Influence of Suspended Solids on Silica Removal by Coagulation with Aluminum Salts

Effluents with high silica content can exceed the discharge limits and/or limit the application of reuse treatments based on reverse osmosis membranes. In these cases, silica removal is usually carried out in the effluent by coagulation. In this work, silica is removed from process waters in dissolved air flotation (DAF) units used as internal treatments. It is hypothesized that the presence of a high content of small suspended solids and colloids in DAF units should favour the rate of precipitation of Al(OH)3 and the orthokinetic flocculation, thus the removal efficiency of contaminants. Results confirmed that the coagulant required for silica removal in the process water is 20-50% lower than in the effluent, especially in DAF2, where the amount of suspended solids is higher and their size is smaller. If the main aim is silica removal, the most efficient coagulants are PAC-HB in DAF1 and PAC-MB in DAF2. If a simultaneous high removal of turbidity and soluble COD is required, the recommended treatment is PANS-PA2.