Bioelectrochemical partial-denitrification coupled with anammox for autotrophic nitrogen removal

The denitrification at low C/N ratio is challenging, usually requiring large amounts of energy and chemical inputs. Here we present a new method of denitrification by coupling autotrophic bioelectrochemical partial-denitrification (PD) with anammox. Nitrite was demonstrated to be stably accumulated by cathodic electrotrophic bacteria, and the small bias voltage of 0.1 V can improve nitrate-to-nitrite transformation ratio (NTR) from 44 ± 12% (no energy input) to 60 ± 9%. Then the nitrite together with ammonium was removed through anammox with a total nitrogen removal of 90 ± 3%, where the contribution of anammox accounted for 78 ± 5%. The self-alkalization of catholyte and the external voltage were found important on achieving PD. Microbial community analysis showed that Thiobacillus, a genus of autotrophic denitrifying bacteria, may play a key role in nitrite production. The log2FC values of napA was 83% higher than nirS, and nirK was significantly (p < 0.01) downregulated, contributing to the occurrence of nitrite accumulation. This proof-of-concept study demonstrated the feasibility of bioelectrochemical PD/A as a novel technology for controllable denitrification in wastewater treatment.