Membrane Bioreactor (MBR) – Principle, Types, Process, Uses

What is Membrane Bioreactor?

• A Membrane Bioreactor (MBR) is a wastewater treatment technology that combines a biological treatment process with membrane filtration.
• In an MBR system, microorganisms break down organic matter in the wastewater, and then the treated water is passed through a membrane filter to remove any remaining suspended solids and microorganisms. This results in a highly treated effluent that is suitable for reuse or discharge to the environment.
• MBR systems are commonly used for treating municipal and industrial wastewater, as well as for water reuse applications.
• The sewage effluent shall be collected in a chamber with a screen. This manually cleaned screen is offered to remove large and free-floating particles that could otherwise clog pumps and pipe lines.
• After screening, wastewater is collected in an underground equalisation tank, where it is mixed using an air diffusion system. Non-clogging submersible pumps transport effluent from the equalisation tank to the biological chamber for further degradation of organic pollutants.

• Up to 8000 mg/l of MLSS (mixed liquor suspended solids) are maintained in the Biological-MBR tank. A greater and more comprehensive removal of organic materials from raw sewage effluent is facilitated by the high bacterial concentration in a relatively small region.
• The air blower with two lobes provides the bacteria with the oxygen they require. The air is utilised for both cleaning membranes and oxygenating bacteria.
• The filtration is performed by a vacuum pump drawing penetrated water directly from PMTR modules. The resulting permeate water is ultra-filtered and has turbidity 1.0 NTU.
• Water and wastewater treatment rely heavily on membrane filtration, which is preferable to conventional water technologies due to its higher performance and cost-effectiveness. Microfiltration, ultrafiltration, nanofiltration, and reverse osmosis are the fundamental membrane processes (RO).
• These membranes’ separation ranges are as follows: MF is 100 to 1000 nm, UF is 5 to 100 nm, NF is 1 to 5 nm, and RO is 0.1 to 1 nm. Using membranes in the range of MF and UF, MBRs have developed as a viable secondary treatment method for more than a decade.
• Utilizing microfiltration (MF) or ultrafiltration (UF) and merging them with a biological process such as a suspended growth bioreactor, Membrane bioreactor (MBR) processes are mostly utilised for wastewater treatment (WWT). The membranes are used as a filter to remove the particles produced during the biological process, resulting in a product that is clear and devoid of pathogens. Figure depicts an immersed MBR (iMBR) to provide a visual illustration.

Working Principle of Membrane Bioreactor

The sewage waste and industrial effluent first enter the stilling room before transferring to the screening chamber. In this chamber, Hyper Filteration-manufactured tools are used to remove plastic, coarse particles, metals, etc. from the water in order to protect the equipment for subsequent processing and to prevent the water pipes from becoming clogged. In the process of water treatment, this procedure or treatment is known as sewage treatment. After the elimination process is complete, the sewage waste and effluent are sent to the equalisation tank. The equalisation tank is used to keep the rate of sewage and effluent combination steady and uniform. It is also utilised to homogenise sewage and effluent mixtures. Due to aeration in the equalisation tank, the detonation velocity of wastewater has dropped from 10% to 20%. Both the MBR tank and the Equalization tank utilise blowers. Send homogeneous raw sewage and industrial effluent to the Membrane Bioreactor after equalisation. Membrane bioreactors are utilised in secondary or primary biological treatment. Similar to an activated sludge tank, Membrane Bioreactor has an aeration system. In the Membrane Bioreactor, the aeration system is continuously used to give continuous circulation to the sewage in order to maintain the dissolved oxygen concentration in the chamber. A microbe, also known as a biolayer, has established a layer on the plastic media, and with the assistance of correct aeration, the microorganism can be grown, hence increasing the Membrane Bioreactor’s effectiveness.

Membranes

Solid–liquid separation is done by Microfiltration (MF) or Ultrafiltration (UF) membranes during MBR wastewater treatment. A membrane is merely a two-dimensional material that is typically used to separate fluid components based on their relative size or electrical charge. Semi-permeability is the capacity of a membrane to permit the passage of only particular molecules (sometimes also permselective). This is a physical process in which separated components remain unaltered chemically. Components that flow through the pores of a membrane are known as permeate, whereas those that are rejected create a concentrate or retentate.

