MBR Design Considerations

More than 10 years operating experience with full-scale MBRs has shown that the use of membranes in the activated sludge process requires an adaptation of the overall process— consisting of (1) pretreatment, (2) aeration tank, and (3) filtration—to the specific strengths and weaknesses of membranes for biomass separation. As the specific measmes required may differ with the module design, the various options will be discussed in general in the following sections. 

2 Oil and Grease As nondegradable oil and grease may reduce the membrane flux, proper design of the oil and grease traps is crucial (Cornel and Krause, 2006). Screens remove only about 50% of the mineral oil present (van der Roest et al., 2002). 

3 Equalization Tank As membranes are costly and the membrane surface area has to be designed for the maximum flow, it has to be checked whether hydraulic peaks can be leveled out. Equalization tanks and/or an aeration tank operation mode with alternating water levels are measures to be considered in the design phase. At municipal plants, the maximum stormwater flow at low temperatures has to be taken in account. 

Oxidized forms of iron or aluminum salts—which are used in the wastewater treatment process for phosphate precipitation—have no significant influence on the membrane 

The hydraulic retention time (HRT) should be long enough to ensure hydrolysis and avoid short cuts. According to van der Roest et al. (2002), the use of a selector tank to improve the sludge characteristics may be beneficial. 

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