Improving the Properties of Slurry Fuel Preparation To Recycle Municipal Sewage Sludge by Alkaline Pretreatment

Abstract

Municipal wastewater sludge (denoted as sludge hereafter) can be economically and effectively used by mixing it with coal to prepare slurry fuels. However, the abundant extracellular polymeric substances (EPSs) and strong water-holding capacity of raw sludge cause the viscosity of slurry fuel to increase remarkably, which is disadvantageous for industrial applications. The slurrying properties of sludge can be improved by alkaline treatment. This study examined the effects of calcium oxide (CaO) on EPSs, water-holding capacity, floc structure of sludge, and co-slurrying properties of sludge with coal. Results showed that, after CaO modification, the relative amount of EPSs with a number-average molecular weight (Mn) > 5000 Da noticeably decreased, whereas the relative amount of EPSs with Mn < 5000 Da increased. Moreover, the soluble chemical oxygen demand (SCOD) of sludge increased, and its volatile suspended solid (VSS) content decreased. Raw sludge had a SCOD of 16.2 mg of O2/g of dry sludge (DS) and VSS of 58.19%. After treatment with 0.2 g of CaO/g of DS for 24 h, SCOD increased to 153.6 mg of O2/g of DS, VSS decreased to 45.61%, the water-holding capacity of the sludge decreased, and the saturated water content of raw sludge decreased from 84.71 to 79.10%. The characteristic viscosity ηc of the raw sludge–coal slurry was 1635.3 mPa s, which decreased to 1082.2 mPa s after treatment with 0.2 g of CaO/g of DS for 24 h. These findings strongly suggested that sludge solubilization can help improve slurrying.