Operation operated under optimal conditions and membrane compartment or tank including those to achieve are not independent. High Dependency biological N and P removal, if membrane tank recycle required required. (e.g., recycle ratio 4) to limit tank VSS concentration build-up. Membrane Performance Consistency

Recovery of Membrane Performance

Membrane Life or Replacement Requirements

Less susceptible to wastewater and characteristics.

changing biomass

More susceptible to changing wastewater and biomass characteristics requiring alteration in membrane cleaning strategy and/or cleaning frequency.

Off-line cleaning required every 1 to 2 months. Simple, automated procedure normally requiring less than 4 hours.

Off-line “recovery” cleaning required every 2 to 6 months. A more complex procedure requiring significantly more time and manual activity, at least on occasion may be required (i.e., physical membrane cleaning).

Results to-date imply an operating life of 7 years or more can be achieved with polymerics prior to irreversible fouling. Operating life of ceramics much longer.

Results to-date imply an operating life of 5 years may be possible prior to irreversible fouling and/or excessive membrane physical damage.

Full Scale Application Status

Economics

Conventional membrane based systems have a very long track record. Few non- conventional systems in operation in the U.S.

Full scale application widespread in the U.S.

Non-conventional designs translate Power and capital cost advantage at to comparable power costs. higher wastewater feed rates. Comparable capital cost at least at lower wastewater feed rates (e.g., approaching 1893 m3/day).

7. DESIGN FEATURES 7.1 Pre-treatment To reduce the chances of membrane damage, wastewater should undergo a high level of debris removal prior to the MBR. Primary treatment is often provided in larger installations, although not in most small to medium sized installations, and is not a requirement. In addition, all MBR systems require 1- to 3-mm-cutoff fine screens immediately before the membranes, depending on the MBR manufacturer. These screens require frequent cleaning. Alternatives for reducing the amount of material reaching the screens include using two stages of screening and locating the screens after primary settling. 7.2 Membrane Location

MBR systems are configured with the membranes actually immersed in the biological reactor or, as an alternative, in a separate vessel through which mixed liquor from the biological reactor is circulated. 7.3 Membrane Configuration MBR manufacturers employ membranes in two basic configurations: hollow fiber bundles and plate membranes. Siemens/U.S.Filter’s Memjet and Memcor systems, GE/Zenon’s ZeeWeed and ZenoGem systems, and GE/Ionics’ system use hollow-fiber, tubular membranes configured in bundles. A number of bundles are connected by manifolds into units that can be

readily changed for maintenance or replacement. The other configuration, such as those provided by Kubota/Enviroquip, employ membranes in a flat-plate configuration, again with manifolds to allow a number of membranes to be connected in readily changed units. Screening requirements for both systems differ: hollow-fibre membranes typically require 1- to 2-mm screening, while plate membranes require 2- to 3-mm screening. 7.4 System Operation All MBR systems require some degree of pump-ing to force the water flowing through the membrane. While other membrane systems use a pressurized system to push the water through the membranes, the major systems used in MBRs draw a vacuum through the membranes so that the water outside is at ambient pressure. The advantage of the vacuum is that it is gentler to the membranes; the advantage of the pressure is that throughput can be controlled. All systems also include techniques for continually cleaning the system to maintain membrane life and keep the system operational for as long as possible. All the principal membrane systems used in MBRs use an air scour technique to reduce buildup of material on the membranes. This is done by blowing air around the membranes out of the manifolds. The GE/Zenon systems use air scour, as well as a back-pulsing technique, in which permeate is occasionally pumped back into the membranes to keep the pores cleared out. Backpulsing is typically done on a timer, with the time of pulsing accounting for 1 to 5 percent of the total operating time. 7.5 Downstream Treatment The permeate from an MBR has low levels of suspended solids, meaning the levels of bacteria, BOD, nitrogen, and phosphorus are also low. Disinfection is easy and might not be required, depending on permit requirements. The solids retained by the membrane are recycled to the biological reactor and build up in the system. As in conventional biological systems, periodic sludge wasting eliminates sludge buildup and controls the SRT within the MBR system. The waste sludge from MBRs goes through standard solids-handling technologies for thickening, dewatering, and ultimate disposal. Hermanowicz et al. (2006) reported a decreased ability to settle in waste MBR sludges due to increased amounts of colloidal-size particles and filamentous bacteria. Chemical addition increased the ability of the sludges to settle. As more MBR facilities are built and operated, a more definitive understanding of the characteristics of the resulting biosolids will be

achieved. However, experience to date indicates that conventional biosolids processing unit operations are also applicable to the waste sludge from MBRs. 7.6 Membrane Care The key to the cost-effectiveness of an MBR system is membrane life. If membrane life is curtailed such that frequent replacement is required, costs will significantly increase. Membrane life can be increased in the following ways: 

Good screening of larger solids before the membranes to protect the membranes from physical damage.



