Fe²⁺-Coupled Organic-Substrate-Enhanced Nitrogen Removal in Two-Stage Anammox Biofilm Reactors

Anammox is a novel and energy-efficient biological nitrogen removal technology. Enhancing its performance in treating low-strength nitrogen wastewater is essential for expanding its practical applications. In response to challenges such as low nitrogen removal efficiency (NRE), poor operational stability, limited environmental resistance, and the interference of organic compounds commonly found in real wastewater, this study developed a two-stage upflow anammox biofilm reactor system (R1 and R2) enhanced by an Fe²⁺-coupled organic substrate strategy for deep nitrogen removal under low-nitrogen conditions. Results showed that sodium acetate at a chemical oxygen demand (COD) concentration of 40 mg/L provided the greatest enhancement to anammox activity, achieving an average total nitrogen removal efficiency (NRE) of 90.02%. However, the reactor performance was significantly inhibited under higher COD conditions (e.g., COD = 60 mg/L). Under an influent Fe²⁺ concentration of 10 mg/L, the reactors’ NRE increased and then decreased as the COD concentration rose from 0 to 100 mg/L, resulting in the highest efficiency being achieved at an average NRE of 94.11%, observed under 10 mg/L Fe²⁺ coupled with 60 mg/L of COD in the two-stage anammox system. Scanning electron microscopy revealed that the co-addition of Fe²⁺ and organic substrates led to the formation of granular protrusions and pores on the sludge surface, which favored the structural stability of the biomass. At a COD level of 40 mg/L, the contents of extracellular polymeric substances and heme c in anammox biofilm were significantly higher compared to the addition of 10 mg/L Fe²⁺ alone, whereas excessive COD inhibited both indicators. These findings suggest that moderate levels of Fe²⁺ coupled with organic matter can promote anammox activity for deep nitrogen removal, while excessive organics have inhibitory effects. This study provides theoretical support for enhancing nitrogen removal from low-strength wastewater using Fe²⁺ and organic-substrate-assisted anammox processes.