|Fig. 1: A multi-stage flash desalination Plant. (Source: Wikimedia Commons)|
Recently, a water desalination project has been proposed in the Monterey Bay area. The "Deep Water Desal" project was proposed to be built in Moss Landing, and the project aims to draw sea water from the Monterey Canyon, which is about 1,000 feet off-shore and has a depth of 130 feet. A key number to consider here is that the goal and projected outcome of the project is to produce 13,400 acre-feet of water per year. The project has been estimated to cost about $230 million dollars. 
The multistage "flash" evaporation (MSF) technique is the ideal method to use when a desalination plant is near a power plant. A multi-stage flash desalination Plant is pictured in Fig. 1.
It is important to note again that the proposed Moss Landing Desalination plant will be located next to the moss landing power plant, making it cheaper to transfer energy from the power plant to the desalination plant. MSF distillation requires a lot of energy, and utilizes both thermal and electrical energy. Thermal energy in this system is in the form of low-pressure bleed steam (1 to 3 bars) for the feed-brine heating, and medium-pressure steam for the ejectors to generate the required vacuum in different sections of the unit.  The electrical energy is used in the units various pumps such as recycle, cooling water, distillate product, brine blow down, condensate, and chemical dosing pumps.  MSF units range from 10,000 to 35,000 m3/day and consist of a series of stages, ranging from 4 to 40 each, with successively lower temperature and pressure that cause flash evaporation of the hot.  Also, it is about $0.15 kWh for electricity in California. 
We can define freshwater for human consumption as based on the World Health Organization's guidelines for the level of Total Dissolved Solids (TDS) that should be allowed in potable drinking water as no more than 1,000 parts per million (ppm).  For agricultural use the level of TDS needs to be much lower at ~50 ppm. 
From these statistics above, we can assume that the cost to desalinate water, especially to a level that can be used for drinking water and agriculture would be very high. It seems that as of right now, since there is not a cheaper option, and the cost just to build the plant in the first place is so high, it may be best to avoid building the plant altogether until a cheaper or more efficient method is created.
© Jack Richardson. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 J. Johnson, "Moss Landing Desal Proposals Expect Environmental Certification This Year," Monterey County Herald, 6 Apr 16.
 A. Al-Karraghouli and L. L. Kazmerski, "Energy Consumption and Water Production Cost of Conventional and Renewable-Energy-Powered Desalination Processes," Renew. Sust. Energy Rev. 24, 343 (2013).
 J. Jiang, "The Price Of Electricity In Your State," National Public Radio, 28 Oct 11.
 "Guidelines for Drinking-Water Quality, 3rd Ed." Volume 1: Recommendation, World Health Organization (2008).
 L. Summers, "Desalination Processes and Performance," Lawrence Livermore National Laboratory, UCRL-ID-120367, June 1995.