AWWA WQTC59014

Water that is retained longer within storage has been related to taste and odor problems, loss of disinfectant residual, and increased disinfection byproducts (DBPs). In Champlain Water District's (CWD's) case, an oscillating thermocline at the plant intake introduces buoyant water into system storage, possibly displacing the older water from storage into the distribution system. CWD modeled and reconfigured a standpipe storage tank to enhance our operational approach to retention time and DBP formation management. The project was intended to model, design, build and test a standpipe mixing system. Contracted computational fluid dynamic (CFD) modeling visualized that stratification would occur under existing negative buoyancy and isothermal conditions with "pockets" of higher DBP and lower disinfectant residual water throughout the tank and indicated the existence of both a retained zone, and a turnover zone within the tank. A proposed tank modification was modeled showing that the retained zone would likely be eliminated. A mixing system was retrofitted to the previous single inlet/outlet configuration. A grab sampling plan to investigate and understand any possible turnover zone water quality changes was implemented. The testing plan determined HAA5s, TTHMs, and disinfectant residuals at several tank draw down stages during four separate trials in July 2002, before tank modification, and July 2003, after tank modification, under similar operational and raw water DOC and temperature conditions. Background data was collected to gauge similarity in before and after tank modification background water quality conditions. Includes 11 references, tables, figures.