Active mixing has been shown to eliminate thermal stratification, prevent the formation of dead zones, reduce disinfectant residual loss and improve overall water quality in potable water storage tanks. 

But how much mixing is enough? And how should an engineer or an operator address mixer performance when writing a specification?

This issue came up recently when an engineering firm asked us how to specify mixing performance for a 1.5 MG standpipe with a common inlet/outlet. The tank showed signs of pronounced thermal stratification during much of the year, and water quality was generally low.

Based on our experience, we gave the engineer three pieces of advice:

1. Don't use "pumping rate" as the sole criterion. 

Some equipment manufacturers claim various pumping rates for their equipment, with very little data to substantiate it.  (One should ALWAYS ask manufacturers for the PROOF of their pumping rates!)  It might be tempting to assume that, if a piece of equipment pumps 1250 GPM, that it would turn over a 1.5MG tank in 20 hours.  But this assumes the pumping action is thoroughly mixing the tank. If the flow pattern created by such equipment fails to reach some parts of the tank, then this assumption is invalid.

2. Temperature is a reliable way of measuring mixing performance. 

If a tank is not uniform in temperature, it is by definition, poorly mixed. (Unfortunately, tanks that are uniform in temperature may still be chemically stratified - with little or no disinfectant residual in some places and high residual in other places.) We often install submersible temperature sensors along with the PAX Water Mixer to demonstrate to a customer that the entire volume of a tank is completely mixed.  We recommend a temperature specification to measure mixing effectiveness: "Temperatures in the tank will be maintained to within 1 degree F at all times" is an example of such a temperature-based specification.  In addition, we recommend a specification for chemical uniformity, such as "disinfectant residual levels within the top five feet of water and bottom five feet of water will converge to within 0.20 ppm".

3. CFD can clarify the flow field created by the mixer.

We often use advanced Computational Fluid Dynamics (CFD) calculations to model the action of a mixer in a tank. CFD enables engineers to visualize the flow field created by the mixer and to quantitatively assess the overall turn-over achieved inside the tank. We recommend that engineers require CFD validation that a mixing system will create a flow field that will fully mix the tank. Any reputable manufacturer should be able to provide such analysis.

Interested in learning more about specifying mixing systems?  Join us tomorrow for a free webinar - Specifying Active Mixing Systems.

Our purpose at PAX Water Technologies is to bring together the latest science, technology and engineering to create energy efficient products that meet the changing demands of the water industry.

We welcome your comments and questions.

Sincerely,

PeterS. Fiske, Ph.D.

CEO, PAX Water Technologies