Online monitoring and control of drinking water corrosion potential at a full-scale plant

To mitigate pipe and water quality degradation in distribution, treatment targets were established to continuously control the corrosion potential of drinking water towards cement, lead and iron materials at a 225 MLD surface water treatment plant. Indicators were identified to estimate the corrosion potential of water for each of the 3 materials: the Langelier Saturation Index (LSI) for cement, pH for lead and a novel corrosion index for iron. Linear Polarization Resistance (LPR) probes equipped with cast iron and lead electrodes provided direct and continuous measurements of actual lead and iron corrosion rates for finished water. Corrosion control was achieved by adjustments to caustic soda doses. The set-point for finished water pH was determined with a site-specific algorithm using online values for temperature, pH, and conductivity as surrogate for calcium and alkalinity. Over the 7 months of implementation, measured iron corrosion rates were maintained well below the established threshold of 50 μm/year. Lead corrosion rates varied from 20 to 40 μm/year with higher levels occurring when pH dropped below 7.5. The full-scale experience validated a treatment strategy and demonstrated the feasibility of using LPR probes for online monitoring of corrosion potential on drinking water matrices.