Virginia Law

Article 5. Subsurface Soil Absorption System

12VAC5-610-900. General.

Article 5
Subsurface Soil Absorption System

Subsurface soil absorption systems are sewage disposal systems which utilize the soil to further treat and dispose of effluent from a treatment works in a manner that does not result in a point source discharge and does not create a nuisance, health hazard or ground or surface water pollution.

Statutory Authority

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

Historical Notes

Derived from VR355-34-02 § 4.25, eff. February 5, 1986; amended, eff. May 11, 1988.

12VAC5-610-910. Scope.

For the purpose of this chapter, a "subsurface soil absorption system" shall refer to that part of a sewage disposal system beginning at the flow splitting device and extending through the absorption area or areas.

Statutory Authority

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

Historical Notes

Derived from VR355-34-02 § 4.26, eff. February 5, 1986; amended, eff. May 11, 1988.

12VAC5-610-920. Distribution methods.

The term distribution methods refers to the piping, flow splitting devices, gravel, and other appurtenances beginning at the point of flow splitting and ending at the application of effluent to the soil absorption area. Two basic methods are considered:

A. Gravity; and

B. Pressure.

Statutory Authority

§§ 32.1-12, 32.1-164, and 32.1-164.9 of the Code of Virginia.

Historical Notes

Derived from VR355-34-02 § 1.3, eff. February 5, 1986; amended, eff. May 11, 1988; amended, Virginia Register Volume 16, Issue 16, eff. July 1, 2000; Volume 32, Issue 24, eff. August 25, 2016.

12VAC5-610-930. Gravity distribution.

Gravity distribution is the conveyance of effluent from a distribution box through the percolation lines at less than full flow conditions. Flow to the initial distribution box may be initiated by pump, siphon or gravity.

A. Enhanced flow distribution. Enhanced flow distribution is the initiation of the effluent flow to the distribution box by pump or siphon for the purpose of assuring more uniform flow splitting to the percolation lines. Enhanced flow distribution shall be provided on systems where the flow is split more than 12 times or the system contains more than 1200 linear feet of percolation lines. For the purpose of this chapter, enhanced flow distribution is considered to produce unsaturated soil conditions.

B. System size. Distribution systems containing 1800 or more linear feet of percolation piping shall be split into multiple systems containing a maximum of 1200 linear feet of percolation piping per system.

C. Distribution boxes. The distribution box is a device for splitting flow equally by gravity to points in the system. Improperly installed distribution boxes are a cause for absorption field malfunction.

1. Materials. The preferred material for use in constructing distribution boxes is concrete (3000 psi). Other materials may be considered on a case-by-case basis. All materials must be resistant to both chemical and electrolytic corrosion and must have sufficient structural strength to contain sewage and resist lateral compressive and bearing loads.

2. Design. Each distribution box shall be designed to split the influent flow equally among the multiple effluent ports. All effluent ports shall be at the same elevation and be of the same diameter. The elevation of the effluent ports shall be at a lower elevation than the influent port. The placement of the influent ports shall be such as to prevent short circuiting unless baffling is provided to prevent short circuiting. The minimum inside width of a gravity flow distribution box shall be equal to or greater than 12 inches. The inside bottom shall be at least four inches below the invert of the effluent ports and at least five inches below the invert of the influent port. A minimum of eight inches freeboard above the invert of the effluent piping shall be provided. The distribution box shall be fitted with a watertight, removable lid for access.

3. Installation. The hole for placement of the distribution box shall be excavated to undisturbed soil. The distribution box shall be placed in the excavation and stabilized. The preferred method of stabilizing the distribution box is to bond the distribution box to a four inch poured in place Portland cement concrete pad with dimensions six inches greater than the length and width dimensions of the distribution box. The box shall be permanently leveled and checked by water testing. Conduits passing through the walls of a distribution box shall be provided with a water stop.

D. Lead or header lines. Header or lead lines are watertight, semirigid, or rigid lines that convey effluent from a distribution box to another box or to the percolation piping.

1. Size. The lead or header lines shall have an internal diameter of four inches.

2. Slope. Minimum slope shall be two inches per 100 feet.

3. Materials. The lead or header lines shall have a minimum crush strength of 1500 pounds per foot and may be constructed of cast iron, plastic, vitrified clay, or other material resistant to the corrosive action of sewage.

