When a public water system (PWS) monitors for chlorine in the distribution system, should the residual disinfectant be measured as free chlorine or total chlorine?
A PWS must measure residual disinfectant concentrations with one of the analytical methods approved by EPA for this purpose (40 CFR 141.74(a)(2)). The table in 40 CFR 141.74(a)(2) includes EPA approved analytical methods for residual disinfectant based on both free chlorine and total chlorine.
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