Oxidative stress induced membrane biofouling and its implications to on-line chemical cleaning in MBR

On-line chemical cleaning with sodium hypochlorite (NaClO) offers an effective option for maintaining sustainable permeability in MBR. However, inevitable exposure of activated sludge to NaClO leads to the generation of oxidative stress in microorganisms. Therefore, this study filled up the knowledge gap by investigating the potential effects of the NaClO-induced oxidative stress on biofouling development. For this purpose, stressed and non-stressed microorganisms with the same cell viability were prepared and their fouling propensities were further examined in a crossflow microfiltration system. It was found that the membrane fouling potential of stressed biomass was significantly increased due to the faster colonization of survived bacteria on membrane surface. In the absence of oxidative stress, more dead bacteria preferentially adhered onto membrane surface, followed by the attachment of live cells. However, a stratified structure of fouling layer appeared during the filtration of stressed bacteria, i.e. viable bacteria more situated at the membrane surface on the top of which a layer of dead bacteria was formed. These suggested that the oxidative stress prompted survived bacteria to rapidly and predominantly adhere onto the membrane surface, which in fact could be considered as a protective strategy for bacteria to survive under harsh conditions. Consequently, the NaClO-induced oxidative stress may negatively impact on the long-term effectiveness of on-line chemical cleaning in MBR.