4. Appurtenances.

a. Joints. Lead or header lines shall have joints of the compressions type with the exception of plastic lead or header lines that may be welded sleeve, chemically fused, or clamped (noncorrosive) flexible sleeve.

b. Adapters. Joining of lead or header lines of different size or material shall be accomplished by use of a manufactured adapter specifically designed for the purpose.

c. Valves. Valves shall be constructed of materials resistant to the corrosive action of sewage. Valves placed below ground level shall be provided with a valve box and a suitable valve stem so that it may be operated from the ground surface.

5. Construction.

a. Bedding. All lead or header lines shall be bedded to supply uniform support and maintain grade and alignment along the length of the lead or header lines. Special care shall be taken when using semirigid pipe.

b. Backfilling and tamping. Lead and header lines shall be backfilled and tamped as soon as possible after the installation of the lead or header lines has been approved. Material for backfilling shall be free of large stones and debris.

6. Termination. Header or lead lines shall extend for a minimum distance of two feet into the absorption trenches.

E. Gravity percolation lines. Gravity percolation lines are perforated or open joint pipes that are utilized to distribute the effluent along the length of the absorption trenches.

1. Size. All gravity percolation lines shall have an internal diameter of four inches.

2. Slope. The slope of the lines shall be uniform and shall not be less than two inches or more than four inches per 100 feet.

3. Design. Effluent shall be split by the distribution system so that all gravity percolation lines installed shall receive an equal volume of the total design effluent load per square foot of trench, i.e., the fraction of the flow received by each percolation line divided by the length of the gravity percolation lines shall be equal for all gravity percolation lines in a system.

4. Length. No individual gravity percolation line shall exceed 100 feet in length.

5. Materials.

a. Clay. Clay tile shall be extra-strength and meet current ASTM standards for clay tile.

b. Perforated plastic drainage tubing. Perforated plastic drainage tubing shall meet ASTM standards. At not greater than 10 feet intervals the pipe shall be plainly marked, embossed, or engraved thereby showing the manufacturer's name or hallmark and showing that the product meets a bearing load of 1,000 lb. per foot. In addition, a painted or other clearly marked line or spot shall be marked at not greater than 10 feet intervals to denote the top of the pipe.

The tubing shall have three holes, 1/2 to 3/4 inch in diameter evenly spaced and placed within an arc of 130 degrees, the center hole being directly opposite the top marking.

Spacing of each set of three holes shall be at four inch intervals along the tube. If there is any break in the continuity of the tubing, an appropriate connection shall be used to join the tubing.

6. Installation.

a. Crushed stone or gravel. Clean gravel or crushed stone having a size range from 1/2 inch to 1-1/2 inches shall be utilized to bed the gravity percolation lines.

Minimum depth of gravel or crushed stone beneath the percolation lines shall be six inches. Clean course silica sand (does not effervesce in presence of dilute hydrochloric acid) may be substituted for the first two inches (soil interface) of the required six inches of gravel beneath the percolation lines. The absorption trench shall be backfilled to a depth of two inches over the gravity percolation lines with the same gravel or crushed stone. Clean sand, gravel, or crushed stone shall be free of fines, clay, and organic materials.

b. Grade boards or stakes. Grade boards or stakes placed in the bottom or sidewalls of the absorption trench shall be utilized to maintain the grade on the gravel for placement of the gravity percolation lines. Grade stakes shall not be placed on centers greater than 10 feet.

c. Placement and alignment. Perforated gravity percolation piping shall be placed so that the center hole is in the horizontal plane and interfaces with the minimum six inches of graded gravel. When open joint piping is utilized the upper half of the top of the 1/4-inch open space shall be covered with tar paper or building paper to block the entrance of fines into the pipe during the backfilling operation. All gravity percolating piping shall be placed in the horizontal center of the absorption trench and shall maintain a straight alignment and uniform grade.

d. Backfilling. After the placement of the gravity percolation piping the absorption trench shall be backfilled evenly with crushed stone or gravel to a depth of two inches over the piping. Untreated building paper or other suitable material shall be placed at the interface of the gravel and soil to prevent migration of fines to the trench bottom. The remainder of the trench shall be backfilled with soil to the ground surface.

