State and federal water quality regulators, and the operators of drinking water systems, face a common challenge: how to achieve compliance with increasingly strict state and federal water quality regulations.

To reduce the risk of contamination by water-borne pathogens, the Long Term 2 Enhanced Surface Water Treatment Rule calls for better protection through increased treatment efficiency and maintenance of disinfectant residual throughout the system.  At the same time, the Stage 2 Disinfectant and Disinfectant Byproducts Rule calls for lower disinfectant by-products throughout the system.

One rule seems to call for increasing the use of disinfectant while the other seems to call for less disinfectant!  It turns out that this seemingly paradoxical set of mandates can be achieved by improving the effectiveness of the disinfectant residual itself through active mixing.  [See our article, Doing More With Less In The Distribution System: A Mandate of Stage 2 Regulations, from Water Online Magazine.]

Potable water storage tanks can be a major source of disinfectant loss in the distribution system.  Biofilms, which like to grow on tank walls and in the nooks and crannies around flanges, corners and appurtenances, react with disinfectant residual and can "use up" a tank's remaining residual.  Tanks that are stagnant can form dead zones where disinfectant is entirely used up, allowing biofilms to proliferate.  These "high-demand tanks" are like a Roach Motel for disinfectant: healthy levels of disinfectant goes in, but nothing comes out.

Active mixers help in several ways.  First, by completely mixing the water inside a tank, the mixer ensures that disinfectant residual reaches every nook and cranny inside a tank, preventing the growth of biofilms.  Furthermore, by completely circulating the water, mixing eliminates dead zones where disinfectant can become depleted.  As one experienced operator told us: "Water is like a kid: if you give it something to do, it'll stay out of trouble!"

Old Town, Maine, is one municipality that has installed submersible active mixers in a majority of their storage tanks.  The results were dramatic: disinfectant residual levels throughout their system improved greatly, and the operator was able to reduce disinfectant concentrations leaving the plant by 30% while experiencing increased levels of residual in their distribution system. (Figure 1).

PAX Water will be hosting a webinar aimed at water quality regulators at the state and federal level on June 9th, 2010.  Registration is free, but space is limited. 

We will discuss these latest results, and other benefits of using active mixing in your tanks and reservoirs.

We welcome your comments and questions.

Sincerely,

Peter S. Fiske, Ph.D.

CEO, PAX Water Technologies