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A membrane is defined as a material that forms a barrier capable of selectively resisting the transfer of specific particles, molecules or substances when exposed to the action of a driving force, thus effecting a separation. The membranes should there for be produced from a material of reasonable mechanical strength than can maintain a high throughput and retention of the desired permeate and retentate to a high degree. The ideal physical structure of the membrane is a thin layer of material with a small range of pore size and a high surface porosity. Membranes have the ability to separate dissolved solutes in liquid stream and separate gas mixtures in the form of filtration.
Different types of membranes can be classified depending on their design objective:
The driving force used for the separation, such as temperature, concentration gradient, pressure, partial pressure etc.
Mechanism of separation.
Structure and chemical composition.
Construction geometry of the membrane.
Membrane filtration is used to remove particles that are too small for ordinary filters to remove. An application for this is to prevent waste particles from exiting with the permeate to be digested by microbes, which too cannot filter out. This simplified concept is typically called a Membrane Bio Reactor (MBR).
Fig1.1 Diagram of how membrane filtration works.1
The basic types of membrane filtration systems are: microfiltration, ultrafiltration, nanofiltration and reverse osmosis. These types differ mostly in the size of the pores, which prevent a size of particle from permeating. Listed below are descriptions of these filtration systems:
This type of filtration retains particles in the range of 0.1 to 10 microns. Particles in the size range include suspended solids and bacteria. Part of the viral contamination can be caught in this process even though viruses are smaller than the pores o f microfiltration membrane, because viruses can attach themselves to the bacteria biofilm. Examples of microfiltration are: separation of bacteria from wastewater, separation of oil/water emulsions and the pre-treatment of water for nanofiltration or reverse osmosis.
This type of filtration retains particles in the range of 0.001-0.1 microns. Proteins and sugar molecules are in this size range and even viruses can be completely removed. This type of membrane filtration is not as common as microfiltration in the waste water processes, but the need to remove viruses my call for ultrafiltration.
Reverse osmosis removes the smallest particles compared to micro and ultra filtration, retaining particles smaller than 0.001 microns. It also can retain ionic substances such as dissolves salts and metals. This process has been extensively used for desalination of salt water.
Removal of Suspended Solids
Suspended solids are described as small solid particles which remain in suspension as colloids in water due to water motion. Suspended solids are dangerous to human health as they are known to carry bacteria and diseases such as cholera as well as being highly displeasing to the eye. These solids are almost always removed currently with the use of settling tanks or simple filters but this does not guarantee maximum removal, this is where membranes come into their forte.
The removal of suspended solids in this context uses microfiltration or ultrafiltration membranes in a hollow fibre set up to remove suspended solids from the fluid. Both ultra and microfiltration remove suspended solids from the wastewater feedstock to give a turbidity of less than 0.1 NTU (Nephelometric Turbidity Units) and a 15 minute silt density index of less than 3. 4 These membranes are used in water treatment as a polishing stage to take the water quality to a suitable level for irrigation or as a pre-treatment step to reverse osmosis. There is also an option to submerge micro or ultrafiltration membranes inside the activated sludge basin of a wastewater treatment plant. This use of membranes effectively eliminates the need for a secondary clarifier in the treatment system.
Removal of Dissolved Solids
Dissolved solids are removed from wastewater for more aesthetic reasons than health reasons. Dissolved solids do not affect human health in any way but do affect the taste and clarity of the water. For this reason it is deemed essential to remove dissolved solids from the water so as to keep up the appearance of good clean water for the customer. Dissolved solids are different from suspended solids as they cannot be removed from the water by a sieve of 2 micrometers porosity. Due to their small particle size these dissolved solids are very difficult to remove from the water and therefore the suggestion of a Reverse Osmosis membrane for the task is one that is potentially economically viable.
A reverse osmosis membrane in a wastewater treatment system would remove the majority of dissolved solids to produce water with a very low TDS (Total Dissolved Solids) for reuse in industrial processes and boilers as well as potable reuse after further treatment. Coupling the reverse osmosis system with the ultra or microfiltration steps described previously would give extremely high quality water which could be made potable with UV treatment. This entire system would give a minor carbon footprint as membrane separation processes use very little energy in comparison to common separation processes used today such as aeration tanks.