F. Gravelless material is a proprietary product specifically manufactured to disperse effluent within the absorption trench of an onsite sewage system without the use of gravel. Gravelless material may include chamber, bundled expanded polystyrene, and multi-pipe systems. The division shall maintain a list of all generally approved gravelless material. Gravelless material on the generally approved list may be used in accordance with Table 5.4 of 12VAC5-610-950.

1. Gravelless material that received general approval as of December 12, 2013, shall retain such status when used in accordance with the requirements of this chapter. After December 12, 2013, the division shall review and evaluate new applications for general approval pursuant to the requirements of this chapter.

a. Any manufacturer of gravelless material may submit an application for general approval to the division using a form provided by the division. A complete application shall include the manufacturer's contact information, product specifications, product approvals in other states or territories, installation manual, and other information deemed necessary by the division to determine compliance with this chapter.

b. The manufacturer of gravelless material shall identify in the application for general approval any recommendation that deviates from the requirements of this chapter. If the recommendation is approved by the division, then the manufacturer shall include the deviation in the gravelless material's installation manual.

2. Gravelless material shall have the following minimum characteristics for general approval:

a. The minimum exterior width shall be at least 90% of the total width of the absorption trench. The exterior width of a chamber system shall be measured at the edge or outer limit of the product's contact with the trench bottom unless the division determines a different measurement is required based on the gravelless material's design. The exterior width of bundled expanded polystyrene and multi-pipe systems shall be measured using the outside diameter of the bundled gravelless material unless the division determines a different measurement is required based on the gravelless material's design. The division shall establish the exterior width of any gravelless material that is not considered a chamber, bundled expanded polystyrene, or multi-pipe system.

b. Gravelless material shall have a minimum height of eight inches to provide a continuous exchange of air through a permeable interface.

c. Gravelless material shall have a permeable interface that shall be located along the trench bottom and trench sidewalls within the absorption trench.

d. Gravelless material shall provide a minimum storage capacity of 1.3 gallons per square foot of trench bottom area.

e. Gravelless material shall pose no greater risk to surface water and groundwater quality than gravel in absorption trenches. Gravelless material shall be constructed to maintain structural integrity such that it does not decay or corrode when exposed to effluent.

f. Gravelless material shall have a minimum load rating of H-10 or H-20 from the American Association of State Highway and Transportation Officials or equivalent when installed in accordance with the manufacturer's specifications and minimum specified depth of cover in nontraffic or traffic areas, respectively.

3. For designs using gravelless material, the absorption trenches shall receive an equal volume of effluent per square foot of trench. Trench bottom area shall be equal to or greater than the minimum area requirements contained in Table 5.4 or Table 5.5 of 12VAC5-610-950. Trench sidewall shall not be included when determining minimum area requirements. When open-bottom gravelless material is utilized, it shall provide a splash plate at the inlet of the trench or other suitable method approved by the manufacturer to reduce effluent velocity.

4. Installation of gravelless material shall comply with this chapter and the approved installation manual unless the department grants a deviation pursuant to 12VAC5-610-660 or the division has granted a deviation identified in the installation manual.

5. Gravelless material shall contain a pressure percolation line along the entire length of the trench when low pressure distribution is utilized pursuant to 12VAC5-610-940 D.

6. When pumping effluent to overcome gravity, any open-bottom gravelless material shall provide a high-flow splash plate at the inlet of the trench or other suitable method approved by the manufacturer to reduce effluent velocity.

7. When enhanced flow distribution is used, open-bottom gravelless material shall contain a percolation pipe that extends a minimum of 10 feet from the trench's intersection with the header line. The percolation pipe shall be installed in accordance with the manufacturer's approved installation manual. The dosing volume shall be a minimum 39 gallons per 100 linear feet of absorption trench.

8. Gravelless material may be substituted for gravel in accordance with this chapter, provided that the certifying licensed professional engineer or onsite soil evaluator approves the substitution. The certifying licensed professional engineer or onsite soil evaluator shall document the substitution and related design changes on the inspection report submitted in accordance with 12VAC5-610-330. A new construction permit pursuant to 12VAC5-610-310 is not required for the substitution.

