Wastewater collection and drainage has been prevalent in various parts of the world since ancient period. Cloaca Maxima, a vaulted channel sewer of the ancient Roman drainage system is the oldest existing Roman engineering monument that was used to transport drainage water to the Tiber River. However, unplanned drainage of wastewater in water bodies raised concerns about environment and public health. Urban development, improved quality of life and expanding cities lead to the adoption of more advanced and efficient treatment techniques of wastewater. The treatment involved construction of large number of treatment units in a process treatment train which led to the sprouting of another challenge scarcity of land for the establishment of these treatment facilities. This made it essential to develop engineered treatment methods to meet our needs. Wastewater treatment process comprise unique units grouped together to provide primary, secondary, tertiary and advanced treatment. Wastewater treatment schematic:.Among these various stages, this article focuses on the membrane bioreactors (MBR) technology, which is utilized in the secondary treatment stage of wastewater. Urban wastewater secondary treatment usually consist of conventional activated sludge treatment process (ASP) which utilizes heterotrophic bacteria in aerobic environment. Sludge produced from the process is removed by gravity settling. However, building up of sludge in this process imposes construction of large size of aerated bioreactor and also sludge treatment needs to be provided. MBR is an improved version of the conventional ASP where secondary clarifier has been replaced by the membrane unit to isolate treated water from the influent. 2. Membranes Bio ReactorMBR consist of an activated sludge bioreactor built with a membrane separation module to retain the biomass and sludge coming with the influent wastewater. Since, the effective pore size of the membrane is close to 0.04 μm or below, the effluent produced from the MBR is considered highly treated and clarified. There are basically two main process configurations of MBR with pumped and airlift hydraulic operations:1. Submerged or immersed MBR (iMBR)2. Side stream MBR (sMBR)Out of these two MBRs, iMBR is considered more energy efficient as compared to sMBR due to the fact that in side stream MBR, the membrane module requires pumped crossflow to counter high pressure and volumetric flows which consumes significant amount of energy. In sMBRs, there always has to be a balance between the pumping energy demand and the flux. To achieve higher flux, a high transmembrane pressure is required with a high retentate velocity since energy demand is directly proportional to the flowrate times pressure. On the contrary, if flowrate is decreased by reducing cross sectional area of the membrane, it would result in the pressure drop along the length of the module on the retentate side. This is due to the inverse relationship between resistance to flow and area. Among the two MBRs, sMBR is more susceptible to fouling than iMBR, since it operates under higher flux and fouling increases with increasing flux and lowers permeability especially above the critical flux..2.1 MembranesMembranes are solid semi permeable material medium which selectively allows transport of physical or chemical components through it, depending upon the membrane pore sizes and their structural variations. The membrane materials used for MBR are majorly categorized into two types-polymeric and ceramic. However, a variety of polymeric or ceramic materials are used to form membranes such as: Polymeric - Polyacrylonitrile (PAN), high density polyethylene (HDPE), polyethylsulphone (PES), polysulphone (PS), polytetrafluoro ethylene (PTFE) and polyvinylidine difluoride (PVDF). Ceramic- Aluminum oxide/ Alumina (Al2O3), silicon carbide (SiC), Titanium dioxide/ Titania (T iO2) and Zirconium dioxide/Zirconia (ZrO2) Membranes are manufactured such that they have high porosity, narrow pore size distribution to provide higher selectivity and throughput, be mechanically, thermally and chemically strong to resist temperature and pH concentrations. Ceramic membranes perform better than polymeric to resist fouling and chemical attack. However, their application is limited due to its high cost. Majority of MBR membrane modules available in the market are polyvinylidene difluoride (PVDF) based, after PVDF most selling is PES and PE. There are 4 key membrane separation processes utilized, which includes reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF). Any of these membrane separation processes can be identified based on the following factors:1. Membrane Configuration2. Material of Membrane3. Type of Driving Force4. Separation Mechanism5. Nominal Size of the separationTypical operating ranges (in terms of particle size separation) of these membrane separation processes are as follows:1. Microfiltration - 0.07 −2.0 μm2. Ultrafiltration - 0.008 −0.2 μm3. Nanofiltration - 0.0009 −0.01 μm4. Reverse osmosis - 0.0001 −0.002 μmFor MBR, the separation processes mainly utilized is ultrafiltration along with the biological treatment in