|
|
Potential release of lead (Pb) from scale should be a primary concern for some water utilities thinking of switching to chloramine disinfection. If the distribution system has had significant accumulation of lead, especially in the higher oxidation state of Pb(IV), a switch to chloramine disinfection may cause some of this lead to be released back into the water. There are known cases where switching to chloramine disinfection has caused lead levels to far exceed federal drinking water standards at the tap even when lead level in the raw water was far below the standard. Old pipes in water distribution systems can have a mixture of scale coatings on their inside surfaces. This scaling is beneficial up to the point where it begins to significantly impede flow because it provides pipe corrosion control benefits. Scales are generally composed of carbonates, phosphates, silicates, oxides and even metal hydroxides in some cases. Within many distribution systems it is not unusual to have highly mixed scales that contain lead [Pb(II) and Pb(IV)] salts or oxides as part of the scale. Pb(IV) scales are associated with waters that have had a persistently high redox potential. For example, a chlorination system with a pH of 7.5 or higher would cause any lead in solution to precipitate as PbO2. This lead dioxide would remain very insoluble as long as the system maintained a high redox potential. A switch to chloramines for secondary disinfection would reduce this redox potential and cause some of the lead from the PbO2 or possibly other lead (PbIV) salts to be released back to solution. |