Due to their small pore sizes, ultra and microfiltration membranes mentioned above remove bacteria from the water system. Bacteria are simply too big to fit through the pores of a membrane and with the smaller pore sizes of ultrafiltration membranes they can even reject smaller sized viruses and possibly endocrine disruptors.
A lot of current membrane research is focusing on the use of membranes as a proven removal system for endocrine disruptors. These disruptors, such as DDT, are highly hazardous to human health as they interfere with all actions of hormones in the human body, including those for development and normal cell function 7 and so must be thoroughly removed from any water system.
Membrane Bioreactor systems (MBR)
Bioreactors are reactors which use living organism to convert or produce materials. Typically bioreactors can use immobilized enzymes, microorganisms, animal, or plant cells and new mythologies which include cell fusion or genetic engineering. For the living organisms to survive, bioreactors operate under milder conditions of temperature and pressure compare to reactors.
Membrane bioreactors are essentially membrane and biological reactor systems combined, this is a relatively new concept for waste water treatment. MBR applications for wastewater treatment are classified in this report in the later sections. With recent technical innovation and cost reduction have made MBR to become an established process option for wastewater treatment. The advantage of utilizing MBRs compared with conventional wastewater processes include smaller footprint, easy retrofitting and upgrade of exsisting wastewater treatment plants in to MBRs. However the main drawback limiting the application of MBRs is membrane fouling.
Fig3.1. A Simple schematic of a) submerged MBR, b) side stream MBR with a separate filtration unit.3
The first series of MBRs for municipal waste water treatment in Europe was in 1998. Previously medium sized have been employed successfully however larger sized projects have been slow to be implemented. Many companies were deterred by the lack of full scale experience and complexity, life expectancy and high membrane costs. It was only in recently years that the use of large scale MBR for municipal waste water applications were being used. This can be attributed to experience gained from pilot scale projects, drastic decrease in membrane costs, and improved performances of membranes.
Advantages of using MBR
MBRs can operate effectively when the food to organism (F/M) ratio is low. When FM ratio is low, the organisms are under food limiting condition, increasing the sludge settleability. In conventional wastewater treatment the sludge may fail to settle and overflow in the settling tank. In a MBR the membrane prevents suspended flocs (biomass) from leaving the digester. Further advantages of using the MBR are summarized as follows:2
MBR are proven to be efficient in removing both organic and inorganic contaminates as well as microbiological organisms from waste water. Since this process proven to be good, possible further treatment from reverse osmosis is possible with drastically minimized biofouling and chemical scaling.
Suspended particles are not lost, the solid retention time (SRT) and hydraulic retention time (HRT) can be controlled.
MBR hydraulic retention time is lower (8-10 hours) compared with conventional waste water treatment processes (15-28 hours) for the same size tank volume. Thus reducing tank volume is the same throughput is desired.
Compared to conventional wastewater treatment MBR requires a smaller footprint per BOD loading per unit feed flow rate. This is ideal for expanding existing facilities where space is limited.
MBRs operate at low F/m and long SRT, resulting in less sludge generation. This reduces sludge needed for disposal and the total cost to do so.
Due to membrane separation, the need for a settling tank is avoided. The slow growing microbes are retained, such as nitrifying bacteria and bacteria capable of degrading complex organic compounds. Also extra cellular enzymes and soluble oxidants produced by these organisms, thus creating a process where a wider range of organic compounds can break down.
Phosphorus as suspended solids are better removed my MBRs.
MBRs are able to handle fluctuations in nutrient concentration because the extensive biological assimilation and retention of biomass. Any complex organics that are unable to be digested shall be retained by the membrane and cleaned out as sludge.
A MBR is easier to control as a whole and more amenable to automation compared to conventional waste water treatment processes.
