Stable nitrification under sulfide supply in a sequencing batch reactor with a long fill period

Ammonia-containing wastewater often also contains sulfide, which severely inhibits biological nitrification. In this study, the applicability of a sequencing batch reactor (SBR) with a long fill period to achieve stable nitrification of sulfide-containing anaerobic digestion effluent was evaluated. During the start-up period, in which no sulfide was supplied to the reactor (Phase I, experimental days 0-18), ammonium was oxidized to nitrate stably after day 6. During the period in which sulfide was supplied at 32 mg-S L−1 d−1 (Phase II, experimental days 18-30), the nitrification efficiency temporarily decreased to 79%, but recovered rapidly to 100% within two days. The sulfide tolerance of the sludge in the SBR was evaluated by batch bioassays at the end of the start-up period. The results showed that the half-maximum inhibitory concentration (IC50) for the sludge was 0.73 mg-S L−1, which was a typical value. Therefore, pre-existing sulfide tolerance of the sludge was not the main factor enabling stable nitrification during the sulfide supply period. Throughout the experimental period, the reactor was operated using a long sludge retention time of 139 days. The effective sludge retention in the SBR with a long fill period probably promoted the acclimatization of the microbes to sulfide, allowing the nitrification performance to remain stable even under sulfide supply.