Virginia Law

A. A waterworks utilizing surface water sources or GUDI sources, in whole or in part, shall be disinfected in accordance with subsection C of this section and 12VAC5-590-1000.

B. The owner of a groundwater system subject to the requirements of 12VAC5-590-421 A 1 d shall provide primary disinfection treatment by means of one of the following:

1. A residual disinfectant concentration (C) and contact time (T) to achieve a 4-log removal or inactivation of viruses. CT shall be calculated in accordance with subsections C and D of this section, which contain information on calculation methods and contact tank baffling factors.

2. UV treatment to achieve a 4-log removal or inactivation of viruses. Log inactivation shall be determined in accordance with 12VAC5-590-401 E 7 c. A secondary disinfection residual in the distribution system may be required by the department.

C. Disinfection criteria.

1. An owner of a waterworks utilizing surface water sources, in whole or in part, or GUDI sources shall provide a minimum 3-log (99.9%) removal of Giardia cysts and a 4-log (99.99%) removal of viruses, respectively.

2. Additional inactivation levels that must be achieved by disinfection shall be in accordance with Table 500.1 for waterworks employing the filtration processes listed.

D. A disinfection profile shall be developed in accordance with the procedures in subdivisions D 1, D 2, and D 3.

1. The owner shall monitor at least weekly for a period of 12 consecutive months to determine the total log inactivation for Giardia lamblia and viruses. If an owner monitors more frequently, then the monitoring frequency shall be evenly spaced. An owner of a waterworks that operates for fewer than 12 months per year shall monitor weekly during the period of operation.

2. The owner of a waterworks with a single point of disinfectant application before the entrance to the distribution system or with more than one point of disinfectant application shall conduct the monitoring in subdivisions D 2 a through D 2 e of this section.

a. For a waterworks using a disinfectant other than UV, the temperature of the disinfected water shall be measured at each residual disinfectant concentration sampling point during peak hourly flow or at an alternative location approved by the department.

b. For a waterworks using chlorine, the pH of the disinfected water shall be measured at each chlorine residual disinfectant concentration sampling point during peak hourly flow or at an alternative location approved by the department.

c. The residual disinfectant concentration (C) of the water before or at the first customer and before each additional point of disinfectant application shall be measured at peak hourly flow.

d. The disinfectant contact times (T) of the water before or at the first customer and before each additional point of disinfectant application shall be determined during peak hourly flow. The disinfectant contact time to be used for calculating CT is T₁₀, which is the detention time at which 90% of the water passing through a unit is retained within that unit. T₁₀ shall be determined either by calculations that involve the theoretical hydraulic detention time and baffling factors that account for the degree of short-circuiting that might be expected through any given unit or by tracer studies. The baffling factors listed in Table 500.15 shall be used in determining contact time if tracer studies are not performed.

e. Inactivation credits for ozone contactors will be based on only the chambers that have a measured ozone residual. A minimum of two dedicated online monitors per ozone contactor shall be installed at locations suited to the CT calculation method used. Ozone residual levels shall be monitored continuously and recorded. Methods for computing log inactivation of Giardia lamblia and virus shall be approved by the department. Tracer studies shall be required to verify T₁₀ values before receiving inactivation credit.

3. Instead of conducting new monitoring under subdivision D 2 of this section, an owner may elect to meet the requirements of subdivision D 3 a or D 3 b of this section.

a. The owner who has at least one year of existing data that are substantially equivalent to data collected under the provisions of subdivision D 3 of this section shall use these data to develop disinfection profiles if the owner has neither made a significant change to the treatment practice nor changed sources since the data were collected. The owner may develop a disinfection profile using up to three years of existing data.

b. The owner may use a disinfection profile developed previously in accordance with the procedures in subdivisions D 1, D 2, and D 3 of this section, if the owner has neither made a change to the treatment practice nor changed sources since the profile was developed. An owner that has not developed a virus profile shall develop a virus disinfection profile using the same monitoring data on which the Giardia lamblia profile is based.

E. The owner shall calculate the total inactivation ratio for Giardia lamblia and viruses as specified in subdivisions E 1 through E 4 of this subsection based on CT_99.9 (3-log) values using the appropriate values in Tables 500.2 through 500.14. Note that the 3-log values in the tables for Giardia lamblia also indicate that a 4-log virus inactivation can be achieved. pH and temperature values between the indicated values in Tables 500.2 through 500.14 shall be determined by linear interpolation, or the CT value at the lower temperature and at the higher pH shall be used. All parameters necessary to determine the total inactivation ratio shall be monitored during peak hourly flow.

1. The owner using only one point of disinfectant application shall determine the total inactivation ratio for the disinfection segment based on either of the following methods:

a. Determine one inactivation ratio (CT_calc/CT_99.9) before or at the first customer during peak hourly flow.

b. Determine successive CT_calc/CT_99.9 values, representing sequential inactivation ratios, between the point of disinfectant application and a point before or at the first customer during peak hourly flow. The owner shall calculate the total inactivation ratio by determining (CT_calc/CT_99.9) for each sequence and then adding the (CT_calc/CT_99.9) values together to determine total inactivation (∑ (CT_calc/CT_99.9)).

2. The owner using more than one point of disinfectant application before the first customer shall determine the CT value of each disinfection segment immediately before the next point of disinfectant application, or for the final segment before or at the first customer, during peak hourly flow. The (CT_calc/CT_99.9) value of each segment and (∑ (CT_calc/CT_99.9)) shall be calculated using the method in subdivision E 1 a or E 1 b of this subsection.

3. The owner shall determine the total logs of inactivation of Giardia lamblia by multiplying the value calculated in subdivision E 1 a or E 1 b of this subsection by 3.0.

4. The owner shall determine the total logs of inactivation of viruses by multiplying the value calculated in subdivision E 1 a or E 1 b of this subsection by 4.0.

F. A disinfection benchmark shall be calculated following the procedures in subdivisions F 1, F 2, and F 3 of this subsection.

1. For each year of profiling data collected and calculated, an owner shall determine the lowest mean monthly level of both Giardia lamblia and virus inactivation. The owner shall determine the mean Giardia lamblia and virus inactivation for each calendar month for each year of profiling data by dividing the sum of daily or weekly Giardia lamblia and virus log inactivation by the number of values calculated for that month.

2. The disinfection benchmark is the lowest monthly mean value (for waterworks with one year of profiling data) or the mean of the lowest monthly mean values (for waterworks with more than one year of profiling data) of Giardia lamblia and virus log inactivation in each year of profiling data.

3. The owner of a waterworks using chloramines, ozone, or chlorine dioxide for primary disinfection shall calculate the disinfection benchmark for viruses from the data collected in the same manner used to calculate the Giardia lamblia disinfection benchmark.

G. The owner shall retain the disinfection profile in graphic form, as a spreadsheet or in some other format acceptable to the department for evaluation as part of sanitary surveys conducted by the department.

H. Before making a significant change to the waterworks disinfection practice, the owner shall review the disinfection benchmark and consult with the department.

1. Significant changes to disinfection practice are (i) changes to the point of disinfectant application, (ii) changes to the disinfectants used in the treatment plant, (iii) changes to the disinfection process, and (iv) any other modification identified by the department.

2. The owner shall submit the following information to the department as part of the consultation process: (i) a description of the proposed change; (ii) the disinfection profile and benchmarks established for Giardia lamblia and, if necessary, viruses; (iii) an analysis of how the proposed change will affect the current levels of disinfection; and (iv) any additional information to justify the change.