The Surface Water Treatment Rule requires the residual disinfectant concentration in water entering the distribution system to be no less than 0.2 mg/L for more than four consecutive hours (40 CFR 141.72(a)(3) and (b)(2)). How did EPA arrive at the four-h
EPA believes that some time allowance should be allotted for systems to restore the disinfectant residual rather than categorically defining this absence as a treatment technique violation. Once systems are aware that the disinfectant concentration level is low or absent, four hours is a reasonable amount of time for operators to adjust and/or repair the disinfection or monitoring equipment or to bring backup disinfection or monitoring units on-line (54 FR 27486, 27494; June 29, 1989).
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