(495f) High-Efficiency Seawater Desalination Via Nf/Ro Multi-Pass Arrays

In recent years, reverse osmosis (RO) seawater desalination technology has undergone a remarkable transformation. The number and capacity of large RO plants have increased significantly. In a parallel shift, the capital and operating costs have decreased such that estimates of total desalted seawater cost in the U.S. ranges from $600 to $1,200 per acre-foot (af). In California, the Metropolitan Water District of Southern California has initiated a program to subsidize member agencies up to $250 /af for potable water produced from seawater. Nonetheless, production of potable water via seawater desalination remains 2 to 3 times the cost of importing water from Northern California or the Colorado River and treating local brackish and reuse waters (~$200 to $400 /af). Given the diminishing energy savings from increased membrane permeability and the high efficiency of pumping and energy recovery in seawater RO, further cost reductions to seawater desalination must involve increased product water recovery, decreased operating pressure, and decreased RO membrane fouling.

In the meanwhile, potential environmental impact remains another major limitation of seawater desalination and is likely the principle reason for whether or not a permit to build will be granted. Environmental issues include feed water intake, energy consumption/fossil fuel combustion, and concentrate disposal. Energy consumptions and potential global warming impact is directly related to operating pressure of seawater RO processes. Environmentalists and regulators are also concerned about the potential impact of disposed concentrate (residual) on the local marine environment. Residual from an RO plant is high in salinity as well as chemicals used in pretreatment processes (acid, caustic, polymers, etc.); therefore, it is important to reduce the volume and (potential) environmental burden of RO concentrate disposed directly into the ocean. Additional concerns about impingement and entrainment of sea creatures in seawater intake structures is another environmental concern. Use of beach wells as intake source has been popular due to the difficulty of getting a water intake source permit. Beach wells are also being considered as potential replacement for traditional seawater RO pretreatment (e.g., granular or membrane filtration), but the efficacy of beach well extraction on fouling reduction is not clear. It is critical to remove insoluble microbial, colloidal, organic, and mineral matter before the feed water enters the RO membrane.

The modern approach to seawater desalination by RO membrane technology is the use of integrated membrane systems consisting of feed water pre-treatment processes (beach wells, media or membrane filtration, pH adjustment), a one-pass RO stage, and product water post-treatment (stabilization, boron removal, disinfection). In such a system, the energy requirements to drive the single stage RO process comprise ~40 percent of the overall cost of produced water. The RO product water recovery cannot be driven beyond about 50 to 60 percent because increasing retentate osmotic pressure at high recovery produces a diminishing economic benefit. In addition, the higher retentate concentration increases salt passage, surface fouling, and residual concentration. Novel approaches to reduce the energy required in seawater desalination include the use of seawater RO membranes with different permeability (to balance flux and pressure through the system) or the use of multi-stage NF/RO integrated membrane systems (e.g., the ?Long Beach method?).

We hypothesize that reducing TDS, organic, and mineral concentrations of seawater through NF pre-treatment would allow use of low pressure RO membranes at higher flux (reduced ?footprint? and capital cost), lower operating pressure (reduced energy cost), and higher water recovery (more product water), thus, reducing the overall cost of water produced. In addition, with reduced scaling concerns the RO process could be operated at high pH, which would enable high rejection of deprotonated borate ions. Selective removal of minerals in the NF pre-treatment stage further allows utilization of efficient brackish water concentrate treatment processes such as accelerated chemical precipitation, as well as the option to redirect various concentrate and permeate flows to reduce pressures, enhance recovery, and minimize concentrate.

In this study, we explore the combination of true nanofiltration (NF), brackish water RO (BWRO), low pressure RO (LPRO), and seawater RO (SWRO) membranes to more efficiently and effectively produce potable quality water from seawater. A rigorous numerical model is used to predict and optimize product water quality, overall waer recovery, and specific energy consumption. Based on predictions by the model, bench scale experiments are being conducted to verify the results. Synthetic seawater solutions (with and without model foulants) are desalted in a bench scale cross flow membrane filtration system using commercial NF, BWRO, LPRO, and SWRO membranes. Feed and permeate concentrations of major seawater ions were determined by using a combination of ion-selective electrodes and ICP-MS analyses. Membrane surfaces are imaged by SEM and EDS analysis to confirm elemental composition of eventual fouling/scaling layers formed and the limiting scaling salts. Results of bench scale studies will be presented with further analysis of the technical and economic feasibility of high-efficiency seawater desalination via NF/RO multi-pass arrays.