The performance and microbial community in response to MnO₂ nanoparticles in anammox granular sludge

The response of anaerobic ammonium oxidation (anammox) process and granular sludge in the upflow anaerobic sludge blanket reactor was analyzed under long-term exposure to MnO₂ nanoparticles (NPs) (1-200 mg L^-1). The addition of 200 mg L^-1 MnO₂ NPs had a significantly positive effect on nitrogen removal and this system exhibited excellent performance, with a total nitrogen removal efficiency of 93.1%. Moreover, the specific anammox activity enhanced with increasing concentrations of MnO₂ NPs up to the maximum value of 657.3 ± 9.3 mg TN g^-1 VSS d^-1 under MnO₂ NPs concentration of 200 mg L^-1. This value was approximately 1.6-fold higher than that of the reactor in the absence of MnO₂ NPs. The extracellular polymeric substances and settling velocity were both increased with MnO₂ NPs addition. Meanwhile, the high-throughput sequencing results revealed that MnO₂ NPs increased the relative abundance of dominant bacteria (Candidatus Kuenenia) from 17.3% at the absence of MnO₂ NPs to 23.9% at 200 mg L^-1 MnO₂ NPs, which resulted in a higher efficiency of biological nitrogen removal on the anammox system. These results indicated that MnO₂ NPs enhanced nitrogen removal performance of anammox process.