It is well known that disinfectant residual levels drop as potable water makes its way through the distribution system. It is also generally believed that the rate of disinfectant loss is a function mainly of time – as water sits in the distribution system, the residual is consumed as it reacts with naturally occurring organic matter.
The Old Town Water District, which serves the municipality of Old Town, Maine (near Bangor) presents data in the latest issue of Opflow titled, Fire and Ice: Tame Temperature Extremes in Water Tanks, that shows that this simple water age/disinfectant loss model may need revision.
In their study (which PAX participated in), they installed PAX mixers in two of the three tanks in their distribution system. Initially, their motivation for installing mixers was simply to prevent ice damage during winter. But during the summer months they noticed a substantial decrease in residual loss in their distribution system.
Before installing the first mixer, they would typically lose around 1.2 mg/l of chlorine in their distribution system. But after installing the mixer, chlorine consumption dropped by more than half, which allowed them to reduce the amount of chlorination they did at the plant. After installation of a second mixer, the chlorine loss dropped again.
No other changes were made to the distribution system. The average water age in the distribution system remained the same. The big improvement in residual in the distribution system appears to be the result of the mixers.
How could this happen? We at PAX have worked with many customers who find that some of their storage tanks appear to consume MORE disinfectant than would be predicted based on water age alone.
We suspect that these tanks suffer from poor hydraulics, which prevents fresh, residual-rich inlet water from mixing in the tank. When inlet water does not mix, zones within the tank (principally the sides near the water line) can allow biofilms to grow. These biofilms introduce additional organic carbon to the tank, and tax the remaining disinfectant residual. When the tank is thoroughly mixed, the biofilms are burned back and the overall residual demand in the tank drops.
We believe this will also have a direct impact on DBP levels. As you know, DBPs are the product of the chemical reaction between organic matter and disinfectant. By lowering the consumption of disinfectant in these tanks, you lower DBP production. Water in a well-mixed tank is also cooler than water in an unmixed tank, and lower temperatures slow down the chemical reactions that lead to DBPs.
We welcome your comments and questions.
Peter S. Fiske, Ph.D.
CEO, PAX Water Technologies