Biological nitrogen removal from nitrate sewage via novel CANDAN process in continuous-flow UASB reactor with municipal wastewater as co-substrate

The innovative Complete Ammonium and Nitrate removal via Denitratation-Anammox over Nitrite (CANDAN) process represents a promising low-carbon technology for wastewater treatment, has been widely studied in sequential batch reactors (SBRs) treating synthetic wastewaters. This work focuses on the development of the CANDAN process within a continuous-flow Upflow Anaerobic Sludge Blanket (UASB) reactor for nitrate sewage treatment (NO3–-N: 100–1000 mg N/L), featuring varying proportions of real municipal wastewater as a co-substrate (61.3 mg NH4+-N/L, 191.5 mg COD/L). The results demonstrate consistently high N removal performance, with over 84.0 % of total nitrogen (TN) removed from the mixed wastewaters at a rate of approximately 0.6 Kg N/m3/d. As the proportion of municipal wastewater in the feed increased, the activity of denitrifying microorganisms improved. However, their growth was constrained by exacerbated endogenous respiration resulting from prolonged Hydraulic Retention Time (HRT) under a constant loading rate. The ideal nitrite production with the nitrate-to-nitrite transformation ratio (NTR) of above 84.6 % was always obtained. This sustained the dominance of the anammox pathway at around 80 % throughout the operation, underscoring its pivotal role in supporting the exceptional performance of the CANDAN process within the UASB reactor. Microbial analysis revealed an increase in species richness and diversity with higher proportions of municipal wastewater, while anammox functional bacteria exhibited slight enrichment in the later stages, further emphasizing the sustainability of the CANDAN process even with increased proportions of municipal wastewater. The outcomes of this study provide valuable insights into the successful development of a continuous-flow CANDAN process for highly efficient nitrate sewage treatment by incorporating real municipal wastewater as a co-substrate, advancing the goal of achieving carbon–neutral wastewater treatment practices.