Desalination an alternative to overcome chronic water scarcity in the region

California has faced droughts the last three years and there are many possibilities we could continue facing some more in the future. Combined with the potential impacts of climate change and a growing population, it makes sense then to balance our water supply with a source of water that doesn’t rely on rain.
Water desalination is the process of taking salt and other minerals out of seawater to make freshwater. Municipalities, water districts, and private companies in California and other parts of the U.S. are primarily considering using reverse osmosis (RO) technology to develop new seawater desalination.
Reverse osmosis and other membrane technologies force liquid at high pressure through a membrane with pores that block the passage of larger salt and mineral molecules.
The membranes used formed a dense barrier layer designed to allow only water to pass while preventing the passage of salt ions. Reverse osmosis is the final category of membrane filtration “Hyperfiltration” capable of removing particles larger than 0.1 nanometers.
Despite the promise of desalination technology to help rid the world of water scarcity, significant challenges exist including high energy consumption making it an expensive option, impact in marine life at water intakes and outfalls, operational issues, and social and political considerations.
*Water Intake
One of the main environmental concerns associated with seawater desalination is the feedwater intake which may affect the biota of marine life surrounding the system. Impingement and entrainment of fish and other aquatic life may occur when aquatic organisms are trapped against intake screens by the velocity and force of flowing water.
Seawater wells are a viable option to minimize ecological impacts and provide more reliable intake water due to natural filtration suspended solids, organics, reduce turbidity, and lower salt density but intakes depends on the hydrogeology and substrates characteristics associated with the subsurface system and may not be practical for large desalination plants.
Determining the appropriate location and type of intake should include a thorough site assessment and evaluation that will help implementing mitigating measures and reduce environmental impact.
*Energy use
Desalination plants are expensive to construct and operate. The process requires that high pressure be exerted on the high concentration side of the membrane, usually 600-1000 (psi) for seawater, requiring a substantial amount of energy and making the technology more costly than other treatment processes. For example, State Water Project requires an intensive 6.75 kwh per1000gal to transport water more than 3,000 vertical ft from the Delta (Sacramento – San Joaquin River). A typical reverse osmosis plant uses 22kwh per 1000 gal.
Another concern about the high rate of energy which desalination consumes is that plants could lead to increased carbon emissions and contribute to global climate, depending on the energy source used to operate. Renewable energy, such as wind and solar, provides an opportunity for desalination plants to be carbon neutral by reducing fossil fuel use and associated greenhouse gas emissions.
Regardless of the high energy costs, California governments are under pressure to look for new water sources including desalination. The state’s long-standing water supply problems have continued to worsen in recent years due to drought, contamination of groundwater, increasing requirements to maintain water instream for endangered species protection, and population growth.
*Brine (Concentrate) disposal
Among the more challenging issues with respect to desalination processes is disposing of the waste concentrates. Desalination concentrate have higher salinity than seawater, and is denser sinking to the seabed and negatively impacting the environment surrounding the outfall. Several considerations may help facilities develop environmental disposal such as co-discharge with other wastewater effluent that is currently discharging to the ocean; locate concentrate outfalls in a tidal zone; and/or add diffusers to improve mixing.

Regulatory and permitting process
The most significant hurdles to implement desalination technologies come from the complexity of regulations, and local/state/federal agencies limited permitting experience. Agencies involve in the permitting process are the California Regional Water Quality Control Board, the City, the California Department of Health Services, the California Coastal Commission, the State Lands Commission, and EPA.
Multiple agency involvement may contribute to unproductive project time and costs.

California has over a dozen desalination plants and plans for at least four new ones. The Carlsbad Plant is scheduled to begin operating in 2012 with a capacity of 50 MGD ($300M); Huntington Beach Desalination Plant starts this year with a capacity of 50 MGD ($250M); Camp Pendleton with a capacity of 100 MGD ($1.9 Billion); and the Marin Municipal Water District approved construction of what would be the first desalination plant in the San Francisco Bay area, is expected to open in 2014 ($105M).