Potable Water Storage Tanks Need Mixing
There are more than 50,000 municipal water systems in the United States and most of these systems rely on water towers, above-ground, and below-ground tanks to store their treated potable water. Many operators rely on mixing via normal cycling of water in and out of the tank to limit water age and deterioration of disinfectant chemicals.
This reliance on 'passive mixing' is not enough in most cases to prevent thermal stratification, which can lead to a drop in residual chlorine and a resultant growth of bacteria.
When bacteria contamination occurs in a tank, a common response is to set the tank for deep drawdown and in extreme cases, completely drain, flush, and refill the tank. Not only is this strategy a big waste of water resources, it also raises cost for labor and chemicals.
When continuous mechanical mixing is combined with frequent sampling & periodic chlorine boosting (when needed), water quality is maintained and less disinfectant is required. This paper will address GridBee® & SolarBee® active mixing technologies developed to completely mix potable water storage tanks up to 100 million gallons or more while virtually eliminating thermal stratification, sampling inconsistencies, and other tank water quality issues. These technologies have the additional benefits of reduced labor and chemical expenditures as well as providing a means for EPA regulatory compliance.
Mixing Can Eliminate Temperature Stratification
Thermal stratification promotes deterioration of water quality especially in the upper part of the tank. When water warms and stratifies, newly treated cold water coming into the tank will not mix with the existing water and tends to stay near the bottom. This cold bottom water is also the first to exit the tank creating a usage 'short-circuit' leaving the bulk of the warmer top water unmixed and unused. (See Figure 1.) Thermal stratification can start to occur with temperature gradients as small as 0.1 °C. If stratification persists, the chlorine residual may decline in the warmer unmixed volume allowing bacterial growth and an overall decrease in water quality. Stratification also introduces sampling inconsistencies that can lead to over- or under- boosting of chemical disinfectant. Since water quality testing is based on sampling from pre-selected locations within a tank, it is possible water quality testing may not reflect the real health of the entire water volume especially in a stratified condition.
However, a fully and consistently mixed storage system provides a reliable means for true testing of water quality parameters including temperature and residual disinfectant from any sample point.
Mixing Can Reduce Trihalomethanes
In water systems with natural organic matter (NOM) content, chlorine added at the treatment facility can react with NOM to form a disinfection byproduct (DBP) called trihalomethane (THM). THM formation is a concern for many water systems due to of potential health risks over time. The THM compliance limit is currently 80 ppb (locational running annual average) but this may be lowered at some point in the future.
Unmixed tanks with old, stratified water will tend to have high levels of THM in the upper, warmer part of the tank. Good floor to surface mixing homogenizes the water into a lower THM 'blend' as existing, high THM water is mixed with the new, lower THM water. The well mixed, lower THM blend can then pass out of the tank under the compliance limit and as the average water age declines with mixing, new formation of THM in the tank is further minimized.
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