Self-sealing Evaporation Ponds

Jean-Philippe Nicot, principal investigator

Because the Texas population is expected to grow tremendously in the coming decades, municipalities and other water-supplying entities will have to supplement or replace their current fresh-water sources. Desalination of brackish water is part of the mix and could become the sole source of water for some West Texas cities. However, concentrate disposal could impede further consideration of desalination because of legal, technical, and cost challenges. An option for small facilities (<1 MGD—million gallons a day) is to use evaporation ponds. Evaporation ponds legally require at least a geomembrane or clay liner on the bottom and on the sides, which can be prohibitively expensive for small communities, especially if a leak occurs. A solution that may allow less reliance on conventional liners and that would limit leaks is to use self-sealing evaporation ponds. Water chemical composition of those ponds is such that the precipitating material creates a low-permeability layer that would heal itself if punctured. This project will examine the possibility of incorporating this low-permeability layer (precipitant) into the pond liner system as a liner component (e.g., the low-permeability layer will be used as a liner in a double-liner system or as a component in a composite liner) or possibly as the liner itself.

By design, material accumulates at the bottom of the evaporation pond. The sludge consists primarily of less-soluble salts, such as calcite and gypsum, and its chemical composition is a function of that of feed water. The first objective of the work, supported by the Texas Water Development Board, is to understand the chemical evolution of desalination concentrates as they become more concentrated by evaporation. The second objective is to understand flow characteristics of the sludge/precipitates. We propose to look at selected concentrate compositions, examine whether they can generate material with self-sealing properties and, if not, identify what additives or additional material would have to be provided for it to acquire those properties. We will also look at the ensuing legal issues at the Federaland State levels.

Executive Summary

The State of Texas has taken a renewed interest in desalination of brackish water. Because the Texas population is expected to grow tremendously in coming decades, many municipalities and other water-supplying entities will need to supplement their current fresh-water sources. Desalination of brackish water is high on the list of water-source alternatives for supplying some or all of the increased water needs in many communities. However, disposal of desalination concentrates may pose legal, technical, and economic barriers, especially for smaller communities with water supplies of less than one million gallons per day (MGD). In this report, we examine evaporation ponds and the possibility of incorporating a low-permeability layer (precipitant) into the pond-liner system as a liner component or possibly as the liner itself. One aspect of this analysis was to investigate the regulatory requirements and barriers of using self-sealing ponds, if this strategy proves to be a technically viable alternative to standard pond liners. Another part of the work consisted of understanding the favorable chemical conditions, natural or induced, for the precipitation of such a compound(s). The third and last facet of this work was to investigate the savings or extra costs of this approach. 

The following observations characterize the regulatory issues relating to self-sealing pond liners. (1) No significant regulatory barriers currently exist that would prevent the permitting of self-sealing evaporation pond-liner technologies at desalination facilities in Texas. (2) No Federal authorizations are required, but a Texas Land Application Permit (TLAP) must be obtained from the Texas Commission on Environmental Quality (TCEQ) Water Quality Division. (3) TCEQ has considerable latitude for approving alternative permit requirements for industrial permits. Rules for municipal wastewater treatment are used as guides for the evaluation of industrial evaporation ponds but do not impose strict regulatory requirements. Currently approved pond liners include a 3-foot-thick layer of in situ clay or compacted clay (with a maximum hydraulic conductivity of 10-7 cm/s) or a geomembrane liner (polyvinyl chloride [PVC], high-density polyethylene [HDPE], butyl rubber, polypropylene, etc) of 30 mils (0.76 mm) or more with leak-detection monitoring. An alternative liner technology may be approved by TCEQ if it can be demonstrated to achieve and maintain equivalent containment capabilities to the pre-approved liners and that the resulting liner material(s) will not deteriorate because of reactivity with salinity or other compounds in the effluent stream or other ambient conditions. Supporting demonstration information may include previous research, pilot projects, and monitoring data from existing operational facilities currently utilizing the proposed technology. Regulatory processing for the permitting of an evaporation pond could be simplified if the self-sealing technology were recognized by the TCEQ as an accepted type of liner, equivalent to compacted clay or geomembrane liners. No statutory change or rulemaking would be required to revise the permit instructions to add self-sealing pond liners to the list of acceptable methods, although compelling scientific and engineering evidence would be necessary to justify such a modification.