Statutory Authority

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

Historical Notes

Derived from VR355-34-02 § 4.28, eff. February 5, 1986; amended, eff. May 11, 1988; Virginia Register Volume 16, Issue 16, eff. July 1, 2000; Volume 32, Issue 24, eff. August 25, 2016; Volume 39, Issue 4, eff. November 24, 2022.

12VAC5-610-940. Low pressure distribution.

Low pressure distribution is the conveyance of effluent through the pressure percolation lines at full flow conditions into the absorption area with the prime motive force being a pump or siphon. Low pressure systems are limited to a working pressure of from one to four feet of head at the distal end of the pressure percolation lines. For the purpose of this chapter low pressure distribution is considered to provide unsaturated soil conditions.

A. Dosing cycle. Systems shall be designed so that the effluent volume applied to the absorption area per dosing cycle is from seven to 10 times the volume of the distribution piping, however, the volume per dosing cycle should not result in a liquid depth in the absorption trench greater than two inches.

B. Manifold lines. Manifold lines are watertight lines that convey effluent from the initial point of flow splitting to the pressure percolation lines.

1. Size. The manifold line shall be sized to provide a minimum velocity of two feet per second and a maximum velocity of eight feet per second.

2. Materials. All pipe used for manifolds shall be of the pressure type with pressure type joints.

3. Bedding. All manifolds shall be bedded to supply uniform support along its length.

4. Backfilling and tamping. Manifold trenches shall be backfilled and tamped as soon as possible after the installation of the manifold has been approved. Material for backfilling shall be free of large stones and debris.

5. Valves. Valves for throttling and check valves to prevent backflow are required wherever necessary. Each valve shall be supplied with a valve box terminating at the surface.

C. Pressure percolation lines. Pressure percolation lines are perforated pipes utilized to distribute the flow evenly along the length of the absorption trench.

1. Size. Pressure percolation lines should normally have a 1-1/4 inch inside diameter.

2. Hole size. Normal hole size shall be 3/16 inch to 1/4 inch.

3. Hole placement. Center to center hole separation shall be between three and five feet.

4. Line length. Maximum line length from manifold should not exceed 50 feet.

5. Percent flow variation. Actual line size, hole size and hole separation shall be determined on a case-by-case basis based on a maximum flow variation of 10% along the length of the pressure percolation lines.

6. Materials and construction. The preferred material is plastic, either PVC or ABS, designed for pressure service. The lines shall have burr free and counter sunk holes (where possible) placed in a straight line along the longitudinal axis of the pipe. Joining of pipes shall be accomplished with manufactured pressure type joints.

7. Installation.

a. Crushed stone or gravel. Clean gravel or crushed stone having a size range from 1/2 inch to 3/4 inch shall be utilized to bed the pressure percolation lines. Minimum depth of gravel or crushed stone beneath the percolation lines shall be 8-1/2 inches. Clean course silica sand (does not effervesce in the presence of dilute hydrochloric acid) may be substituted for the first two inches (soil interface) of the required 8-1/2 inches of gravel beneath the pressure percolation lines. The absorption trench shall be backfilled to a depth of two inches over the pressure percolation lines with the same gravel or crushed stone. Clean sand, gravel or crushed stone shall be free of fines, clay and organic materials.

b. Grade boards or stakes. Grade boards or stakes placed in the bottom or sidewalls of the absorption trench shall be utilized to maintain the gravel level for placement of the pressure percolation lines. Grade stakes shall not be placed on centers greater than 10 feet.

c. Placement and alignment. Pressure percolation lines shall be placed so that the holes face vertically downward. All pressure percolation piping shall be placed at the same elevation, unless throttling valves are utilized, and shall be level. The piping shall be placed in the horizontal center of the trench and shall maintain a straight alignment. Normally the invert of the pressure percolation lines shall be placed 8-1/2 inches above the trench bottom. However, under no circumstance shall the invert of the pressure percolation lines be placed closer than 16-1/2 inches to the seasonal water table as defined in 12VAC5-610-470 D. When the invert of the pressure percolation lines must be placed at an elevation greater than 8-1/2 inches above the trench bottom, landscaping over the absorption area may be required to provide the two inches of gravel and six inches of fill over the pressure percolation lines required in subdivision 7 a of this subsection.

d. Backfilling. After the placement of the pressure percolation piping the absorption trench shall be backfilled evenly with crushed stone or gravel to a depth of two inches over the opening. Untreated building paper or other suitable material shall be placed at the interface of the gravel and soil to prevent migration of fines to the trench bottom. The remainder of the trench shall be backfilled with soil to the ground surface.

