Internal hydraulic recirculation enhances spatially coupled nitrification-denitrification in a passive downflow...

Rapid urbanization has intensified the demand for compact, energy-efficient decentralized wastewater treatment technologies. This study investigated the long-term performance of a laboratory-scale primary settling tank-down-flow hanging sponge reactor (PST-DHS) system operated for over 500 days, benchmarked against a conventional trickling filter (TF) under progressively decreasing hydraulic retention times. The DHS reactor consistently outperformed the TF, achieving 91-95 % COD removal, >94 % TSS removal, and ∼95 % NH₄⁺-N oxidation at an ammonia loading rate of 0.254 ± 0.060 kg-NH₄⁺-N/m³·d. Although complete nitrification was achieved, nitrate accumulation in DHS effluent limited total nitrogen (TN) removal. Effluent recirculation enhanced nitrogen removal by recycling nitrate-rich effluent to the carbon-rich upper reactor zones. At a recirculation ratio (R) of 1:1, the effluent TN decreased to 9.4 ± 1.9 mg/L, meeting the National Green Tribunal effluent discharge standard of 10 mg/L. Residence time distribution analysis revealed that R = 1 provided a balance between reactor hydraulics (P_e≈24 and N≈12) and apparent half-order TN removal (k_1/2= 7.82 ± 1.91). Depth-resolved profiling coupled with 16S rRNA amplicon sequencing revealed pronounced vertical microbial stratification, with denitrification-associated taxa (Dechloromonas, Aquabacterium, and Pseudomonas) enriched in the top and nitrifying taxa (Nitrospira and Nitrosomonas) predominating in the lower sponge media layers. This spatial organization was consistent with the proposed spatial coupling of nitrification and denitrification within a single reactor. Collectively, the hydraulic, biochemical, and microbial findings demonstrate that optimized effluent recirculation improves flow distribution and nitrogen transformation, establishing the DHS process as a robust and potentially energy-efficient solution for decentralized wastewater treatment.