Increased solubilization of excess sludge does not always result in enhanced anaerobic digestion efficiency

“Solubilization (SCOD/TCOD, chemical oxygen demand (COD), %)” is generally used as an indicator for the pretreatment effectiveness in anaerobic digestion (AD) of excess sludge. In the present work, ultrasonic (irradiation 5–60 min) and alkaline pretreatments (pH 9–13) were applied to excess sludge individually and in combination. Biological methane potential tests were carried out using the whole part of pretreated sludge and only the soluble fraction. Although solubilization increased with pretreatment intensity increase, methane production was inconsistent with increased solubilization, due to the lower methane yield of soluble fraction. While the soluble fraction obtained under mild pretreatment condition (pH 9 + ultrasonication 5 min) showed 91% of methane yield, it was only 61% in case of severe pretreatment condition (pH 13 + ultrasonication 60 min). 25.8% of the pretreated sludge at (pH 13 + ultrasonication 60 min) consisted of soluble but non-biodegradable portion. These findings demonstrate that increased solubilization does not always result in enhanced AD efficiency.