Types of MBR in Waste Water Treatment
Extractive Membrane Bioreactors
Use of membrane bioreactors in the treatment of wastewater as a method of extraction exploits the membranes high degree of separation to enhance the biological treatment step. A microfiltration or ultrafiltration membrane is used in the system to maintain the optimal conditions within the reactor for maximum biological degradation of wastewater pollutants. The difference in concentration across the membrane forces the organic pollutants across the membrane to be degraded by biological means. These reactors can operate in 2 modes. 5
The membranes are immersed in the bio-medium tank and wastewater is circulated around the membranes. The organic components are selectively transported out of the main water flow across the membranes to the biomass for degradation due to differences in concentration. Specialist cultures are then located across the membrane which will degrade the organic mass effectively as a method of cleaning the water.
In this method the membrane forms an external circuit with the tank. The wastewater medium is pumped along the shell side of a hollow fibre membrane set up with the bio-medium pumped tube side in the membrane lumens. Again, the organic compounds transfer across the membrane due to a difference in concentration in the bio-medium side. The pollutants are then degraded by specialist microbes in the medium while the remaining processed fluid flows on through the shell side to the next process.
Fig 5.1 Different modes of Extractive Membrane Bioreactor Operation 5
Bubble-less Aeration Membrane Bioreactors (MABRs)
MABR stands for Membrane Aeration Bioreactor. These reactors use gas-permeable membranes to supply high quality oxygen to a biofilm of microbes without bubbles. Conventional aerobic reactors use sparging and bubbling as a method of supplying oxygen to microbes and their efficiency relies on the availability of air in the system. Conventional systems typically lose 80 to 90% of the sparged air to the atmosphere. 5
Current research focuses on using hollow fibre modules for this process due to their high surface are and small volume. By flowing the gas on the lumen side and attaching the microbes on the shell side with the water flow there is high support for the biofilm which reduces potential for bubbling and loss of oxygen. This method gives maximum oxygen to the microbial biofilm and hence gives maximum processing of organic components in the wastewater.
Recycle Membrane Bioreactors
Recycle MBRs work on an external membrane module that separates any remaining substrate or biomass from the end products. The remaining fluid is then retained and recycled back into the reactor for further processing. External membrane systems are most commonly used in continuous processes and are considered as the best option for removing inhibitory products from a bioreactor system. These systems can be used in a beaker or tube configuration. The beaker configuration effectively uses a tube as a reaction vessel with U-shaped fibres constantly removing the product from the system while the tube configuration acts in a similar way to a shell and tube heat exchanger. Current research in these systems is aimed towards the use of biocatalysts on the membrane surface to aid degradation and lower diffusional resistances caused by cake build up on the surface which is accountable for between a 10 and 90% loss of activity. 5
Fig 5.3 Schematic of a wastewater treatment plant with and without a membrane separation unit 6
Membrane Separation Bioreactors (MSBRs)
Current activated sludge treatment steps rely quite heavily on the hydrodynamic conditions of the system and sludge settling properties for a high quality effluent. MSBRs are used in the activated sludge step of wastewater treatment to aid the biological treatment in the yield of a higher quality effluent in any conditions. The membrane (ultra or microfiltration) is used to remove suspended solids from the effluent. Using membranes in this way ensures almost completely bacterial, viral, colloidal and solid free effluent in the system output.
Another option of this system is to remove the secondary sedimentation step in the processing of wastewater and replace it with a microfiltration or ultrafiltration membrane unit. The effect of this system would be to remove suspended solids from the water and withdraw a fairly clean effluent from central membrane housing in the middle of the system. The water withdrawn from the system would simply leave any solids on the outside of the membrane to be cleaned off and disposed off in the normal manner.
Fig 5.4 Evolution of the MSBR 5
This review of MBRs for the application of waste water treatment has proved that this niche technology can be implemented with relative ease. The main problem with current conventional waste water processes is the large footprint the facility imposes. MBR are proven to be reliable and have shown ease of use. Effluent quality remains consistently high and generally independent to the influent quality. Current waste water research efforts have been aimed towards MBRs, and the next stage is in the development of a robust and efficient MBR process for various waste water applications.
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