Temperatures are expected to top 100 degrees across much of the Eastern seaboard this week and municipalities large and small will be contending with managing their water quality. 

In general, higher temperatures mean higher water use for gardens and recreation. This means higher turn-over in water tanks, which is a good thing.  But for areas with large capacity, or low summer demand, high temperature can lead to thermal stratification – which spells trouble for water quality.

Thermal stratification can happen any time of the year, but is often most pronounced during the summer.  Water from the treatment plant is typically much colder than the daytime temperature in the summer, and when it enters a storage tank, it typically remains on the bottom of the tank.  During daylight hours the sun beats down on the walls of the tank, heating the water inside and causing it to rise.  This hot water rides on top of the colder water at the bottom of the tank and can remain in the tank for many days.  The graph below shows the water temperature in a 500,000 gallon tank in Redwood City, CA at one foot below the surface (in blue), 3 feet below the surface (in red), 5 feet below the surface (in green) and 8 feet below the surface (in purple).  With each day, the temperatures in the upper parts of the tank rise and things don’t cool down at all during the night.

Thermal stratification leads to a host of water quality problems including high water age, loss of residual, and taste and odor complaints.

Active mixing can eliminate thermal stratification in storage tanks.  By completely mixing the water and keeping it circulating, operators can be assured that fresh water entering the tank is thoroughly circulated, and temperatures and residual levels will be consistent.  Better still, by keeping the tank mixed, air temperatures in the headspace above the water will be lower, reducing the rate of corrosion inside the tank.

We welcome your comments and questions.

Sincerely,

Peter S. Fiske, Ph.D.

CEO, PAX Water Technologies