There are 5 types of membrane configuration for MBRs:

1. Hollow fibre (HF)
2. Spiral-wound
3. Plate-and-frame (i.e. flat sheet (FS))
4. Pleated filter cartridge
5. Tubular

Other topologies, such as spiral-wound (SW), are unsuitable for MBR applications due to their sensitivity to the presence of suspended particulates.

Parts of Membrane Bioreactors

A membrane bioreactor (MBR) system typically includes the following components:

1. Pretreatment: This includes equipment such as screens, grit chambers, and oil/water separators to remove large debris, heavy particles, and oil/grease from the wastewater.
2. Biological treatment: This includes an aeration tank or other type of reactor where microorganisms are used to break down the pollutants in the water.
3. Membrane filtration: This includes the actual membrane module, which can be made up of various types of membranes, such as microfiltration (MF), ultrafiltration (UF), or reverse osmosis (RO) membranes.
4. Disinfection: This includes equipment such as ultraviolet (UV) disinfection systems or chlorine injection systems to kill any remaining microorganisms in the water.
5. Pumps and piping: This includes the pumps and piping that are used to transport the water through the different stages of the system.
6. Control and monitoring: This includes the equipment and systems used to monitor and control the operation of the MBR, including sensors, controllers, and computer systems.
7. Maintenance: This includes all the equipment and tools that are needed for regular maintenance and cleaning of the system, such as membrane cleaning systems, chemical dosing systems, and sludge handling equipment.
8. Electrical and mechanical equipment: This includes the electrical and mechanical equipment needed to operate the system, such as motors, blowers, and electrical panels.

It is important to note that the specific components used in an MBR system will depend on the type of system and the specific treatment goals.

Operating Procedure of Membrane Bioreactor

Using a membrane bioreactor (MBR) involves several steps:

1. Pretreatment: The wastewater is first screened to remove large debris and then passed through a grit chamber to remove sand and other heavy particles.
2. Biological treatment: The pretreated wastewater is then passed through a biological treatment unit, such as an aeration tank, where microorganisms are used to break down the pollutants in the water.
3. Membrane filtration: The biologically treated water is then pumped through a membrane filtration unit, which uses a semi-permeable membrane to remove small particles and microorganisms.
4. Disinfection: The treated water is then disinfected to kill any remaining microorganisms and reduce the risk of disease.
5. Monitoring and control: The entire process is continuously monitored to ensure that the water is being treated to the required standards, and adjustments are made as necessary to optimize the process.
6. Maintenance: Regular maintenance is required to ensure that the system is operating efficiently and to replace or clean the membranes when necessary.

It is important to note that the exact process will vary depending on the type of MBR system, the quality of the wastewater, and the specific treatment goals. Also, it is important to have a professional team to operate the system, as they have the knowledge and skills to properly maintain and troubleshoot the system.

Treatment Process

• Combining conventional biological treatment (e.g. activated sludge) with membrane filtration, Membrane Bioreactors provide an enhanced level of organic and suspended particles removal. These systems can also provide an advanced level of nutrient removal when constructed appropriately.
• Membranes are submerged in an aerated biological reactor in an MBR system. Depending on the manufacturer, the membranes have porosities ranging from 0.035 microns to 0.4 microns, which is between microfiltration and ultrafiltration.
• This level of filtration allows for the passage of high-quality effluent through the membranes and removes the sedimentation and filtering processes commonly employed in wastewater treatment. Since sedimentation is no longer required, the biological process can run at a significantly greater mixed liquor concentration.
• This drastically decreases the necessary process tankage and permits many existing facilities to be updated without the addition of additional tanks.
• To enable optimal aeration and scouring around the membranes, the mixed liquor is normally maintained in the range of 1.0-1.2% solids, which is four times that of a standard plant.

Types of Membrane Bioreactors

There are several types of membrane bioreactors (MBRs), each with its own unique features and advantages. Some of the most common types include:

1. Submerged Membrane Bioreactors (SMBR)

• A Submerged Membrane Bioreactor (SMBR) is a type of membrane bioreactor (MBR) where the membrane module is submerged in the biological treatment tank, and the water is pumped through the membrane.
• The purpose of an SMBR is to combine the biological treatment of wastewater with membrane filtration to achieve a high level of treatment efficiency and water quality. The microorganisms in the biological treatment stage break down the pollutants in the water, while the membrane removes small particles and microorganisms to produce a high-quality effluent.

