Academic journal article Journal of Environmental Health

Heterotrophic Bacteria Control in a Residential Reverse-Osmosis Drinking-Water Filter

Academic journal article Journal of Environmental Health

Heterotrophic Bacteria Control in a Residential Reverse-Osmosis Drinking-Water Filter

Article excerpt


In 1992, a project involving drinking-water treatment for over 50 homes and a commercial office was begun in a subdivision of Suffolk, Virginia. The primary goal was to reduce the naturally occurring fluoride levels in the municipal well water source from 6 milligrams per liter (mg/L) to below 2 mg/L. The federally established maximum contaminant level (MCL) for fluoride in drinking water is 4 mg/ L, and the U.S. Environmental Protection Agency (U.S. EPA) has established a secondary MCL of 2 mg/L. The primary MCL for fluoride is set at a level at which no known adverse health effects occur or are anticipated. The secondary MCL has been set to assure good aesthetic water quality. The municipal water utility considered three options for reducing the fluoride to an acceptable level:

* a pipeline from the city central system,

* central treatment at the subdivision, and

* treatment in each home using a point-of-use filter.

After cost, waste disposal, maintenance, and timeliness factors were studied, the point-of- use filters were chosen as the preferred solution.

When this project began, point-of-use treatment was not acceptable for permanent remediation of high fluoride levels, but could be used as an interim method of treatment if the utility retained control and consistently demonstrated conformance to all required regulatory conditions (1). The local water utility for the City of Suffolk, the Virginia Department of Health, U.S. EPA, and three manufacturers of point-of-use reverse osmosis filtration products were involved (2). These groups jointly determined that the performance of the filters should be evaluated by measuring conductivity, fluoride, total dissolved solids (TDS), pH, sodium, residual chlorine, total coliform, and heterotrophic bacteria as measured by heterotrophic plate count (HPC). The municipal water supplier retained responsibility and control of the treatment systems, even though the equipment was installed, serviced, and maintained by local representatives of the equipment manufacturers.

From the start, the filters performed satisfactorily except for variable HPC results, with some units testing higher than desired. These results brought all interested parties together to determine the future direction of the project and to define an upper limit for HPC in potable water. Since there was no drinking-water MCL for HPC, U.S. EPA's formulation process for other drinking water microbiological requirements and the history of that process were studied. During the development of the drinking-water Total Coliform Rule, U.S. EPA considered using HPC levels of less than 500 colony-forming units per milliliter (CFU/mL) as a "negative" acceptance criteria for questionable total-coliform test results. Levels above 500 CFU/mL were considered a "positive" for questionable total-coliform test results (3). In other words, when a total-coliform test for a water sample had questionable results, an HPC of less than 500 CFU/mL was sufficient reason to record the coliform test as negative and the water was considered safe. If levels above 500 CFU/mL were found, then the test was positive and the water was not acceptable for consumption. In addition, U.S. EPA, in developing the Surface Water Treatment Rule, decided that an HPC of less than 500 CFU/mL was acceptable as evidence of adequate disinfection of drinking water (4,5).

The above considerations relate to municipal and small drinking-water systems only. For these systems, the Safe Drinking Water Act permits use of bottled water as a substitute for municipal water when the water supply does not conform to any of the health requirements (6). Bottled water is monitored by the Food and Drug Administration (FDA), and FDA has established no HPC standards for bottled water. This inconsistency between governmental agencies leaves open the question of the health effects of heterotrophic bacteria in drinking water. …

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