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Membrane Boi Reactor ( MBR ) Introduction : This paper will discuss the application of membrane bioreactors (MBRs) to treat domestic waste for small communities, either for discharge or for reuse. The use of membrane bioreactors for both domestic and industrial wastewater treatment has expanded significantly in the last few years. This paper will discuss the MBR process, and then highlight two case studies that demonstrate the applicability of the MBR to small communities. Membrane Bioreactors : The application of MBRs to a variety of wastewater sources has expanded from a few systems in the late 1980’s to thousands of systems in operation today. They are now being used on wastes ranging from domestic effluent with Biological Oxygen Demand (BOD) ranges of 200-600 mg/l to industrial waste with influent BODs of 18,000 mg/l. This technology has served customers that need effluent of consistent quality with low BOD and Total Suspended Solids (TSS) for reuse or discharge, for sites where space is limited, for sites where there is a need to increase the capacity of an existing system, or to reduce operating requirements. In a conventional wastewater treatment plant, the secondary clarifier limits the solids concentration in the aeration tank. Typical mixed liquor suspended solids (MLSS) concentrations are 1,500 mg/l to 5,000 mg/l. The MBR replaces secondary clarification in a conventional wastewater treatment plant. MBRs separate biologically treated effluent from the mixed liquor utilizing membranes to perform the separation. The membranes allow the purified water to pass through the pores (filtrate), while creating a complete barrier to the passage of any solid greater than 0.4 microns, which includes almost all bacteria and suspended solids. In an MBR, the membranes create a solids barrier and therefore the process is not subject to gravity settling solids limitations, as in conventional clarifiers. MBRs are limited instead by the fluid dynamics of high solids mixed liquor, and the effect on oxygen transfer. Typical MLSS concentrations in MBR systems are 10,000 mg/l to 12,000 mg/l. Figure 1 shows a basic flow sheet of a typical MBR chamber. Effluent is passed through a screen to remove solids. An equalization tank is typically included to handle variation in flows so that the MBR can be sized to treat the average daily flow rather than the peak daily flow. The membranes are submerged in the aeration tank. Filtrate is drawn through the membranes using a suction pump. Biomass is removed using a sludge pump as needed. Air blowers supply air for the biological rocess, to scour the membranes and to uniformly distribute suspended solids throughout the aeration tank. The continual agitation caused by the flow of air and water over the membrane surface scours the membrane surface to control fouling. There are normal modes of operation, a filtration mode and a resting mode. In the filtration mode, the suction pump operates to pull water through the membranes to produce treated effluent. In the resting mode, the suction pump is switched off. This allows the membranes to relax, and with no vacuum on the membranes, the air scour has a greater impact. Typically, the filtrate pump operates for 8 to 13 minutes and then is off for 2 minutes. Despite this continuous cleaning, a gradual accumulation of organic substances can occur at the membrane surface. This can increase the transmembrane pressure (TMP) across the membrane. To control the TMP, in-situ chemical cleaning of the membranes is used about every three months. The in-situ cleaning consists of reverse flow of a 3,000 ppm sodium hypochlorite solution through the membranes over two hours. Typical performance parameters monitored for an MBR include flowrate, TMP, temperature,dissolved oxygen (DO), feed BOD and TSS and effluent BOD and TSS. Monitoring is typically performed manually on a daily basis or automatically via a SCADA system. Hydraulic retention times (HRT) for MBRs are typically 4-20 hours. On most domestic wastes this is enough time to allow for the oxidation of organic material and ammonia (nitrification). Sludge retention times (SRT) are 15-45 days. The older sludge age and the higher MLSS concentrations in the MBR process compared to conventional systems enable the MBR to produce a lower volume of sludge for disposal than conventional treatment systems. Older sludge contains a greater concentration of nitrifying bacteria, so the MBR has an advantage in achieving complete nitrification over conventional WWTPs. Older sludge tends to be more difficult to settle, which can be a problem for conventional WWTPs that rely on a settling process to separate the effluent from the sludge. This is not an issue for the MBR since the membrane performs the separation step. Advantages of MBRs for Small Communities The characteristics of the MBR system make it ideal for a small community that needs to guarantee effluent quality for discharge or reuse. The MBR produces water from domestic effluent with less than 10 mg/l BOD and less than 2 mg/l TSS. Since the membranes provide a barrier to solids, the process is not subject to the upsets that can lead to effluent being discharged that does not meet environmental regulations. For non-potable reuse applications, the MBR can provide water of less than 0.1 NTU, which meets most standards to comply with water recycling quality criteria. The MBR system discussed in this paper has been accepted in California for Title 22 reclaim applications. For reuse applications that require a reverse osmosis (RO) system, the MBR provides water with a 15-minute silt density index (SDI15) less than 2. The MBR filtrate can be directly treated by RO without any need for additional pretreatment. The system is very simple to operate, with a minimum of pumps and controls and low requirements for operator attention. Typical operator requirements for an MBR for a small community would average less than one hour per day. Typical daily maintenance includes monitoring of performance parameters. On approximately a weekly basis, waste from the pre-screens would be disposed of, and sludge would be pumped from the aeration tank to a sludge holding tank. Typical in-situ cleanings take place approximately four times a year. For a small community, these low maintenance requirements can be performed by on-site maintenance personnel in addition to other duties, or as an external service by personnel who visit the plant on an asneeded basis.