Engineered manganese redox cycling in anaerobic-aerobic MBBRs for utilisation of biogenic manganese oxides to efficiently remove micropollutants

Freshly formed manganese oxides (MnOx) can oxidise various organic pollutants, but its reactivity is short-lived and therefore continuous pollutant removal by MnOx is not feasible. In this study, an engineered cycle of manganese reduction followed by oxidation was implemented in a two-stage moving bed biofilm reactor (staged MBBR), aiming to replenish MnOx for continuous micropollutant removal. In the anaerobic reactor, added MnO₂ was reduced to Mn²⁺ by microbially-mediated reaction with organic matter in the sewage. The reduced Mn2⁺ was then re-oxidised to biogenic manganese oxides (BioMnOx) in the aerobic reactor, which precipitated into the suspended biofilm. Part of the BioMnOx was regularly recycled to the anaerobic reactor for the next round of manganese reduction. Mass balance assessment shows that manganese redox cycling functioned well during continuous operation, and BioMnOx was continuously regenerated within the aerobic reactor. A specific Mn-oxidising bacteria belonging to Stenotrophomonas sp. was isolated from suspended biofilm of the aerobic reactor and identified, and Illumina high-throughput sequencing analysis reveals a microbial community shift in the staged MBBR during manganese redox cycling. Furthermore, both batch and continuous-flow experiments show that specific micropollutants such as bezafibrate, diclofenac and sulfamethoxazole were efficiently removed (over 80%) by the staged MBBR.