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CE 421/521- Environmental Biotechnology

 Design of Aerobic Membrane Bioreactors for the Treatment of Wastewater

Shawn Nauman

Abstract

There is a large amount of interest in the use of membrane bioreactors (MBR) for wastewater treatment because they have some potential advantages over the conventional activated sludge system.  The replacement of the clarifier with a membrane would reduce the area needed for the treatment plant.  MBR have a more controllable effluent suspended solid concentration even with bulking.  MBR have longer solids retention time (SRT) increase the biomass in the reactor and also reduce the amount excess sludge produced.   The biggest draw back of a MBR is the problem with membrane fouling that reduces the time of MBR operation before the membrane needs to be cleaned.  This paper examines some the critical design concerns for a MBR. The aspects to be look at are the membrane placement, aeration system and aeration rates, membrane material and diameter size, membrane configuration, and operational fluxes. The kinetics of the MBR can be modeled and hydraulic retention time, SRT, biomass concentration, and substrate residual can be predicted given some of the operation parameters. 

Keywords: membrane bioreactor, membrane, wastewater, fouling, kinetics

 Introduction

The one of the most widely used process for wastewater treatment is the conventional activated sludge system.  However, there are some disadvantages that come with a conventional activated sludge; mainly the effluent suspended solids is dependent on the settling of the mixed liquor suspended solids (MLSS) in the clarifier.  The suspended solids concentration of the conventional activated sludge system must remain low enough to enable the clarifier to settle enough solids to meet the effluent standards. In order to get an acceptable suspended solids concentration the area of the clarifier must be large.  Even with a large clarifier area a conventional activated system is still susceptible to shock loads, in which a large increase in the biomass occurs and the clarifier may not be able to settle all the excess suspended solids.  In order to keep the biomass concentration low, the excess sludge must be removed and disposed of and that adds to the operational cost.  Bulking is also another concern for a conventional activated sludge system; both filamentous and non-filamentous biomass may flow over the weir of the clarifier and cause the effluent to exceed the standards for suspended solids (Choi et al., 2002).

The standards for suspended solids are becoming more stringent and the rising cost of disposing of the excess sludge has made other treatment alternatives more attractive for wastewater treatment.   MBR have become a promising new alternative to the conventional activated sludge system and is being increasingly studied.  A MBR replaces the clarifier of a conventional activated sludge system with a membrane filter that is used to separate the suspended solids from the effluent.  The advantages of the MBR is that the effluent suspended solids concentration is not dependent on the settling of the MLSS, so a MBR is able to operate with a much higher biomass concentration than a conventional activated sludge system.  The area of the plant is reduced because the clarifier is no longer needed and with the higher biomass concentrations the area of the reactor itself may be reduced.  The quality of the effluent is not affected by shock loads or by bulking.  MBR are able to have a longer (SRT) and lower the observed yield of biomass and produce less sludge lowering disposal costs.  The longer SRT may also improve nitrification (Choi et al., 2002).

The main disadvantages of MBR are the fouling and cleaning of the membrane.  During operation of MBR the membrane collects a cake or gel layer of organic and inorganic material that forms on the membrane and reduces the flux across the membrane. Once the fouling becomes too extensive to continue operating the membrane must be cleaned before operation of the system can resume.  The fouling must be slowed to give the MBR a reasonable operational time between cleanings, if it is to be use for large-scale wastewater treatment.  The fouling can be slowed by the design, operational parameters, and materials used for the MBR.  This paper will look at the factors of design and operation for a MBR.