Coagulation performance of polyaluminum silicate chloride (PASiC) for water and wastewater treatment

A series of polyaluminum silicate chloride (PASiC) composite coagulants were prepared by two approaches: (1) hydroxylation of the mixture of AlCl3 and fresh polysilicate (PASiCc); (2) hydroxylated polyaluminum chloride (PAC) combined with fresh polysilicate (PASiCm). Laboratory experiments were conducted to evaluate PASiC in comparison with PAC for the coagulation of water and wastewater and to investigate the effects of preparation technique, [OH]/[Al] ratio (i.e. γ value), pH, Al/Si ratio, and temperature on the coagulation performance of PASiC. The experimental results showed that PASiC performed more efficiently than PAC did in removing algae, turbidity, oil, chemical oxygen demand (COD) and total phosphate (TP). At the same Al/Si ratios, PASiCc achieved slightly greater removal of turbidity than PASiCm did. However, the difference became less significant at higher dosages. The results also demonstrated that the coagulation efficiency of PASiC was better at greater γ values and that PASiC became unstable during aging at very high γ values. The optimum coagulation pH range of PASiCc at a γ value of 2.0 and an Al/Si ratio of 10 was found to be 6.0–8.5, slightly wider than that of PAC (6.0–8.0).