Effect of hydraulic selection pressure on the characteristics of partial nitritation/anammox granular sludge in a continuous-flow reactor

Strong hydraulic selection pressure is a control co-factor for maintaining the stability of granular sludge in a continuous-flow bioreactor. In this study, efficient nitrogen removal was achieved in a continuous partial nitritation/anammox (PN/A) granular reactor in a short hydraulic retention time (HRT), decreasing from 100 to 40 min, where more than 95% of ammonium and 80% of total inorganic nitrogen (TIN) were removed at a constant nitrogen loading rate of 2.0 kg (m(3) d)(-1). Results showed that the decreased HRT substantially influenced the observed biomass growth curve, with a trade-off between the growth and washout of microbial populations. Further, sludge characterization indicated that shorter HRT improved granular compactness with the accumulation of extracellular polymeric substances and increased the specific nitrogen removal rate, reaching q(TIN) = 39.9 mg N g(-1) VSS.h(-1). These observations were consistent with shifts in the bacterial community structure of the PN/A sludge with different particle sizes. High-throughput pyrosequencing revealed that strong hydraulic selection pressure promoted the niche segregation of aerobic and anaerobic ammonium-oxidizing bacteria (AOB and AnAOB) and washout of nitrite-oxidizing species in the granules. The effective retention of k-strategists AnAOB (dominated by genus Candidatus Kuenenia) occupying the interior layer of the granules exhibited a solid retention time (SRT) 10-fold longer than that of AOB (identified as Nitrosomonas) enriched in the aerobic zone. A strategy involving HRT and SRT coordination in operation can enhance the efficiency and stability of PN/A systems, considering the potential development of small aggregates into mature granules. (C) 2021 The Authors. Published by Elsevier B.V.