Membrane Bioreactors for RO Pretreatment

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.