8. Appurtenances. The distal (terminal) end of each pressure percolation lines shall be fitted with a vertical riser and threaded cap extending to the ground surface. Systems requiring throttling valves will be supplied with couplings and threaded riser extensions at least four feet long so that the flow may be adjusted in each line.

D. Gravelless material with general approval may be used for low pressure distribution in accordance with the manufacturer's approved installation manual, Table 5.4 of 12VAC5-610-950, and the applicable requirements of this chapter.

Statutory Authority

§§ 32.1-12, 32.1-164, and 32.1-164.9 of the Code of Virginia.

Historical Notes

Derived from VR355-34-02 § 4.29, eff. February 5, 1986; amended, eff. May 11, 1988; Virginia Register Volume 16, Issue 16, eff. July 1, 2000; Volume 32, Issue 24, eff. August 25, 2016.

12VAC5-610-950. Absorption area design.

A. The absorption area is the undisturbed soil medium utilized for absorption of the effluent. The absorption area includes the infiltrative surface in the absorption trench and the soil between and around the trenches when trenches are used.

B. Suitability of soil horizon. The absorption trench bottom shall be placed in the soil horizon or horizons with an average estimated or measured percolation rate less than 120 minutes per inch. Soil horizons are to be identified in accordance with 12VAC5-610-480. The soil horizon must meet the following minimum conditions:

1. It shall have an estimated or measured percolation rate equal to or less than 120 minutes per inch;

2. The soil horizon or horizons shall be of sufficient thickness so that at least 12 inches of absorption trench sidewall is exposed to act as an infiltrative surface; and

3. If no single horizon meets the conditions in subdivision 2 of this subsection, a combination of adjacent horizons may be utilized to provide the required 12-inch sidewall infiltrative surface. However, no horizon utilized shall have an estimated or measured percolation rate greater than 120 minutes/inch.

C. Placement of absorption trenches below soil restrictions. Placement of the soil absorption trench bottom below soil restrictions as defined in 12VAC5-610-490 D, whether or not there is evidence of a perched water table as indicated by free standing water or redoximorphic features, including concentrations, depletions, or stains, nodules, or concretions of iron and manganese, requires a special design based on the following criteria:

1. The soil horizon into which the absorption trench bottom is placed shall be a Texture Group I, II, or III soil or have an estimated or measured percolation rate of less than 91 minutes per inch.

2. The soil horizon shall be a minimum of three feet thick and shall exhibit no characteristics that indicate wetness on restriction of water movement. The absorption trench bottom shall be placed so that at least two feet of the soil horizon separates the trench bottom from the water table or rock. At least one foot of the absorption trench side wall shall penetrate the soil horizon.

3. A lateral ground water movement interceptor (LGMI) shall be placed upslope of the absorption area. The LGMI shall be placed perpendicular to the general slope of the land. The invert of the LGMI shall extend into, but not through, the restriction and shall extend for a distance of 10 feet on both sides of the absorption area (See 12VAC5-610-700 D 3).

4. Pits shall be constructed to facilitate soil evaluations as necessary.

D. Sizing of absorption trench area for septic tank effluent.

1. Required area. The total absorption trench bottom area required shall be based on the average estimated or measured percolation rate for the soil horizon or horizons into which the absorption trench is to be placed. If more than one soil horizon is utilized to meet the sidewall infiltrative surface required in subsection B of this section, the absorption trench bottom area shall be based on the average estimated or measured percolation rate of the "slowest" horizon. The trench bottom area required in square feet per 100 gallons (Ft²/100 Gals) of sewage applied for various soil percolation rates is tabulated in Table 5.4. The area requirements are based on the equation:

log y = 2.00 + 0.008 (x)

where y = Ft²/100 Gals

x = Percolation rate in minutes/inch

Notwithstanding the above, the minimum absorption area for single family residential dwellings shall be 400 square feet.

