Two-stage start-up to achieve the stable via-nitrite pathway in a demonstration SBR for anaerobic co-digestatetreatment.

The supernatant effluent from full scale anaerobic codigestion of waste-activated sludge and the organic fraction of municipal solid waste was treated by a demonstration sequencing batch reactor for short-cut nitrogen removal. After inoculation with conventional municipal activated sludge, the strategy of two-stage start-up allowed us the fast speciation of ammonia oxidizing bacteria (achieved within 20 days), then the achievement of the maximal treatment potential (0.8 kgN m−3 d−1) for nitritation−denitritation. By automatic indirect control of the free ammonia and the free nitrous acid concentrations, the via nitrite pathway was fully and stably achieved under aerobic conditions (DO of 1.5 mg L−1) and T of 15 °C. Under ordinary operation, the specific nitritation rates were 15−20 mgN gMLVSS−1h−1, while the denitritation rate was 45−50 mgN gMLVSS−1h−1. In-situ and ex-situ respirometry and gene-based molecular techniques demonstrated the stable presence of a dominant, restricted ammonia oxidizing bacterial population after the first-stage aerobic start-up. We therefore demonstrated that codigestion and advanced nitrogen removal from anaerobic supernatant may optimize the performances of an integrated municipal treatment plant.