Administrative Code

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Virginia Administrative Code
Title 12. Health
Agency 5. Department of Health
Chapter 640. Alternative Discharging Sewage Treatment Regulations for Individual Single Family Dwellings

12VAC5-640-460. Design requirements for system components.

A. All discharging systems shall be equipped with a means of disinfecting the effluent which is acceptable to the division and meets the performance requirements of this chapter.

1. All discharging systems utilizing chlorine as a disinfectant shall be equipped with a chlorinator and contact chamber. Dechlorination is to be supplied if required by the General Permit.

a. Chlorinator capacity shall be based on the degree of treatment, flow variations, and other variables in the treatment processes. For disinfection, the capacity shall be adequate to maintain a total chlorine residual between 1.0 mg/l and 3.0 mg/l in the effluent after the required contact period. All chlorinators shall be designed to provide the appropriate dose of chlorine and mix the chlorine with the effluent. All chlorine products used to disinfect effluent from a discharging system shall be approved by the U.S. Environmental Protection Agency for use as a sewage disinfectant; products unapproved for wastewater disinfection are not acceptable. Use of unapproved products shall constitute a violation of this chapter.

b. The chlorine contact chamber shall have a length to width ratio of 20:1 and shall provide a contact time of 30 minutes based on peak hourly flow, or 60 minutes based on peak daily flow. The length to width ratio may be reduced on a case-by-case basis when increased chlorine contact times are utilized.

c. When required by the General Permit, dechlorination capacity shall be adequate to dechlorinate the maximum chlorine residual anticipated and achieve the required General Permit effluent limits for total residual chlorine by providing at least 1-1/2 parts sulfite salt to one part chlorine. Provisions shall be made to thoroughly mix the dechlorinating agent with the contact tank effluent within a period of approximately one minute.

d. To meet Reliability Class I or Class II, all chlorination and dechlorination units shall be alarmed to notify the operator when tablets are not present in the dosing chamber or equipped with duplicate units that automatically switch over to the redundant unit if the primary unit is not operating.

2. Disinfection can be achieved through exposure of microorganisms to a sufficient level of ultraviolet light (UV) irradiation at the germicidal wavelength for an adequate period of time.

a. UV disinfection equipment shall be capable of providing a minimum average calculated dose of 50,000 microwatt-seconds per square centimeter after the UV lamps have been in operation for 7,500 hours or more and at a 65% transmissivity. The dosage may be reduced on a case-by-case basis when sufficient information is provided to demonstrate that the required level of disinfection can be obtained at a lower dose level through test data.

b. UV lamps shall produce 90% or more of their emitted light output at the germicidal wavelength of 253.7 nanometers.

c. UV lamp assemblies shall be so located as to provide convenient access for lamp maintenance and removal.

d. UV lamps should not be viewed in the ambient air without proper eye protection as required by VOSH and other applicable regulations. The system design should prevent exposure of bare skin to UV lamp emission for durations exceeding several minutes.

e. An elapsed time meter shall be provided to indicate the total operating time of the UV lamps.

f. UV systems are sensitive to color and suspended solids. Precautions should be taken to protect the UV system from both color and excessive suspended solids.

g. To meet Reliability Class I or Class II, all UV units shall be equipped with a sensor to detect bulb failure with an alarm or equipped with duplicate units that automatically switchover if the primary unit is not operating.

B. Post-aeration as required by the General Permit shall be provided to ensure that the final effluent complies with the dissolved oxygen effluent limits in the General Permit. Post-aeration may involve diffused aeration or cascade type aeration. All post-aeration designs shall assume a zero dissolved oxygen concentration in the influent wastewater to the post-aeration unit.

