Promoting nitrogen removal in low C/N wastewater treatment by S⁰ packed-bed process: Post polishing or via disproportionating sulfidogenesis enhancement

Sulfur autotrophic denitrification (SADN) is frequently implemented as a polishing step for secondary effluents, typically through an elemental sulfur packed-bed reactor (S⁰PBR) placed downstream of the conventional activated sludge (CAS) process. Recent studies show that S⁰PBRs can also function as sulfidogenic units via sulfur disproportionation (SD), it may offer benefits if placed in a side-stream of the CAS process. However, the feasibility of a side-stream S⁰PBR operated via SD in an Anoxic/Oxic process (SD-AO) has not been investigated, and its nitrogen removal performance relative to the conventional downstream SADN configuration (AO/SADN) has not been explored. This study developed and evaluated A/O process coupled with S⁰PBRs and operated in two distinct patterns to treat low C/N wastewater: side-stream SD and downstream SADN. At a lower S⁰PBR-to-A/O volume ratio (3.75), SD-AO achieved better nitrogen removal performance, with a total nitrogen removal efficiency (TNRE) 13 % higher than AO/SADN, and met the discharge standard. At a higher volume ratio (5.63), both configurations met the standard, with AO/SADN preferable in regions where stricter limits apply. Notably, at equivalent volume ratios, elemental sulfur consumption in SD-AO was approximately 48 % lower than in AO/SADN due to reduced sulfur oxidation under low DO, despite the higher TNRE observed in SD-AO. Further analysis of nitrogen mass balance and microbial data revealed that anaerobic ammonium oxidation could be a key contributor to the higher nitrogen removal observed in SD-AO. This study proposes an innovative S⁰PBR application pattern for treating low C/N wastewater and may facilitate broader application of biological sulfur disproportionation in wastewater treatment.