The Michigan Department of Environmental Quality says levels of orthophosphate, a corrosion inhibitor being used in Flint to restore the protective barrier on the walls of water piping, drops too low on “occasions … at one or more locations.”
"Fortunately, these excursions were not significantly below the recommended level and they were of a short duration,” Bryce Feighner, chief of the department's Office of Drinking Water and Muncipal Assistance said in a Dec. 17 letter to Flint officials Based on the demonstrated improvement in overall water quality, these excursions did not have a substantial impact on restoration of the protective coating.
Adding the chemical should reduce the amount of lead dissolving from service lines and household plumbing. Feighner recommended that the city develop a standard operating procedure for responding to low levels of orthophosphate.
More details here.