Administrative Code

Virginia Administrative Code
5/27/2024

Article 2. Design Requirements

12VAC5-640-430. Performance requirements.

Article 2
Design Requirements

A. All systems operated under this chapter shall meet the effluent limitations set forth by the State Water Control Board in the General Permit. All systems operated under this chapter shall maintain the treatment system in accordance with the approved construction permit or as modified by the final construction permit in accordance with the operation permit, "as built" plans, and the operation and maintenance manual.

B. No system shall be approved for use which provides a bypass pipe, or otherwise allows untreated or partially treated effluent to discharge in the event of a system failure.

Statutory Authority

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

Historical Notes

Derived from VR355-34-400 § 3.5, eff. July 30, 1992; amended, Virginia Register Volume 32, Issue 6, eff. December 16, 2015.

12VAC5-640-432. Treatment unit and additional system component classifications.

A. Biological treatment units will be classified by the division according to the data available to demonstrate the performance limits and reliability of those treatment units. The division may classify treatment units as generally approved or not generally approved. The type and frequency of testing for each approval class is designed to reflect the certainty with which the system has demonstrated its ability to meet the limits of the General Permit or the performance requirements of this chapter.

1. General approval may be issued by the division for both TL-2 and TL-3 treatment units in accordance with the current policies of the division. Generally approved units shall be listed on the division's website.

2. Nongenerally approved biological treatment unit designs shall be properly supported with design calculations and one or more of the following:

a. Documentation from applicable engineering standards, texts, or other publications;

b. Relevant peer-reviewed research;

c. Technical guidance from other states (may be considered on a case-by-case basis); or

d. Technical guidance from the U.S. Environmental Protection Agency.

Scale drawings of the treatment unit, appropriate design calculations, and control system details shall be provided that demonstrate the ability of the unit to meet the required effluent limits and reliability standards at the proposed design flow.

B. Additional system components for discharging systems will be classified by the division as generally approved or not generally approved.

1. The division shall consider additional system components such as post-filtration, disinfection, dechlorination, and post-aeration to be generally approved if the unit has been tested and approved under a National Sanitation Foundation (NSF) or other recognized protocol for the proposed wastewater use or if the design complies with the design standards in 12VAC5-640-460.

2. Nongenerally approved system component designs shall be properly supported with design calculations and one or more of the following:

a. Documentation from applicable engineering standards, texts, or other publications;

b. Relevant peer-reviewed research;

c. Technical guidance from other states (may be considered on a case-by-case basis); or

d. Technical guidance from the U.S. Environmental Protection Agency.

Scale drawings of the treatment unit, appropriate design calculations, and control system details shall be provided that demonstrate the ability of the unit to meet the required effluent limits and reliability standards at the proposed design flow.

C. Discharging systems that are comprised entirely of generally approved biological treatment units and system components as described in this section are considered generally approved treatment systems.

Statutory Authority

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

Historical Notes

Derived from Virginia Register Volume 32, Issue 6, eff. December 16, 2015.

12VAC5-640-434. Reliability.

A. Reliability is a measure of the ability of a component or system to perform its designated function without failure or interruption of service. Overflow criteria, such as the allowable period of noncompliant discharge, are utilized solely for the establishment of reliability classification for design purposes and are not to be construed as authorization for, or defense of, an unpermitted discharge to state waters. The reliability classification shall be based on the water quality and public health and welfare consequences of a component or system failure.

B. Reliability Class I is required for dry ditch and intermittent stream discharges with 250 feet of easement available and wetland discharges with 100 feet of easement available.

1. For biological treatment processes, Reliability Class I shall be met by providing one of the following:

a. A passive, backup biological treatment system (e.g., an intermittent sand, peat, or media filter or a constructed wetlands);

b. A generator for the treatment system with automatic transfer switch;

c. A 24-hour holding tank for raw wastewater with telemetry system to immediately notify the operator of system failure; or

d. Any alternative means that limits the discharge of a noncompliant effluent to a maximum of 24 hours.

