Silica and metals removal by pretreatment to prevent fouling of reverse osmosis membranes

Reverse osmosis is being increasingly used for desalination of seawater, brackish water and as well the marginal and polluted wastewaters. The operation of waters bearing substantial amount of silica is only feasible if there is a chemical pretreatment in process to prevent excessive and unbearable fouling by this refractory deposit. In this study, model solutions representing tailings wastewater which usually has high silica content was used in standard jar tests using the Boltac Coagulation and Flocculation simulator to determine the effectiveness of chemical pretreatment (by precipitation and coagulation) for removal of silica and other species such as magnesium, calcium, iron and manganese which affect silica fouling. Two precipitants were tested: lime and soda ash, and caustic soda. Precipitant aids (alum and ferric chloride) were also examined in combination with optimum precipitant doses to observe their differential effects. The optimum precipitant dose of caustic soda was 200 mg/L. Ferric chloride and alum were tested with this optimum caustic soda dose, but did not improve removal significantly, if at all. The optimum precipitant dose of lime and soda ash was 150 mg/L lime and 450 mg/L soda ash. Ferric chloride and alum were tested with this optimum lime and soda dose, but again did not improve removal enough to warrant their use. Neither of the methods were as effective in removal of magnesium and calcium as in removal of other components; lime and soda ash treatment as expected actually increased the calcium content of solutions.