In the past, many utilities and regulators evaluated the quality of drinking water within the distribution system based on and water entering the system and system averages. This paradigm is changing. The trend is now towards a concept of "environmental equity" where all consumers should receive similar water quality. This new paradigm is reflected in upcoming regulations. For example, the Stage I Disinfection By-Product (DBP) Rule will require monitoring at a location representing maximum residence time. To go a step further, Stage II DBP Rule compliance will be based on the running annual average at each monitoring location. The reason for this change is exemplified in Figures 1 and 2 which show the running annual average trihalomethane (THM) and haloacetic acid (HAA) levels at the filter effluent (filt), clearwell effluent (fin), a distribution system location near the plant (DSE), two sites of average residence time (AVG), and the location of maximum residence time (MAX) from a surface water plant which uses free chlorine for disinfection.
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