Domestic wastewater treatment by down-flow hanging sponge (DHS) system: A sustainable and techno-economic approach

The biological wastewater treatment processes utilize excess energy for aeration and pumping, and the generated sludge is associated with severe waste disposal issues. Hence, this study represents a cost-efficient and reliable method for treating wastewater without the requirement of external artificial aeration or additional sludge treatment steps. A down-flow hanging sponge (DHS) system occupied by sponge pieces, i.e., as a biofilm carrier, was established to treat domestic wastewater over 100 days of continuous feed. The DHS reactor was operated at a food-to-micro-organism (F/M) ratio of 0.27 kg COD/kg MLVSS/d, a sludge residence time (SRT) of 89 d, and a fixed hydraulic retention time (HRT) of 1.2 h. The results demonstrated that the removal efficiencies of chemical oxygen demand (COD), total suspended solids (TSS), ammonia (NH₃), and total dissolved solids (TDS) were 78.71±4.23%, 81.67±4.76%, 63.20±3.84%, and 9.95±0.81%, respectively. An additional DHS unit was used as a post-treatment process to improve the effluent quality, giving final concentrations of 38.42±2.21 mg/L for COD, 6.12±0.42 mg/L for TSS, 2.79±0.14 mg/L for NH₃, and 411.31±25.70 mg/L for TDS. The first DHS unit was responsible for the degradation of COD compounds as an organic carbon source, with NH₄⁺-N as the main N-source, for heterotrophic growth. The second DHS was responsible for eliminating a large portion of NH₃, probably due to the occurrence of autotrophic nitrification, converting NH₄⁺ to NO₃⁻. Improving the performance of sequential DHS/DHS units could be attributed to operating the entire system at an F/M ratio of 0.18 kg COD/kg MLVSS/d, SRT of 161 d, and HRT of 2.4 h. The system performance to treat wastewater was successfully predicted by the Monod model, giving kinetic parameters of µ_max= 0.65 1/h and Ks= 12.13 g/L (R² 0.943). Because natural ventilation was used to supply the sponge with oxygen, treating sewage by the dual DHS/DHS system would cost 2.23 US$/m³. It’s suggested that an additional unit, such as membranes, could be used to reduce the remaining TDS concentrations.