Rice straw biochar modified by aluminum chloride enhances the dewatering of the sludge from municipal sewage treatment plant

Enhancement of the dewatering of the sludge by using rice straw biochar (RSB) modified by aluminum chloride (AlCl3) was investigated, and the possible enhancing mechanisms were discussed. Results showed that the settled volume after 30 min (SV30%), specific resistance to filtration (SRF), moisture content (MC) and capillary suction time (CST) of the sludge were decreased and the net sludge solids yield (YN) was increased by the increasing raw or modified RSB, which indicated a higher sludge dewaterability. When the dosage of the modified RSB was adjusted to 0.3 g(RSB)/g(dry sludge), SV30%, SRF, MC and CST were decreased to 79.8%, 1.2 × 1012 m/kg, 81.4% and 38 s, respectively, YN was increased to 19.4 kg/(m2·h). Furthermore, performance of the modified RSB in the dewatering of the sludge was significantly better than that of the raw RSB. For the enhancing mechanisms, charge neutralization occurred when the modified RSB (loaded with positively charged aluminum species on its surface) was dosed into the sludge system, thus destroying the stable sludge colloidal system, thus far easier to congregate the sludge particles, which enhanced the dewatering of the sludge. Another main enhancing mechanism was that after conditioned by the modified RSB, certain skeleton structures were formed in sludge cake to make water pass through easily by decreasing the extracellular polymeric substances (EPS) of the sludge. We found that the effectiveness of using the modified RSB to enhance the dewatering of the sludge is substantial and promising.