Operating Procedure of SMBR

The mechanism of an SMBR involves the following steps:

1. Pretreatment: The wastewater is screened and grit-removed to remove large debris and heavy particles.
2. Biological treatment: The pretreated wastewater is then passed through an aeration tank or other type of reactor where microorganisms are used to break down the pollutants in the water.
3. Membrane filtration: The biologically treated water is then pumped through a membrane filtration unit, which uses a semi-permeable membrane to remove small particles and microorganisms.
4. Disinfection: The treated water is then disinfected to kill any remaining microorganisms and reduce the risk of disease.

Uses of Submerged Membrane Bioreactors (SMBR)

Uses of Submerged Membrane Bioreactors (SMBR) include:

• Municipal wastewater treatment
• Industrial wastewater treatment
• Water reuse
• Sludge treatment
• Desalination

Advantages of Submerged Membrane Bioreactors (SMBR)

Advantages of Submerged Membrane Bioreactors (SMBR) include:

• High treatment efficiency
• High-quality effluent
• Compact and space-saving design
• Low energy consumption

Disadvantages of Submerged Membrane Bioreactors (SMBR)

Disadvantages of Submerged Membrane Bioreactors (SMBR) include:

• High maintenance and membrane replacement costs
• Membrane fouling
• Require skilled personnel to operate and maintain
• Potential for clogging of the membrane.

It is important to note that the specific use, advantages, and disadvantages of an SMBR will depend on the specific characteristics of the wastewater and the treatment goals.

2. External Membrane Bioreactors (EMBR)

• An External Membrane Bioreactor (EMBR) is a type of membrane bioreactor (MBR) where the membrane module is located outside of the biological treatment tank, and the water is pumped through the membrane after it has been biologically treated.
• The purpose of an EMBR is similar to that of a Submerged Membrane Bioreactor (SMBR), which is to combine the biological treatment of wastewater with membrane filtration to achieve a high level of treatment efficiency and water quality. The microorganisms in the biological treatment stage break down the pollutants in the water, while the membrane removes small particles and microorganisms to produce a high-quality effluent.

Operating Procedure

The mechanism of an EMBR involves the following steps:

1. Pretreatment: The wastewater is screened and grit-removed to remove large debris and heavy particles.
2. Biological treatment: The pretreated wastewater is then passed through an aeration tank or other type of reactor where microorganisms are used to break down the pollutants in the water.
3. Membrane filtration: The biologically treated water is then pumped through a membrane filtration unit, which is located outside of the biological treatment tank, and uses a semi-permeable membrane to remove small particles and microorganisms.
4. Disinfection: The treated water is then disinfected to kill any remaining microorganisms and reduce the risk of disease.

Applications of EMBR

Uses of External Membrane Bioreactors (EMBR) include:

• Municipal wastewater treatment
• Industrial wastewater treatment
• Water reuse
• Sludge treatment
• Desalination

Advantages of EMBR

Advantages of External Membrane Bioreactors (EMBR) include:

• High treatment efficiency
• High-quality effluent
• Reduced fouling and clogging of the membrane
• Easy maintenance

Disadvantages of EMBR

Disadvantages of External Membrane Bioreactors (EMBR) include:

• Space-consuming design
• High maintenance and membrane replacement costs
• Require skilled personnel to operate and maintain
• Higher energy consumption compared to SMBRs

It is important to note that the specific use, advantages, and disadvantages of an EMBR will depend on the specific characteristics of the wastewater and the treatment goals.

3. Hybrid Membrane Bioreactors (HMBR)

A Hybrid Membrane Bioreactor (HMBR) is a type of membrane bioreactor (MBR) that combines the features of Submerged Membrane Bioreactors (SMBR) and External Membrane Bioreactors (EMBR). In this type of MBR, the membrane module is partially submerged in the biological treatment tank and partially outside of it.

The purpose of a HMBR is similar to that of SMBR and EMBR, which is to combine the biological treatment of wastewater with membrane filtration to achieve a high level of treatment efficiency and water quality. The microorganisms in the biological treatment stage break down the pollutants in the water, while the membrane removes small particles and microorganisms to produce a high-quality effluent.

