(176b) Optimizing Coagulant Selection and Dosing Strategy for Enhanced RO Feedwater Pretreatment

The longevity and efficiency of reverse osmosis (RO) membranes depend on effective RO feedwater pretreatment to avert membrane fouling. Pretreatment via microfiltration (MF) and ultrafiltration (UF) are typically utilized prior to the RO process, and the performance of these filtration steps in removing fine suspended solids can be improved by various coagulation strategies prior to filtration. It is critical to select the most effective coagulant and optimize the coagulation dose (whether coagulation is conducted in coagulation tanks or inline) and dosing strategy to enable effective removal of total suspended solids (TSS) and achieve low treated water turbidity. Accordingly, an approach was developed for coagulant selection and dose optimization that includes sequential co-dosing of multiple coagulants. Various coagulants were evaluated, including ferric chloride (FeCl₃), aluminum chloride (AlCl₃), and polymeric coagulants including polyacrylamide (PAM). The study included various water sources including, but not limited to, brackish groundwater (turbidity ~2 NTU), and secondary treated municipal wastewater (turbidity ~4 NTU). The study demonstrated that effective coagulation and removal of TSS alleviates the burden on downstream filtration while leading to high quality pretreated RO feedwater. One of the important findings is that sequential dosing of FeCl₃ followed by PAM resulted in the production of large and stable flocs of low supernatant turbidity. The above coagulation strategy significantly reduced the feedwater TSS, and filtrate turbidity compared to other conventional approaches. The highest TSS removal was achieved with the above coagulant dosing strategy which was shown to lessen the burden on prefiltration and leads to treated water quality that meets the required quality for RO feedwater.