2. For disinfection, a Reliability Class I design shall ensure that the effluent is continually disinfected by providing electronic or mechanical means of monitoring the process such that failure of disinfection systems may be corrected within 24 hours.

C. Reliability Class II is required for dry ditch and intermittent stream discharges with 500 feet of easement available and wetland discharges with 250 feet of easement available. Reliability Class II is also required for the reduction of the distance between discharge points to 250 feet on an all weather stream.

1. For biological treatment processes, Reliability Class II shall be met by providing:

a. A fixed film biological treatment process such as an intermittent sand filter, recirculating media filter, or a peat filter;

b. A suspended growth biological system followed by post-filtration;

c. Telemetry to relay alarm conditions to the operator; or

d. Any alternative means that limits the discharge of a noncompliant effluent to a maximum of 36 hours.

2. For disinfection, a Reliability Class II design shall ensure that the effluent is continually disinfected by providing electronic or mechanical means of monitoring the process such that failure of disinfection systems may be corrected within 36 hours.

D. Reliability Class III is required for all weather stream discharges with a separation distance between discharge points of 500 feet or greater. For the purposes of this chapter, noncompliant discharges must be limited to a maximum of 48 hours.

Statutory Authority

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

Historical Notes

Derived from Virginia Register Volume 32, Issue 6, eff. December 16, 2015.

12VAC5-640-440. Special factors affecting system design.

Each type of discharging system has its own unique advantages and disadvantages. These unique characteristics define the situations where a system may be used to advantage. The design of the system must be appropriate for the intended use and the site conditions where the system is to be installed. Subdivisions 1 through 6 of this section contain a list of factors that will require special design consideration. This list should not be considered all encompassing. There may be other design factors that require special consideration. A preliminary engineering conference may be scheduled with the department to discuss such factors prior to submitting designs for department review.

1. When a discharge is proposed to a wetland, dry ditch, or intermittent stream, restricted access to the wetland, dry ditch, or intermittent stream in accordance with 12VAC5-640-450 to protect public health.

2. Intermittent use for the purposes of this chapter constitutes use of the system for less than three consecutive months. Systems serving weekend cottages or other intermittent uses require special design, operation, and maintenance consideration.

3. When a discharging system is proposed to be located in an area subject to infiltration by surface water or shallow groundwater, the department may require additional protection from infiltration, inflow, and flotation, including placement of the system above natural grade.

4. Erosion must be controlled by the owner of the discharging system in accordance with any local erosion control ordinances.

5. All systems shall normally be designed to treat the BOD5 loading rate of 0.4 lbs/day per bedroom and a flow of 150 gallons per day per bedroom for systems up to three bedrooms. Systems serving single family dwellings having more than three bedrooms shall be permitted and designed to treat the anticipated loading rate based on BOD5 and be capable of handling anticipated peak loading and flow rates. All systems shall be designed to operate over the range of anticipated flow and loading rates. When a system is permitted with a design less than the maximum capacity of the dwelling, the owner shall have the construction permit recorded and indexed in the grantor index under the owner's name in the land records of the clerk of the circuit court having jurisdiction over the site of the discharging system.

6. All system designs must include protection of the components from freezing or other adverse weather conditions and ensure that the system will function properly year round.

Statutory Authority

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

Historical Notes

Derived from VR355-34-400 § 3.6, eff. July 30, 1992; amended, Virginia Register Volume 32, Issue 6, eff. December 16, 2015.

12VAC5-640-450. Design criteria for the use of intermittent streams, dry ditches, or wetlands.

