The performance of an upflow anaerobic sludge blanket (UASB) reactor was investigated for the treatment of domestic wastewater at different operational temperatures (28, 20, 14 and 10°C) and loading rates. For each temperature studied a constant CODt removal was observed as long as the upflow velocity was lower than 0.35 m/h: 82% at 28°C, 68% at 14°C and 44% at 10°C. At 20°C the COD removal increased with the HRT, reaching similar values as at 28°C for long HRT. At upflow velocities higher than 0.35 m/h, a reduction in total COD removal was observed due to washout of influent TSS. At 28°C, a constant 200 g sludge mass was observed and COD removal was attributed to biological degradation only. At lower temperatures, COD removal resulted from degradation and solids accumulation in the reactor. The increase in reactor sludge was greater as the temperature decreased and explains the similar overall COD removal efficiency at 28°C, 20°C and 14°C. During the transition from winter to summer conditions (10°C to 28°C), methane production initially increased due to the degradation of accumulated solids. Afterwards, methane production gradually declined and an increase in COD removal was observed, indicating that the TSS accumulated during the winter was exhausted and influent degradation remained.

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