Siphon downflow hanging sponge reactor with influent bypass system for improved simultaneous nitrification–denitrification efficiency

The siphon downflow hanging sponge (siphon DHS) was developed by integrating a siphon tube to create anoxic and anaerobic zones for enhanced denitrification. However, limited aerobic volume resulted in low nitrification rates. In this study, the siphon DHS reactor volume was modified (50:25:25 = aerobic: anoxic: anaerobic zones), and an influent bypass system (20 % to the anoxic zone) was integrated to enhance simultaneous nitrification-denitrification. The integrated design achieved 76 % nitrification and 20 % total nitrogen (TN) removal, representing a threefold improvement over conventional DHS while maintaining 86 % soluble chemical oxygen demand removal. The bypass system enhanced denitrification by providing sufficient organic matter to the anaerobic zone, while modified volume ratios enhanced nitrification. Microbial analysis revealed increased denitrifier abundance (Comamonadaceae: 14 %) and reduced nitrite accumulation. The integrated design achieved 44 % cost reduction for nitrogen removal while maintaining high treatment efficiency, providing a sustainable decentralized wastewater treatment solution.