Pretreatment of reverse osmosis (RO) feedwater for removal of suspended matter, organic, inorganic and biological foulants is essential for averting and reducing the potential for RO membrane fouling. Removal of suspended solids, which is the focused of this study, is typically accomplished via microfiltration (MF) and ultrafiltration (UF) to provide RO feed water of low turbidity. Therefore, it is critical to improve filtration efficacy to enable removal of submicron size suspended material even prior to filtration. Accordingly, RO feedwater pretreatment train was developed that comprises of inline coagulation upstream of a hydrocyclone separator, followed by MF and optional UF stage. Through selection of the optimal coagulant type, sequential co-dosing of a mix of coagulants, and convective residence time for coagulation, rapid floc formation kinetics and sufficiently large floc size enabled high removal efficiency of suspended matter via hydrocyclonic separation. The pretreatment train was evaluated using brackish field water and secondary-treated municipal wastewater having total suspended solids (TSS) concentration of 31 mg/L and 5 mg/L, respectively, and corresponding turbidity range of 0.3-0.6 NTU and 3.5-4 NTU. Initial coagulant jar tests served to select the coagulant type and dose range. Hydrocyclone separation post-inline coagulation were conducted allowing for a convective residence time (for coagulation) in the range of 5-32 seconds) depending on the set feed flow rate and the length of the tubular coagulation tubing. Dosing with ferric chloride followed with polyacrylamide dosing significantly enhanced floc formation resulted in hydrocyclonic total suspended solids (TSS) mass removal exceeding 50% . Subsequent microfiltration (1 – 2 µm) of the hydrocyclone-treated water yielded filtrate turgidities well below 0.1 NTU , meeting the recommended level for RO feedwater, which then served to deploy effective pilot-scale testing of the proposed approach.
