Anaerobic treatment of chocolate-processing industry wastewater at different organic loading rates and temperatures

The objective of the present study was to determine the optimum operating temperature of laboratory-scale upflow anaerobic sludge blanket (UASB) reactors during the treatment of a chocolate-processing industry wastewater at medium applied organic loading rates (OLR_appl). Four UASB reactors were operated at different temperature (15, 20, 25 and 30 °C) and three OLR_appl (2, 4 and 6 kg soluble chemical oxygen demand (COD_s)/(m³ d)). The flowrate and the hydraulic retention time were constant (11.5 L/d and 6 h, respectively). The monitored parameters were pH, temperature, COD_s, and total and volatile suspended solids. The COD_s removal efficiency (RE) and biogas production rate (BPR) were calculated. The 15 °C UASB reactor had the lowest RE (39 to 78%) due to the low operating temperature. Regardless of the OLR_appl, the RE of the 20, 25 and 30 °C reactors was high and similar to each other (between 88 and 94%). The BPR of the four UASB reactors had the same behaviour as the RE (BPR of 15 °C: 0.3 to 0.5 L_biogas/(L_reactor d) (L_b/(L_r d)) and BPR of 20, 25 and 30 °C: 0.5 to 1.9 L_b/(L_r d)).