Sewage sludge disruption through sonication to improve the co-preparation of coal–sludge slurry fuel: The effects of sonic frequency

Abstract

The sewage sludge volume has been increasing annually, and if not treated properly, this sludge endangers the environment and human health. Sludge can be considered as a carbon-containing material, and the energy within is utilized easily and economically by blending it with coal to prepare a slurry fuel called coal–sludge slurry (CSS). However, sludge is always disrupted before CSS preparation because of its high bound water content and viscosity. In this study, sonication was performed to disrupt sludge and to enhance co-slurrying with coal. The effects of sonic frequency were highlighted, and the results showed that low-frequency sonication significantly reduces viscosity. Moreover, this process improved CSS slurrying. The characteristic viscosity (the apparent viscosity measured at a shear rate of 100 s−1) of CSS prepared with raw sludge was 1663.6 mPa⋅s; following sludge disruption via sonication (the specific energy was 30 kJ/g dry sludge) at 15, 25, and 35 kHz, the characteristic viscosities of prepared CSSs decreased to 1121.6, 1194.8, and 1234.3 mPa⋅s, respectively. On the basis of the dynamic model of a sonic cavitation bubble, low-frequency sonication intensified cavitation such that the radius and impact velocity of a cavitation bubble increased. This finding was consistent with experimental results.