Operating Procedure of HMBR

The mechanism of a HMBR involves the following steps:

1. Pretreatment: The wastewater is screened and grit-removed to remove large debris and heavy particles.
2. Biological treatment: The pretreated wastewater is then passed through an aeration tank or other type of reactor where microorganisms are used to break down the pollutants in the water.
3. Membrane filtration: The biologically treated water is then pumped through a membrane filtration unit, which is partially submerged in the biological treatment tank and partially outside of it, and uses a semi-permeable membrane to remove small particles and microorganisms.
4. Disinfection: The treated water is then disinfected to kill any remaining microorganisms and reduce the risk of disease.

Applications of HMBR

Uses of Hybrid Membrane Bioreactors (HMBR) include:

• Municipal wastewater treatment
• Industrial wastewater treatment
• Water reuse
• Sludge treatment

Advantages of HMBR

Advantages of Hybrid Membrane Bioreactors (HMBR) include:

• High treatment efficiency
• High-quality effluent
• Reduced fouling and clogging of the membrane
• Easy maintenance
• Reduced energy consumption compared to SMBRs

Disadvantages of HMBR

Disadvantages of Hybrid Membrane Bioreactors (HMBR) include:

• Higher construction and maintenance costs
• Require skilled personnel to operate and maintain

It is important to note that the specific use, advantages, and disadvantages of a HMBR will depend on the specific characteristics of the wastewater and the treatment goals.

4. Integrated Membrane Bioreactors (IMBR)

An Integrated Membrane Bioreactor (IMBR) is a type of membrane bioreactor (MBR) that is designed to provide both biological treatment and membrane filtration in a single compact unit. IMBRs typically have a smaller footprint and lower energy consumption compared to traditional MBR systems.

The purpose of an IMBR is similar to that of other MBR types, which is to combine the biological treatment of wastewater with membrane filtration to achieve a high level of treatment efficiency and water quality. The microorganisms in the biological treatment stage break down the pollutants in the water, while the membrane removes small particles and microorganisms to produce a high-quality effluent.

Operating Procedure of IMBR

The mechanism of an IMBR involves the following steps:

1. Pretreatment: The wastewater is screened and grit-removed to remove large debris and heavy particles.
2. Biological treatment: The pretreated wastewater is then passed through an aeration tank or other type of reactor where microorganisms are used to break down the pollutants in the water.
3. Membrane filtration: The biologically treated water is then pumped through a membrane filtration unit, which is integrated within the same unit as the biological treatment stage, and uses a semi-permeable membrane to remove small particles and microorganisms.
4. Disinfection: The treated water is then disinfected to kill any remaining microorganisms and reduce the risk of disease.

Uses of IMBR

Uses of Integrated Membrane Bioreactors (IMBR) include:

1. Municipal wastewater treatment
2. Industrial wastewater treatment
3. Water reuse
4. Sludge treatment
5. Desalination

Advantages of IMBR

Advantages of Integrated Membrane Bioreactors (IMBR) include:

1. High treatment efficiency
2. High-quality effluent
3. Reduced fouling and clogging of the membrane
4. Easy maintenance
5. Compact and space-saving design
6. Low energy consumption

Disadvantages of IMBR

Disadvantages of Integrated Membrane Bioreactors (IMBR) include:

1. High maintenance and membrane replacement costs
2. Require skilled personnel to operate and maintain
3. Limited flexibility in terms of scalability

It is important to note that the specific use, advantages, and disadvantages of an IMBR will depend on the specific characteristics of the wastewater and the treatment goals.

5. Membrane Aerated Biofilm Reactor (MABR)

A Membrane Aerated Biofilm Reactor (MABR) is a type of membrane bioreactor (MBR) that uses air bubbles to provide oxygen to the microorganisms in the biological treatment stage. The bubbles are introduced through the membrane itself, which acts as a support for the biofilm, providing a large surface area for the microorganisms to grow on.

The purpose of a MABR is to combine the biological treatment of wastewater with membrane filtration to achieve a high level of treatment efficiency and water quality. The microorganisms in the biofilm break down the pollutants in the water, while the membrane removes small particles and microorganisms to produce a high-quality effluent.

