Effects of calcium oxide on the surface properties of municipal wastewater sludge and its co-slurrying ability with coal

Urbanization has generated large volumes of municipal wastewater sludge (sludge for short) that threaten the environment and human health. As such, utilization of the sludge in a cost-effective manner is an important concern. The sludge, as a carbon-containing material, can be made into coal-sludge slurry (CSS) by mixing it with pulverized coal. However, the slurryability of the raw sludge is relatively poor due to the abundance of hydrophilic groups in the sludge and its loose floc structure. In this study, modification of the sludge with calcium oxide (CaO) showed positive effects on the sludge slurryability. The characteristic viscosity of the coal raw-sludge slurry (R-CSS) was 1635.3   mPa   s. After modification of the sludge with 2   wt.% CaO for 24   h, the CSS viscosity decreased to 1184.4   mPa   s. The hydrophobic group (C C and C H) content of the sludge increased from 61.5% to 74.9% as the CaO dosage increased from 0% to 4%, and the zeta potential increased from –55.1   eV to –36.2   eV. The optimum CaO dosage for improved slurryability was found to be 2   wt.%. Compared with R-CSS, the coal modified-sludge slurry (M-CSS) showed lower yield stress, weaker pseudoplastic and thixotropic behaviors, and poorer static stability.