Facts about South Central Texas desalination
• An acre-foot is a volume of one acre (660 by 66 feet) one foot deep, or about 325,853.4 gallons.
• Most domestic water planning is done in acre-feet and acre-feet per year.
• By 2060, the South Central Texas Region (Region L) water plan indicates we will need 436,751 acre-feet per year of additional water to meet forecast demand for our region.
• Desalination membrane pores are really tiny, 1 to 10 angstroms across. An angstrom is one ten-billionth of a meter. For comparison, a water molecule which is not symmetrical, is 3.2 angstroms at its widest.
Earlier columns discussed drought and options for conserving and collecting water for non-drinking use. Texas' Water Development Board, Regional Water Planners and forward-thinking area governments have developed plans for filling the projected-increasing gap between available potable or drinkable water and forecast demand. The gap for our area is huge, more than 400,000 acre-feet of water per year by 2060.
Filling 29 percent of that gap uses two types of non-traditional water supplies. Reverse osmosis desalination plants to process brackish and seawater would be built. Three brackish water plants (combined capacity 42,220 acre-feet of water per year) and a seawater plant (capacity 84,012 acre-feet of water per year) would provide 16.5 percent of the region's water supply. Brackish water would come from the Wilcox aquifer underlying Bexar, Atascosa and Wilson counties, and the Edwards aquifer in southern Bexar county.
What are economics of desalination?
Texas has no seawater desalination plants. We currently have 12 municipal desalination plants (the largest in El Paso) with a total 134,400 acre-feet of water per year capacity - 58 percent of that from brackish groundwater and 42 percent from brackish surface water.
What does producing that water cost today?
According to Texas' Water Development Board, current costs for untreated drinking water range from $357 to $782 per acre-feet of water per year. Planned brackish-water facilities will produce water-treatment-plant-ready water for $1,245 to $1,883 per acre-feet of water per year. The seawater facility cost would be $2,284 per acre-feet of water per year when the plant is finally built.
For comparison, output from a million-gallon-per-day seawater plant planned in the Laguna Madre Water District (bond election approved 2011) will cost $800 to $1,400 per acre-feet of water per year when it opens.
Why is desalination expensive?
Reverse osmosis has been around a while. First demonstrated in 1867 by Moritz Traube and commercially developed starting in the 1960s, costs have dropped - but it's still expensive compared to treating well or river water if it were available. Why? First, reverse osmosis is very energy-intensive and, unfortunately, carbon-intensive if we use hydrocarbons to produce electricity to do reverse osmosis.
Powerful high-pressure electric pumps are required to force water molecules - but not larger salt molecules - through a special membrane with very tiny pores. Second, energy required is directly related to the salinity of the raw water. Desalination of seawater with salt concentrations more than 35 parts per thousand is a lot more expensive than processing lower-concentration subsurface brine.
Third, reverse osmosis membrane is easily damaged by impurities in raw brine. Silt or organics in the water - such as bacteria, algae, plankton, proteins from plants and animals in the water, hydrocarbons - all degrade or stop up the membrane. To prevent this, extensive and expensive pretreatment of raw-water is required. In addition, if chlorine is used as a biocide or sterilizer, it must be removed after pre-treating because chlorine destroys all commercially-available membranes.
Finally, the process also produces very-high-salt brine that must be disposed of in ecologically-safe manner. It can be evaporated to produce crystal salt; be injected into the ground; or, as in the case of seawater, returned to open oceans - but not estuaries because high salinities will damage our sensitive bay ecologies, destroying fish, crab and shrimp nurseries; and aquatic plants that thrive in brackish bay water. Each disposal choice costs money and can be energy-intensive as well.
Is desalination worth it?
Ask someone without water how much they would pay for a gallon. If Texas's choice is no water to support growth, agriculture, industry and tourism, the issue may not be cost but can we get necessary supplies through non-traditional sources like desalination? The Texas Legislature is grappling with that issue right now.
Sources: Desalination and Membrane Technologies: Federal Research and Adoption Issues, Nichole Carter, Congressional Research Service, Jan. 8, 2013;
Cost Guidance for Seawater Desalination Facilities in Texas, Jonathan Dietrich P. E. and Christophe Robert, on the web at twdb.state.tx.us;
Fundamentals of Membranes for Water Treatment, Alyson Sagle and Benny Freeman, on the web at twdb.state.tx.us;
A Survey of Subsurface Saline Water of Texas, Texas Water Development Board Report 157, Volume 1, Oct. 1972, on the web at twdb.state.tx.us;
2011 South Central Texas Regional Water Plan, Texas Water Development Board;
Water or "Water Treatment Estimation Routine," U.S. Bureau of Reclamation (USBR) "Water Treatment Engineering and Research Group," on the web at usbr.gov/pmts/water/awtr.html.
Paul and Mary Meredith are master naturalists. Contact them at firstname.lastname@example.org.