Environmental and Economic Evaluation of Downflow Hanging Sponge Reactors for Treating High-Strength Organic Wastewater

This study evaluated the performance of a downflow hanging sponge (DHS) in reducing the concentrations of chemical oxygen demand (COD), ammonia (NH3), total suspended solids (TSS), and total dissolved solids (TDS) in high-strength organic wastewater (HSOW). The DHS unit was composed of three segments connected vertically and operated under different organic loading rates (OLRs) between 3.01 and 12.33 kg COD/m(sponge)(3)/d at a constant hydraulic retention time (HRT) of 3.6 h. The results demonstrated that the DHS system achieved COD, NH3, TSS, and TDS removal efficiencies of 88.34 +/- 6.53%, 64.38 +/- 4.37%, 88.13 +/- 5.42%, and 20.83 +/- 1.78% at an OLR of 3.01 kg COD/m(sponge)(3)/d, respectively. These removal efficiencies significantly (p < 0.05) dropped to 76.39 +/- 6.58%, 36.59 +/- 2.91%, 80.87 +/- 5.71%, and 14.20 +/- 1.07%, respectively, by increasing the OLR to 12.33 kg COD/m(sponge)(3)/d. The variation in COD experimental data was well described by the first-order (R-2 = 0.927) and modified Stover-Kincannon models (R-2 = 0.999), providing an organics removal constant (K-1) = 27.39 1/d, a saturation value constant (K-B) = 83.81 g/L/d, and a maximum utilization rate constant (U-max) = 76.92 g/L/d. Adding another DHS reactor in a secondary phase improved the final effluent quality, complying with most environmental regulation criteria except those related to TDS concentrations. Treating HSOW with two sequential DHS reactors was economically feasible, with total energy consumption of 0.14 kWh/m(3) and an operating cost of about 7.07 USD/m(3). Accordingly, using dual DHS/DHS units to remove organics and nitrogen pollutants from HSOW would be a promising and cost-efficient strategy. However, a tertiary treatment phase could be required to reduce the TDS concentrations.