AWWA WQTC58830

Research was done to evaluate the use of reverse osmosis water treatment technology to remove arsenic from groundwater supplies for small communities in North Dakota. Point-of-use reverse osmosis (POU-RO) technology was evaluated. Twenty-one POU-RO units were installed in homes and other buildings in Oakes, North Dakota for a pilot test. After installation, the units were monitored for arsenic removal for two years. In addition to evaluating arsenic removal, questions about the management, maintenance, monitoring, and cost of treatment were addressed by this research. The City of Oakes is a predominantly residential community located in southeast North Dakota. The city has a population of about 1,760. Oakes drinking water is supplied by three wells situated in different parts of the city. Typical arsenic concentrations in raw water pumped from these wells range from 10 to 21 ppb. The water delivered to residences in Oakes is not disinfected or treated in any way. Arsenic removal was evaluated by periodically collecting and analyzing samples of treated water (i.e., permeate) and untreated water from each of the units installed for the pilot test. A total of 105 permeate samples and 115 untreated water samples were collected and analyzed over the two- year test period. The arsenic concentrations in the untreated drinking water ranged from 6 to 40 ppb and the average arsenic concentration was 13.5 ppb. With one exception, the arsenic concentrations in the permeate samples ranged from 0 to 4 ppb and the average arsenic concentration was 0.25 ppb. Even though most of the permeate samples collected during the test period contained no measurable amounts of arsenic, a few of the units did exhibit signs of reduced membrane performance over the course of the monitoring period. Reduced membrane performance was indicated by a gradual increase in the conductivity of the permeate over time. However, even when permeate conductivity exceeded one half of the untreated water conductivity, the unit still produced substantial arsenic removal. Although reverse osmosis was believed to be the main removal mechanism for arsenic, it may not have been the only significant mechanism. The POU-RO units also contained two activated carbon cartridges. Since these were both carbon block cartridges, they also serve as filters. When the prefilters were changed after one year of service, it was observed that they were coated with iron. It is possible that adsorption of arsenic onto the iron may have been an additional removal mechanism. Includes 9 references, table, figures.