Membrane Bioreactors for RO Pretreatment

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ABSTRACT
Currently, treated municipal wastewater is typically discharged to the environment in most parts of the world. It is, however, a potential water resource from which high quality water for reuse can be produced. Reuse applications include both non-potable applications such as irrigation and production of industrial process water, and indirect potable reuse including aquifer recharge. For many reuse applications, effluent from conventional wastewater treatment plants is frequently further treated by reverse osmosis (RO) to reduce the total dissolved solids in the effluent for water reuse applications.

There are some major challenges for RO systems operating on biologically treated wastewater. One challenge is plugging of the brine spacer due to high levels of suspended solids in the feed. Many RO systems today use ultrafiltration (UF) as pretreatment to remove suspended solids. The UF system does an excellent job of providing water with low suspended solids as to feed RO. However, the UF system requires additional space and does not effectively reduce the amount of dissolved solids such as organics that are fed to an RO system. The UF system can also be susceptible to upsets from a conventional wastewater treatment plant.

Instead of separating the biological treatment process and the ultrafiltration step, the latest wastewater treatment plants combine both processes in a membrane bioreactor (MBR). The membranes are integrated right into the aeration tank of the biological process. Advantages of this process are that a clarifier is no longer needed, and the effluent quality is considerably better, which improves RO performance. Additionally, the MBR process reduces footprint significantly
compared to the combination of wastewater treatment followed by membrane filtration.

This paper discusses the challenges of using RO membranes for water reuse and the benefits of using MBR in comparison to UF as pretreatment for RO. The paper highlights examples of MBR systems providing RO pretreatment today. This paper also reviews cost factors for different pretreatment alternatives technology, and discusses factors that should be considered when deciding the optimum treatment train for a water reuse plant.

INTRODUCTION
Currently, biologically-treated municipal and industrial wastewater is typically discharged to the environment in most parts of the world. It is, however, a potential water resource from which high quality water for reuse can be produced. For example, at the City of Scottsdale, Arizona, Water Campus a 14 MGD (million gallons per day) water reuse plant has been in operation since 1999, and other even larger plants have recently been built around the world. Reuse applications include both non-potable applications such as irrigation and production of industrial process water, and indirect potable reuse including aquifer recharge. For many reuse applications, effluent from conventional wastewater treatment plants is frequently further treated by reverse osmosis (RO) to reduce the total dissolved solids in the effluent for water reuse applications.

CHALLENGES OF RO SYSTEMS FOR WATER REUSE
There are some major challenges for RO systems operating on biologically treated wastewater. One challenge is plugging of the brine spacer due to high levels of suspended solids in the feed. RO membranes used for most water reuse applications today are typically spiral-wound elements, where membrane leaf units are rolled around a central permeate tube. Feed water flows into a brine spacer, typically made of vexar-type netting with a thickness of between 28 and 31 thousandths of an inch. If there is a high level of suspended solids in the feed water, the brine spacer can become plugged.

A second challenge is organic fouling, since many biologically treated wastewaters contain high levels of organics. The organics in the feed water are rejected by the RO membrane and are progressively concentrated as the water flows across the membranes. Towards the outlet of the RO system, the concentration of organics can lead to excessive fouling of the membrane.

A third challenge is biofouling. Biofouling requires both the presence of microorganisms and a food source. The organics in wastewater make an excellent food source for microorganisms. In addition, some treated wastewaters have high levels of bacteria present and so biogrowth occurs quickly in the RO system. Even when the RO feed water is disinfected, an RO system is not sterile and so biogrowth can still occur, albeit at a slower pace. In the past, RO systems treating wastewater used cellulose-acetate (CA) membranes, since these membranes can withstand some free chlorine. Feeding water with a chlorine residual minimized biogrowth. However, since CA membranes have limited pH stability and operate at high pressures, the trend today is to use thin film composite (TFC™) membranes. These membranes are not chlorine-tolerant and so strategies other than chlorine addition to avoid biofouling must be used when treating wastewater. These strategies may include use of an alternate disinfectant such as chloramine, or the minimization of organics and microorganisms in the feed by providing more extensive pretreatment.

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