Throughput rates that are not excessive, i.e., that do not push the system to the limits of the design. Such rates reduce the amount of material that is forced into the membrane and thereby reduce the amount that has to be removed by cleaners or that will cause eventual membrane deterioration.



Regular use of mild cleaners. Cleaning solutions most often used with MBRs include regular bleach (sodium) and citric acid. The cleaning should be in accord with manufacturer-recommended maintenance protocols.

7.7 Membrane Guarantees The length of the guarantee provided by the membrane system provider is also important in determining the cost-effectiveness of the system. For municipal wastewater treatment, longer guarantees might be more readily available com-pared to those available for industrial systems. Zenon offers a 10-year guarantee; others range from 3 to 5 years. Some guarantees include cost prorating if replacement is needed after a certain service time. Guarantees are typically negotiated during the purchasing process. Some manufacturers’ guarantees are tied directly to screen size: longer membrane warranties are granted when smaller screens are used.

8. WORKING THEORY Normally, systems are built with two different compartments. The first section is the screening stage where the wastewater enters the unit. In this area; heavy solids are first separated subsequently traversing to another compartment which houses

the membranes. The initial screening is of high importance, as the larger molecules (scum and grit) will not trap the surface of the membrane and lead to fouling. In the second compartment, the biological process takes place involving vigorous agitation, coming from air bubbles generated from a blower system. This acts to scour and clean the surface of the membrane to prevent buildup of material on the and also to provide sufficient oxygen concentration for biological action that supports growth of bacteria Depending on how the system is designed to ensure efficient air to water oxygen transfer, the household MBR is capable to support up to 4000ppm of MLSS level while large-scale industrial wastewater treatment plant bioreactor scan handle up to 20000ppm. A complete unit usually comes equipped with a backflush system whereby discharged wastewater will now move counter flow from the permeate side back again to the system to dislodge trapped material accumulating on the surface During this process, air scouring will still continue to run to help increase removal efficiency. Fig 7: Working theory

Fig 8: Schematic diagram of the Membrane Bioreactor

9. WATER RECLAMATION The use of MBR technology for reclamation is a rapidly expanding application. MBR technology is well suited for reuse treatment due to its small footprint and relatively easy operation. Small MBR systems can be designed to pull wastewater directly from the sewer at the remote points of reuse, eliminating the need for large central treatment plants and redistribution. MBR effluent is ideal for further treatment by reverse osmosis. The high quality of the MBR permeate allows increased RO flux with reduced fouling. Following RO treatment the water generally meets or exceeds all drinking water standards and may be even higher in quality than virgin water. Despite the high water quality public acceptance within the US is difficult. Studies have suggested that a hierarchy of acceptable use exists.

Treatment Reuse Hierarchy: 1. Forest Irrigation 2. Forage Crop Irrigation 3. Food Crop Irrigation 4. Park and Garden Irrigation 5. Livestock Watering 6. Cooling 7. Industrial Cleaning 8. Industrial Process 9. Fishery Use 10. Recreational Water Supplies 11. Public Grey Water 12. Public Drinking Water In many cases the public fears are unfounded or irrational as “de-facto” reuse of drinking water supplies occurs already, often with less treatment than direct reuse designed systems. In areas with ample water supplies the reuse of economic cost of wastewater reuse cannot usually be justified. In areas with water scarcity such as Singapore, which relies on Malaysia to supply it’s water the reuse of water is highly accepted. Reused water is sold under the name NeWater and all wastewater treatment plants are being retrofitted with MF, RO and UV systems for production.

TABLE 3: Typical MBR Effluent Quality S.NO. 1. 2. 3. 4. 5. 6.

PARAMETERS BOD TSS Ammonical nitrogen as NH3-N Nitrogen as TKN Fecal coliform count pH

UNITS mg/l mg/l mg/l

VALUES <2 <1 <0.5

mg/l MPN/100ml

<1 <2 6.8-7.8

10. APPLICATIONS OF MEMBRANE BIOREACTOR: 10.1 Applications in municipal wastewater treatment MBR systems were initially used for municipal wastewater treatment, primarily in the area of water reuse and recycling. Compactness, production of reusable water, and trouble-free operation made the MBR an ideal process for recycling municipal wastewater in water and space limited environments. the development of less expensive submerged membranes made MBRs a real alternative for high flow, large scale municipal wastewater applications. Over 1,000 MBRs are currently in operation around the world with approximately 66% in Japan, and the remainder largely in Europe and North America. Out of these installations, about 55% use submerged membranes while the rest have external membrane modules. 10.2 Applications in industrial wastewater treatment High organic loadings and very specific and difficult to treat compounds are two major characteristics of industrial waste streams that render alternative treatment techniques such as the MBR desirable. Since, traditionally wastewater with high COD content was treated under anaerobic conditions, initial attempts of MBR applications for industrial wastewater were in the field of anaerobic treatment.

