Influence of unsteady membrane bioreactor operation on EPS formation and filtration resistance

Extracellular polymeric substances (EPS) are considered as the major cause of membrane fouling in membrane bioreactors. Recent studies have revealed a linear relationship between fouling rate (increase of filtration resistance over time) and polysaccharide (PS) concentration [1]. Several factors like the type of wastewater, sludge loading rate, sludge age, MLSS concentration, and mechanical stress are known to influence the concentration of dissolved EPS. Also, unsteady states like intermittent feeding or shifts in the oxygen supply have been identified as additional factors leading to an increase in EPS formation or to a change in its fouling propensity. However, no systematic investigation to quantify such influences has been undertaken so far. This study is aimed at determining the influence of discontinuous excess sludge withdrawal often applied in small decentralised plants and resulting unsteady loading rates on EPS formation and filtration resistance. The general trend of increased fouling rate at higher PS concentrations is confirmed but data show a larger scatter which could be due to a change in the PS structure and hence their fouling potential. The levels of dissolved oxygen and nitrate which also fluctuate more strongly in MBRs with irregular sludge wastage appear to have a profound impact on EPS uptake rate and thereby on EPS concentration.