12 Va. Admin. Code § 5-590-500 – Disinfection criteria, determination of CT, disinfection profiles, and disinfection benchmarks for Giardia and virus inactivation.

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. 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. 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.

TABLE 500.1 Maximum Log Removal Credits for Various Filtration Technologies and the Resulting Minimum Required Logs of Inactivation by Disinfection
FILTRATION PROCESS^a	MAXIMUM LOG REMOVAL CREDITS FOR FILTRATION	ADDITIONAL LOG INACTIVATION REQUIRED BY DISINFECTION
Giardia lamblia	Viruses	Giardia lamblia	Viruses
Conventional	2.5	2.0	0.5	2.0
Direct	2.0	1.0	1.0	3.0
Pre-engineered package	2.5	Zero	0.5	4.0
Diatomaceous Earth	2.0	1.0	1.0	3.0
Slow Sand	2.0	2.0	1.0	2.0
Membrane (MF or UF)	3.0	Zero	0.5	4.0
Bag or Cartridge	2.0	Zero	1.0	4.0
^aRefer to Part III of this chapter for further description of the filtration processes.

D. A disinfection profile shall be developed in accordance with the procedures in subdivisions D 1, D 2, and D 3.
1. 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. 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.
  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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. 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.
  1. 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.
  2. 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. 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:
  1. a. Determine one inactivation ratio (CT_calc/CT_99.9) before or at the first customer during peak hourly flow.
  2. b. Determine successive CT_calc/CT_99.9values, 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. 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. 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. 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. 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. 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. 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.

TABLE 500.2 CT Values for 3-Log Inactivation of Giardia lamblia by Free Chlorine at Less than 0.5^oC
FREE RESIDUAL (mg/L)	pH
≤ 6.0	6.5	7.0	7.5	8.0	8.5	≤ 9.0
≤ 0.4	137	163	195	237	277	329	390
0.6	141	168	200	239	286	342	407
0.8	145	172	205	246	295	354	422
1.0	148	176	210	253	304	365	437
1.2	152	180	215	259	313	376	451
1.4	155	184	221	266	321	387	464
1.6	157	189	226	273	329	397	477
1.8	162	193	231	279	338	407	489
2.0	165	197	236	286	346	417	500
2.2	169	201	242	297	353	426	511
2.4	172	205	247	298	361	435	522
2.6	175	209	252	304	368	444	533
2.8	178	213	257	310	375	452	543
3.0	181	217	261	316	382	460	552

TABLE 500.3 CT Values for 3-Log Inactivation of Giardia lamblia by Free Chlorine at 5^oC
FREE RESIDUAL (mg/L)	pH
≤ 6.0	6.5	7.0	7.5	8.0	8.5	≤ 9.0
≤ 0.4	97	117	139	166	198	236	279
0.6	100	120	143	171	204	244	291
0.8	103	122	146	175	210	252	301
1.0	105	125	149	179	216	260	312
1.2	107	127	152	183	221	267	320
1.4	109	130	155	187	227	274	329
1.6	111	132	158	192	232	281	337
1.8	114	135	162	196	238	287	345
2.0	116	138	165	200	243	294	353
2.2	118	140	169	204	248	300	361
2.4	120	143	172	209	253	306	368
2.6	122	146	175	213	258	312	375
2.8	124	148	178	217	263	318	382
3.0	126	151	182	221	268	324	389

TABLE 500.4 CT Values for 3-Log Inactivation of Giardia lamblia by Free Chlorine at 10^oC
FREE RESIDUAL (mg/L)	pH
≤ 6.0	6.5	7.0	7.5	8.0	8.5	≤ 9.0
≤ 0.4	73	88	104	125	149	177	209
0.6	75	90	107	128	153	183	218
0.8	78	92	110	131	158	189	226
1.0	79	94	112	134	162	195	234
1.2	80	95	114	137	166	200	240
1.4	82	98	116	140	170	206	247
1.6	83	99	119	144	174	211	253
1.8	86	101	122	147	179	215	259
2.0	87	104	124	150	182	221	265
2.2	89	105	127	153	186	225	271
2.4	90	107	129	157	190	230	276
2.6	92	110	131	160	194	234	281
2.8	93	111	134	163	197	239	287
3.0	95	113	137	166	201	243	292

