Long-term operation of a two-reactor partial nitrification/anammox system with reverse sequence configuration and internal recirculation: Process performance and microbial community dynamics

Despite the higher investment costs of two-stage partial nitrification/anammox (PN/AMX) systems compared to single-reactor configurations, creating more favorable conditions for specific N-cycling microbial groups in different reactors can help more easily maintain the stability of the overall system. In this work, an unusual two-reactor PN/AMX system with brush-type carriers, reverse sequence configuration and internal recirculation was operated for 500 d using both high strength synthetic wastewater and real reject water. Feasibility of the proposed system was confirmed by relatively high (>70%) total N removal without strict control over the nitrification process due to synergistic activity of nitrifying, denitrifying and anammox bacteria. The limiting level of COD in the influent was ∼600 mg O2/L, exceeding which sharply deteriorated the process of nitrogen removal. The microbial community of the PN reactor naturally evolved into clearly distinguishable free-floating sludge and 2 types of biofilms (red and brown), which had different relative abundances of key N cycling groups in each phase of PN/AMX system operation. While the AMX community was dominated by the genus Candidatus Jettenia ecosi. Considering the ease of operation and relatively high N removal efficiency, the proposed PN/AMX system may merit further study for process optimization and practical use. [Display omitted] •Unusual PN/AMX system with reverse sequence configuration and internal recirculation.•N removal of more than 70% at HRT of 1.06–1.36 d and recirculation ratio of 1.0.•COD level of more than 600 mg /L sharply deteriorated N removal.•Clearly distinguishable free-floating sludge and 2 types of biofilms developed.•No need of strict control over nitrification due to self-regulation N-cycle groups.