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(1) Full-time disinfection with chlorine, chlorine dioxide, chloramines, or a disinfectant that maintains a residual is mandatory where the source of water is from lakes, reservoirs, or streams, or ground water sources under the direct influence of surface water, or where the water may be exposed to a potential source of contamination including, but not limited to:

(a) losses of pressure within the system that could result in backflow or infiltration conditions;

(b) substandard distribution, pumping, or storage facilities; or

(c) other circumstances where the department determines that the history and nature of contamination indicates a residual is required for a safe water.

(2) Full-time microbial treatment that provides adequate inactivation or removal of harmful pathogens is mandatory whenever the water may be exposed to a potential source of contamination through:

(a) treatment processes, as determined by the department; or

(b) unprotected or poorly protected ground water sources.

(3) Methods of full-time microbial treatment must be reviewed and approved by the department prior to the installation or use of any form of treatment.

(4) When the department determines a residual is required in a ground water system, the residual disinfectant concentration measured as free chlorine, total chlorine, combined chlorine, chlorine dioxide, or other department approved disinfectant(s) must not be less than 0.2mg/l using the DPD method or 0.1mg/l using the amperometric titration method. A heterotrophic bacteria concentration in water in the distribution system less than or equal to 500 per milliliter, measured as heterotrophic plate count (HPC), is an acceptable substitute for disinfectant residual for purposes of determining compliance with this rule.


History: 75-6-104, MCA; IMP, 75-6-104, MCA; NEW, 1977 MAR p. 1187, Eff. 12/24/77; AMD, 1982 MAR p. 594, Eff. 3/26/82; AMD, 1991 MAR p. 1492, Eff. 8/16/91; TRANS, from DHES, 1996 MAR p. 1499; AMD, 1998 MAR p. 1167, Eff. 5/1/98; AMD, 2003 MAR p. 1279, Eff. 6/27/03; AMD, 2006 MAR p. 540, Eff. 2/24/06; AMD, 2009 MAR p. 1794, Eff. 10/16/09; AMD, 2024 MAR p. 253, Eff. 2/10/24.

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