Key factors improving the stability and the loading capacity of nitrogen removal in a hydroxyapatite (HAP)-enhanced one-stage partial nitritation/anammox process

Anammox is attracting attention as the most promising nitrogen removal technology. Recently, the combination of the anammox process and the hydroxyapatite (HAP) granules comes with the advantages, including simul-taneous removals of nitrogen and phosphorus and better settleability of the sludge. In this study, the key factors for a stable and efficient nitrogen removal in a one-stage partial nitritation/anammox (PNA) process with HAP -based granules was explored. Under a fixed hydraulic retention time of 12 h, in the case that HAP was not properly formed and phosphorus removal efficiency (PRE) was low, the highest nitrogen removal rate (NRR) achieved stably was only 1.3 +/- 0.07 kg N/m3/d, while a stable nitrogen removal efficiency of 81 +/- 3.3 % and a higher NRR of 2.0 +/- 0.1 kg N/m3/d were obtained with the proper formation of HAP and higher PRE. The achievement of the high NRR benefited by a high microorganism concentration with improved settleability due to the formation of HAP-based granules. To achieve a high NRR in a long-term operation, a necessary phosphorus removal rate/nitrogen removal rate ratio above 0.02 was confirmed, which allowed for the desired sludge set-tleability and concentration. The average diameter of the formed micro-granules was 92-159 mu m. The spatial distribution of the functional bacteria in the granules was analyzed with fluorescence in situ hybridization. Anammox bacteria and the ammonium oxidizing bacteria the two most abundant bacteria during the whole operation. The results of this study are evidence of the necessity and potential of proportionable HAP formation for highly-efficient nitrogen removal in the one-stage PNA process.