The AWWA held a webinar on October 6, 2010 entitled Sustainable Water Quality Management within the Storage Tank: Part 2 (you can listen to a rebroadcast here).

Randy Moore from Utility Service Company (the largest contract tank maintenance company in the US), Mark Johnson Director of Metropolitan Water Operations and Maintenance, Massachusetts Water Resources Authority, and I discussed approaches to lowering cost and improving reliability in the distribution system, with a focus on tank operation and maintenance. More than 400 water operators, managers and engineers signed in from across the US and beyond.

While active mixing was only one part of the discussion the topic attracted a number of questions. Here are some of them, along with my answers:

Q. Does separating your inlet from your outlet in the tank provide enough mixing?

A. Many times, it does not. Separate inlets and outlets are a class of passive mixing system that uses the momentum of the water during the fill cycle to try and mix the entire contents of the tank.  There are several drawbacks to this approach.

First – retrofitting your tank with separate inlets and outlets, or with multiple nozzles, is expensive, and requires that you take your tank out of service for an extended period of time.

Second – this approach “mixes” only during the fill cycle.  If turn-over is low, or if the tank is already thermally stratified, there isn’t enough energy to mix the entire volume in the tank.

Even separating your inlet so that water enters at the top of the tank may not be sufficient to mix. We have seen some tanks where cold water enters at the top of the tank and then falls to the bottom without mixing.

Q. How does active mixing help reduce biofilm formation?

A. Biofilms will naturally form on the surfaces inside your distribution system where disinfectant is low.  The top water inside a tank is typically the first place where disinfectant is lost, mainly due to evaporation. If the tank is unmixed, the warm water at the top of the tank loses all its residual and biofilms are free to grow unrestrained – and you will see the result in the form of lost residual and higher DBP levels.  

An active mixer keeps all the water in the tank moving, and ensures that fresh, residual-rich water touches all surfaces where biofilms can grow. By actively circulating the water, biofilms have no safe place to grow. The result is higher residual levels, more stable water quality, and lower DBPs.

Q. Can active mixing reduce the humidity in a tank?

A. Absolutely.  We have shown that actively mixed tanks have much cooler air temperatures inside. By lowering the air temperature the humidity level drops, and corrosion rates drop too.

We welcome your questions and comments.

Sincerely,

Peter S. Fiske, Ph.D.

CEO, PAX Water Technologies