Pilot-scale comparison of sodium silicates, orthophosphate and pH adjustment to reduce lead release from lead service lines
Kimia Aghasadeghi, Sigrid Peldszus, Benjamin F. Trueman, Anushka Mishrra, Mitchell G. Cooke, Robin M. Slawson, Daniel E. Giammar, Graham A. Gagnon, Peter M. Huck, Water Research, Volume 195,2021,116955,ISSN 0043-1354, https://doi.org/10.1016/j.watres.2021.116955
In 2018, the Chair team undertook a collaborative pilot-scale study to compare three different corrosion control treatments (i.e. sodium silicates, orthophosphate and pH adjustment) for reducing lead release from lead service lines. At that time very few studies were available on using sodium silicates for controlling lead release. The pilot set up was commissioned in August 2018 at the Lake Huron water treatment plant (Grand Bend, ON) and was operated continuously for 68 weeks. In this study a harvested service line was used that was excavated by the City of London. The pilot study concluded at the end of 2019 after which scale analysis at Washington University (St. Louis, Missouri, USA) and biofilm accumulation analysis at Wilfrid Laurier University (Waterloo, ON) were completed. Lead analysis was done at Dalhousie University (Halifax, NS) and it included the detection of colloidal dispersions containing lead which provided important insights for the interpretation of the lead release data. The results were published in February 2021 in Water Research as open access (journal subscription is not needed for accessing the paper). The paper abstract which outlines the key findings is provided below. The full paper can be accessed and downloaded via this link: https://doi.org/10.1016/j.watres.2021.116955
If you have any questions about this study, please contact Kimia Aghasadeghi at kimia.aghasadeghi@uwaterloo.ca.
Abstract:
Sodium silicate is thought to mitigate lead release via two mechanisms: by increasing pH and by forming a protective silica film. A pilot-scale study using an excavated lead service line (LSL) fed with water from a Great Lakes source was undertaken to: (1) clearly distinguish the pH effect and the silica effect; (2) compare sodium silicate to orthophosphate and pH adjustment; (3) determine the nature of silica accumulation in the pipe scale. The LSL was cut into segments and acclimated with water at pH 7.1. Median dissolved lead was 197 μg/L in the last 8 weeks of acclimation and dropped to 16 μg/L, 54 μg/L, and 85 μg/L following treatment with orthophosphate (dose: 2.6 mg-PO4/L, pH: 7.9), pH adjustment (pH: 7.9) and sodium silicate (dose: 20 mg-SiO2/L, pH: 7.9), respectively. When silica dose was increased from
20 mg-SiO2/L to 25 mg-SiO2/L (pH: 8.1), lead release destabilized and increased (median dissolved lead: 141 μg/L) due to formation of colloidal dispersions composed mainly of lead- and aluminum-rich phases as detected by field flow fractionation used with inductively coupled plasma mass spectrometry. Si was present in the scale at a maximum of 2.2 atomic % after 17 weeks of silica dosing at 20 mg- SiO2/L. Under the conditions tested, sodium silicate did not offer any benefits for reducing lead release from this LSL other than increasing pH. However, sodium silicate resulted in lower levels of biofilm accumulation on pipe walls, as measured by heterotrophic plate counts, when compared to orthophosphate.