2. Area reduction. See Table 5.4 for area reduction when gravelless material or low pressure distribution is utilized. A reduction in area shall not be permitted when flow diversion is utilized with low pressure distribution. When gravelless material is utilized, the design width of the trench shall be used to calculate minimum area requirements for absorption trenches.

E. Minimum cross section dimensions for absorption trenches.

1. Depth. The minimum trench sidewall depth as measured from the surface of the mineral soil shall be 12 inches when placed in a landscape with a slope less than 10%. The installation depth shall be measured on the downhill side of the absorption trench. When the installation depth is less than 18 inches, the depth shall be measured from the lowest elevation in the microtopography. All systems shall be provided with at least 12 inches of cover to prevent frost penetration and provide physical protection to the absorption trench; however, this requirement for additional cover shall not apply to systems installed on slopes of 30% or greater. Where additional soil cover must be provided to meet this minimum, it must be added prior to construction of the absorption field, and it must be crowned to provide positive drainage away from the absorption field. The minimum trench depth shall be increased by at least five inches for every 10% increase in slope. Sidewall depth is measured from the ground surface on the downhill side of the trench.

2. Width. All absorption trenches utilized with gravity distribution shall have a width of from 18 inches to 36 inches. All absorption trenches utilized with low pressure distribution shall have a width of eight inches to 24 inches.

F. Lateral separation of absorption trenches. The absorption trenches shall be separated by a center to center distance no less than three times the width of the trench for slopes up to 10%. However, where trench bottoms are two feet or more above rock, pans, and impervious strata, the absorption trenches shall be separated by a center to center distance no less than three times the width of the trench for slopes up to 20%. The minimum horizontal separation distance shall be increased by one foot for every 10% increase in slope. In no case shall the center to center distance be less than 30 inches.

G. Slope of absorption trench bottoms.

1. Gravity distribution. The bottom of each absorption trench shall have a uniform slope not less than two inches or more than four inches per 100 feet.

2. Low pressure distribution. The bottom of each absorption trench shall be uniformly level to prevent ponding of effluent.

H. Placement of absorption trenches in the landscape.

1. The absorption trenches shall be placed on contour.

2. When the ground surface in the area over the absorption trenches is at a higher elevation than any plumbing fixture or fixtures, sewage from the plumbing fixture or fixtures shall be pumped.

I. Lateral ground water movement interceptors. Where subsurface, laterally moving water is expected to adversely affect an absorption system, a lateral ground water movement interceptor (LGMI) shall be placed upslope of the absorption area. The LGMI shall be placed perpendicular to the general slope of the land. The invert of the LGMI shall extend into, but not through, the restriction and shall extend for a distance of 10 feet on either side of the absorption area.

J. Controlled blasting. When rock or rock outcroppings are encountered during construction of absorption trenches the rock may be removed by blasting in a sequential manner from the top to remove the rock. Percolation piping and sewer lines shall be placed so that at least one foot of compacted clay soil lies beneath and on each side of the pipe where the pipe passes through the area blasted. The area blasted shall not be considered as part of the required absorption area.

K. Trenches receiving TL-2 effluent or TL-3 effluent are exempt from the increase in trench depth with slope and the soil cover requirements as found in 12VAC5-610-950 E. The following additional requirements shall apply.

1. Soil dispersal loading rates shall not exceed the values in Table 5.5.

2. The minimum vertical separation to a limiting feature shall be maintained under the entire infiltrative surface in accordance with 12VAC5-613-80.

3. The minimum soil cover, after settling, shall be six inches as measured from the finished ground surface to the uppermost limit of the dispersal media (or gravelless material) utilized in the absorption area. On sloping sites, cover shall be tied back into the existing slope to facilitate stabilization of the slope and maintenance of the site. The soil cover, with amendments as needed, shall be of a quality, character, and fertility suitable to establish a vegetative cover that is uniform and sufficiently mature to survive and inhibit erosion.

4. The minimum installation depth is equal to the sidewall of the dispersal system construction as described in 12VAC5-610-930 F (gravelless), 12VAC5-610-940, and subdivision E 1 of this section. On sloping sites, the minimum installation depth is measured on the downhill side of the absorption trench.

