Probiotic strategy for biofouling control through direct injection of quorum-quenching bacteria into membrane bioreactors

Abstract

In membrane bioreactors (MBRs), N-acyl homoserine lactones (AHLs)-related quorum sensing (QS) has been described as one of the main causes of biofouling during wastewater treatment. Quorum-quenching (QQ) bacteria for the mitigation of QS in MBRs have been applied but limited to entrapping methods only. Thus, this study investigates the direct injection of different doses (0–77.2 mg/d) of unentrapped QQ strains (Rhodococcus sp. BH4) into a probiotic QQ MBR (i.e., bioaugmented reactor) while monitoring the changes in fouling rates as well as microbial communities, biopolymers, signal molecules, and treatment performances. The QQ bacteria show effective biofouling mitigation in diverse MBR phases, but a halt in QQ bacteria injection leads to increased fouling rates in the probiotic QQ MBR. The QQ bacteria exhibit a noticeable AHL degradation rate during BH4 inoculation as opposed to when inoculation is halted. They also have significant inhibitory effects on the generation of soluble carbohydrates and proteins. However, cell-bound extracellular polymeric substance (EPS) concentrations are relatively higher in the probiotic QQ reactor despite decreased fouling rates. This can be ascribed to the production of EPS by injected BH4, as revealed in pure BH4 batch cultures. Probiotic QQ has impacts on both the relative bacterial abundance and the overall microbial composition. Thiothrix is identified as the most dominant genus in the mixed liquor of both MBRs; however, its relative abundance becomes low in the probiotic MBR during QQ bacteria injection. These findings provide valuable information that will foster the development of improved QQ strategies in MBRs.