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 (OLRappl). Four UASB reactors were operated at different temperature (15, 20, 25 and 30 °C) and three OLRappl (2, 4 and 6 kg soluble chemical oxygen demand (CODs)/(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, CODs, and total and volatile suspended solids. The CODs 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 OLRappl, 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 Lbiogas/(Lreactor d) (Lb/(Lr d)) and BPR of 20, 25 and 30 °C: 0.5 to 1.9 Lb/(Lr d)).

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