...Most important, the treated Flint River water lacked one chemical that the treated Detroit water had: phosphate. “They essentially lost something that was protecting them against high lead concentrations,” Giammar says. Cities such as Detroit add orthophosphate to their water as part of their corrosion control plans because the compound encourages the formation of lead phosphates, which are largely insoluble and can add to the pipes’ passivation layer.
Flint didn’t use orthophosphate despite a recommendation to do so from Veolia, an environmental services company that studied the quality of the treated Flint River water after the switchover. In a March 2015 report, Veolia suggested that the city spend $50,000 annually to add the corrosion inhibitor.* By press time, C&EN was unable to get a comment from Flint city officials about why a corrosion inhibitor wasn’t added to the river water.
The entire Flint water crisis could have been avoided if the city had just added orthophosphate, Edwards says. He bases his opinion, in part, on experiments his group ran on the treated Flint River water. The researchers joined copper pipes with lead solder and then placed the pieces in either treated Flint River water or treated Detroit water. After five weeks in the Flint water, the joined pipes leached 16 times as much lead as those in the Detroit water, demonstrating just how corrosive the treated Flint water was. But when the scientists added a phosphate corrosion inhibitor to the Flint water, the factor went down to four.
Still, orthophosphate isn’t the only corrosion solution. Some water utilities treat water so it has a high pH and high alkalinity, Giammar says. Such conditions decrease the solubility of lead carbonates, which also contribute to the pipe’s protective mineral layer.
The treated Flint River water had a relatively low pH that decreased over time.
According to monthly operating reports from the Flint treatment plant, the city’s water had a pH of about 8 in December 2014, but then it slowly dropped to 7.3 by August 2015. Environmental engineers say that if water pH drifts too low in the absence of orthophosphate, the water can start to leach high levels of lead from pipes.In the comments (keep scrolling), a very interesting statement from a former operator at the Flint water plant:
I was an operator at the Flint water plant in 2014-15. To answer your question about a target pH, it was 8.5-9.0. I would add that Flint utilized lime softening so the pH was raised to a target range of 11.0-11.6 and subsequently reduced through the addition of carbon dioxide.
As far as the decision to not implement optimized corrosion control, that was a decision by committee including the city, their engineering firm and the Michigan DEQ. It was the DEQ that misinterpreted the Lead and Copper Rule requirement for communities with over 50,000 residents to implement corrosion control immediately after a source water change. Instead, the DEQ allowed Flint to conduct the two consecutive six month monitoring periods first....As a parent that is incredibly paranoid about lead, this is pretty awful. It seems to me that whatever systems are in place to attempt to avoid these sorts of issues is not robust enough.
(We don't usually think about infrastructure like water treatment very much, but it probably is an area that we, as a country, should think a little more about (and probably pay a little more financial and scientific attention to.))
*UPDATE: Michael Torrice writes in the comments that this sentence has been corrected by C&EN. Veolia recommended the addition of polyphosphate, which would not have prevented the corrosion issue.