Characteristics of Metal-Polysilicate Coagulants

The authors propose metal-polysilicate complexes as new inorganic polymer coagulants for the removal of colloidal suspensions. In the first part of this paper, procedures of silicic acid polymerization to attain very higher molecular weight polysilicates in a much more stable state than conventional methods are discussed. Then, a procedure for introducing metal ions such as aluminum and iron into the polymerized silicic acid is presented to give coagulation capability, i.e. charge-neutralizing capacity, to the polysilicic acid. Several new inorganic polymers are thus prepared as cationic coagulants. In the final part, the effectiveness of the proposed aluminum- or ferric-polysilicate coagulants for coagulation and flocculation is evaluated by means of conventional jar tests and pilot plant experiments. These studies show that both the resulting water quality and the rate of removal after sedimentation are greatly improved in comparison with conventional inorganic coagulants. Unlike conventional organic coagulants, these inorganic polymer coagulants are considered to be safe for human health because the safety of silicates and metal coagulants has been demonstrated by many years of utilization.