Membrane Bioreactors, Applications to Wastewater Treatment and Reuse

Abstract

AbstractMembrane bioreactor (MBR) technology for wastewater treatment has attracted enormous attention worldwide over the past 15 years. MBRs combine biological wastewater treatment with membrane technology. Ultra‐ or microfiltration membranes separate the biomass from the mixed liquor during the activated sludge process (ASP). Herein, the membrane is employed as a filter and rejects the solids to provide a clarified product. The main advantages of the process are the superior effluent quality as a result of complete solid removal and the small footprint of the plant due to more compact aeration tanks and the absence of a final sedimentation tank. The disadvantages can be the membrane fouling and the higher energy demand of the process. Worldwide, MBR systems have been implemented in more than 200 countries. Key drivers for this technology are the legislation relating to environmental protection and wastewater management, local water scarcity, the cost (return on investment), and the increase in the acceptance of MBR technology and wastewater reuse.To optimize the MBR process, the design of the conventional ASP needs to be modified at several points. The mechanical pretreatment, particularly the screening process, has to be adapted as classical screens of around 6 mm rating are insufficient for an MBR. Most MBRs are operated with 1 mm up to 3 mm screen gap size. The biological stage can be operated with higher mixed liquor suspended solids (MLSS) concentration. The content can be increased from about 3000 mg/l in conventional ASP to about 12,000 mg/l in MBR systems resulting in a lower foot print and lower hydraulic retention time (HRT). Also, the control of the MBR plant is more complex in comparison with conventional ASP as the membrane process must be embedded into the control system. In addition, the cleaning of the membrane must be implemented into design, operation, and control.