The King County Wastewater Treatment Division initiated an investigation in 2001 of the potential use of ferric chloride addition for enhanced primary treatment at the South Treatment Plant in Renton, Washington. The South Treatment Plant treats an annual average flow of 70 million gallons per day (mgd). The objective of the study was to determine if the benefits of enhanced solids removals using ferric chloride (FeCl3) for advanced primary treatment (APT) in the primary clarifiers, and resultant energy savings in the secondary and solids processes, outweighed the cost of purchasing ferric chloride and flocculant aid polymer. The project began in March 2001 with jar testing of various doses of ferric chloride and polymer. The target primary clarifier removal rates were 50 percent biochemical oxygen demand (BOD) and 80 percent total suspended solids (TSS). These removal rates were seen during jar testing at doses of 30 mg/L ferric chloride and higher (the stoichiometric demand based on influent phosphorus concentration was approximately 35 mg/L). However, the cost of the chemicals at a dose of 30 mg/L would likely make enhanced primary treatment infeasible. The study initially targeted a dose of 15 to 20 mg/L ferric chloride, with a dose of 0.2 mg/L anionic polymer to see if some enhancement in the removal rates would provide a benefit to the plant operations budget. Two trial periods were planned: one under partial flow conditions (1 to 4 primary clarifiers) and one under full-flow conditions (8 primary clarifiers). The first trial (2 weeks long) performed in July 2001 (using one primary clarifier at a ferric chloride dose of 15 to 20 mg/L) did not show removal rates near the target levels in the primary clarifiers. However, a change in the influent wastewater characteristics negatively affected plant performance overall, including the ferric chloride trial. The plant influent was monitored over the next few months. Once it returned to historically “normal” conditions, another trial would be scheduled. The second trial (Round 2) was run in winter 2001 at the same doses using four clarifiers. The results were near the target levels for BOD and TSS removal. The conclusion of the second trial was to wait until summer 2002 and run a trial. In addition, a series of settling tests indicated that the target removals could be achieved with less chemical addition, but longer settling time.
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