Nitrate removal by anammox biomass with intracellular carbon source as electron donors via DNRA pathway

Abstract In this work, a novel nitrate (NO3−) reduction pathway by anaerobic ammonium oxidation (anammox) biomass was firstly discovered with the intracellular carbon sources as the only electron donors. And the possible reaction mechanism was deduced to be intracellular dissimilatory nitrate reduction to ammonium (DNRA) pathway according to the experimental results. In batch experiments, without any external electron donors, NO3−-N (about 50 mg/L) was reduced to N2 within 48 h, and a small amount of NO2−-N was detected (the maximum of 2 mg/L) with the anammox biomass concentration of 4400 mg/L. Acetylene (4.46 mmol/L) addition resulted in obvious NH4+ accumulation during NO3− degradation by anammox biomass, since acetylene mainly interfered in hydrazine (N2H4) generation from NH4+ and NO. Without HCO3− addition, the NO3−-N degradation rate was slower than that with HCO3− addition. Simultaneously, glycogen contents inside anammox biomass decreased to 133.22 ± 1.21 mg/g VSS and 129.79 ± 1.21 mg/g VSS with and without HCO3−, respectively, from 142.20 ± 0.61 mg/g VSS. In the long-term experiment, anammox biomass stably degraded NO3−-N without external electron donors addition, and the maximum removal efficiency of NO3−-N reached 55.4%. The above results indicated the anammox bacteria utilized the DNRA pathway to reduce NO3− to NO2− and further NH4+, then normal anammox metabolism would continue to convert the produced NO2− and NH4+ to N2. The intracellular stored carbon sources (e.g., glycogen) were supposed to be electron donors for NO3− degradation. This capability would enhance the viability and living space of anammox bacteria in different natural ecosystems, and make it plausible that complete nitrogen removal could be implemented only by the anammox process.