Split treatment evaluation, optimization, and operation of the Ames water treatment plant

The objective of this study was to test the hypothesis that split treatment at the City of Ames lime softening plant could significantly reduce or eliminate reliance on carbon dioxide for pH control. The need for this study resulted from uncertainty surrounding carbon dioxide availability as a direct result of COVID-19 supply chain disruptions and increased costs. Split treatment was implemented by bypassing a portion of the influent raw water around the typical lime softening process and rejoining the raw water with the lime softened effluent in the recarbonation basin. The present study has value for existing and future lime softening plants having demonstrated the efficacy of split treatment to reduce the dependence on carbon dioxide to reduce pH to stop the softening reaction prior to filtration while maintaining water quality objectives. A 10% split resulted in a 67% reduction in carbon dioxide usage during the study period. This translates into significant cost savings in both chemical cost (lime and carbon dioxide) and sludge disposal costs. Annual chemical cost savings alone were approximately $95,000 if a 10% split were practiced throughout the year. If sludge disposal costs were included, the savings would approach $159,000 annually. Due to the success of the trial, piping modifications are being considered to allow split treatment operation as a regular part of the water treatment flow scheme.