5. When trenches are installed at less than 12 inches from the ground surface, timed dosing shall be used to disperse the effluent.

6. For slopes up to 15% slope, there are not any soil texture group limitations for shallow placed trenches receiving TL-2 effluent or TL-3 effluent. For slopes over 15%, trench systems installed in Texture Group III and IV soils shall have a 12-inch or greater installation depth.

7. Designs supported by division-approved manufacturer design manuals may deviate from subdivisions K 4 and K 5 of this section.

8. Notwithstanding subdivisions K 1 through K 7 of this section, the minimum absorption area for a single-family residential dwelling receiving TL-2 effluent or TL-3 effluent shall be 400 square feet.

Statutory Authority

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

Historical Notes

Derived from VR355-34-02 § 4.29, eff. February 5, 1986; amended, eff. May 11, 1988; Virginia Register Volume 16, Issue 16, eff. July 1, 2000; Volume 32, Issue 24, eff. August 25, 2016; Volume 39, Issue 4, eff. November 24, 2022.

12VAC5-610-955. Drip dispersal.

A. Drip dispersal applies wastewater in an even and controlled manner over an absorption area. Drip dispersal system components may include treatment components, a flow equalization pump tank, a filtration system, a flow measurement method, supply and return piping, small diameter pipe with emitters, air/vacuum release valves, redistribution control, and electromechanical components or controls.

B. Drip dispersal system tubing shall be color coded and certified by the manufacturer as designed and manufactured for the dispersal of wastewater. All drip dispersal system tubing shall be equipped with emitters approved for use with wastewater. For the application of septic tank effluent, the tubing must have self-cleaning emitters.

1. The minimum linear feet of tubing in the system shall be one-half of the minimum soil absorption area in square feet.

2. All tubing shall be placed on contour.

3. Except as provided by 12VAC5-613, drip systems dispersing septic tank effluent shall comply with the requirements of 12VAC5-610-594.

4. Drip systems dispersing secondary effluent or better require a minimum of six inches of cover over the tubing. Cover may be achieved by a combination of installation depth and Group II or Group III soil cover or other approved material over the drip field.

5. The discharge rate of any two emitters shall not vary by more than 10% in order to ensure that the effluent is uniformly distributed over the entire drip field or zone.

6. The emitters shall be evenly spaced along the length of the drip tubing at not less than six inches or more than 24 inches apart.

7. The system design shall protect the drip emitters and system from the effects of siphoning or backflow through the emitters.

C. Drip dispersal systems shall comply with the following minimum soil absorption area requirements:

1. For the dispersal of septic tank effluent, the minimum soil absorption area for a drip system shall be calculated by multiplying the trench bottom area required for a low pressure distribution system in Table 5.4 of 12VAC5-610-950 by three.

2. For the dispersal of TL-2 effluent or TL-3 effluent, the minimum soil absorption area shall be in accordance with Table 5.5 of 12VAC5-610-950.

3. Air/vacuum release valves shall be located at the high points of the supply and return manifolds to each zone.

D. All drip dispersal systems shall be equipped with devices or methods to restrict effluent from draining by gravity to portions of a zone or laterals lower in elevation. Variable distribution due to gravity drainage shall be 10% or less within a zone.

E. A minimum of six hours of emergency storage above the high water alarm in the pump chamber shall be provided. The equalization volume shall be equal to 18 hours of storage. The equalization volume shall be measured from the pump off level to the high water alarm level. An audio/visual alarm meeting the requirements of 12VAC5-610-880 B 8 shall be provided for the pump chamber.

F. Each drip dispersal zone shall be time-dosed over a 24-hour period. The dose volume and interval shall be set to provide unsaturated flow conditions. Demand dosing is prohibited. Minimum dose volume per zone shall be 3.5 times the liquid capacity of the drip laterals in the zone plus the liquid capacity of the supply and return manifold lines (which drain between doses) accounting for instantaneous loading and drain back.

1. At each dosing cycle, the system design shall only allow a full dose volume to be delivered.

2. For design flows greater than 1,000 gallons per day, a means to take each zone off line separately shall be provided. The system shall have the capability to bypass each zone that is taken out of service such that each subsequent dose is dispersed to the next available zone in sequence.

