AWWA WQTC64163

A full-scale case study is presented where coagulation of water characterized by high total organic carbon (TOC) concentrations is optimized using measurements of zeta potential. Each spring, snowmelt runoff water percolates through the forest floor of the Cache la Poudre River (Poudre River) watershed resulting in high TOC, low alkalinity water that must be treated by the City of Fort Collins, Colorado, Water Treatment Facility (FCWTF). During the 2005 snowmelt runoff period, the raw Poudre River TOC ranged up to 13 mg/L, with the TOC at or above 8 mg/L for 23 days. The increased TOC is predominantly made up of humic substances that control the coagulant demand. Modification of the coagulation process to treat this water at the FCWTF consists of conditioning the raw water with a 30% lime slurry and carbon dioxide gas prior to alum addition in order to achieve a coagulated water pH within the range of 6.5 to 6.8. The rapidly changing water quality conditions that occur during the spring snowmelt runoff period require the operators to make rapid decisions about alum dose changes. During the 2005 snowmelt runoff, the alum dose was adjusted to keep the zeta potential within the ¿¿¿¿ 3 mV range as the TOC rose to its peak and then descended back down towards its baseline value. A new generation of zeta potential instruments allows for frequent measurements of zeta potential that are reliable and that are not user-dependent. The adjustment of the alum dose based on zeta potential resulted in the alum dose closely following the TOC, with the alum dose being adjusted by about 3.5 mg/L for each mg/L change in raw water TOC. While maintaining the postcoagulation targets for pH and zeta potential, the full-scale process performance was evaluated with respect to the ability to meet the multiple goals of a combined filter effluent turbidity of less than 0.1 ntu, a TOC removal greater than 50%, and a residual aluminum less than 100 µg/L. The results of this full-scale case study showed that zeta potential measurements can be successfully used by plant operators as a key process control and optimization tool during challenging conditions of high TOC water, allowing them to meet their multiple plant effluent goals. Includes 21 references, tables, figures.