Membrane bioreactors for wastewater treatment are combination of a suspended growth biological treatment method, usually activated sludge, with membrane filtration equipment, typically low-pressure microfiltration (MF) or ultrafiltration (UF) membranes.
The membranes are used to perform the critical solid-liquid separation function
Membrane Bioreactors combine conventional biological treatment (e.g. activated sludge) processes with membrane filtration to provide an advanced level of organic and suspended solids removal. In an MBR system, the membranes are submerged in an aerated biological reactor. The membranes have porosities ranging from 0.035 microns to 0.4 microns (depending on the manufacturer), which is considered between micro and ultrafiltration.
Membrane modules are immersed in the aerobic tank where the organic content (BOD) is biologically degraded by activated sludge. The MLSS concentration in the MBR system is 10 to 20 g / L compared to 3 to 4 g / l in conventional activated sludge systems, so the retention time is only 30% of a conventional system. The membranes also separate the suspended solids from the liquid through the filtration process.
Whereas the pore size of the membrane is usually 0.1 micron, in addition to the suspended solids, the bacteria such as coliform are removed. The immersed membrane filtration process also eliminates the need for the gravity or clarifier tank required by conventional activated sludge systems. Nitrate is removed by moving the MLSS from the aerobic tank to the oxygen tank. An additional coagulants and flocculants can also be added to eliminate phosphorus.
Membrane technology used in MBR system:
Basically the membranes have five shapes: 1) Plate-and-frame, 2) Spiral-wound, 3) Hollow fiber 4) Tubular 5) rotary disc and cylindrical, but mainly two types of tubular and hollow fiber membranes are used in MBR systems. Tubular membranes are like tubes with internal pores. The pressurized wastewater enters the pipe and discharged perpendicular to the membrane. The disadvantage of tubular membranes is the low surface-to-volume ratio, but they are appropriate for the treatment of high solids wastewater due to its Anti-clogging feature. Hollow fiber membranes are made of compact, flexible, categorized fibers. In these membranes, the wastewater enters through the fiber, and the treated water comes out of the other end of the fiber.
Fiber membranes have a high surface-to-volume ratio and, as a result, are more likely to become clogged. The flexibility of the fibers allows them to be backwashed without damage to the membrane. Fiber membranes are mainly used in submerged MBR.
MBR is used in two types of external and internal design in wastewater treatment. External MBRs are located after activated sludge (aerobic or anaerobic) systems and are used for high pollution and low flow wastewater such as sanitary landfill wastewater, pharmaceutical industry, etc., while internal MBR is embedded in the activated sludge reactor and is known as submerged MBR. This type of MBR is suitable for medium and high-flow wastewaters and is competitive with conventional and advanced systems. They are used for high-flow wastewater such as municipal and industrial wastewater.
MBR Process Basics:
Typical schematic for membrane bioreactor system:
MBR membrane filtration has two major configurations:
1) vacuum-driven membranes immersed directly into the bioreactor (iMBR)
2) pressure-driven filtration in side-stream MBRs (sMBR)
They typically use hollow fibers or flat sheet membranes mounted on the bioreactor.
Although sMBRs are more energy intensive than iMBRs, they offer a number of advantages:
• Reduced membrane area requirement
• Operational flexibility for operation & cleaning cycle
• Maintenance and plant downtime costs, particularly for membrane module replacement, are generally slightly lower; the modules are readily accessible and so can be replaced in a much shorter time than for the immersed membranes
• Operation at higher solids concentrations is possible
• Operation at a lower energy demand is possible if the pressure and flow rates are reduced
In the MBR process, the parameter ranges are different from the conventional activated sludge process:
• SRT for the conventional system is 5 to 20 days and for the MBR system is 20 to 30 days
• MLSS for the conventional process is 2000 mg / l and for the MBR process 5000 mg / l to 2000 mg / l.
• Retention time is 30% of conventional activated sludge process.
Schematic comparison of conventional activated sludge (CAS) and membrane bioreactor (MBR) processes:
In general MBRs have three distinct membrane configurations:
• flat sheet (FS)
• hollow fibre (HF)
• multitube (MT)
In general MBRs have been applied to treat effluent in a number of industrial sectors, like:
• food and beverage − high in organic loading
• petroleum industry − exploration, refining and petrochemical sectors
• pharmaceutical industry – have active pharmaceutical ingredients (APIs)
• pulp and paper industry − high levels of suspended solids, COD and BOD
• textile industry effluent − re-biodegradability, toxicity, FOG content and color
MBR process specification at a glance:
Parameters which are effective in MBR membranes fouling:
Modern systems are maintained with chemicals, i.e. it is not necessary to remove the membranes from the membrane tank. Organic fouling can be cleaned with sodium hypochlorite and inorganic fouling with oxalic acid.
Most MBR processes are cleaned and washed weekly with chemicals. However chemical cleaning is always required at some points since there remains a residual resistance which can be defined as ‘irrecoverable fouling’ which may build up over an amount of time and decrease the membrane life.
Fouling occurs as a consequence of interactions between the membrane and the mixed liquor, and is one of the principal limitations of the MBR process. Fouling of membranes in MBRs is a very complex phenomenon with diverse interlinkages among its causes, and it is very difficult to localize and define membrane fouling clearly. The main causes of membrane fouling are:
• Adsorption of macromolecular
• Growth of biofilms on the membrane surface
• Precipitation of inorganic matter
• Aging of the membrane
Mechanism of membranes fouling in the MBR process:
When an air/ liquid stream flows parallel to the membrane surface it creates a shear force which helps cleaning and limiting the degree of fouling on the membrane.
Operation of MBR systems
The operation of the membrane is relatively straightforward, a pump is required to supply pressure and circulate the membrane module flow, as well as a valve to maintain the pressure of the concentrated material. Filtered water is usually released at atmospheric pressure.
As the solid materials in the wastewater accumulate on the filter (membrane fouling) the pressure on the feed portion increases and as a result, the membrane flux and the percentage of rejection are reduced.
When the membrane recovery is reduced, the membrane is removed from service and cleaned by backwash and chemicals. In order to avoid membrane fouling and deposition of suspended solids at the membrane surface, turbulent flow conditions (Reynolds number greater than 2000) in the membrane is necessary.
Parameters such as pressure, temperature, membrane compression density, flux (flow rate per unit area), recovery factor, solute rejection, membrane lifetime, pH, turbidity, energy consumption and flow rate are all effective in MBR operation. Parameters related to MBR utilization include α factor and energy consumption and membrane fouling. In summary, the Fouling can be divided into two groups (Microfouling) deposition, biological clogging, adsorption / clogging by organic matter, piercing obstruction and (Macrofouling) cake formation on the surface of membrane, junk arrival, etc.
The applications of the MBR process are mostly:
• Limited space
• high quality treated water required (e.g. for water reuse)
Advantages of MBR system:
• Independent control of HRT and SRT
• Need low space
• Improve biological treatment
• Secondary clarifiers and secondary purification processes are not required. In some cases, other process units, such as biological process or ultraviolet disinfectants, can also be eliminated and minimized.
• longer sludge age and less sludge production
• High effluent quality
• High loading rate capability
• High operation and capital costs (membranes)
• Membrane complexity and fouling
• Membrane age
• Energy costs