Critical Review on the Effects of Mixed Liquor Suspended Solids on Membrane Bioreactor Operation

Abstract

One of the characteristics of MBRs is that they typically operate with higher mixed liquor suspended solids (MLSS) concentration than activated sludge with a conventional settling tank. While higher MLSS has obvious benefits in terms of increasing the volumetric loading or the solids retention time, it can have negative impacts on system operation and economics. We critically evaluate three hypotheses on how high MLSS may adversely affect MBR operation: (1) reduced membrane flux with high MLSS, (2) decreased aeration alpha (α) value with high MLSS, and (3) poorer thickening characteristics of excess sludge wasted from an MBR based on the Sludge Volume Index (SVI) and the Capillary Suction Time (CST). The results support the first and second hypotheses, but not the third. Increasing MLSS decreases the critical permeate flux, but the effect is strong only for MLSS<∼5 g/L. For the typical MLSS zone (>∼5 g/L), flux‐management techniques to prevent serious cake formation are more important than MLSS. The aeration α decreases with increasing MLSS concentration, although the strength of the correlation depends on system‐specific factors that are poorly understood. Thickening properties of IMBR sludge are not significantly poorer than those of traditional activated sludge, based on available CST tests.