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
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.
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
4. knowledge of whether it is likely to occur with other membrane
5. an awareness of regulatory and technical options to deal
with its occurrence, so that appropriate regulatory requirements
and policies could be achieved.
& 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.
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.
The project tasks:
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.
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
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.
that deficit potassium is a likely contributor to toxicity in
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
a procedure to predict the potential for concentrate major ion
toxicity from analysis of the raw water quality.
protocols for determining presence and specific causes of major
ion toxicity in membrane concentrates.
the major ion toxicity protocol into a broad protocol for determining
the cause of toxicity in membrane concentrates.
single ion LC50 values for several major ions at different TDS
and confirmed the likelihood of major ion toxicity occurring
at other locations in the United States (besides Florida).
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
that major ion toxicity in membrane concentrate is not caused
by the membrane process itself, but results from the nature
of the groundwater.
and evaluated regulatory and technical approaches to facilitate
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