Characterization of Activated Sludge and Biofilm in MBR with Bacterial Quorum Quenching

Abstract

Recently, quorum sensing (QS) has been found to play a key role in biofilm formation on the membrane surface in membrane bioreactors (MBRs). Thus quorum quenching (QQ) which interrupts QS systems has received great attention as a fundamental solution to control biofouling in MBRs. Previous studies have proved that the bacterial intra-species QQ with lactonase producing bacteria, Rhodococcus sp. BH4, could efficiently alleviate the biofouling in MBRs. In addition to delay in transmembrane pressure (TMP) which is a fouling index, it has been also reported that QQ led to change in the production of extracellular polymeric substances (EPS) in biofilm. However, the analyses of previous studies were only performed considering the EPS in biofilm. In this study, we aimed to further characterize the effect of QQ considering both EPS in biofilm and mixed liquor while applying Rhodococcus sp. BH4 in two different types of MBRs. The first set of experiment was conducted with an anoxic/oxic combined MBR. In this set, soluble microbial product (SMP) was investigated in priority. In addition to examination of protein and polysaccharide, size exclusion chromatography (SEC) equipped with fluorescence detector was used to qualitatively analyze protein-like substances (Ex/Em wavelengths: 280/350 nm). Significant decrease in aromatic protein-like substances with molecular weight range of 100-1000 kDa was observed. Also, the filterability of sludge supernatant which depends on fouling tendency of SMP was evaluated by dead-end filtration with 150 kDa membrane. It was observed that QQ resulted in better filterability, shown by 2-3 times lower cake layer resistance. Another set of experiment with aerobic MBR was designed to study the QQ effect of Rhodococcus sp. BH4, considering not only SMP but also bound EPS of floc and biocake. Aromatic protein-like and humic acid-like substances (Ex/Em wavelengths: 280/350 nm and 345/443 nm, respectively) were analyzed with SEC. In case of EPS bound to floc, neither aromatic protein-like nor humic acid-like substances showed apparent difference. However, both components have decreased in biocake EPS, where humic acid-like substances were mostly removed by QQ.