Anammox bacteria enrich naturally in suspended sludge system during partial nitrification of domestic sewage and contribute to nitrogen removal

Achieving the anammox process in suspended sludge system without supplementing carriers or inoculating anammox bacteria is a challenging topic, especially inmunicipal wastewater treatment. Long-termmaintenance of partial nitrificationwas realized in this study bymaintaining dissolved oxygen (DO) below0.5 mg/L, by using a DO meter and controlling the oxygen diffuser apparatus in anaerobic-aerobic-anoxicmode in a sequencing batch reactor, leading to successful enrichment of anammox bacteria. Under 134 maintenance days of partial nitrification with an average nitrite accumulation rate of 94.5%, anammox bacteria gradually enriched in suspended sludge with an increasing relative abundance from 0% on 18th day to 0.44% on 124th day, and the nitrogen removal efficiency improved from 30% to 85%. (NO2-)-N-15 N isotope labeling test showed that activities of anammox and denitrification were 1.48 and 1.22 mu molN/(gVSS.h), respectively. The contribution of anammox in the anoxic stage to nitrogen removal was 20%, but anammox N removal continued in the aerobic stage, as well. Long-term maintenance of partial nitrification and lowdissolved oxygen levelswere the key factors leading to enrichment of anammox bacteria. (C) 2021 Elsevier B.V. All rights reserved.