Feasibility and Mechanism of Combined Conditioning with Coagulant and Flocculant To Enhance Sludge Dewatering

Coagulation or flocculation is often individually used or in hybrid conditioning with other preconditioning for sludge dewatering. However, combined conditioning with coagulants and flocculants for sludge dewatering is rarely reported. In this study, dewatering performances of sludge with individual use of poly(aluminum chloride) (PACl; as the coagulant) and a biopolymer named “BF-ADSW” (as the flocculant) were investigated, and the feasibility of applying the coagulation–flocculation process using PACl and BF-ADSW for sludge dewatering and the related mechanism were explored. BF-ADSW was harvested from anaerobically digested swine wastewater (ADSW), and its yield was 2.98 g/L, meaning that 2.98 g of BF-ADSW can be harvested from 1 L of ADSW. After conditioning by PACl with a dose of 10% dry solids (DS), the sludge moisture content and settled volume after 30 min were decreased from the raw values of 98.5% and 96.1% to 80.8% and 74.2%, respectively, which were further decreased to 62.3% and 49.2% after further conditioning by 15% DS of BF-ADSW, respectively, indicating that sludge dewaterability and settleability can be obviously improved by PACl coagulation followed by BF-ADSW flocculation. Compared to sludge conditioning by individual PACl and BF-ADSW, reduction of bound water in the coagulation–flocculation process was the most efficient (2.24 g/g of DS), and release of sludge extracellular polymeric substances was the largest (32.2 mg/g of volatile suspended solids), resulting in the best dewatering performance. The synergistic effect of coagulation and flocculation, including charge neutralization and bridge aggregation, was the main mechanism that efficiently enhanced sludge dewatering. These findings demonstrated that coagulation–flocculation is a feasible process to improve sludge dewatering.