Removal of strontium by ion exchange and lime softening at eight drinking water treatment plants

Abstract

The United States (US) Environmental Protection Agency (EPA) included strontium (Sr) in the Contaminant Candidate List 3 as a potential chemical for future regulation. Sr occurrence data at US public water systems were gathered under the Unregulated Contaminant Monitoring Rule 3, and the EPA assessed the potential to improve public health as well as the costs of promulgating a Sr regulation. The EPA delayed issuing a final regulatory determination for Sr to gather more information, including treatment research on removing Sr from drinking water for which there is very limited data, but the health reference level was lowered to 1.5 mg L⁻¹. The objective of this research was to evaluate the effectiveness of full-scale ion exchange (IX) and lime softening on Sr removal from groundwater at eight drinking water treatment plants in Ohio. IX softeners removed 92–98% of Sr which utilities blended to achieve Sr concentrations ranging from 2.7–7.0 mg L⁻¹ resulting in an overall removal including blending of 67–81%. Lime softening plants achieved 44–95% Sr removal on average, resulting in finished water Sr concentrations ranging from 0.2–4.9 mg L⁻¹. Sr removal appeared to parallel calcium removal, and the Sr concentration in the finished water might be limited by the Sr concentration in the raw water. Under normal operating conditions, only 2 of the 8 softening plants consistently achieved finished water Sr concentrations less than the health reference level. Although treatment modifications likely could improve Sr removal at increased operating costs, the finished water concentration might be limited by blending at IX plants and the raw water Sr concentration at lime softening plants.