Recently Completed AWWA Research Foundation Project

A report entitled "Major Ion Toxicity in Membrane Concentrate" has been recently published by American Water Works Association Research foundation (AWWARF) as a result of a four-year research project funded by AWWARF. The Florida Department of Environmental Protection (FDEP) and the Southwest Florida Water Management District (SWFWMD) and South Florida Water Management District (SFWMD) provided supplemental funding.

BACKGROUND: When the project began in late 1995, a handful of Florida membrane drinking water plants were not able to renew their concentrate disposal permit due to the failure of whole effluent toxicity (WET) tests. The cause of the test failures was not known; however, ion-related toxicity was suspected. The situation affected existing membrane plants having failed WET tests in terms of cost and public perception, and not knowing whether they would be permitted beyond their temporary operating permits. The situation also affected existing plants not yet having gone through a full WET test process, in terms of not knowing what fate might lie ahead for them. The situation also affected utilities considering new membrane plants, in terms of the feasibility of such plants. Until the issues surrounding the failed toxicity test were resolved, more and more plants might fail their WET tests, and some new plants might not be able to obtain their permits. The unsettled nature of these issues represented increased risks to utilities considering membrane processes, such that this otherwise attractive processing option might not be considered. Thus while membrane drinking water processes were experiencing high growth rates, the increasing concern surrounding concentrate toxicity, unless addressed, threatened to significantly dampen this growth rate.

RESEARCH OBJECTIVES AND TECHNICAL APPROACH: The objectives of the project were to develop:

1. an understanding of the nature of major ion toxicity,
2. protocol for determining its presence and cause(s),
3. models to predict when it is likely to occur given water quality analyses,
4. knowledge of whether it is likely to occur with other membrane processes,
5. an awareness of regulatory and technical options to deal with its occurrence, so that appropriate regulatory requirements and policies could be achieved.

Mickley & Associates undertook several well-defined laboratory and non-laboratory tasks to achieve these objectives. Laboratory work consisted of conducting many WET tests on actual concentrate, mocks of concentrate, and on other test solutions. Water quality analyses were done to characterize the solutions used. Many manipulations and modifications of concentrate and mock solutions were done in the process of clarifying the nature of, determining the cause of, and developing test protocols for determination of concentrate toxicity.

Non-laboratory work included analysis of historical data to determine patterns in feedwater and concentrate data and toxicity occurrence and analyzing all laboratory results. In addition, groundwater data were examined to determine the likelihood of major ion toxicity in states other than Florida. An approach was developed for predicting major ion toxicity occurrence and technical and regulatory options were reviewed for dealing with major ion toxicity. Finally, physiological models were reviewed that might offer some explanation as to the nature of the toxicity.

CONCLUSIONS: The project tasks:

  • Confirmed the role of major ion toxicity in nine out of nine membrane concentrates studied, not just as one cause of toxicity but also as the cause of most if not all of the toxicity occurring.
  • Determined that excessive calcium levels are the major and possibility only contributor to concentrate toxicity in several of the concentrates, and likely contribute to toxicity in the remaining nine concentrates studied.
  • Determined that excessive fluoride levels are the major and possibly only contributor to toxicity in two of the nine concentrates and likely contribute to toxicity in two other cases.
  • Determined that deficit potassium is a likely contributor to toxicity in one case.
  • Characterized the general nature of major ion toxicity with respect to threshold limits, dilution requirements for non-toxicity, single ion versus multi-ion contributions, and the effect of test TDS levels on toxicity.
  • Developed a procedure to predict the potential for concentrate major ion toxicity from analysis of the raw water quality.
  • Developed protocols for determining presence and specific causes of major ion toxicity in membrane concentrates.
  • Incorporated the major ion toxicity protocol into a broad protocol for determining the cause of toxicity in membrane concentrates.
  • Developed single ion LC50 values for several major ions at different TDS levels.
  • Addressed and confirmed the likelihood of major ion toxicity occurring at other locations in the United States (besides Florida).
  • Determined that the possible occurrence of major ion toxicity is for all practical purposes restricted to groundwater sources and brackish water reverse osmosis, membrane softening, and electrodialysis/electrodialysis reversal processes.
  • Determined that major ion toxicity in membrane concentrate is not caused by the membrane process itself, but results from the nature of the groundwater.
  • Reviewed and evaluated regulatory and technical approaches to facilitate concentrate permitting.

In sum, the project results provide the drinking water utilities and the regulatory community with valuable understanding and tools for addressing the testing and permitting of membrane concentrate. Specific practical applications of the results include 1) predicting the presence and cause(s) of major ion toxicity from consideration of concentrate or even groundwater analyses and 2) the use of the laboratory protocols to guide determination of the presence and cause(s) of major ion toxicity for compliance purposes. Interim reports are available. The full report will be published later in 2000.

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