Effect of coagulation treatment on sludge dewatering performance: Application of polysilicate and their mechanism

• The preparation conditions of polysilicate were optimized by single factor and RSM. • PSF, PSAF, and PSAFM were amorphous polymers with high polymerization degree. • PSAFM showed superior performance in sludge dewatering treatment. • The dewatering was due to adsorption bridging and charge neutralization. Coagulation is an extensive and effective pretreatment technology for improving sludge dewatering performance. In this study, polysilicate ferric (PSF), polysilicate aluminum ferric (PSAF), and polysilicate aluminum ferric magnesium (PSAFM) were prepared by copolymerization reaction, and their preparation conditions include pH, temperature (T), and mole ratio were optimized by the single factor and response surface experiments. The optimal preparation conditions of the above coagulants are shown as follows: PSF (pH = 1.0, T = 20 °C, Si/Fe = 1.00), PSAF (pH = 1.0, T = 20 °C, Si/M = 0.40, Fe/Al = 1.562), PSAFM (pH = 1.0, T = 20 °C, Si/M = 1.08, Fe/Al = 2.00, Fe/Mg = 2.00). The characterization of dewatering agents was analyzed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD), and Fe-Ferron time-by-time complexation colorimetric method. Also, the particle sizes, molecular weights and Zeta potentials were analyzed. The results indicated that Fe 3+ , Al 3+ , Mg 2+ and polysilicon acid underwent polymerization to form macromolecular amorphous polymers. The performance of polysilicate in sludge dewatering pretreatment were investigated by coagulation experiments, meanwhile, in order to study the influence of polysilicic acid on sludge dewatering performance, commercial polyferric sulfate (PFS) was taken as a comparison. The results showed that polysilicate improved sludge dewatering performance through adsorption bridging effect and charge neutralization. In addition, the content of tightly bound EPS (TB-EPS) and bound water was reduced by polysilicate, which resulted in excellent sludge dewatering performance.