TABLE 500.5 CT Values for 3-Log Inactivation of Giardia lamblia by Free Chlorine at 15^oC
FREE RESIDUAL (mg/L)	pH
≤ 6.0	6.5	7.0	7.5	8.0	8.5	≤ 9.0
≤ 0.4	49	59	70	83	99	118	140
0.6	50	60	72	86	102	122	146
0.8	52	61	73	88	105	126	151
1.0	53	63	75	90	108	130	156
1.2	54	64	76	92	111	134	160
1.4	55	65	78	94	114	137	165
1.6	56	66	79	96	116	141	169
1.8	57	68	81	98	119	144	173
2.0	58	69	83	100	122	147	177
2.2	59	70	85	102	124	150	181
2.4	60	72	86	105	127	153	184
2.6	61	73	88	107	129	156	188
2.8	62	74	89	109	132	159	191
3.0	63	76	91	111	134	162	195

TABLE 500.6 CT Values for 3-Log Inactivation of Giardia lamblia by Free Chlorine at 20^oC
FREE RESIDUAL (mg/L)	pH
≤ 6.0	6.5	7.0	7.5	8.0	8.5	≤ 9.0
≤ 0.4	36	44	52	62	74	89	105
0.6	38	45	54	64	77	92	109
0.8	39	46	55	66	79	95	113
1.0	39	47	56	67	81	98	117
1.2	40	48	57	69	83	100	120
1.4	41	49	58	70	85	103	123
1.6	42	50	59	72	87	105	126
1.8	43	51	61	74	89	108	129
2.0	44	52	62	75	91	110	132
2.2	44	53	63	77	93	113	135
2.4	45	54	65	78	95	115	138
2.6	46	55	66	80	97	117	141
2.8	47	56	67	81	99	119	143
3.0	47	57	68	83	101	122	146

TABLE 500.7 CT Values for 3-Log Inactivation of Giardia lamblia by Free Chlorine at 25oC and Higher
FREE RESIDUAL (mg/L)	pH
≤ 6.0	6.5	7.0	7.5	8.0	8.5	≤ 9.0
≤ 0.4	24	29	35	42	50	59	70
0.6	25	30	36	43	51	61	73
0.8	26	31	37	44	53	63	75
1.0	26	31	37	45	54	65	78
1.2	27	32	38	46	55	67	80
1.4	27	33	39	47	57	69	82
1.6	28	33	40	48	58	70	84
1.8	29	34	41	49	60	72	86
2.0	29	35	41	50	61	74	88
2.2	30	35	42	51	62	75	90
2.4	30	36	43	52	63	77	92
2.6	31	37	44	53	65	78	94
2.8	31	37	45	54	66	80	96
3.0	32	38	46	55	67	81	97

TABLE 500.8 CT Values for Inactivation of Viruses by Free Chlorine, pH 6.0-9.0
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
0.5	1	2	3	4	5	6	7	8
2	6.0	5.8	5.3	4.9	4.4	4.0	3.8	3.6	3.4
3	9.0	8.7	8.0	7.3	6.7	6.0	5.6	5.2	4.8
4	12.0	11.6	10.7	9.8	8.9	8.0	7.6	7.2	6.8
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
9	10	11	12	13	14	15	16	17
2	3.2	3.0	2.8	2.6	2.4	2.2	2.0	1.8	1.6
3	4.4	4.0	3.8	3.6	3.4	3.2	3.0	2.8	2.6
4	6.4	6.0	5.6	5.2	4.8	4.4	4.0	3.8	3.6
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
18	19	20	21	22	23	24	25
2	1.4	1.2	1.0	1.0	1.0	1.0	1.0	1.0
3	2.4	2.2	2.0	1.8	1.6	1.4	1.2	1.0
4	3.4	3.2	3.0	2.8	2.6	2.4	2.2	2.0

