Radionuclides in Drinking Water
Lime SofteningLime softening is a process where hydrated lime or quicklime is added to raise pH and precipitate calcium. In enhanced softening, the pH is increased further in a second stage, to at least 10.6 to also remove magnesium.
Lime softening has been identified by EPA as a “best available technology” (BAT) and Small System Compliance Technology (SSCT) for uranium and radium. Conventional single stage softening can remove between 50 and 80 percent of radium. Enhanced lime softening can remove up to 90 percent of radium and uranium and may also remove arsenic, iron, and manganese.
The pH must be greater than 10.6 and sufficient magnesium must be present (or added) for effective removal of uranium.
Enhanced softening is a relatively expensive technology and requires careful monitoring and understanding of the chemistry to ensure proper operation. Although package plants are available, additional operator training will likely be needed unless a softening plant exists already.
Because softening removes hardness and alkalinity, water from softening plants may change the corrosivity of the water. It may be necessary to add corrosion-inhibiting materials to the finished water to protect the distribution system and prevent possible simultaneous compliance issues with other regulations like the Lead and Copper Rule.
Treatment residuals generated by lime softening will include backwash, sludge, and aged/ineffective media. The concentration of radionuclides in these residuals may impact disposal options. Water softening plants are generally not allowed to discharge solid or liquid wastes to water bodies. Removal of radionuclides will likely complicate the disposal process. However, several states have sludge disposal programs allowing for land application of softening sludge. Aged/ineffective media and sludge will need to be disposed of in an appropriate class of landfill. Refer to the disposal section of the Web site for more detailed information. Disposal Issues.