Investigation of impact of split treatment on finished water quality for the Ames water treatment plant

In response to limited carbon dioxide availability and increasing costs during the COVID-19 pandemic, an investigation into split treatment was performed. A 2020 pilot study showed that a 5% raw water bypass and target caustic alkalinity range of 26-36 mg/L as CaCO₃ resulted in 25% and 7% carbon dioxide and lime savings, respectively. Considering this promising outcome, the potential for scaling and corrosion within the plant was evaluated during a subsequent split treatment trial in this study. Mild steel scale coupons were assessed for scaling and corrosion at four different treatment conditions. These results found that a 5% raw water bypass is predicted to have a minimal impact of scaling and corrosion on the water plant infrastructure when compared to full lime softening. Results are significant since split treatment can save over $150,000 annually with no apparent change in water quality, improving the plant's resilience and sustainability. PRACTITIONER POINTS: A 5% raw water bypass produced scale and corrosion similar to full lime softening. Localized build-up results in surface corrosion. Relocation of SHMP did not show appreciable sequestration of hardness ions. Adjusting dosage and/or using a polyphosphate blend is recommended. A 5% raw water bypass can be implemented at a lime softening facility to reduce chemical usage and cost. A similar procedure can be conducted to analyze the impacts of a larger percent bypass for additional chemical savings.