ASP..3. DiscussionsThe concept of MBR was introduced in late 1960s when, a microfiltration membrane was installed in an aeration tank of an ASP. However, the first successful installation of MBR plant was achieved in mid 1990s. With the replacement of secondary clarifier from ASP to MBR, more than 50% of the footprints of wastewater treatment system are reduced. Submerged membranes proved to be highly efficient in trapping the contaminants and allowing smooth passage of biologically treated water which led to the elimination of clarification and tertiary filtration needs. MBR has proven to be an advantage over the conventional ASP treatment in the following ways:1. High quality effluent.2. Higher volumetric loading rates3. Reduced hydraulic retention time4. Longer solid retention time5. Reduced sludge production6. Potential for nitrification/denitrification simultaneously.This membrane technology is an efficient compact technology for municipal and industrial wastewater treatment. However, the major challenge or drawback associated with this technology is membrane fouling. This drawback significantly and adversely affects the performance of the membrane and its operating life which results in increase in its maintenance and operation cost. Membrane fouling in general, depends upon the physical, chemical and biological characteristics of wastewater being fed into the membrane which also varies from site to site, type of membrane being utilized and operating conditions. A membrane is exposed to fouling due to following reasons:• Deposition of particulate matter, present in the feed water over the membrane.• Scaling caused by the precipitation of inorganic salts• Organic matter deposition on the membrane• Biological fouling caused by the micro- organisms present in feed water.• Simultaneous occurrence of any of the above.Membrane fouling in MBR is typically caused by the suspended particulates (cell debris or micro-organisms), sludge flocs, colloids or solutes. These foulants deposit on the membrane pores, clogs and reduces the permeability of the membrane. The fact that mixed liquor suspended solids (MLSS) present in feed water are heterogenous in nature, makes membrane fouling an inevitable challenge in MBR applications. Fouling of membrane has been a significant topic of research in wastewater treatment to enhance the applications of MBR in wastewater treatment industry..3.1 Membrane Fouling MechanismFouling of membrane is defined as an interaction between the foulants and membrane material by physical or chemical means. Various forms of fouling include pore narrowing, pore clogging which depends on the size of the particle and membrane pore size, and cake formation (continuous deposition of bacteria, bio polymers and inorganic matter). Biofouling is defined as deposition of biological material on the membrane surface that influences the hydraulic performance in MBR systems. Soluble microbial products (SMP) bond with the membrane surface through adhesive forces during the filtration. When MBRs are under operation, the bacteria cohesively bonds with these SMPs. As the mixed liquor flows through these clogged membrane pores, dissolved oxygen and nutrients become available to attached bacteria. This stimulates the bacteria to generate extracellular polymeric substances (EPS) along with the biofilm. EPS are found inside and outside of microbial cell. They are secreted by microorganisms and formed by cellular lysis and hydrolysis of macromolecules. EPS has been found to be the key cause of membrane fouling as it acts as a binding material to aggregate microbes.In terms of operation, fouling of membrane causes decline in the flux through the permeable membrane when the MBR is maintained under constant transmembrane pressure (TMP). When the permeate flux is kept constant, it results in the increase of TMP. When a sharp increase in the TMP is observed under a constant operating flux, then this condition is called as severe membrane fouling and the sudden rise of pressure is known as TMP jump. A TMP jump can be understood in three stages- First stage- initial conditional fouling; Second stage- a linear or weekly exponential gradual rise in TMP due to clogging of pores by micro-organisms: Third stage- sudden increase in the rate of TMP rise. Due to gradual clogging of pores and decline in flux, results in the accumulation of particles and formation of cake layer. This layer is thick enough to make the bacteria present in the inner biofilm to suffocate and die due to oxygen deficiency.In situation when membrane reaches stage 3, cleaning of membrane becomes mandatory. The main aim of fouling control is to slow down the TMP jump which can be done by modifying the sludge characteristics or lowering the operating flux..3.2 Preventive Measures Against FoulingMembrane cleaning is a critical factor to carry out smooth performance of MBR system. It can be done either by physical or chemical means. Physical cleaning utilizes reversed flow through the membrane or scouring the membrane with air bubbles while ceasing the permeation. This is also called membrane relaxation.3.2.1 Membrane RelaxationIt is considered as