Operating Procedure of MABR

The mechanism of a MABR involves the following steps:

1. Pretreatment: The wastewater is screened and grit-removed to remove large debris and heavy particles.
2. Biological treatment: The pretreated wastewater is then passed through the membrane module, where air bubbles are introduced to provide oxygen to the microorganisms in the biofilm.
3. Membrane filtration: The biologically treated water is then pumped through the membrane module, which uses a semi-permeable membrane to remove small particles and microorganisms.
4. Disinfection: The treated water is then disinfected to kill any remaining microorganisms and reduce the risk of disease.

Uses of MABR

Uses of Membrane Aerated Biofilm Reactor (MABR) include:

• Municipal wastewater treatment
• Industrial wastewater treatment
• Water reuse
• Sludge treatment

Advantages of MABR

Advantages of Membrane Aerated Biofilm Reactor (MABR) include:

• High treatment efficiency
• High-quality effluent
• Reduced fouling and clogging of the membrane
• Low energy consumption
• Low maintenance costs
• Increased surface area for biofilm growth
• Compact and space-saving design

Disadvantages of MABR

Disadvantages of Membrane Aerated Biofilm Reactor (MABR) include:

• High initial costs
• Requires skilled personnel to operate and maintain
• Limited flexibility in terms of scalability
• Potential for membrane damage due to air bubbles

It is important to note that the specific use, advantages, and disadvantages of a MABR will depend on the specific characteristics of the wastewater and the treatment goals. While MABR systems are relatively new, they are becoming more popular because of their high treatment efficiency and low energy consumption, making them a promising technology for wastewater treatment.

6. Moving Bed Bioreactor (MBBR)

A Moving Bed Bioreactor (MBBR) is a type of membrane bioreactor (MBR) that uses plastic carriers in the biological treatment stage to provide a large surface area for the microorganisms to grow on. The carriers are suspended in the treatment tank and are constantly in motion, providing a high level of mixing and oxygen transfer.

The purpose of a MBBR is to combine the biological treatment of wastewater with membrane filtration to achieve a high level of treatment efficiency and water quality. The microorganisms growing on the carriers break down the pollutants in the water, while the membrane removes small particles and microorganisms to produce a high-quality effluent.

Operating Procedure of MBBR

The mechanism of a MBBR involves the following steps:

1. Pretreatment: The wastewater is screened and grit-removed to remove large debris and heavy particles.
2. Biological treatment: The pretreated wastewater is then passed through the treatment tank where the plastic carriers are suspended and provide a large surface area for the microorganisms to grow on.
3. Membrane filtration: The biologically treated water is then pumped through a membrane filtration unit, which uses a semi-permeable membrane to remove small particles and microorganisms.
4. Disinfection: The treated water is then disinfected to kill any remaining microorganisms and reduce the risk of disease.

Applications of MBBR

Uses of Moving Bed Bioreactor (MBBR) include:

• Municipal wastewater treatment
• Industrial wastewater treatment
• Water reuse
• Sludge treatment

Advantages of MBBR

Advantages of Moving Bed Bioreactor (MBBR) include:

• High treatment efficiency
• High-quality effluent
• Reduced fouling and clogging of the membrane
• Low maintenance costs
• Increased surface area for biofilm growth
• Compact and space-saving design
• Low energy consumption

Disadvantages of MBBR

Disadvantages of Moving Bed Bioreactor (MBBR) include:

1. High initial costs
2. Requires skilled personnel to operate and maintain
3. Limited flexibility in terms of scalability
4. Potential for clogging of the carriers

It is important to note that the specific use, advantages, and disadvantages of a MBBR will depend on the specific characteristics of the wastewater and the treatment goals. MBBR systems are becoming more popular because of their high treatment efficiency and low maintenance costs, making them a promising technology for wastewater treatment.

Each type of MBR has its own advantages and disadvantages, and the best choice will depend on the specific treatment goals and the characteristics of the wastewater.

