Removal of hardness and silicon by precipitation from reverse osmosis influent: Process optimization and economic analysis

The removal of hardness and silicon in the reverse osmosis influent is urgently desirable in view of wastewater purification and reuse. However, the development of cost-effective method reducing the hardness and silicon to low level is challenging. Herein, the single step process and double step precipitation process were proposed, aiming at minimize the adverse effect of influent wastewater on reverse osmosis membranes. The optimal dosages of precipitants (FeCl3 and MgCl2) and the effects of pH values and time were investigated. In the double step process, the concentrations of total hardness and silicon reached the lowest values (11.0 mg·L−1 and 2.1 mg·L−1) by adding 400 mg·L−1 MgCl2 and 300 mg·L−1 FeCl3. Subsequently, four schemes for the wastewater treatment were proposed, which scheme D using both FeCl3 and MgCl2 as precipitants obtained the lowest cost (1.20 $·t−1). This work could provide technological support for removing total hardness and silicon from industrial wastewater and industrial wastewater recycling.