Membrane Biological Reactor (MBR)
Process known as MBR had its first experiences in the ‘80s. The first membrane of industrial scale was the flat sheet in 1991. Some years later hollow fibre membranes were developed. Nowadays it is a widely used process globally for sewage and industrial effluents treatment.
MBR means bio membrane reactor or membrane biological reactor and is the combination of a biological treatment such as activated sludge and the solid-liquid filtering via microfiltration or ultrafiltration membranes. The real disruptive change generated by this process is the replacement of the secondary clarifier used in activated sludge. Therefore, this allows the operation of the system with higher concentration of solids suspended in mixed liquor (≥8000 mg/l) and hence it considerably reduces the volume in the biological reactor, improving substantially the quality of the treated water.
In short, the main benefits of a biomembrane reactor treatment are the following:
High quality of the treated effluent: membranes can be ultra or microfiltration, reaching much higher
values for treated water than the activated sludge. It can even be installed a tertiary treatment or reverse osmosis if
Required space: the concentration of mixed liquor suspended solids (MLSS) in a MBR process is generally of 8000 – 12000 mg/l, reducing the volume needed for aeration in 40-50% at least. Costs of civil work are considerably less.
Less sludge generation: operating with such high values of mixed liquor suspended solids means a low
value of F/M ratio. The consequence is the operation in extended aeration levels and thus less sludge generation.
Easy adjustment of an activated sludge to a biomembrane reactor.
Simple operation: since filtration is made by membranes, there are no requirements for sludge settling. This
way there is no need of secondary clarifiers or polishing filters. Sludge is discarded directly from the aeration tank to
a solid concentration in a range of 0.8-1.0%. This is the result of a simple system, operated in a simple way.
Reliable and robust process: MBR process usually operates with low levels of organic load, and membranes put a barrier for the particles. The quality of the effluent is not susceptible to increases in organic or hydraulic contamination, which can influence negatively the effluent in traditional plants.
Easy way to increase the capacity.
Membranes can be installed submerged in the biological reactor or at an independent basin. There are also manufacturers of non-submerged external membranes of crossed flow. The major membranes traded worldwide are submerged, and they differ according to type:
Generally hollow fibre membranes are ultrafiltration with a nominal pore of 0.04 μm.
Membrane is made and assembled inside units called “modules”. These are the basic units of the membrane system which are joined to form a “cassette”.
The cassette can contain up to 48 modules. Each module has a membrane area of 31.58 m 2 , with a total membrane area of 1.517 m 2 for the biggest cassette.
Membrane cassettes are submerged directly into the mixed liquor in separated membrane tank. A cassette series connected to the permeated common manifold that is in turn connected to the suction side of the permeated pump, is called “membrane train”. Permeated pump applies a little vacuum to the permeated manifold to lead the water from the outside to the inside of the fibre membrane, leaving the solids of the mixed liquor in the external part of the membrane.
A highly efficient bubble aeration system especially designed for this purpose is used to clean the exterior surface of the membranes and to detach the solids of the mixed liquor from the fibres. This is achieved by a cyclic aeration system that uses a screen for coarse bubble aeration put at the base of each cassette.
Membrane aeration also provides a part of the oxygen required for the biological process, the rest being supplied by an efficient fine bubble aeration system.
Membrane operates by a repeated filtering cycle. This cycle consists of “filtering” clean water from the outside to the
inside through the membrane, followed by a short period of backwash/ retropulse chemical free (reversing the flow
through the membranes) or of relax (no flow).
Filtering or Production
Filtering o permeated process consists of driving clean water from the mixed liquor through the fibres of the membranes to the permeated pump and then discharge it in a manifold. As a rule, water is produced from each train in periods from 10 to 15 minutes, followed by a period of retropulse and relax of 30 to 60 seconds. This frequency and its lasting can vary depending on the appliances and the type of water.
Backwash and relax
Membrane system has the ability of operating in any of the relaxing or backwash modules. During the backwash mode, membranes are cleaned with water from the outside to the inside for 30-60 seconds every 10-15 minutes (this frequency and its length can vary depending on the appliances and type of water). The water used for the backwash is the permeated water stored in the backwash tank. During relaxing phase the backwash is not used, but the membranes are aerated without carrying permeate for 30-60 seconds every 10-15 minutes.
Relaxing mode takes place when there is no filtration, while the rest of the functions (aeration, recirculation) continue. This relaxing period allow the membrane to be aerated and clean its surface. This is the most efficient use of energy. Generally, it will not be necessary to regularly turn to a backwash. The backwash system is used basically during the membrane cleaning (maintenance and recovery).
Recovery cleaning (CIP)
Recovery cleaning is necessary to restore membrane permeability when the membrane is fouled. Generally, a recovery cleaning must be done if the permeability diminishes to less than 50% of the initial permeability, or if it is less than 62 lmh/bar, whichever comes first. This can take place when transmembrane pressure (TMP) systematically exceeds 0.35 bar (empty) with average flow conditions. Chemicals usually used for CIP are sodium hypochlorite (NaOCl) to clean organic fouling and citric acid to clean inorganic pollutants. Cassettes are cleaned in situ, one train at a time. The train to be cleaned is isolated from the rest of the system. The filtering process stops but the aeration continues for a time to detach the solids from the fibres of the membranes. Mixed liquor at the membrane tank is driven to the bioreactor distribution tank through the sludge recirculation pipes.
