Special Topics in Environmental Management

Drinking Water: Phosphate Corrosion Inhibitors

Analyses of the 2014 Flint, Michigan, drinking water crisis indicate that the high levels of lead in the public supply could have been prevented if the city had added phosphate corrosion inhibitors to its supply. Adding phosphate to domestic water makes it harder for lead compounds in pipes to dissolve and has been used to reduce lead contamination for decades. But there are different types of phosphate corrosion inhibitors, and water systems should consider which one is optimal for lead corrosion control.

Lead and Copper Rule

Under the 1991 federal lead and copper rule (LCR) and its revisions, large and certain small water systems must recommend to the Safe Drinking Water Act primacy agency (usually the state) an optimum corrosion control treatment (OCCT). Federal regulations at 40 CFR 141.2 define the “OCCT” as “the corrosion control treatment that minimizes the lead and copper concentrations at users’ taps while insuring [sic] that the treatment does not cause the water system to violate any national primary drinking water regulations.”

The OCCT requirements differ depending on the system size (i.e., population served) with 50,000 people served as the threshold. Most systems serving more than 50,000 people were required to meet deadlines in 1993 to determine and 1997 to install OCCT. Systems serving 50,000 or fewer people are not required to conduct corrosion control treatment (CCT) steps unless they exceed the lead and/or copper action level.

Phosphate Corrosion Inhibitors

In a 2016 guidance document, the EPA looked a range of OCCTs, including pH/alkalinity/dissolved inorganic compound (DIC) and silicate corrosion inhibitors as well as phosphates. The Agency’s observations on phosphates may be useful to water systems that must recommend OCCTs to their state drinking water offices.

  • Phosphate-based corrosion inhibitors are chemicals that have orthophosphate in their formulation. Orthophosphate reacts with lead and copper to form compounds that have a strong tendency to stay in solid form and not dissolve into water. The extent to which orthophosphate can control lead and copper releases depends on the orthophosphate concentration, pH, DIC, and the characteristics of the existing corrosion scale (e.g., whether it contains other metals, such as iron or aluminum).
  • Orthophosphate is available as phosphoric acid, in salt form (potassium or sodium), and as zinc orthophosphate. Phosphoric acid is a common form available in concentrations between 36 percent and 85 percent. Because it is an acid, it requires special handling. Recent research has found that zinc orthophosphate did not provide additional lead and copper control compared to orthophosphate. Zinc formulations did, however, provide better corrosion protection for cement at low alkalinity/hardness/pH conditions.
  • Polyphosphates are polymers containing linked orthophosphate ions in various structures and are used mainly for sequestering iron and manganese. They work by binding or coordinating the metals into their structures so they cannot precipitate on sinks or clothes. But polyphosphates can also sequester lead and copper, keeping them in the water and actually increasing the risk of exposure. Polyphosphates can revert to orthophosphate in the distribution system, but it is difficult to predict if and when this occurs. Research has confirmed that polyphosphates are generally noteffective on their own for controlling the release of lead and copper.
  • Blended phosphates are a mix of orthophosphate and polyphosphate. It is possible that blends can provide both sequestration of metals and reduce metals’ release. Vendors often recommend blended phosphates as a lead and copper control strategy for systems with elevated iron and manganese. But the EPA says blended phosphates should be used with caution because they may not function as corrosion inhibitors strictly based on concentration and relative amount of orthophosphate. Also, blended phosphates should contain minimum orthophosphate concentrations as a starting point for evaluation. The orthophosphate ratio in the blend and/or the dose may need to be increased to provide adequate lead control. But simply adding more blended phosphate may not be effective because if there is excess polyphosphate available beyond what is bound up with other constituents in the water, it can sequester the lead and copper. The EPA recommends a demonstration study.
  • Changes in the inhibitor chemical used for treatment can affect lead and copper release. For example, changing from an orthophosphate to a blended phosphate is significant because the mechanisms by which the two chemicals control lead release are different, and the effectiveness of blended phosphates depends on other constituents in the water (e.g., calcium). Furthermore, changing to a different manufacturer of blended phosphates can impact lead and copper release, even if the percentage of orthophosphate in the blend is similar.

The EPA guidance, Optimal Corrosion Control Treatment Evaluation Technical Recommendations for Primacy Agencies and Public Water Systems, is here.