Development of a Membrane Desalination and Coprecipitation System for Enhanced Reclamation of Cooling Tower Water: Mechanism Evaluation and Pilot-Scale Study

The raw water in southern Taiwan has high hardness and high silica (SiO2) characteristics. If the high SiO2-content water is used as a makeup water source for cooling towers (CT), the SiO2 concentration in the tower blowdown (BD) would exceed the saturation level (120 mg/L) within a short operational period. The objective of this study was to develop a combined coprecipitation (first stage) and membrane desalination [ultrafiltration (UF)/reverse osmosis (RO) (second stage)] system to enhance the water reclamation efficiency of cooling tower (CT) in the petrochemical industry. Flue gas desulfurization (FGD) wastewater containing magnesium ions (Mg2+) and cooling tower blowdown (CTBD) were mixed, and Mg2+ and silica (SiO2) were removed through the coprecipitation process (core mechanism). The supernatant was directed to the UF/RO for reclamation. Results show that alkaline environments promoted the creation of Mg2+─SiO2 coprecipitates [Mg3Si2O5(OH)4, Mg2SiO4 and MgSiO3], which allowed for the effective removal of SiO2. Under alkaline conditions, when the molar ratio of Mg2+/calcium ions (Ca2+)∶SiO2 was 1∶1, Mg2+ exhibited more pronounced advantages for SiO2 removal. The maximum SiO2 removal rate reached 99% in the Mg2+∶SiO2 group, compared to 75% in the Ca2+∶SiO2 group. Mg2+ showed a stronger correlation (R2=0.95) with SiO2 removal than Ca2+ (R2=0.61355). Following the coprecipitation process, the supernatant was transferred to the UF/RO for desalination. The pilot-scale study results indicate that up to 92% of SiO2 in CTBD could be removed. When the volume ratio of FGD wastewater to CTBD was 1∶5 (pH=10.5), the SiO2 concentration was reduced to below 25 mg/L. The UF/RO treatment results show that the conductivity, alkalinity, and Mg2+ concentrations were reduced to 179 μS/cm, 15 mg as CaCO3/L, and not detectable, respectively. The regenerated water is suitable for reuse as makeup water for CT.