PAX Water Technologies presented a technical talk at the 25th annual Association of State Drinking Water Administrators meeting last week in Pittsburgh, PA entitled Stabilization and Improvement of Distribution System Water Quality in Reservoirs and Tanks – a Key to Stage 2 Compliance.

We got a slew of great questions after the presentation - here are the top ones:

1. How does active mixing lower DBP levels in the distribution system?

Answer:  Active mixing helps in three ways.

First, active mixing ensures that disinfectant residual is distributed throughout a tank, reducing the growth of new biofilms that introduce additional organic carbon into the tank.

Second, active mixing eliminates thermal stratification that can produce zones of warm, residual-depleted water - a breeding ground for bacteria.

Third, active mixing circulates the water up to the surface where volatile DBPs can evaporate into the air.

2. Why does active mixing lower residual demand inside a tank?

Answer:  Residual is consumed by chemical reactions with organic matter (this also produces DBPs). Unmixed tanks often have a few places where organic matter in the form of biofilms can grow. Once these biofilms start growing, they further challenge the remaining residual in the tank. We have seen some (chloraminated) tanks that experience a cycle of biofilm growth, followed by residual depletion, followed by a rise in nitrite levels (indicating the beginnings of nitrification). Operators of these tanks repeatedly dose their tank to restore disinfectant levels, but after a few weeks the cycle starts over again. Once the tanks are actively mixed, the circulation of residual-rich inlet water keeps biofilms at bay and ends the cycle of residual depletion.  In the case of Old Town, Maine, the installation of active mixers resulted in a dramatic lowering of residual demand not only in the tank but throughout the distribution system, allowing the utility to lower chemical feed rates.

3. Small systems often have the most challenges with DBPs - and they are the least able to pay for new mitigation technologies such as active mixers - what can they do?

Answer:  The first thing we encourage any small system facing DBP compliance issues to do is weigh all their options. Compliance with Stage 2 rules is not a choice. How they address their issues is ALL ABOUT CHOICES.  While there are a number of mixing technologies on the market, only the PAX Water Mixer comes with a performance and money-back guarantee.  If some systems are too small to afford ANY additional technology, they may be eligible for SRF or USDA funding to offset the cost of the equipment. Finally, PAX and its partner Utility Service Company offer financing to spread the cost out over several years.  In the end analysis, we believe that the PAX Water Mixer is the most economical way to improve hydraulics inside water storage tanks and ensure that biofilm growth is kept in check.

We encourage municipalities to carry out their own analysis of their options. 

To learn more about active mixing as a tool for Stage 2 compliance, join us on November 4th for a free webinar for operators.

We welcome your thoughts and comments.

Sincerely,

Peter S. Fiske, Ph.D.

CEO, PAX Water Technologies