Top Subjects of Interest: Desalination for Public Water Systems

Texas’ population is expected to reach almost 46 million by 2060, and because of the recent long-lasting drought, 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.

Desalination at Public Water Systems

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) membranes
• electrodialysis (ED)/electrodialysis reversal (EDR) membranes
• distillation

Reverse Osmosis (RO) Membranes

In Texas, RO 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 (125 psi to 300 psi range) RO membranes. However, for higher concentrations of total dissolved solids (TDS) and other dissolved salts, medium pressure (350 psi to 600 psi range) RO membranes are sometimes required. The pressure reference is the required pressure on the feed water side of the RO membranes.

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. Interfering chemical constituents can require more frequent chemical cleaning, and may result in premature fouling and replacement, of the membranes.

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 (800 psi to 1200 psi range) RO membranes. 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.

Rules:

Desalination continues to gain attention as communities seek to treat brackish water. For this reason, the TCEQ streamlined the desalination approval process for public water systems in 2015. The revised rules for using either reverse osmosis or nanofiltration membranes for groundwater sources do not require a public water system to submit an exception request for these treatments, but instead will follow the plan review process.

Electrodialysis and distillation are considered innovative technology, and are reviewed as exceptions to the rules [30 TAC 290.42(g) 30 TAC Chapter 290, Subchapter D

Disposal of By-products

The TCEQ requires proper disposal of the concentrated brine by-products of the desalination process. Sometimes these by-products are disposed of in underground injection wells under waste rules pertaining to class I wells in 30 TAC Chapter 331, Subchapter D

The brine by-products may also be authorized to be discharged to waters in the state under a Texas Pollutant Discharge Elimination System permit and as outlined in the Procedures to Implement Texas Surface Water Quality Standards.