The performance and microbial communities of Anammox and Sulfide-dependent autotrophic denitrification coupling system based on the gel immobilization

•Gel immobilization was applied in the Anammox-SDAD coupling system successfully.•The TN removal efficiency achieved 85.69% under the optimal S2−/NO3– of 0.6.•The removal performance was stable under the rapid changes of substrate loading rate.•Gel immobilization boosted the adaptability of functional bacteria to the temperature. Anammox and sulfide-dependent autotrophic denitrification (ASDAD) coupling system can improve the nitrogen removal, but high sulfide concentration will affect the activity of anaerobic ammonia-oxidizing bacteria (AnAOB). Gel immobilization technology can enhance the survivability of microorganisms in unsuitable environments. Therefore, in this investigation, gel immobilization technology was applied into the ASDAD coupling system to explore the removal performance and microbial communities. The results showed that the optimal S2−/NO3– was 0.6, under which the best TN removal efficiency was 85.69%. The removal performance of ASDAD coupling system was stable under rapid variations of nitrogen loading rate and sulfide loading rate. Immobilized sludge cubes could attenuate the effects of temperature on AnAOB and sulfide-oxidizing bacteria. Observations of SEM and stereoscope suggested that AnAOB was more likely to exist on the surface of the sludge cubes. Thiobacillus, Candidatus Brocadia, and Candidatus Kuenenia were the main functional bacteria in the coupling system.