All owners of systems discharging to an intermittent stream, dry ditch, or wetland shall ensure the following conditions are met:

1. Direct contact between minimally diluted effluent and insects, animals, and humans must be restricted for the life of the system. This will be achieved by reducing the chance of ponding and run-off and limiting access to the effluent. The department shall require fencing, rip-rap, or other barriers to restrict access to effluent discharging to a dry ditch, intermittent stream, or wetland as deemed necessary to protect public health. This determination shall be made by the department on a case-by-case basis.

a. For dry ditch and intermittent stream discharges, the restricted access area shall begin at the point where the effluent is discharged and continue for 500 feet, until the effluent discharges into an all weather stream or is no longer visible during the wet season. The design shall provide justification for the length of the restricted access channel if less than 500 feet.

b. For wetland discharges, the restricted access shall extend for a distance of 250 feet along the flow path of the discharge unless a 10:1 dilution with the wetland can be achieved. If the flow path cannot be established and a 10:1 dilution cannot be obtained, then access shall be restricted for 100 feet radially from the point of discharge. For wetland discharges, the access barrier may be a subsurface discharge point, but in no case shall the discharge point and diffuser be greater than 18 inches below the natural wetland surface.

2. When effluent is discharged to a dry ditch, intermittent stream, or wetland, the owner shall own the land or acquire an easement from the downstream or downgradient land owner to discharge on all land below the point of discharge for the distance shown in Table 3.2. To allow for system construction and repair within the restricted access area, and to facilitate maintenance and monitoring, the easement shall be a minimum of 25 feet wide and approximately centered on the low point of the dry ditch or intermittent stream for the entire length of the restricted access area. For wetlands, the easement shall be measured radially from the point of discharge unless flow direction can be established. In those cases where flow direction can be established, the easement shall be a minimum of 25 feet wide and approximately centered on the discharge path and extend for a distance along the flow path as described in Table 3.2. If the slope across the discharge site is equal to or greater than 10%, the flow direction can be determined by observation. For slopes less than 10%, a site specific study must be conducted to document the direction of flow. All easements must be in perpetuity and shall be recorded by the owner with the clerk of the circuit court having jurisdiction over the property prior to issuance of the construction permit. For the purposes of complying with this chapter, written approval to utilize an easement owned by the Virginia Department of Transportation shall be recorded by the owner with the clerk of the circuit court office having jurisdiction over the property.

3. Each discharging system that discharges to a dry ditch, intermittent stream, or wetland must receive additional treatment beyond that required by the General Permit in order to reduce the increased potential for public health problems which may result when partially treated effluent is not diluted. Such additional treatment shall be capable of producing an effluent with a quality of 10 mg/l of BOD5, 10 mg/l of suspended solids and a fecal coliform level of less than or equal to 100 colonies per 100 ml. Treatment units approved as TL-3 are recognized as having the ability to meet this BOD5 and TSS standard, but have not been tested for compliance with the fecal coliform standard. Therefore, the following reliability classifications in Table 3.2 must be met when designing discharge systems intended to discharge into dry ditches, intermittent streams, or wetlands.

TABLE 3.2
REQUIREMENTS FOR RELIABILITY CLASSIFICATION AND OWNERSHIP OR EASEMENTS DOWNSTREAM FROM SYSTEMS THAT DISCHARGE TO DRY DITCHES, INTERMITTENT STREAMS, OR WETLANDS

Reliability Class

Downstream or Down Channel Distance for Dry Ditches or Intermittent Streams (feet)

Wetlands from Discharge Point along Flow Path or Radially from Discharge Point

No spring below

Spring below

Reliability Class I

250 ft

1,500 ft

100 ft

Reliability Class II

500 ft

1,500 ft

250 ft

Statutory Authority

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

Historical Notes

Derived from VR355-34-400 § 3.7, eff. July 30, 1992; amended, Virginia Register Volume 32, Issue 6, eff. December 16, 2015.

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

Cs

=

Dissolved oxygen saturation (mg/l)

Ca

=

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

Cb

=

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

n

=

The number of equal size steps

r

=

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

where: T

=

Water temperature (°C)

h

=

Height of one step (ft)

a

=

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)

b

=

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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