Code of Colorado Regulations
1000 - Department of Public Health and Environment
1002 - Water Quality Control Commission (1002 Series)
5 CCR 1002-11 - REGULATION NO. 11 - COLORADO PRIMARY DRINKING WATER REGULATIONS
Section 5 CCR 1002-11.23 - MAXIMUM RESIDUAL DISINFECTANT LEVELS RULE

Universal Citation: 5 CO Code Regs 1002-11 ยง 23

Current through Register Vol. 47, No. 5, March 10, 2024

11.23(1) Chlorine and Chloramines MRDL

(a) Applicability for Chlorine and Chloramines MRDL

For all community and non-transient, non-community water systems that supply water treated with chlorine or chloramines, the supplier must comply with the requirements specified in this section, 11.23(1).

(b) MRDL Requirements for Chlorine and Chloramines
(i) The chlorine and chloramines MRDLs are as follows:

TABLE 11.23-I MRDLs FOR CHLORINE AND CHLORAMINES

Disinfectant

MRDL (mg/L as Cl2)

Chlorine

4.0

Chloramines

4.0

(ii) The BATs for achieving compliance with the MRDLs for chlorine and chloramines are specified in 40 CFR 141.65(c).

(iii) To protect public health, the supplier may increase residual disinfectant concentration in the distribution system to a level greater than (>) the MRDL for a time necessary to address specific microbiological contamination problems caused by circumstances including but not limited to:
(A) Distribution system line breaks.

(B) Storm run-off events.

(C) Source water contamination events.

(D) Backflow contamination events.

(c) Monitoring Requirements for Chlorine and Chloramines
(i) To determine compliance with the MRDLs for chlorine and/or chloramines, the supplier must monitor the residual disinfectant concentration in the distribution system at the same time and at the same locations that total coliform samples are collected under 11.16, as identified in the supplier's sample siting plan under 11.16(3).
(A) The supplier may use the results of samples collected under 11.8(3)(c)(i)(B) or 11.11(2)(c)(i)(B) to satisfy both the requirements specified in this section, 11.23(1), and 11.8(3)(c)(i)(B) or 11.11(2)(c)(i)(B).

(d) Compliance Determination for Chlorine and Chloramines
(i) Compliance with the MRDL for chlorine or chloramines is determined quarterly based on the RAA of all sample results collected.
(A) If the supplier collects more than one sample in a month, the supplier must average all sample results collected that month to get the monthly average.
(I) The supplier must use the monthly average in the RAA calculation.

(B) If the supplier switches between the use of chlorine and chloramines for disinfection during the year, the supplier must include all sample results for both chlorine and chloramines in calculating the RAA and determining compliance.

(C) If the supplier collects samples under the conditions specified in 11.23(1)(b)(iii), the supplier should not include those sample results in calculating the RAA.

(e) MRDL Violations for Chlorine and Chloramines
(i) The following constitute chlorine and/or chloramines MRDL violations:
(A) The RAA is greater than (>) the MRDL for chlorine and/or chloramines.

(B) The RAA, calculated before four consecutive quarters of samples have been collected, is greater than (>) the MRDL for chlorine and/or chloramines regardless of the subsequent sample results.

(f) Response to MRDL Violations for Chlorine and Chloramines
(i) In the event of a chlorine and/or chloramines MRDL violation, the supplier must:
(A) Notify the Department no later than 48 hours after the violation occurs.

(B) Distribute Tier 2 public notice as specified in 11.33.

(g) Reporting Requirements for Chlorine and Chloramines

(i) The supplier must submit all of the following information no later than the 10th of the month following the end of each quarter in which samples were collected.
(A) The number of samples collected during each month of the last quarter.

(B) The monthly average of all sample results collected in each month for the last 12 months.

(C) The RAA of the monthly averages.

(D) Indicate which residual disinfectant was analyzed for each sample.

(E) Whether the MRDL was violated.

(ii) The Department may complete the calculations and determine whether the MRDL was violated, instead of having the supplier report that information.

11.23(2) Chlorine Dioxide MRDL

(a) Applicability for Chlorine Dioxide MRDL

For all systems that use chlorine dioxide for disinfection or oxidation, the supplier must comply with the requirements specified in this section, 11.23(2), when using chlorine dioxide.

(b) MRDL Requirements for Chlorine Dioxide
(i) The chlorine dioxide MRDL is as follows:

TABLE 11.23-II MRDL FOR CHLORINE DIOXIDE

Disinfectant

MRDL (mg/L as ClO2)

Chlorine dioxide

0.8

(ii) The BATs for achieving compliance with the MRDLs for chlorine dioxide are specified in 40 CFR 141.65(c).

(c) Monitoring Requirements for Chlorine Dioxide
(i) To determine compliance with the chlorine dioxide MRDL, the supplier must monitor the residual disinfectant concentration daily at each entry point during normal operating conditions and as identified in the monitoring plan developed under 11.5(3)(a)(v).
(A) If any daily entry point sample result is greater than (>) the MRDL, the next day, the supplier must collect three chlorine dioxide residual disinfectant concentration samples in the distribution system. The three samples must be collected at least six hours apart and at a location that is as close to the first customer as possible unless:
(I) The supplier uses chlorine to maintain a residual disinfectant concentration in the distribution system and there are one or more points where disinfection is added after the entry point (i.e., booster chlorination), then the supplier must collect one sample at each of the following locations:
(a) As close to the first customer as possible.

(b) At a location representative of average residence time.

(c) At a location representative of maximum residence time.

(d) Compliance Determination for Chlorine Dioxide

If the supplier collects more than the required number of samples, all sample results must be used to determine compliance.

(e) MRDL Violations for Chlorine Dioxide
(i) The following constitute acute chlorine dioxide MRDL violations:
(A) Any daily entry point sample result is greater than (>) the MRDL and on the next day one or more of the three additional distribution system sample results is greater than (>) the MRDL.

(B) The supplier fails to collect any of the three additional distribution system samples the day after a daily entry point sample result was greater than (>) the MRDL.

(ii) The following constitute non-acute chlorine dioxide MRDL violations:
(A) Any two consecutive daily entry point sample results are greater than (>) the MRDL and all three additional distribution system results are less than or equal to ([LESS THAN EQUAL TO]) the MRDL.

(B) The supplier fails to collect a daily entry point sample the day after a daily entry point sample result was greater than (>) the MRDL.

(f) Response to MRDL Violations for Chlorine Dioxide
(i) In the event of an acute chlorine dioxide MRDL violation, the supplier must:
(A) Notify the Department of the violation and initiate consultation as soon as possible but no later than 24 hours after the violation occurs.

(B) Distribute Tier 1 public notice as specified in 11.33.

(C) For an acute MRDL violation specified in 11.23(2)(e)(i)(A), take immediate corrective action to lower the level of chlorine dioxide to less than (<) the MRDL.

(ii) In the event of a non-acute chlorine dioxide MRDL violation, the supplier must:
(A) Notify the Department no later than 48 hours after the violation occurs.

(B) Distribute Tier 2 public notice as specified in 11.33.

(C) For an MRDL violation specified in 11.23(2)(e)(ii)(A), take corrective action to lower the level of chlorine dioxide to less than (<) the MRDL at the entry point.

(g) Reporting Requirements for Chlorine Dioxide

(i) The supplier must submit the following information no later than the 10th of the month following the end of each quarter in which samples were collected:
(A) The dates, results, and locations of samples collected during the last quarter.

(B) Whether an MRDL violation occurred and whether the resulting violation was acute or non-acute.

(ii) The Department may complete the calculations and determine whether the MRDL was violated, instead of having the supplier report that information.

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