Full-Scale Comparison of Single-Pass Reverse Osmosis to Dual-Pass Nanofiltration for Seawater Desalination

摘要

The Long Beach Water Department (LBWD), similar to many Southern California utilities, has been facing the issue of decreased potable water supplies. Because of its location, the City of Long Beach is ideally situated to take advantage of desalinated seawater as a source of drinking water and is interested in incorporating desalinated seawater into its potable water portfolio. However, before seawater desalination can become a reality, additional advances are needed to further reduce the cost of desalination. LBWD is currently exploring feasibility of using nanofiltration membranes (NF) in a dual-pass configuration to evaluate the possibility of achieving additional cost reduction as well as meeting all water quality objectives.rnNF membrane was primarily developed as a membrane-softening process, offering an alternative to chemical softening (1). NF is also effective in the removal of disinfection byproduct (DBP) precursors (2-4) and other synthetic organic chemicals (SOCs) (5). There are currently several full-scale NF plants worldwide that target the removal of hardness and/or DBP precursor removal from surface or groundwater supplies.rnIn the late 1990s through early 2000s, the Saline Water Conversion Corporation in Saudi Arabia (SWCC, Saudi Arabia) evaluated the effectiveness of NF as a pretreatment to seawater reverse osmosis (SWRO) membranes (6-9). The main objectives for the NF pretreatment of SWRO feed were to:rna) minimize particulate and microbial fouling of the SWRO membranes by removal of turbidity and bacteriarnb) prevent scaling by removal of the hardness ions, andrnc) lower the operational pressure of SWRO process by reducing the feedwater's total dissolved solids (TDS) concentrationrnSeveral publications presented by SWCC addressed various issues related to the application of NF pretreatment for SWRO. Early work was performed at a pilot facility (6-8) in which Persian Gulf seawater (TDS = 44,000 mg/L) was first pretreated through multimedia filters, cartridge filter, and NF membranes. The NF permeate was subsequently treated through the SWRO process, which produced potable water. The concentrate was further processed through a multistage flash (MSF) distillation unit. Various unspecified NF and SWRO membrane types were evaluated in the study.rnSubsequently, SWCC converted one of two SWRO parallel trains (Umm Lujj SWRO plant, Saudi Arabia) from a single SWRO into a NF-SWRO train to evaluate the NF pretreatment (9). The second identical train was maintained as