1. Effluent post-aeration may be achieved by the introduction of diffused air into the effluent.

a. Diffused aeration basins shall be designed to eliminate short-circuiting and the occurrence of dead spaces. For maximum efficiencies, sufficient detention time shall be provided to allow the air bubbles to rise to the surface of the wastewater prior to discharge from the basin.

b. When the detention time in the aeration basin exceeds 30 minutes, consideration shall be given to the oxygen requirements resulting from biological activity in the aeration unit.

c. Diffused air aeration systems shall be designed utilizing Fick's Law (the rate of molecular diffusion of a dissolved gas in a liquid) in the determination of oxygen requirements. Supporting experimental data shall be included with the submission of any proposal for the use of diffusers that are considered nonconventional. Such proposals will be evaluated on a case-by-case basis by the division.

d. Alternatively, an airflow of one cubic foot per minute at a diffuser submergence of one foot is sufficient to increase the dissolved oxygen of 1000 gallons per day of effluent to greater than five mg/l dissolved oxygen at 25°C.

e. If airflow is to be siphoned off the blower for the biological treatment unit, calculations shall be submitted to verify that there is sufficient air for both uses.

2. Effluent post-aeration may be achieved through a turbulent liquid-air interface established by passing the effluent downstream over either a series of constructed steps that produces a similar opportunity for transfer of dissolved oxygen to the effluent, otherwise known as cascade or step aeration.

a. The following equation shall be used in the design of cascade/step type aerators:

rn = (Cs-Ca)/(Cs-Cb)

where: r


Deficit ratio



Dissolved oxygen saturation (mg/l)



Dissolved oxygen concentration above the weir, assumed to be 0.0 mg/l



Dissolved oxygen concentration in the effluent from the last or preceding step



The number of equal size steps



1 + (0.11) (ab) (1 + 0.046 T) (h)

where: T


Water temperature (°C)



Height of one step (ft)



1.0 for effluents (BOD5 of less than 15 mg/l) or
0.8 for effluents (BOD5 of 15 mg/l to 30 mg/l)



1.0 for free fall and 1.3 for step weirs

b. The equation for determining the number of steps is dependent upon equidistant steps, and if unequal steps are used, transfer efficiencies must be determined for each separate step.

c. The effluent discharge to a cascade type aerator shall be over a sharp weir to provide for a thin sheet of wastewater. Consideration shall be given to prevention of freezing.

d. The final step of the cascade type aerator shall be above normal stream flow elevation and the cascade aerator shall be protected from erosion damage due to storm water drainage or flood/wave action.

e. When pumping is necessary prior to discharge over the cascade aerator, the range of the flow rate to the post-aeration unit must be accounted for in the design.

f. A step aerator with multiple steps each less than or equal to one foot and a total drop of five feet is sufficient to increase the dissolved oxygen in an effluent at 25°C to greater than five mg/l.

C. Post-filtration may be used to ensure compliance with the reliability standards in 12VAC5-640-434 and generally follow the biological treatment unit and are prior to disinfection in the treatment process. For granular media filters, the media depth shall not be less than 30 inches. Sand media for intermittently dosed and recirculated effluent, shall have an effective size of 0.30 mm to 1.0 mm and 0.8 mm to 1.5 mm, respectively. The uniformity coefficient should not exceed 4.0. No more than 2.0% shall be finer than 0.177 mm (80 mesh sieve) and not more than 1.0% shall be finer than 0.149 mm. No more than 2.0% shall be larger than 4.76 mm (4 mesh sieve). Larger granular media up to five mm in effective size may be considered on a case-by-case basis. The filter shall be equipped with an underdrain. The surface of the filter shall be accessible for maintenance. For the purposes of a filtration unit, the maximum surface hydraulic loading rate is 15 gpd/sf.

D. Constructed wetlands that are used as a passive backup biological treatment unit for the purposes of meeting Reliability Class I requirements of 12VAC5-640-434 B shall be lined with a minimum surface area of 100 square feet, a depth of 18 inches, a length to width ratio of about four to one, and shall have subsurface flow. Wastewater shall be disinfected prior to entering the constructed wetlands and sampling ports shall be provided to allow monitoring of the influent to the wetlands. Effluent dechlorination prior to entering the wetlands may be necessary to protect the plants from toxic levels of chlorine.

Statutory Authority

§§ 32.1-12 and 32.1-164 of the Code of Virginia.

Historical Notes

Derived from VR355-34-400 § 3.8, eff. July 30, 1992; amended, Virginia Register Volume 32, Issue 6, eff. December 16, 2015.

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