Mixing in distribution system storage tanks: Its effect on water quality

Nearly all distribution systems in the United States include storage tanks and reservoirs. They are the most visible components of a water distribution system but are generally the least understood in terms of their impact on water quality. Long residence times in storage tanks can have negative impacts on water quality in drinking water distribution systems, including the loss of chlorine residuals and the regrowth of microorganisms. It is generally accepted that complete mixing maximizes water quality but little is understood about mixing regimes in general. This paper demonstrates the use of compartment models to characterize mixing in three tanks. It was found that the mixing regimes in these tanks were well characterized by compartment-type models and that these tanks were not completely mixed, contrary to conventional wisdom. It is clear that much more research is needed in order to truly understand mixing regimes in tanks and reservoirs and that this understanding, if properly applied, can have a positive impact on water quality.