Effect of Ultrasonication on Excess Sludge Bioavailability for Methane Production

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The effects of ultrasonication on methane production from excess activated sludge were analyzed in relation to the bioavailability of the solubilized organic matter and the solid fraction. The solubilized organic matter was divided into three categories according to biodegradability: easily-, less-, and un-degradable organic matter. The rapid increase in easily-degradable fraction was observed at a specific sonication power of about 4,000 kJ/kg-TS but the fraction leveled off at specific power inputs above 9,000 kJ/kg-TS. The un-degradable fraction began to increase at specific power inputs of about 6,000 kJ/kg-TS and leveled off at about 9,000 kJ/kg-TS. The effect of ultrasonication on methane production was pronounced by the initial methane production rate, indicating boosted hydrolysis rate that is usually a limiting step of methane production. The methane production rate from the solubilized organic matter leveled off at the specific power input around 6,000 kJ/kg-TS, suggesting that about 60% of solubilized organic matter was converted into methane. Methane production from whole sonicated sludge showed that the amount of methane converted from the solid fraction was larger than that from the solubilized organic matter, and that ultrasonication enhanced the bioavailability of solid. The rationale for the optimal ultrasonication conditions was presented.