TABLE 500.9 CT Values for Inactivation of Giardia lamblia by Chlorine Dioxide, pH 6.0-9.0
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
1	2	3	4	5	6	7	8	9
0.5	10.0	8.6	7.2	5.7	4.3	4.2	4.2	4.1	4.1
1	21.0	17.9	14.9	11.8	8.7	8.5	8.3	8.1	7.9
1.5	32.0	27.3	22.5	17.8	13.0	12.8	12.6	12.4	12.2
2	42.0	35.8	29.5	23.3	17.0	16.6	16.2	15.8	15.4
2.5	52.0	44.5	37.0	29.5	22.0	21.4	20.8	20.2	19.6
3	63.0	53.8	44.5	35.3	26.0	25.4	24.8	24.2	23.6
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
10	11	12	13	14	15	16	17	18
0.5	4.0	3.8	3.7	3.5	3.4	3.2	3.1	2.9	2.8
1	7.7	7.4	7.1	6.9	6.6	6.3	6.0	5.8	5.5
1.5	12.0	11.6	11.2	10.8	10.4	10.0	9.5	9.0	8.5
2	15.0	14.6	14.2	13.8	13.4	13.0	12.4	11.8	11.2
2.5	19.0	3.8	3.7	3.5	3.4	3.2	3.1	2.9	2.8
3	23.0	7.4	7.1	6.9	6.6	6.3	6.0	5.8	5.5
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
19	20	21	22	23	24	25
0.5	2.6	2.5	2.4	2.3	2.2	2.1	2.0
1	5.3	5.0	4.7	4.5	4.2	4.0	3.7
1.5	8.0	7.5	7.1	6.7	6.3	5.9	5.5
2	10.6	10.0	9.5	8.9	8.4	7.8	7.3
2.5	2.6	2.5	12.2	11.4	10.6	9.8	9.0
3	5.3	5.0	14.2	13.4	12.6	11.8	11.0

TABLE 500.10 CT Values for Inactivation of Virus by Chlorine Dioxide, pH 6.0-9.0
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
1	2	3	4	5	6	7	8	9
2	8.4	7.7	7.0	6.3	5.6	5.3	5.0	4.8	4.5
3	25.6	23.5	21.4	19.2	17.1	16.2	15.4	14.5	13.7
4	50.1	45.9	41.8	37.6	33.4	31.7	30.1	28.4	26.8
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
10	11	12	13	14	15	16	17	18
2	4.2	3.9	3.6	3.4	3.1	2.8	2.7	2.5	2.4
3	12.8	12.0	11.1	10.3	9.4	8.6	8.2	7.7	7.3
4	25.1	23.4	21.7	20.1	18.4	16.7	15.9	15.0	14.2
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
19	20	21	22	23	24	25
2	2.2	2.1	2.0	1.8	1.7	1.5	1.4
3	6.8	6.4	6.0	5.6	5.1	4.7	4.3
4	13.3	12.5	11.7	10.9	10.0	9.2	8.4

TABLE 500.11 CT Values for Inactivation of Giardia lamblia by Chloramines, pH 6.0-9.0
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
1	2	3	4	5	6	7	8	9
0.5	635	568	500	433	365	354	343	332	321
1	1,270	1,136	1,003	869	735	711	687	663	639
1.5	1,900	1,700	1,500	1,300	1,100	1,066	1,032	998	964
2	2,535	2,269	2,003	1,736	1,470	1,422	1,374	1,326	1,278
2.5	3,170	2,835	2,500	2,165	1,830	1,772	1,714	1,656	1,598
3	3,800	3,400	3,000	2,600	2,200	2,130	2,060	1,990	1,920
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
10	11	12	13	14	15	16	17	18
0.5	310	298	286	274	262	250	237	224	211
1	615	592	569	546	523	500	474	448	422
1.5	930	894	858	822	786	750	710	670	630
2	1,230	1,184	1,138	1,092	1,046	1,000	947	894	841
2.5	1,540	1,482	1,424	1,366	1,308	1,250	1,183	1,116	1,049
3	1,850	1,780	1,710	1,640	1,570	1,500	1,420	1,340	1,260
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
19	20	21	22	23	24	25
0.5	198	185	173	161	149	137	125
1	396	370	346	322	298	274	250
1.5	590	550	515	480	445	410	375
2	788	735	688	641	594	547	500
2.5	982	915	857	799	741	683	625
3	1,180	1,100	1,030	960	890	820	750

TABLE 500.12 CT Values for Inactivation of Virus by Chloramines, pH 6.0-9.0
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
1	2	3	4	5	6	7	8	9
2	1,243	1,147	1,050	954	857	814	771	729	686
3	2,063	1,903	1,743	1,583	1,423	1,352	1,281	1,209	1,138
4	2,883	2,659	2,436	2,212	1,988	1,889	1,789	1,690	1,590
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
10	11	12	13	14	15	16	17	18
2	643	600	557	514	471	428	407	385	364
3	1,067	996	925	854	783	712	676	641	605
4	1,491	1392	1292	1193	1,093	994	944	895	845
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
19	20	21	22	23	24	25
2	342	321	300	278	257	235	214
3	570	534	498	463	427	392	356
4	796	746	696	646	597	547	497

