Long-term monitoring of a full-scale parallel-unit downflow hanging sponge system with adaptive capacity for fluctuating wastewater flows in Japan

Abstract This study presents a 30-month full-scale evaluation of a parallel-unit downflow hanging sponge (DHS) system with adaptive downsizing capability at the Susaki Sewage Treatment Plant, Japan. The DHS reactor comprised ten identical units operated in parallel, allowing capacity adjustment by varying the number of active units. The system (primary sedimentation tank → DHS → moving bed biological filter) was monitored under normal, low-temperature (winter), partial downsizing, and sponge sheet installation (upper part of the sponge media-filled segment) conditions. Dichromate-based chemical oxygen demand (COD Cr ), biochemical oxygen demand (BOD), total suspended solids (TSS), NH₄⁺–N, NO₃⁻–N, total nitrogen, and Escherichia coli were analyzed weekly and monthly to evaluate organic removal, nitrification, and microbial reduction. DHS effluent (77–85% COD Cr , BOD removal, and a 1.5 log₁₀ reduction in E. coli ) consistently met Japanese discharge standards. Downsizing to nine units, that is, reducing the hydraulic retention time, maintained effluent quality, confirming the system’s flexibility under declining inflows. Statistical comparison among operational periods showed no significant difference in NH₄⁺–N loading rate, whereas NH₄⁺–N removal efficiency was significantly higher after sponge sheet installation. The sponge sheet enhanced hydraulic dispersion and nitrification, reducing NH₄⁺–N from 16–19 to 9 mg N·L⁻ 1 and promoting NO₃⁻–N accumulation. Compared to the conventional activated sludge process previously employed, DHS used 83% less power and reduced excess sludge generation, demonstrating its potential as a sustainable, energy-efficient alternative for small- to medium-scale sewage treatment facilities with aging infrastructure. These findings highlight DHS as a resilient and socially viable solution for regions with declining populations, ensuring reliable wastewater treatment despite demographic and economic challenges.