May2011_ES&E_4_2010 11-06-04 12:03 AM Page 28
Continuous mixing improves sampling accuracy and water quality in storage tanks By Joel Bleth
T
here are thousands of municipal water systems in Canada, many of which rely on water towers and above- and belowground storage tanks to store treated potable water. Drinking water treated with either chlorine or chloramines will normally remain stable and safe for a few days in storage. As such, municipal water system operators have tended to rely on normal cycling of water in and out of the tank to keep the water mixed in order to limit aging and deterioration of disinfectant chemicals. However, this reliance on “passive mixing� is often not enough to prevent thermal stratification that can lead to a drop in residual chlorine and the resultant growth of bacteria. When bacteria contamination occurs, a common practice is to drain and flush the tank, and refill it. This practice wastes water and raises costs of labour and chemicals.
Figure 1: When passive mixing is insufficient, potable water tanks can stratify, stranding warmer water near the top of the tank, where residual disinfectants begin to dissipate.
When continuous mechanical mixing is combined with frequent sampling and periodic chlorine boosting as needed, water quality is maintained and less disinfectant is needed overall, due to elimi-
nation of sampling inconsistencies. New, near-laminar-flow, active mixing technologies have been developed to completely mix storage tanks with up to 100 million gallons of potable water, vir-
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28 | May 2011
Environmental Science & Engineering Magazine