Synergistic effects of nanobubbles and chemicals on backwashing for submerged MBRs treating municipal wastewater

We previously proposed a novel approach to mitigate membrane fouling in membrane bioreactors: nanobubble water (NB water) was used for backwashing of membranes used in MBRs. It was demonstrated in our previous work that nanobubble-assisted backwashing (NBB) was more effective than conventional hydraulic backwashing for fouling mitigation. However, there remains room for further improvement in the cleaning efficiency of NBB. In this work, an attempt was made to improve the cleaning efficiency of NBB by adding a small amount of sodium hypochlorite (NaClO) to NB water used for backwashing. The cleaning efficiency of NBB was improved by the synergetic effect of NaClO and NBs working in different ways when they were co-present in the backwash solution. It was suggested that NBs made the structures of the fouling layer looser and more porous, whereas NaClO chemically degraded the components of the fouling layer. The importance of pH in NBB combined with NaClO was shown in this study. The cleaning efficiency was found to be high at pH 4, followed by 8.5 and 12. This difference was mainly attributed to differences in the sizes of NBs under different pH. Thus, the synergetic effects of NBB combined with NaClO were mostly enhanced under acidic conditions. The application of NBB has still been rarely investigated and this study showed for the first time that the efficiency of NBB could be further improved by mixing NB with chemical reagents. This can lead to a new method for alleviating membrane fouling in MBRs. [Display omitted] •Nanobubble assisted backwashing (NBB) was tested under different pH.•Synergistic effects between nanobubbles (NBs) and NaClO (NB + NaClO) could be confirmed.•NB + NaClO backwashing exhibited high cleaning efficiency under acidic conditions.•The size distribution of NBs is a major factor affecting the cleaning efficiency of NBB.•Enhanced oxidation power of NaClO at low pH contributed to the synergy with NBs.