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Desalination for Public Water Systems

Public water systems are using desalination to treat brackish water with reverse osmosis membranes. TCEQ has rules for these processes and for disposing of treatment waste by-products.

Texas’ population is expected to reach almost 46 million by 2060, and because of more frequent droughts, Texans have had to plan far in advance to sustain communities, businesses, industries, and the environment. Because of these challenges—especially the drought—public water systems have begun to turn to less conventional sources of water.

Types of Desalination Treatment

Public water systems can convert brackish water and seawater to safe and aesthetically pleasing drinking water using desalination treatment processes that include:

  • reverse osmosis (RO) and Nanofiltration (NF) membranes
  • electrodialysis (ED)/electrodialysis reversal (EDR) membranes
  • distillation

Reverse Osmosis (RO) and Nanofiltration (NF) Membranes

In Texas, RO and NF membranes are the most common form of treatment for desalination of brackish water from either a groundwater or surface water source. Brackish water can be treated using low pressure RO or NF membranes—125 psi to 300 psi range. However, for higher concentrations of total dissolved solids (TDS) and other dissolved salts, medium pressure RO membranes—350 psi to 600 psi range—are sometimes required. The pressure reference is the required pressure on the feed water side of the membranes.

Electrodialysis Membranes and Distillation (h3)

Electrodialysis and distillation are considered innovative technology, and are reviewed as exceptions to the rules for public water systems in Title 30, Texas Administrative Code [See Subsection 290.42(g)] 

Water Sources

Brackish Groundwater

Groundwater with a TDS concentration at or above 1,000 milligrams per liter (mg/L) can be considered brackish. Typical brackish groundwater sources have TDS concentrations of 3,000 mg/L and greater. Groundwater sources typically produce stable water qualities and do not exhibit variations that can degrade the treated water quality or quantity during different times of the year.

Groundwater sources are more likely than surface water sources to have levels of chemical constituents that must be removed, because they interfere with the performance of the RO membranes. Those interfering chemical constituents may result in premature fouling—and can require more frequent chemical cleaning and membrane replacement.

Brackish Surface Water

Surface water sources are subject to significant fluctuations in TDS concentrations not seen in groundwater. Pilot studies conducted over all four seasons (one year) can provide very useful information on the seasonal raw water quality variations that will be experienced under full-scale operation. Surface water sources have higher turbidity and silt density index levels that must be reduced in pretreatment.

Seawater

TDS levels in seawater can be in the 30,000 mg/L range and higher. Treating seawater requires the use of high-pressure RO membranes—800 psi to 1200 psi range. The higher the pressure the RO membranes operate at, the higher the energy cost and cost per 1,000 gallons of water.

Besides the higher TDS concentrations, seawater is subject to some of the same variations in raw water quality as brackish surface water sources. Due to these variations in water quality TCEQ requires detailed reviews before approving a seawater desalination water treatment plant. For regulatory guidance on the approval process see Seawater Desalination for Public Water Systems (RG-640).

Public Water System Approval

Groundwater Desalination

Approval for groundwater desalination for RO and NF is a two-step process.

  • A Texas licensed professional engineer must submit an engineering report and sealed plans.
  • After the membrane treatment system is constructed, the second step is to submit documentation that the treatment system meets required drinking water standards.

 Seawater Desalination

For regulatory guidance on this approval process see Seawater Desalination for Public Water Systems (RG-640).

Disposal of By-Products

Concentrated brine by-products generated during brackish or seawater desalination must be disposed of properly. Regulations for discharging or disposing of these by-products may come under rules for: