Pass the Salt

By TRWA Instructor Scott Willeford

Access to fresh water has become a hot topic in the Lone Star State, especially in certain regions experiencing record growth. The influx of households relocating and resettling is forcing water suppliers in Texas to explore different options to supply adequate drinking water to support new development. In searching for options, the scarcity of fresh water can lead entities to consider desalination of seawater along the coast, and groundwater inland to supply future and existing needs of Texas residents. “As populations increase and existing surface water supplies are being tapped out, or groundwater is depleted or polluted, then the problems are acute and there are choices to be made about desalination,” said Michael Kiparsky of the Wheeler Water Institute in a 2019 report from Yale’s School of the Environment. According to the same report, in the past three decades, the cost of desalination has dropped by half.

In the 1960s, the first large-scale desalination water treatment plants were built in the Middle East. There are now some 20,000 facilities globally that turn seawater into fresh water. The Kingdom of Saudi Arabia, which has access to very little fresh water but can maintain lower pricing on fossil fuels used in their desalination plants, produces the most fresh water of any nation — a fifth of the world’s total. As of 2019, more than 300 million people now get their water from desalination plants, according to the International Desalination Association.

The process of desalination removes dissolved salts from the water, with thermal and membrane technologies being the two most common desalination technologies. The thermal process heats salt water to produce water vapor which is then condensed and collected as fresh water. Membrane processes rely on semi-permeable membranes to separate salts from water. Membrane processes can be pressure-driven (reverse osmosis is the most common method), or voltage-driven (electrodialysis and electrodialysis reversal).

Desalination of seawater has downsides, especially with waste disposal, which can impact the ocean’s ecological system through the process of disposal. A 2019 study from the UN showed waste generation during desalination was up to 50 percent higher than previous estimates. The process of desalination takes two gallons of seawater to make a gallon of fresh water, which means a gallon of leftover brine with very high salt content. This brine waste is disposed of by returning it to the ocean. If the waste is not diffused over large areas, the chemical composition of the waste can deplete affected areas in the ocean of oxygen and damage sea life. The unused byproduct of the desalination process, a highly concentrated salt water, is referred to as concentrate. In the case of seawater desalination, this byproduct is reintroduced back into the ocean where the original seawater came from, at a prescribed safe distance from the coastline to reduce the impact on marine ecosystems. The desalination plants currently operating in Texas use only brackish groundwater to create fresh water, with the largest plant – the Kay Bailey Hutchison Plant in El Paso – producing approximately 27.5 million gallons per day. A single desalination plant is currently in production in Texas, with the average cost to produce 1 acre-foot of desalinated water from seawater projected to range from approximately $800 to about $1,400 according to the Texas Water Development Board. In contrast, the average cost to produce 1 acre-foot of desalinated water from brackish groundwater ranges from approximately $357 to $782, according to the same source.

Due to the potential damage to marine life and the obvious difference in cost, recent innovations in desalination have moved toward brackish groundwater. Brackish groundwater is defined as groundwater with a total dissolved solids content of between 1,000 and 10,000 parts per million. The solids in brackish groundwater are one-tenth the amount in seawater which makes the process much cheaper. Brackish groundwater is an integral water supply source in Texas. Texas has more than 2.7 billion acre-feet of brackish groundwater in 27 of the 31 major and minor aquifers. When desalination is used inland with brackish water, the concentrate (byproduct of desalination) is injected into deep formations underground via injection wells; or, using newer technologies such as separating out the minerals from the concentrate, where the waste product can be commercialized. The concentrate or waste may be discharged to a saline water body, sanitary sewer, injection well, or evaporation pond.

Texas has more than 100 desalination plants. Most are small or intermittent type facilities, but Texas has two large facilities: the aforementioned Kay Bailey Hutchinson plant in El Paso, and the Southmost Regional Water Authority Desalination plant, which produces 7.5 million gallons a day for South Texas. TRWA members North Alamo WSC and East Rio Hondo WSC also have brackish desalination plants in South Texas. A new brackish water desalination plant is under construction in San Antonio. Phase 1 of the San Antonio Water System’s (SAWS) desalination plant was set to be complete in late 2016 and will produce 10 million gallons of fresh water daily. By the time Phase 3 is completed in 2026, the system will produce 25 million gallons of fresh water each day. I live in Northeast Texas, which boasts the luxury of an adequate water supply of both groundwater and surface water. There are currently two reservoirs, Bois’D Arc Lake (North Texas Municipal District) and Lake Ralph Hall (Upper Trinity Water District) being built in Northeast Texas to accommodate the future water needs of the Dallas-Ft. Worth Metroplex. Reservoirs themselves are constantly being innovated, but to keep up with the growth throughout the state, innovations like desalination are a crucial part of securing Texas’ water future. The development of new strategies to supply an ample and safe drinking water supply for the future has become imperative to meet the needs of a prospering Texas.