Membrane bioreactor design

• MBR system designers simply need wastewater characteristics such influent characteristics, effluent needs, and flow statistics.
• MBR systems can have additional options depending on effluent needs. If necessary, an MBR system can add chemicals to remove phosphorus before the primary settling tank, the secondary settling tank [clarifier], and the MBR or final filters.
• MBR systems were employed for small-scale treatment when parts of the treatment system were shut down and wastewater was routed around for maintenance. MBR systems are now employed for full-treatment. In these cases, one membrane tank/unit should be added to the design.
• This “N plus 1” approach combines membrane and activated sludge processes. MBR unit selection must take operations and maintenance into account. An extra unit gives operators flexibility and assures appropriate operating capacity.
• Bioreactor numbers and sizing are frequently restricted by oxygen transport, not SRT volume. MBR systems can add or remove units and adjust flow rates, but they have restrictions.
• Membranes need a minimum water surface height to stay moist. Peak design flows should be 1.5 to 2 times the average design flow due to membrane physical features.
• Peak flows above that limit require additional membranes or equalisation. External equalisation uses a basin or maintains water in the aeration and membrane tanks at depths greater than required and removes it to handle larger flows (internal equalization).

Design Features

Pretreatment

Prior entering the MBR, wastewater should undergo an intensive level of debris cleaning in order to limit the likelihood of membrane degradation. Larger installations frequently provide primary treatment, although most small to medium-sized installations do not and it is not required. In addition, depending on the MBR vendor, all MBR systems require 1- to 3-mm-cutoff fine screens right before the membranes. These screens must be cleaned frequently. Utilizing two phases of screening and positioning the screens after initial settling are alternatives for lowering the amount of material reaching the screens.

Membrane Location

In MBR systems, the membranes are immersed in the biological reactor or, alternatively, in a separate vessel through which the biological reactor’s mixed fluid is circulated.

Membrane Configuration

Two fundamental membrane configurations are utilised by MBR manufacturers: hollow fibre bundles and plate membranes. Siemens/Memjet U.S.Filter’s and Memcor systems, GE/ZeeWeed Zenon’s and ZenoGem systems, and GE/Ionics’ system all make use of hollow-fiber, tubular membranes arranged in bundles. A number of bundles are joined by manifolds to form units that are easily replaceable for maintenance. The other version, such as those offered by Kubota/Enviroquip, utilises membranes in a flatplate configuration, again with manifolds that permit a number of membranes to be connected to easily replaceable units. Both methods have different screening requirements: hollow-fiber membranes typically require 1 to 2 mm screening, whereas plate membranes require 2 to 3 mm screening.

System Operation

All MBR systems pump water through the membrane. MBRs draw a vacuum through the membranes to keep the water outside at ambient pressure. Other membrane systems use a pressurised system. Pressure controls throughput while vacuum is friendlier on membranes. All systems incorporate cleaning methods to prolong membrane life and system performance. All MBR membrane systems use air scour to reduce material buildup. The manifolds blow air around the membranes. GE/Zenon systems use air scour and back-pulsing to clean membrane pores. Back-pulsing on a timer takes 1–5% of the operation time.

Downstream Treatment

MBR permeate has low suspended particles and bacteria, BOD, nitrogen, and phosphate. Depending on permission requirements, disinfection is simple. The membrane recycles solids to the biological reactor, which accumulate up. As in conventional biological systems, periodic sludge waste reduces accumulation and controls SRT in MBR systems. Standard solids-handling technologies thicken, dewater, and dispose of MBR waste sludge. Increased colloidal-size particles and filamentous bacteria in waste MBR sludges lowered their ability to settle, according to Hermanowicz et al. (2006). Chemicals helped sludges settle. More MBR facilities will improve biosolids characterization. Conventional biosolids processing unit operations work for MBR waste sludge, according to experience.

Membrane Care

Membrane life is crucial to the cost-effectiveness of an MBR system. If membrane lifespan is shortened to the point that regular replacement is required, expenses will increase dramatically. Increasing membrane life can be done in the following ways:

• Effective screening of bigger materials prior to the membranes to prevent physical damage to the membranes.
• Throughput rates that are not excessive, i.e., do not exceed the system’s design limits. These rates minimise the quantity of material that is driven into the membrane and, consequently, the amount that must be removed by cleaners or will eventually lead to membrane damage. Regular use of gentle cleaning products. Regular bleach (sodium) and citric acid are the two most used cleaning options for MBRs. The cleaning must adhere to the manufacturer-recommended maintenance procedures.

Membrane Guarantees

In considering the cost-effectiveness of a membrane system, the length of the manufacturer’s warranty is also significant. Longer warranties may be given for municipal wastewater treatment systems compared to industrial systems. Zenon provides a 10-year warranty, whereas others range between 3 and 5 years. Some assurances feature a cost reduction if replacement is required after a specified period of service. Typically, guarantees are agreed throughout the purchasing process. Certain manufacturers’ warranties are directly proportional to screen size: membrane warranties are longer for smaller screens. Proper membrane life guarantees can be secured using appropriate membrane procurement procedures.

