Anaerobic ammonium oxidation in sponge-bed trickling filter: Operational conditions and mainstream sewage treatment

Applying anammox process in mainstream wastewater treatment is challenged by low nitrogen levels, temperature variability, and microbial competition. This study evaluated a Sponge-Bed Trickling Filter (SBTF) treating real domestic wastewater under tropical conditions for 306 days (Stage 1) and 119 days (Stage 2), targeting operational simplicity suitable for post-treatment after UASB reactors. Polyurethane sponge promoted biofilm formation, biomass retention, and stable anoxic zones. The reactor operated without effluent recirculation under varying nitrogen loading rates (NLR: 0.6-1.5 kgN.m^-3.d^-1), achieving average removals of 42 mgO₂·L^-1 COD and 11 mgNH₄⁺-N·L^-1 with total nitrogen removal of 25-36%. Organic matter did not inhibit anammox activity. Microbial analyses confirmed spatial stratification of Candidatus Brocadia, ammonia oxidizers, and denitrifiers. Findings indicate NLR as the key driver of nitrogen removal and show that recirculation can be minimized under certain conditions. The demonstrated synergy in sponge-bed filters offers a cost-effective for nitrogen removal under realistic mainstream conditions.