a standard operating technique to prevent fouling of membranes. Various techniques have been utilized to maintain relatively stable membrane permeability by prevention and control of membrane fouling through physical, biological and chemical methods. Some of those techniques includes vibration, air sparging, ultra-sonication, sponge ball cleaning, moving media, magnetic enzymatic carriers, relaxation in operation mode and back washing in operation mode. Relaxation refers to providing an interval of halt or break from continuous operation.A comparative study on effect of backwash cycle run time and relaxation cycle run time on membrane fouling was performed. This study compared the fouling rates and fouling resistance during backwash and relaxation cycle and it was concluded that shorter the cycle run time, longer the membrane will remain operational whether it is relaxation cycle or backwash cycle. However, backwash cycle was found to be more efficient in terms of fouling rate as it was lesser as compared to relaxation. This was due to the fact that during backwashing, the cake layer and pores foulants were removed by the permeate as compared to relaxation.3.2.2 Air scouringIn a MBR, air scouring is beneficial as it provides a way to dislodge the cake layer formed on the membrane surface due to the deposition of organic matter. An increase in the rate of air scouring in a MBR leads to reduction in membrane fouling. However, this positive effect of aeration, significantly reduces with the increase in mixed liquor suspended solid (MLSS) concentration due to increased viscosity. Typical air scouring rates ranges from 1 L air/min m2 to 2L air/ min m2. Higher air scouring rate minimizes fouling through scouring action, it also impacts the biomass concentration. When intensity of air scouring is high, it leads to breaking up of the sludge flocs and forms soluble microbial products. To provide higher scouring intensity, higher energy consumption is also required which leads to increase in operational cost. Hence, it is important to establish a tradeoff between the air scouring intensity and biological and fouling treatment. Some of the impactful chemical cleaning methods includes:3.2.3 Chemical Enhanced BackwashThis process involves use of sodium hypochlorite along with a mineral or organic acid (mostly citric acid). Since, this chemical cleaning process involves back flushing also, it is referred to as chemical enhanced backwash or CEB. During the backwash cleaning, the permeate obtained from filtration is sent back through the hollow fibre membrane for the effective removal of the accumulated particles on the membrane. It is performed when a high TMP or low filtrate flux is observed. It is similar to backwashing where filtrate flows from filtrate side to the feed side. There are three types of chemical backwashes performed: CEB-1: It is performed in two stages- alkaline followed by acid to clean the organic and in- organic deposits. CEB-2: Acid CEB-performed for the removal of inorganic deposits. CEB-3: Bleach CEB- performed always after CEB-2 for disinfection. Chemicals are introduced into the system via a flux rate which is lower than backwash flux rate. Once the chemical is injected a soak period begins. After the soaking period, the chemicals and particles are removed from membrane cleaning are washed out with filtrate. 3.2.4 Clean in Place (CIP)CIP is a method utilized to remove fouling and scaling that are difficult to be removed by conventional techniques like backwash or CEB. During normal operation CIP is not required. For this cleaning a chemical solution is introduced to the membrane modules and the flux system is shut down for a longer period of time as compared to the conventional cleaning methods. One major difference between CEB and CIP is that in CIP a forward flush is provided with the recirculation of different chemicals and their extended soak time. Various cleaning chemicals used in CIP are NaOH, NaOCl (1000mg/L typical) and citric acid (2000mg/L typical). They are diluted with water in a feed tank. Mixing of chemicals in this process is performed by recirculation for at-least 60 minutes via pumps. Chemicals are then sent to the UF membrane module for cleaning. To ensure proper cleaning, temperature, pH and concentration of cleaning solution are monitored. After this 60 minutes, filtrate side is introduced in recirculation process and is performed for another 60 minutes. Soaking time of 60 minutes is also provided before the next circulation. After the completion of recirculation process the solution is drained and CIP tank becomes empty to receive filtrate for next rinsing process. Rinsing is performed until a neutral pH is read in the filtrate and TMP is observed. After this process, permeability of the membrane is examined at constant flux. CIP is provided twice per year with both the chemicals NaOCl and citric acid..4. Advancement in Technology for the Reduction of Fouling in MBRBased on the present demand of MBR technology and the feasibility of the existing other technologies, it became essential to develop advanced technologies through researches that can control or