Fateful mistakes in Flint
Water quality issues in Flint began with the decision of city officials in 2014 to switch from buying treated drinking water from Detroit to treating Flint River water themselves using a city-owned treatment facility.
The switch was considered a temporary money-saving “fix” to provide the city with drinking water until they were able to join a new regional system, the Karegnondi Water Authority. A 10-month, US$171,000 engineering effort was undertaken to equip the Flint plant to treat Flint River water before it was put into service.
Sources of drinking water supply, in general, include groundwater and surface waters, such as lakes and rivers. Among those water sources, rivers present the greatest treatment challenge.
Relative to groundwater, surface waters tend to contain more particles, microorganisms, organic matter, taste- and odor-causing compounds, and many types of trace contaminants. On average, surface water also tends to be more corrosive than groundwater.
Beyond the challenges of designing a treatment approach tailored to the source water, water quality engineers must consider myriad engineering, regulatory and financial constraints during design.
In recent years, the cost of chemicals used to treat water has increased at rates well above inflation. Based on a 2009 report published by the Water Research Foundation, the average price of phosphoric acid, a chemical that can inhibit corrosion, increased by 233 percent in 2008 alone. These anticorrosion chemicals are used to prevent lead and other metals in the pipes from leaching into the water. At the time Flint decided to treat its own water, chemical costs were still increasing.
Many utilities treating surface water are under pressure to look for less costly approaches to perform chemical treatment. Yet particle removal, a critical step used to treat surface waters like the Flint River, is a chemical-intensive operation.
Iron and aluminum salts are typically coagulants added to water supplies to help aggregate particles so they can be effectively removed through settling. There are many types of iron and aluminum coagulants, and they have different degrees of effectiveness depending upon the quality of water being treated.
Coagulant choice is an important design decision; therefore the choice of coagulant should not be based only on cost. For example, each coagulant has to be optimized to enhance removal of natural organic matter in the source water. If too little organic matter is removed, it will react with chlorine disinfectants in the water to form hazardous by-products.
A switch from sulfate-based to chloride-based aluminum or iron coagulant salts also alters the chloride-to-sulfate ratio in water. It was this ratio that Dr. Marc Edwards, a faculty member at Virginia Tech, linked in 2010 to higher lead concentrations in vulnerable distribution systems with pipes made from lead. The Flint treatment plant relied on iron chloride coagulants, which may have contributed to the corrosivity of the water.
Science of corrosion
Because of Flint’s method of treating Flint River water, it experienced problems with elevated trihalomethanes, a regulated class of disinfection by-products that are known carcinogens. A domino series of causes and effects were responsible for this problem.
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