Corrosion control has always been a priority for distributing safe drinking water throughout the world’s networks of pipeline. This has become all the more critical following the outrageous lead poisoning revelations in Flint, MI — an incident caused directly by corrosion of the city’s lead-based infrastructure.
And no water quality analysis metric is relied on more heavily to help keep corrosion under control than pH.
The Issue Of Corrosion
For drinking water management purposes, corrosion can be defined as the deterioration of pipeline by constituents in water as it moves from treatment centers to points of use. Typically, this means the conversion of a refined metal such as lead or copper along the pipeline walls into an oxide, hydroxide, or sulfide that is then transported along with the drinking water, contaminating it and presenting a health risk to consumers. And beyond the significant health risks it poses, corrosion causes a number of other headaches for water systems.
“Corrosion in water distribution systems can impact consumers’ health, water treatment costs, and the aesthetics of finished water,” according to a National Drinking Water Clearinghouse fact sheet. “Corrosion can cause higher costs for a water system due to problems with decreased pumping capacity … decreased water production … water damage to the system … [and] customer complaints of water color, staining, and taste problems.”
That is why drinking water treatment operations are expected to conduct the necessary corrosion control measures, fortifying their finished product so that it will not corrode pipelines or pick up any unwanted constituents.
How To Limit Corrosion Through pH
Typically, corrosion control takes place through three forms of chemical treatment. Corrosion inhibitors like phosphates or silicates may be added to form a protective film along piping. Adding a sufficient level of carbonate (40 mg/L or higher) and alkalinity, along with high pH levels, encourages the formation of calcium carbonate within the pipes, creating a barrier between the pipe metals and the water that travels past it. And pH adjustment is also used to make the water less acidic — reducing the levels of carbonic acid — and therefore less likely to dissolve metal piping.
“Depending on the corrosion issue in the distribution system, the pH level that is required to reduce corrosion can vary based on the water quality,” said Darin Skutt, a technical service manager with Carus Corporation, a water treatment technology provider. “For general corrosion control of iron piping that causes red water or copper issues, raising the pH above 8 is typically sufficient… For lead and, in some cases, copper, raising the pH up to 9 to 9.5 can be required in order to meet the U.S. EPA action levels for those metals. If the system is adding a corrosion inhibitor such as phosphate, those work best at pH [levels of] 7 to 8 for lead and copper control, but will also work at higher pH [levels], up to 9.”
But there are some downsides to relying on pH adjustment as the only effort against corrosion.
“The biggest negative of corrosion control through pH adjustment is that, depending on the buffering capacity of the water, the pH can change based on its interaction with the chlorine residual, piping, and scale of the distribution system,” said Skutt. “Thus, the pH of the water that leaves the plant is not the pH that the consumer gets out of their tap, as the water degrades and picks up dissolved metals and color on its way to them.”
It should also be noted that if a particular drinking water solution is especially hard, high pH can cause excessive carbonate scaling, which can plug water meters and entire pipelines. And, as always, particularly high pH can affect the efficacy of chlorine disinfection, cause higher trihalomethanes formation, and impact taste.
Furthermore, pH adjustment alone is probably not enough to guarantee corrosion will not take place; rather, pH is one significant piece of a larger puzzle.
“The adjustment of pH and/or alkalinity for control of corrosivity is based on an idealized and incomplete model of corrosion by product solubility,” said Abigail Cantor, a chemical engineer and computer programmer with Process Research Solutions, a chemical engineering consulting firm that specializes in drinking water quality issues. “No matter what we do, some corrosion of the various metals [in a drinking water pipeline] will occur. We must prevent the various metals from being transferred into the water at concentrations that can harm human health.”
Cantor cautioned that water systems should not rely solely on EPA and American Water Works Association (AWWA) recommendations for corrosion control, which she said encourage pH adjustment and addition of orthophosphate, and not much else. Still, understanding the pH levels of finished water before it leaves a treatment plant and adjusting it accordingly is a critical step in controlling corrosion and ensuring safe drinking water.
“Typically, pH adjustment can be the most cost-effective [corrosion control method] depending on the water quality,” Skutt said.