High - efficiency seawater desalination via NF / RO multi - pass arrays

In recent years, capital and operating costs of seawater desalination plants have decreased remarkably. However, production of potable water from seawater by reverse osmosis membrane desalination remains 2 to 3 times the cost of desalting brackish water, reclaiming wastewater, or importing fresh water even over long distances. In addition, rejection of boron by seawater RO membranes is generally not adequate in one pass systems without pH elevation. Further, environmental issues remain another major limitation of seawater desalination, e.g., impingement/entrainment, energy consumption, and brine discharge. This study will explore various combinations of 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. INTRODUCTION 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). These costs of treatment include capital cost, energy cost, operation and maintenance (O&M) cost, and material cost. 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