reduce the membrane fouling in MBR or enhance the permeability of the membrane. Following developments in MBR technology have been made and the novel MBRs developed are listed below:1. Hybrid biofilm MBR (HFMBR)2. Vertical submerged MBR (VSMBR)3. Submerged rotating MBR (SRMBR)4. Air sparging MBR (AsMBR)5. Jet loop MBR (JLMBR) 1. Hybrid biofilm MBR: This MBR is called hybrid as it incorporates both biofilm technology and membrane filtration into one reactor. Studies confirmed that HFMBR efficiently prevents membrane fouling as the circulating media on the membrane surface generates a shear force that prevents the cake layer formation and also the biofilm traps the fine particles that may cause membrane fouling. It is very efficient for the treatment of high strength or highly concentrated wastewater treatment due to its high biomass concentration in the reactor.2. Vertical Submerged MBR (VSMBR): The main objective of the innovation of this MBR was to efficiently remove the wastewater organics and at the same time to lessen the membrane fouling problem. In a VSMBR, an MBR with anoxic and oxic zones both in one, separated by a horizontal plate with a hole at its center in a specific optimum volume ratio is set up. Along with this, an internal recycle rate four times higher than usual rate is also set up for nutrient removal consideration. Membrane is installed vertically in the aerobic zone of the reactor. A pilot study was performed with these conditions. MLSS is then allowed to flow from anoxic zone (bottom) to oxic zone (top) through the hole. This flow results in the gravity settlement of microorganisms in anoxic zone increasing the MLSS concentration there. In the aerobic zone, diffusers are used to provide air bubbles to reduce the fouling in membrane and also to oxidize organics and ammonia. It was found that, this MBR could efficiently remove membrane fouling by breaking the bond between the deposited organics. 3. Submerged Rotating MBR: This MBR consists of a rounded membrane module submerged in a bioreactor which continuously rotates to create a turbulence inside the bioreactor. This turbulence generated by the rotation of module improves the mass transfer between the two phases. The turbulence provides agitation to the sludge deposits on the membrane surface and separates it from the surface reducing membrane fouling. A study was performed to analyze the effect on SRMBR on membrane flux and it was found that with the increase in rotation rate from 15 rpm to 25 rpm, the permeate flux increased from 42.5 to 47.5L/m2/hour. The rotation of the membrane module provides agitation to the suspended sludge hence also reduces the power consumption required by the conventional mechanical agitator.4. Air Sparging MBR: This is another technology which is found to be useful in addressing the fouling challenge. It utilizes bubbling technique for the prevention of cake deposition and also for the aeration of the microbial population. In this MBR, air bubbles are injected through the membrane surface to accelerate membrane flux. This air injection generates a variety of flow patterns. Based on the void fraction, the flows are categorized as bubble flow, slug flow, churn flow and annular flow. The slug flow pattern is observed to be efficient in fouling control in membranes. The slug flow generates a shear stress that supports in the removal of fouling layer. The design of membrane module also significantly affects air sparging by holding the bubble in the shell for longer time resulting in better air flow on the membrane surface. 5. Jet loop MBR: In this MBR, two cylinders are arranged such that the inner one is called ‘down-comer’ and outer one is called riser. In these, gas is dispersed through a 2-phase nozzle. This creates a turbulence inside the cylinders which breaks down the large micro-organisms and prevents cake formation hence, controlling the membrane from fouling. JLMBR is efficient in treating a concentrated high strength wastewater, supports in lowering membrane fouling, requires lesser space and is easy to operate. .5 ConclusionsThe various innovative chemical and design advancement in MBR technology have significantly improved the performance of membranes in MBR. Technologies like CEB, CIP, Backwashing, submerged rotating MBR and air sparging have been promising and have given a direction on countering the fouling of membranes. All these design advancements seem to have one thing in common which is the detachment of foulants from the membrane surfaces through either vibrations, agitations or shear stress generation between membrane and the foulants to disintegrate it. Also, breaking up of flocs or cake layer from the membrane surface seems to generate more soluble microbial products which needs to be prevented. Other emerging technologies like Quorum Quenching where the biofilm formation and control have been studied seems to be very useful in the future to prevent fouling as well as formation of SMPs. Hence, regulation and control of the biofilm formation in various environmental conditions and its formation trends has a good scope of research. For article references contact the author.
