The Flint River is naturally high in corrosive chloride. Therefore, iron pipes in the water distribution system began corroding immediately after the initial switch from Detroit water. The iron that was released from the corroding pipes reacted with residual chlorine that is added to kill microorganisms, making it unavailable to function as a disinfectant.
Because chlorine, which reacted with the iron pipes, could not act as as disinfectant, bacteria levels spiked. When coliform bacteria were detected in distribution system water samples, water utility managers were obliged by law to increase the levels of chlorine. The higher levels of chlorine, while reducing coliform counts, led to the formation of more trihalomethanes.
Water utilities need to devise treatments specific to each water source and surface water, such as rivers, requires more chemical treatment.
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Providing adequate disinfection while minimizing disinfection by-products is a challenge faced by most utilities even under the best of circumstances. The problems became intractable in Flint due to the excessive corrosion of the pipes that deliver water to people’s homes.
The science of pipe corrosion in drinking water systems is complex and not completely understood. Corrosion control occurs when naturally forming minerals deposit on pipe walls, thereby protecting the iron pipe surfaces from exposure to oxidants in the water. Changes in water quality sometimes dissolve these mineral coatings, exposing the pipe to corrosion.
In iron pipe systems, the released iron corrosion particles are visible, causing colored and turbid water. In older distribution systems, where lead service lines are often still in place, corrosion then releases lead and copper. Corrosion rates can be affected by many factors that are not well-understood, including the presence of bacteria that colonize the pipe wall, as well as pipe age and water flow rates.
Because of the uncertainties around leaching, the majority of utilities treating surface water add phosphate corrosion inhibitors to control corrosion. They devise doses based on the water industry’s experience, rather than on rigorous scientific calculations.
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