Membrane fouling control and cleaning

Fouling generally refers to a decrease in permeate flux or an increase in transmembrane pressure during a membrane operation. Membrane fouling is one of the most difficult obstacles preventing the widespread implementation of MBRs. These are the primary reasons of membrane fouling:

• feed characteristics
• biomass characteristics
• membrane characteristics
• operational conditions
• Membrane fouling may be due to following mechanisms:
  • Formation of surface layer or filter cake on the membrane
  • Fouling at pore entrance surface.
  • Fouling within the membrane structure. It has been proved that some proteins deposit within membrane pores and surface.
• Regular cleaning of membrane is essential to remove membrane fouling and keep the permeability loss in a given range. Different types of cleaning procedures for membrane regeneration are classified as:
  • Physical methods
  • Chemical methods
  • Physic-chemical methods
  • Biological methods

Physical cleaning of membranes is performed primarily to remove reversible fouling and can be accomplished through back-flushing or relaxing (stopping the permeate flow and continuing to scour the membrane with air bubbles). Although it is not possible to remove all of the material accumulated on the membrane surface, the process is straightforward and brief.

Typically, it lasts about 2 minutes. To eliminate irreversible fouling, chemical cleaning is more effective at removing adsorbed deposits from membrane surface. Typically, sodium hypochlorite and sodium hydroxide are used to remove organic deposits, while acidic solutions are used to remove lime and other inorganic deposits. This approach is utilised weekly and lasts between 30 and 60 minutes.

The physico-chemical cleaning methods combine physical cleaning techniques with chemical agents to improve cleaning efficacy. Kuiper et al. operated a 16 m3/d RO system with turbulent cellulose acetate membranes for 19 months on a severely contaminated supply. The combination of mechanical cleaning with foam balls and acid washing proven to be the most successful cleaning technique. The biological cleaning approach utilises microbes or enzymes to assist the elimination of foulants. This method is gaining popularity for two reasons: first, biological components do not cause membrane damage, and second, it minimises the negative environmental consequences of chemicals.

Applications of Membrane Bioreactors

Membrane bioreactors (MBRs) are widely used in various applications such as wastewater treatment, industrial effluent treatment, and drinking water treatment. They combine the processes of biological treatment and membrane filtration to remove pollutants and microorganisms from water. The main advantage of MBRs over traditional treatment methods is their high efficiency in removing pollutants and producing high-quality treated water. MBRs are also compact and require less land area compared to traditional treatment methods. Additionally, MBRs can be used for the treatment of high-strength or difficult-to-treat wastewaters, such as those from food and beverage processing, pharmaceuticals, and petrochemical industries.

1. Municipal wastewater treatment: MBRs can be used to treat domestic and industrial wastewater to a high level of purity, enabling the water to be recycled or discharged safely into the environment.
2. Industrial wastewater treatment: MBRs are used to treat high-strength wastewaters from various industries, such as food and beverage, pharmaceuticals, and semiconductors.
3. Water reuse: MBRs can be used to treat and recycle wastewater for non-potable uses, such as irrigation or industrial processes.
4. Desalination: MBRs can be used to remove salt and other dissolved solids from seawater or brackish water, making it suitable for drinking and other uses.
5. Sludge treatment: MBRs can be used to dewater and stabilize sludge, reducing the volume and improving the quality of the final product.
6. Landfill leachate treatment: MBRs can be used to treat leachate, a liquid that has percolated through waste in a landfill, to remove pollutants and reduce its impact on the environment.
7. Aquaculture: MBRs can be used to treat the water in fish and shellfish farms, removing waste and improving water quality for the aquatic life.
8. Pharmaceutical and Biotechnology Industry: MBRs can be used to treat wastewater from the production of drugs, vaccines, and other biotechnology products.
9. Pulp and paper industry: MBRs can be used to treat wastewater from the production of paper, removing pollutants and enabling the water to be reused in the production process.
10. Oil and gas industry: MBRs can be used to treat produced water and other wastewater generated in the production of oil and gas, removing pollutants and enabling the water to be reused.
11. Mining industry: MBRs can be used to treat wastewater from mining operations, removing pollutants and enabling the water to be reused.
12. Textile industry: MBRs can be used to treat wastewater from the production of textiles, removing pollutants and enabling the water to be reused in the production process.