The technical part of this study started with the assessment of previous laboratory experiments and natural analogs, such as saline lakes. The assessment suggests that precipitation of a specific clay mineral called sepiolite, which is composed of mostly magnesium oxide and silica, could have many advantages. It has the flow properties of clay minerals but is not expandable when exposed to water or to a change in salinity or aqueous ionic makeup. Despite the absence of sepiolite in the few samples collected from Texas evaporation ponds, geochemical numerical simulations performed using U.S. Geological Survey (USGS) code PHREEQC suggests that sepiolite does precipitate in evaporation ponds but in small quantities. Calcite and gypsum are the two minerals that precipitate in significant amounts. After 5 years of operation, an average precipitate thickness is approximately 0.15 inch, containing about 15% sepiolite, with large geographic variability. Addition of low-cost sepiolite precursors to the concentrate stream has been shown to increase the amount of sepiolite precipitated (with a pH maintained at 8.5), to an average of 0.38 inch after 5 years of operation, with a sepiolite fraction of approximately 60%. Water-chemical composition inputs to the numerical simulations were derived from databases containing information on thousands of brackish groundwater samples at the Texas Water Development Board (TWDB) and surface-water samples from TCEQ. They were categorized into 20 groups corresponding to aquifers or group of aquifers, except for one group comprised entirely of all surface-water samples. Although the range and variety of water composition are large, results are essentially qualitatively similar for all groups: without precursors, mostly calcite, gypsum, and some minor minerals precipitate, whereas with addition of precursors, a significant amount of sepiolite can precipitate. This study did not perform laboratory experiments on the precipitates and, therefore, does not present independent hydraulic conductivity analyses. However, other investigators have measured hydraulic conductivity of a variety of precipitants in laboratory experiments. They have observed that measured conductivity values of the precipitant are still too high and above the threshold value of 10-7 cm/s. Overall, while developing self-sealing ponds is not technically challenging, doing so at a lower cost than that of present simple technology may be difficult. 

The regulation section of this study showed that the practical way to make use of self-sealing properties is to exercise an option to demonstrate that the alternative liner will achieve equivalent containment. Even in the case of pre-approved liners, self-sealing deposition could be advantageous in settings where an additional defense-in-depth layer is needed, such as areas with an underlying unconfined aquifer sensitive to contamination. Cost of a 5-acre evaporation pond, following prescriptive rules for municipal wastewater is in the $250,000-350,000 range, not accounting for land purchase.

Substantial savings can be achieved if waivers are granted. Savings can be as high as 90% if the pond consists of little more than an excavation into the ground. Large savings may also be achieved if the leak-detection system is not required for geomembrane liners. Our analysis suggests that the precipitant, even with a hydraulic conductivity >1×10-7 cm/s could efficiently plug holes and defects of the geomembrane. This analysis also suggests that, because defects and holes can be plugged with no operator intervention, a thinner geomembrane could be used, in combination or not with the leak detection system waiver. This possibility, however, needs to be confirmed by experiments and pilot tests.

On the other hand, equivalent containment can be achieved for clay liners—common in Texas— mostly by sheer accumulation of the precipitated material at the bottom of the pond. The modest thickness of at most a few inches of precipitant after a few years of operation suggests that the precipitated material needs to have a hydraulic conductivity much lower than 1×10-7 cm/s to impart the required properties to a scaled-down liner and to be successfully substituted in part or all of the clay liner. As in the case of geomembranes, precipitant may plug small cracks that could appear throughout the life of the pond, reducing the cost of operator intervention.

 

Presentations and Reports

Self-Sealing Evaporation Ponds for Desalination Facilities in Texas
Final report, [PDF, 10 MB]

Self-Sealing Evaporation Ponds for Desalination Facilities in Texas
Presentation to the Texas Water Development Board, May 15, 2007 [PDF, 4.7 MB]

An [Attractive] Option for Desalination Concentrate Disposal: Self-Sealing Evaporation Ponds in Texas
2007 Ground Water Summit National Ground Water Association, Albuquerque, NM, May 1, 2007 [PDF, 3.6 MB]

 

For more information, please contact Jean-Philippe Nicot, principal investigator.
Telephone 512-471-6246;
e-mail jp.nicot@beg.utexas.edu.

July 2007