Effect of Si/Ti molar ratio on enhanced coagulation performance, floc properties and sludge reuse of a novel hybrid coagulant:polysilicate titanium sulfate

• A new hybrid coagulants-polysilicate titanium sulfate was synthesized.
• PST achieved better NOM removal, especially PST20.-PST generated larger flocs than TS；floc breakage and recovery factor were improved by PST.
• PST5 and PST10 flocs were stronger than TS flocs, while PST20 and PST50 flocs were weaker.
• Sludge can be reused to prepare TiO2.

Polysilicic acid (PSiA) has been used to improve the coagulation behavior of metal coagulants (such as aluminum salts and iron salts), while there is not much research on the combination of polysilicic acid and titanium salts. This study was aimed to evaluate the effect of Ti/Si molar ratios on the titanium salt coagulation performance and floc properties in humic–kaolin water treatment. Polysilicate titanium sulfate (PST) was synthesized by mixing different amounts of PSiA and titanium sulfate (TS) solution. According to experiment results, PST20 (Ti/Si molar ratio = 20) exhibited the best natural organic matter removal efficiency (%). The effluent residual turbidity was also improved by PST at proper Ti/Si molar ratio. Floc size of titanium sulfate was enlarged by the addition of PSiA. Additionally, floc strength was enhanced by PST20 and PST50, while PST10 and PST5 flocs were weaker than TS floc. Floc strength and recovery ability were also improved by PSiA. However, flocs produced by PST were much loosely constructed with lower fractal dimension. Sludge of PST coagulation system was collected and calcined at 700 °C to prepare TiO2.