Mixing Restores Disinfectant Residuals And Prevents Nitrification

The Problem
Redwood City Water operates a 500,000 gallon, ground-level steel tank located in the bay area of Northern California, and purchases chloraminated water from a regional wholesale supplier. During the summertime, the total chlorine residual of samples taken near the bottom of the tank typically remained at the target level of 2.0mg/L, but residual levels in samples taken from the top few feet of the water inside the tank had dropped significantly, sometimes to undetectable levels. This was of particular concern due to the increased risk of nitrification in upper layers of the tank, and the associated mitigation costs and negative impacts on water quality from such an event. Further investigation revealed that thermal stratification in the tank was inhibiting mixing in the upper layers of the tank, leading to excessive water age and residual loss.

The Solution
An effective solution for preventing the onset of nitrification is in-tank mixing. Mixing can prevent stratification and ensure that fresh incoming water with high disinfectant residual mixes with the older, low-residual, stratified water.

more...