Performance evaluation of a down-flow hanging sponge (DHS) reactor as a decentralized domestic wastewater treatment system in tropical regions.

In this study, a pilot-scale down-flow hanging sponge (DHS) reactor was operated in the community plant of Bangkok for the treatment of domestic wastewater (COD 285 mg/L, BOD 105 mg/L) collected by separate sewer to evaluate the reactor's feasibility as a decentralized treatment system. The DHS reactor was operated for 600 days at ambient temperatures of 25-30 °C, both with constant flow conditions and with fluctuating flow conditions that simulated wastewater discharge patterns of the community. The results indicate that under constant flow at an HRT of 5 h, the volumetric loading rates of 0.36 kgBOD/m3-sponge/day and 0.16 kgN/m3-sponge/day were the optimum operational conditions of the DHS reactor in order to satisfy the effluent discharge standards. The DHS achieved removal rates of 89, 95, 91 and 90% for COD, BOD, TSS and NH4-N. Under the fluctuating flow condition, improvement of denitrification was confirmed at volumetric loading rates of 0.50 kgBOD/m3-sponge/day and 0.18 kgN/m3-sponge/day. The fluctuating flow of wastewater positively affects retained sludge activities in terms of homogenizing sludge concentration and stimulating oxygen uptake rates. These results suggest that the DHS reactor can be applied as a decentralized treatment system for domestic wastewater with fluctuating flow rates in tropical regions.