Achieving efficient nitrogen removal through spatial coupling of sulfur disproportionation and sulfur-based...

Polysulfide (S_n^2-) produced by sulfur-disproportionating bacteria (SDB) mediated sulfur disproportionation (SD) could serve as effective electron donors for high-rate nitrogen removal from low C/N wastewaters. The syntrophic interactions between SDB and sulfur-oxidizing bacteria (SOB) have been considered the key for establishing Sulfur disproportionation-driven Polysulfide-Enhanced Expeditious autotrophic DeNitrification (SPEEDN) process. However, the presence of NO₃^- could inhibit SD activity, making it difficult for achieving the stable syntrophic symbiosis between SDB and SOB. To address this issue, sulfur-based porous alkaline carriers (SPAC) were constructed in this study to offer them differentiated ecological niches within microenvironments in the presence of NO₃^-. The long-term operation of laboratory-scale reactor packed with SPAC achieved average nitrogen removal rates of 0.96 kg N/m³-d, and peaked at 2.05 kg N/m³-d with HRT of 0.5 h when treating real domestic wastewater, which were substantially higher than that with commercial carriers (0.45 kg N/m³-d on average). The total relative abundance of bacteria potentially performing SD (e.g., Dissulfurimicrobium, Sulfurimonas) reached 14.3% in the inner layer of SPAC, significantly higher than those in the outer layer (4.8%) and in the flocs (2.6%) as well as in the commercial carriers (4.8%). SOB such as Ferritrophicum, Thiobacillus, and Denitratisoma were widely distributed in the SPAC. Batch experiments indicated that the SD and denitrification processes primarily occurred in the inner and outer layers of SPAC, respectively. The produced S^2-/S_n^2- in the inner layer could be utilized for denitrification, thereby enhancing the nitrogen removal rate. It suggested that the SPAC exhibited spatially stratified collaborative characteristic of functional microorganisms, wherein SDB primarily colonized the inner layer of SPAC, rendering SDB less susceptible to NO₃^- shock, while SOB flourished in both outer and inner layers acted as the dual shield for SDB. Collectively, with the functionally space-confining carriers, this study achieved the efficient coupling of sulfur‑nitrogen cycle reactions in the microenvironments, offering a novel approach for the improvement of system performance and stability of SPEEDN.