TABLE 500.13 CT Values for Inactivation of Giardia lamblia by Ozone
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
1	2	3	4	5	6	7	8	9
0.5	0.48	0.44	0.40	0.36	0.32	0.30	0.28	0.27	0.25
1	0.97	0.89	0.80	0.72	0.63	0.60	0.57	0.54	0.51
1.5	1.50	1.36	1.23	1.09	0.95	0.90	0.86	0.81	0.77
2	1.90	1.75	1.60	1.45	1.30	1.23	1.16	1.09	1.02
2.5	2.40	2.20	2.00	1.80	1.60	1.52	1.44	1.36	1.28
3	2.90	2.65	2.40	2.15	1.90	1.81	1.71	1.62	1.52
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
10	11	12	13	14	15	16	17	18
0.5	0.23	0.22	0.20	0.19	0.17	0.16	0.15	0.14	0.14
1	0.48	0.45	0.42	0.38	0.35	0.32	0.30	0.29	0.27
1.5	0.72	0.67	0.62	0.58	0.53	0.48	0.46	0.43	0.41
2	0.95	0.89	0.82	0.76	0.69	0.63	0.60	0.57	0.54
2.5	1.20	1.12	1.04	0.95	0.87	0.79	0.75	0.71	0.68
3	1.43	1.33	1.24	1.14	1.05	0.95	0.90	0.86	0.81
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
19	20	21	22	23	24	25
0.5	0.13	0.12	0.11	0.10	0.10	0.09	0.08
1	0.26	0.24	0.22	0.21	0.19	0.18	0.16
1.5	0.38	0.36	0.34	0.31	0.29	0.26	0.24
2	0.51	0.48	0.45	0.42	0.38	0.35	0.32
2.5	0.64	0.60	0.56	0.52	0.48	0.44	0.40
3	0.13	0.12	0.11	0.10	0.10	0.09	0.08

TABLE 500.14 CT Values for Inactivation of Virus by Ozone
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
1	2	3	4	5	6	7	8	9
2	0.90	0.83	0.75	0.68	0.60	0.58	0.56	0.54	0.52
3	1.40	1.28	1.15	1.03	0.90	0.88	0.86	0.84	0.82
4	1.80	1.65	1.50	1.35	1.20	1.16	1.12	1.08	1.04
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
10	11	12	13	14	15	16	17	18
2	0.50	0.46	0.42	0.38	0.34	0.30	0.29	0.28	0.27
3	0.80	0.74	0.68	0.62	0.56	0.50	0.48	0.46	0.44
4	1.00	0.92	0.84	0.76	0.68	0.60	0.58	0.56	0.54
LOG INACTIVATION CREDIT	WATER TEMPERATURE (°C)
19	20	21	22	23	24	25
2	0.26	0.25	0.23	0.21	0.19	0.17	0.15
3	0.42	0.40	0.37	0.34	0.31	0.28	0.25
4	0.52	0.50	0.46	0.42	0.38	0.34	0.30

TABLE 500.15 Baffling Classifications
BAFFLING CONDITION	T₁₀/T	BAFFLING DESCRIPTION
Unbaffled (mixed flow)	0.1	None, agitated basin, very low length-to-width ratio, high inlet and outlet flow velocities
Poor	0.3	Single or multiple unbaffled inlets and outlets, no intra-basin baffles
Average	0.5	Baffled inlet or outlet with some intra-basin baffles
Superior	0.7	Perforated inlet baffle, serpentine or perforated intra-basin baffles, outlet weir or perforated launders

Statutory Authority

§§ 32.1-12 and 32.1-170 of the Code of Virginia.

Historical Notes

Derived from VR355-18-005.06 § 2.17, eff. August 1, 1991; amended, Virginia Register Volume 9, Issue 17, eff. June 23, 1993; Volume 18, Issue 19, eff. July 3, 2002; Volume 21, Issue 13, eff. April 6, 2005; Volume 25, Issue 5, eff. December 10, 2008; Volume 28, Issue 5, eff. December 7, 2011; Volume 37, Issue 20, eff. June 23, 2021.