10.3 Applications in fields of landfill leachate, sludge digestion, And human excrement In addition to municipal and industrial wastewater treatment, MBRs have been utilized in a number of other areas. One such area is the treatment of landfill leachates. Landfill

leachates usually contain high concentrations of organic and inorganic compounds. Conventionally, the treatment of leachates involves a physical, biological, or membrane filtration process (or a combination of them). MBR systems have been successfully utilized with an additional treatment step for inorganics and heavy metal removal, such as reverse osmosis (RO). Several industrial scale plants, combining a MBR and a reverse osmosis system, are presently operated. The MBR system was also used in the treatment of human excreta in domestic wastewater. These applications, also known as night soil treatment systems, were typified by the high strength of the waste and the need for on-site treatment. The MBR system replaced a rather complex set of treatment systems which incorporated denitrification, coagulation, filtration, and activated carbon treatment. Another application of the MBR is in the area of sludge treatment. Conventionally, sludge stabilization in wastewater treatment plants is achieved by a single pass, anaerobic digester. Since the HRT and the SRT are identical in these systems, the capacity is limited and long solid retention times are required for effective solids destruction. 10.4 Nitrate removal in drinking water Denitrification and removal of natural organic matter are two main treatment requirements for drinking water. Nitrate is the most common groundwater contaminant in North America and world-wide. Nitrate is a stable and highly soluble nitrogen species, easily transported and accumulated in groundwater systems. These properties, coupled with increased anthropogenic discharges of nitrogen containing compounds from point and non-point sources, have resulted in elevated nitrate concentrations in ground and surface waters. Non-point sources may have a larger impact on ground water and are associated with agricultural and livestock practices and residential septic tank effluents. Nitrates can be removed either biologically or by physicochemical treatment techniques such as reverse osmosis, ion exchange, and electrodialysis. Natural organic matter can be treated biologically or through activated carbon adsorption. Biological removal of nitrates and organic matter is receiving more attention due to the complete conversion of nitrate into nitrogen gas and relative ease of operation (Falk 2002). Conventional physico-chemical treatment methods only concentrate nitrate into solutions which still require disposal. In typical biological denitrification processes, however, post treatment processes such as sand filtration, activated carbon adsorption, and disinfection are required to remove biological entities and

excess organic matter and colour. The number of post-treatment processes can be significantly reduced by using a MBR for biological denitrification. All biological entities as well as some dissolved organic matter will be retained in the bioreactor while long denitrifying culture retention times and short hydraulic retention times can be maintained. 11. LIMITATIONS  The primary disadvantage of MBR systems is the typically higher capital and operating costs than conventional systems for the same through-put.  O&M costs include membrane cleaning and fouling control, and eventual membrane replacement.  Energy costs are also higher because of the need for air scouring to control bacterial growth on the membranes. In addition, the waste sludge from such a system might have a low settling rate, resulting in the need for chemicals to produce biosolids acceptable for disposal (Hermanowicz et al. 2006). Fleischer et al. 2005 have demonstrated that waste sledges from MBRs can be processed using standard technologies used for activated sludge processes.

12. CONCLUSION  The membrane bioreactor technology has great potential in wide ranging applications including municipal and industrial wastewater treatment, groundwater and drinking water abatement, solid waste digestion, and odour control.  The technical feasibility of this process has been demonstrated through a number of pilot and bench scale research studies. Full scale systems are operational in various parts of the world and substantial growth in the number and size of installations is anticipated for the near future.  The MBR process is already considered as a viable alternative for many waste treatment challenges and with water quality issues firmly placed into the forefront of public debate, ever tightening discharge standards and increasing water shortages will further accelerate the development of this technology.  Agricultural activities and related industries constitute a potential source of pollution to the environment. Waste from intensive livestock operations and wastewater generated by the food processing industry are two streams characterized by high organic and nutrient strength. Multiple treatment processes are normally required to ameliorate the waste to levels acceptable for on-site reuse or direct discharge to surface water.  MBRs offer a proven alternative due to their ability to handle high organic loadings and wide fluctuations in flow and strength. Activated sludge scrubbing may also be able to be incorporated into these systems for odour control and air pollution management.  High quality effluent produced by the MBR would provide pathogen and bacteria control and assist the facility in complying with strict environmental regulations. It would also allow extensive process optimization through internal water recycle and significantly reduce dependence to municipal waste treatment facilities or to the availability of crop land for waste application.

13. REFERENCES

 Adham, S., P. et al. 2001. Feasibility of the membrane bioreactor process for water reclamation. Water Science and Technology 43(10): 203-209.  BCC. (2011). Membrane bioreactors: global markets. BCC Report MST047C. March 2011.  Fane, A. G. (1996) Membranes for Water Production and Wastewater Reuse. Desalination., 106, 1.  George Crawford et al (2001) The Evolution of Membrane Bioreactor System Designs for Wastewater Treatment. IWA Sept 2001.  Gupta, K.et al. (1994) Membrane Biological Reactor System for Treatment of Oily

Wastewaters. Water Environment Research, 66 (2), pp.133-139.