Achieving superior nitrogen removal performance in low-strength ammonium wastewater treatment by cultivating concentrated, highly dispersive, and easily settleable HAP-nitritation/anammox granule sludge in an air-lift reactor. bioreactor sludge metagenome

In low-strength ammonium wastewater treatment, the application of anammox-based process is still limited due to extreme instability and the poor performance of nitrogen (N) removal. In this work, granule sludge comprised of functional microbes and in-situ self-formed hydroxyapatite (HAP) were inoculated and cultivated in a one-stage partial nitritation/anammox (PNA) reactor for low-strength ammonium wastewater treatment. The results showed that at hydraulic retention time of about 1.0 h and influent ammonium concentration of 63.0 mg/L, an average N removal rate of 1.18 kg/m3/d was achieved, which far exceeds that in similar researches reported until now. The main inorganic matter in sludge was identified as HAP through the X-ray diffractometer and Raman spectrum analysis. Computer tomography was used to explore the structure of the wet granules, and it uncover that the granule has an uneven interior density and numerous shell structure. Combined with the Scanning Electron Microscope images of the dry granule, it was elucidated that the granule was comprised of sub hollow granules. Since the biomass increased with no obvious increase in the granule size, it is inferred that the sub hollow granules have fragile connections with each other and the easy break down of the granules resulted in the high dispersity of sludge. The Florescence in situ hybridization results also showed that the ammonium-oxidizing bacteria and anammox bacteria were mainly distributed in the outer layer of sub granule shell and the HAP in inner, which structure improved the settleability of microbes, and consequently, made it possible to achieve a high biomass in the reactor. Thus, the formed sludge in the reactor was concentrated, highly dispersive and easily settleable, which were thought to be the crucial factors for achieving the desired N removal performance. This study marks a big leap in the treatment of low-strength ammonium wastewater through the one-stage PNA process and has great potential to expand its application.