Water Quality Reports
Williston Water Quality Frequently Asked Questions
This document was prepared by the City of Williston to provide responses to potential questions by customers prompted by recent media attention to water quality at the consumers tap.
What are the qualities of the water source and how is treated at the Williston water treatment plant?
Missouri River water is treated at the Williston water treatment plant with processes designed to remove contaminants, provide potable water that is safe to drink and comply with the water quality requirements of the Safe Drinking Water Act. The river water contains suspended clay turbidity, dissolved minerals that contribute hardness and alkalinity, microorganisms including bacteria, protozoa, viruses and algae, and dissolved organic matter from surface runoff. Water is treated to remove taste and odor-causing substances, dissolved organic compounds, turbidity and hardness, is disinfected to remove and inactivate potential disease-causing organisms, and is stabilized to inhibit corrosion and deposition in the distribution system. The fluoride content is also adjusted to meet North Dakota Department of Health recommended concentrations.
How is the water disinfected?
Water is treated to remove microorganisms and disinfected to inactivate microorganisms and provide a protecting residual against potential cross-contamination in the distribution system or in a customer’s plumbing system. Organisms are removed by coagulation with chemical coagulants so that they will settle from the water in settling tanks, and then are filtered from the water in the anthracite and sand media filters. The high pH environment in the lime softening process also helps inactivate organisms. The turbidity of filtered water is closely monitored to insure these removal processes are working correctly. The water is disinfected by establishing a chlorine residual and then by passing the water through ultraviolet light reactors. After the ultraviolet reactors, ammonia dosed to the water combines with chlorine to create a chloramine residual. The chloramine residual is present in the distribution system to the customers tap to prevent growth of organisms that may enter the water via backsiphonage or other cross connection in the distribution system or customer’s plumbing system. The treatment processes provide a triple barrier against contamination by microorganisms ensuring the water is safe to drink.
Why is a chloramine residual present in the water?
Disinfectants must be capable of killing or inactivating microorganisms in the water treatment plant but also must maintain a disinfecting residual in the distribution system. The disinfection processes used at the Williston water treatment plant are very effective at inactivating organisms. The primary disinfecting processes are chlorination and ultraviolet light inactivation. Chlorine kills viruses, bacteria and giardia lamblia. Ultraviolet light inactivates protozoans such as cryptosporidium and giardia. Although chlorine is an effective disinfectant, it also can react with naturally occurring organic matter found in the Missouri River water to form disinfectant by-products, the most common of which are trihalomethanes and haloacetic acids, the concentrations of which are regulated by the drinking water regulations. A chloramine residual is formed adding ammonia to chlorinated water. The formation of disinfection by-products is inhibited by chloramine, yet chloramine maintains a protective residual in the distribution system.
If organic matter causes disinfection by-products, does the treatment plant remove organic matter as well?
Yes. Powdered activated carbon dosed to the water adsorbs organic matter. The aluminum sulfate coagulant that is added to the water to remove turbidity also removes organic matter. However, the greatest percentage of organic matter removal occurs in the lime softening process. These three processes typically remove 40 to 50 percent of the naturally occurring organic matter, thereby minimizing the potential to form disinfection by-products.
It has been reported that chloramination can cause nitrosamine formation. What are nitrosamines, and what is done at the water treatment plant to minimize nitrosamine formation?
Nitrosamines have been found when a specific type of organic compound reacts with chlormamine. The most common nitrosamine formed in this reaction is N-nitrosodimethylamine (NDMA). Even though NDMA is not currently regulated by drinking water regulations, the Williston water treatment plant is configured to minimize NDMA formation. Operators control the dosages of coagulants to limit the residuals of organic compounds that form NDMA. NDMA formation is also limited by the presence of a chlorine residual prior to ammonia addition in the chloramination process.
Does water from the Williston water plant contain lead?
As a result of treatment processes applied at the Williston water plant, the treated water does not contain lead. However, since the water may come into contact with lead-bearing pipes, solder and faucets on the way to the customers tap, the drinking water regulations require public water supply systems to monitor for lead and copper at customers taps. Water samples are routinely collected from 30 sample sites in the City of Williston and are measured for lead and copper concentrations. The lead and copper concentrations found in these samples are reported in the Williston annual water quality report. The most recent results indicate the concentrations are less that the regulatory action levels.
What qualities of the treated water help control lead corrosion?
The lower the pH of water, the greater is its corrosiveness to lead. Operators at the Williston water treatment plant control the treated water pH in the range of 8.5 to 9.0. This pH helps control corrosion because the potential to dissolve lead is minimized in this pH range. The water plant also improves the stability of the water against corrosion by adding a phosphate chemical. This phosphate chemical interacts with lead pipe and fittings to form a protective layer on the surface that helps prevent corrosion.
Is there fluoride in the water?
Yes. The North Dakota Department of Health recommends public water supplies add fluoride to the water so that the treated water fluoride concentration falls in the range of 0.6 to 1.2 mg/L, with a desired concentration of 0.7 mg/L. This fluoride concentration in drinking water has been found to help prevent dental caries. In voluntary compliance with this recommendation, the Williston water treatment plant adds fluoride to maintain a treated water fluoride concentration ranging from 0.6 to 0.8 mg/L.