In general, membrane bioreactors are becoming increasingly popular for wastewater treatment due to their high treatment efficiency and ability to recover valuable resources such as water, nutrients, and energy.

Advantages of Membrane Bioreactors

• High quality effluent – because all biomatter, sediments, and bacteria are filtered, the water produced by the MBR is of the highest quality. The effluent can be released into the environment because it is immediately reusable or recyclable.
• Independent HRT and SRT – sludge retention time (SRT) and hydraulic retention time (HRT) are completely independent because sludge solids are completely retained in the bioreactor, unlike in CAS where the biomass must be flocculated into flocs that must then settle, so settlebility relates to the retention of the liquid.
• Small footprint – Compared to conventional activated sludge, MRS use 50% less area (no clarifiers are needed and nor pre-treatment nor tertiary treatment are needed, there are less pipes and valves, less equipment). Additionally, MBRs reduce the footprint of activated sludge treatment by eliminating a portion of the liquid component of the mixed liquor. The waste is then processed by the activated sludge process.
• Consistent performance – mixed liquid suspended solids (MLSS), the water’s organic content, can be significantly greater than in CAS. A greater concentration of biomass results in the efficient removal of biodegradable materials.
• Low sludge production – Less sludge is created and must be disposed of less frequently than in CAS.
• Less sludge dewatering – Less sludge dewatering is required because the sludge has a high solids concentration.

Disadvantages of Membrane Bioreactors

1. High cost: The cost of membrane systems can be high, especially when compared to traditional wastewater treatment methods.
2. Fouling and cleaning: Membranes can become fouled over time, reducing their efficiency and requiring regular cleaning.
3. Membrane replacement: Membranes have a limited lifespan and will need to be replaced periodically, which can be costly and disruptive.
4. Process Complexity: MBR systems are more complex than traditional wastewater treatment methods, which can make them difficult to operate and maintain.
5. High energy consumption: MBR systems require high energy to run the pumps and aeration systems, which can lead to high operating costs.
6. Vulnerability to certain pollutants: Some pollutants such as surfactants and lipids can cause damage to the membrane and reduce the system’s efficiency.
7. Limited availability of skilled operators: MBR systems require skilled operators and maintenance technicians, which can be difficult to find in some areas.

It is worth noting that many of these disadvantages have been reduced or overcome with the advances of the technology and many of the disadvantages are more related to the specific design and operation rather than inherent to the technology itself.

Advantages	Disadvantages
Compact	Aeration limitations
High effluent quality	Stress on sludge in external MBR
High volumetric load possible	Membrane pollution
High rate of degradation	Cost price
Possible to convert from existing conventional active sludge purification

Comparison of External and Internal Membrane Based MBR System Configurations

Comparative Factor	External MBR Systems	Internal MBR Systems
Membrane Area Requirement	Characterized by higher flux and therefore lower membrane    area requirement.	Lower flux but higher membrane packing density (i.e., membrane area per unit volume)
Space or Footprint Requirements	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 Design    and operation Dependency	Bioreactor can be designed and operated under optimal conditions including those to achieve biological N and   P   removal, if required.	Design and operation of bioreactor and membrane compartment or tank are not independent. High membrane tank recycle required (e.g., recycle ratio 4) to limit tank VSS concentration build-up
Membrane Performance Consistency	Less susceptible to changing wastewater and biomass characteristics.	More susceptible to changing wastewater and biomass characteristics requiring alteration in membrane cleaning strategy and/or cleaning frequency
Recovery of Membrane performance	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).
Membrane Life or Replacement Requirements	Results to-date implies an operating life of 7 years or more can be achieved with polymeric prior to irreversible   fouling. Operating life of ceramics much longer	Results to-date implies an operating life of 5 years may be possible prior to irreversible fouling and/or excessive membrane physical damage.
Full Scale Application Status	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.
Economics	Non-conventional designs translate to comparable power costs. Comparable capital cost at least at  lower wastewater feed  rates (e.g., approaching 1893 m3/day).	Power and capital cost advantage at higher wastewater feed rates.

References

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