To make the cleaning, membrane tank is filled with permeate and then drained to take the remaining sludge from the tank. Cleaning chemicals (CIP solution) are pumped inside the tank through the membranes and at last, the permeated liquid is used to fully fill the membrane tank until the fibres are submerged. Membranes will remain submerged in the cleaning solution for 6 to 12 hours, depending on conditions.
Upon completion of the CIP, the level of the membrane tank will be from 25-38 cm approx. less than the normal operating level. To neutralise cleaning solution mixed liquor is allowed to enter into the tank and the contents areaerated to create a mixture. This process lasts 30 minutes approx. and, once completed the train is put in service again.
Usually, the recommendations are: two (2) CIP per year with sodium hypochlorite (1000 mg/L as free chlorine) and two (2) CIP per year with citric acid (2000 mg/L). When the plant is operating these conditions may vary, depending on the real conditions.
The goal of regular maintenance cleaning is to increase the range between recovery cleanings. The procedure to maintenance cleaning is completely automated and is controlled by the operator.
To make the maintenance cleaning the membranes tank should be isolated. The recirculation stops and the train is aerated for 2-5 minutes. For 45 minutes a solution of 100-200 mg/l approx. of free chlorine (sodium hypochlorite base product), or 2000 mg/l of citric acid is pumped through the membrane at regular pulses followed by a rinsing with clear water (permeated) at the end. Sodium hypochlorite maintenance cleaning, depending on the application, will take place once a week.
Usually there are no citric acid maintenance cleanings with urban sewage waters. However, for industrial MBR it is used for cleaning once a week.
Once the system is operating, frequency of cleaning can be optimised.
When maintenance cleaning is completed, water inlet is opened again, recirculation is restored and the train is put back into service.
Flat sheet cartridge area can be 0.8 or 1.45 m 2 . It consists of two membrane sheets welded to an ABS bracket by an effective ultrasound welding system. Between the membranes and the ABS bracket there is a spacer sleeve that enhances the permeated flow inside the cartridge. The cartridge includes 1 suction port for the permeated liquid.
Membrane is made of chlorinated polyethylene on a substrate with a very strong non-woven structure. The sizes of the standard and maximum pores are 0.2 and 0.4 μm respectively (micro filtration range: 0.1-10 μm). This allows the removal of bacteria and colloidal material. However, due to the formation of a layer of proteins and cellular material on the membranesurface during operation, the actual size of the pore is in the ultrafiltration range, allowing the removal of elements smaller than the pore size such as viruses, and reducing operating pressure.
All membrane cartridges fit solidly inside the membrane casing, with a space of 7 mm among them. Combined with continuous cleaning by air, this disposal of the cartridges prevents the formation of sludge cakes on the surface of the membranes.
Permeated Tubes and Manifold
Polyurethane permeated tubes collect permeated water from the suction ports at the sides of each membrane cartridge and take it to a common manifold in which all cartridges are connected in parallel. Permeated tubes can be disconnected separately thus allowing to take the membrane cartridges one by one to maintenance works. Therefore, it is a fully modular and versatile system.
Module design ensures air flow on the surface of the membranes creating a lifting sweep of the biomass among the cartridges and on the surface of the membrane. This way is generated the so called cross flow (biomass flow direction forms a 90o angle with the permeated flow direction) which is effective action against contamination. Diffuser consists of a main manifold, from which 20 medium bubble diffusers held, each one of them with 5 holes in the upper part. The main functions of membrane aeration are:
The system includes the automated cleaning of the diffuser modules to ensure aeration and physical cleaning of the actual membranes. The cleaning of the diffuser is made on a daily basis by opening the valve in the air outlet manifold anfor 5 minutes. This creates a Venturi effect that pushes in the air outlet manifold a mix of sludge-air which takes any clogging in the pipes or holes.
CIP: Chemical in situ Cleaning of the Membrane Cartridge
In order to keep membranes performance it is enough to make chemical cleaning in situ, called CIP (Clean In Place). The frequency of these cleanings is every 3-4 months. In practice, the need of cleaning is determined by the measurement of the transmembrane pressure (TMP). When the TMP rises from 5 to 10 kPa regarding the initial value, a CIP is recommended.
To avoid cleaning there is no need of filling the tank with chemical solutions, taking out the membranes of the reactor of draining the reactor. Typically the CIP consists of dosing a diluted solution of sodium hypochlorite (0.5% m/v) in the permeated manifold to remove the biological film that attaches to it. In case of inorganic clogging a cleaning with citric acid must be made.
The dosage of the cleaning solution is made in a short time period (15-20 minutes) by counter flow pumping, through the general permeate manifold. Once the recommended dosage has been made, the system is put to rest for 2-3 hours approx. to ensure permeability recovery. After this period the system is ready to come into operation again. It is important to repeat that this operation takes place every 3-4 months.
In case of contamination with inorganic material, it is used an acid solution instead of sodium hypochlorite. Citric acid (0.5-1.0%) or hydrochloric acid (>2.0%) are employed in case fouling is associated with calcium (hardness). In case of Ca contamination, oxalic acid must not be used. Tank and dosing pump for the solution are used for both chemicals.
Types of Filtration
Filtration by gravity. In this mode, a little hydrostatic pressure (minimum 0.5 m) above the membrane module is enough to make the filtration process. The main advantage is the reduction of capital and operative costs (because there is no need of
pumps or continuous flow valves). Plant maintenance is even easier and both quality of the filtered liquid and flow can be checked separately in each module. The disadvantage is that plant operability will depend on the hydraulic profile as well as the degree of membrane contamination.
Filtration by suction. This mode implies the use of suction pumps on the filtration side, and is suitable for conditioning or when hydrostatic pressure is not enough to make the filtration.