DBP Measurements Reveal Valuable Information about Distribution Systems
Authors:
- Jeffrey R. Cruickshank, P.E. - Hazen and Sawyer
Most water utilities recently have begun additional sampling for disinfection byproducts (DBPs) in response to EPAs Stage 2 DBP Rule. These measurements reveal valuable information about distribution systems and allow utilities to plan compliance strategies. This paper describes what several utilities have learned by studying their DBP samples and linking them to water age predictions from hydraulic models.
The 2006 Stage 2 DBP Rule required utilities to conduct Initial Distribution System Evaluations (IDSEs) that selected sampling sites where DBPs are expected to be highest. Compliance monitoring must begin by 2012 or 2013, depending on the population supplied, but most utilities have already started sampling at the new sites for informational purposes.
A good way of analyzing the DBP measurements is correlating them with water age predictions from a hydraulic model. By plotting DBP concentrations against predicted water age and comparing the measurements with EPA’s maximum contaminant levels (MCLs), utilities can see where they need to focus their compliance strategy. In addition, measurements that don’t fit correlation trends can indicate problems that can be addressed before compliance monitoring begins.
One utility found that DBP concentrations were high even at sampling sites with low water age, showing little increase at sites with high water age. This showed that changing operating methods or residual disinfectants would not solve the DBP problem, and the focus should be on treatment processes inside the water plant.
Another utility measured DBP concentrations that were acceptable at sites with average water age but too high in outlying areas. This showed that compliance strategies should focus on reducing water age or slowing the rate of disinfectant decay.
Several utilities investigated DBP measurements that were higher than expected from water age correlation trends and discovered closed valves and other operational problems.
This paper will give other utilities ideas about what they can learn from their DBP measurements so they can take proactive steps to ensure compliance with MCLs.
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