New strategy for integration of anaerobic side-stream reactor with mainstream B-stage nitritation for short-cut nitrogen removal with granulation.

This study reported a successful mainstream B-stage nitritation reactor with sludge granulation that incorporated a side-stream anaerobic reactor to treat municipal wastewater A-stage effluent. With influent COD/N and COD/P ratios of 2.60 and 27.1, respectively, the system achieved a stable nitrite accumulating ratio (NAR) of 95.1% via partial nitrification with sludge granulations. Kinetic assessment,16S ribosomal RNA sequencing, and functional gene marker quantification confirmed successful nitrite-oxidizing bacteria (NOB) out-selection (<0.05% relative abundance), while none of the commonly employed approaches for NOB out-selection occurred in our study. Notably, approximately 90% of the total biomass was in the biofilm in the mainstream sequencing batch reactor (SBR), with the remaining 10% of the biomass in suspension as granules under the selective wasting strategy. The substrates and oxygen gradient along the depth of the biofilm's layered structure, alongside the anaerobic conditions in the side-stream reactor, were suggested to play roles in NOB suppression and out-selection. Overall, this study provided evidence for a possible new strategy for achieving stable mainstream B-stage nitritation, which is the prerequisite for the downstream anammox process. The novelty aspect of the systems, including the incorporation of an anaerobic sire-stream reactor, absence of the employment of any previously reported nitritation strategies, and granulation formation, provided possible new feasible routes to achieve mainstream short-cut nitrogen removal for efficient wastewater treatment. PRACTITIONER POINTS: Stable partial nitrification achieved in mainstream B-stage SBR under conditions distinct from previous reports. NOB out-selection confirmed by both activities' tests and molecular analysis. Thick biofilm and anaerobic side-stream reactor likely facilitated NOB suppression. Stable sludge granulation was maintained with selective wasting strategy.