G. Filtration shall be provided to remove suspended solids and prevent clogging of emitters. The filtration design shall meet the drip tubing manufacturer's particle size requirements for protection of the emitters at a flow rate equal to or greater than the rate of forward flushing. Filter flush water shall be returned to the treatment system at a point where the residuals and volume of the flush water do not negatively impact the effluent quality or exceed the hydraulic design capacity of the treatment system.

H. A means for measuring or estimating total flow dispersed to the soil absorption area and to verify field dosing and field flushing rates shall be provided.

I. The system shall provide forward field flushing to achieve scouring velocity as specified by the drip tubing manufacturer. Field flushing shall occur on a routine schedule to prevent excessive solids accumulation and clogging. Flush water shall be returned to the treatment system at a point where the residuals and volume of the flush water do not negatively impact the effluent quality or exceed the hydraulic design capacity of the treatment system.

J. Electrical components shall be Underwriters Laboratory (UL) listed for the intended purpose. The designer shall provide a description with a schematic diagram of the electrical and control functions in the operation and maintenance manual. The electrical control equipment shall be mounted within a National Electrical Manufacturers Association (NEMA) 4X rated enclosure with a rigid latching door. All switches shall be clearly identified, and all internal wiring shall be factory installed. All wiring shall be installed according to applicable electrical safety codes and the manufacturer's installation schematic.

K. All components in a drip dispersal system shall be rated to withstand contact with wastewater and recommended for this application by the manufacturer. All components shall be protected from freezing.

L. The designer of the drip dispersal system shall verify the dosing rates, the flushing rates, and other parameters critical to the proper operation of the system at the startup inspection. A summary of the startup inspection shall be included in the operation and maintenance manual and shall include, at a minimum, the dosing volume, the forward flow flushing rate, the pressure head of the system, and verification of proper cycling between zones.

Statutory Authority

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

Historical Notes

Derived from Virginia Register Volume 32, Issue 24, eff. August 25, 2016; amended, Virginia Register Volume 39, Issue 4, eff. November 24, 2022.

12VAC5-610-960. Elevated sand mound.

A. An elevated sand mound is a soil absorption system that incorporates low pressure distribution and sand filtration to produce treated sewage prior to absorption in the natural underlying soil. The elevated sand mound utilizes less gross soil area than most other soil absorption systems. Elevated sand mounds differ from pads in that elevated sand mounds are always an aboveground system, may receive septic tank effluent, and always require pressure distribution, and the infiltrative surface follows the natural ground surface and contour of the site.

B. Mound systems receiving septic tank effluent shall be designed and constructed in accordance with the Wisconsin Mound Soil Absorption System Siting, Design and Construction Manual prepared by the Small Scale Waste Management Project, School of Natural Resources, College of Agricultural and Life Sciences, University of Wisconsin-Madison dated January 2000. Drip dispersal or low pressure distribution shall be used.

C. The manual referred to in subsection B of this section shall be used for the designated construction of elevated sand mounds. The following criteria are required for all elevated sand mound systems in addition to the requirements found in the manual.

1. The proposed mound site shall be fenced, roped, or otherwise secured, and marked, to prevent damage by vehicular traffic. Activities on the mound site shall be severely limited in order to protect it to the greatest extent possible.

2. The mound must be constructed during dry weather and soil conditions.

3. Wooded sites shall not be used unless it is shown by the applicant that the wooded site is the only site available, and if the applicant can demonstrate that the site can be properly prepared. If a wooded site is used, trees shall be removed by cutting them off at ground level, leaving the stumps in place. The cut trees shall be removed using methods that do not require driving equipment over the mound site and that do not result in the removal of any soil from the site. Larger basal areas may be required on wooded sites.

4. When the depth to a restriction, shrink-swell soils, or a water table is less than 24 inches, pretreatment sufficient to produce TL-2 effluent or TL-3 effluent may be used to reduce these distances in accordance with 12VAC5-613-80.

5. The minimum absorption area for single family residential dwellings shall be 400 square feet.

D. Elevated sand mounds receiving TL-2 effluent or TL-3 effluent shall adhere to the following additional design criteria.

1. The basal area (interface of fill sand and original soil surface) loading rate shall not exceed the values found in Table 5.5.

2. The minimum sand depth under the dispersal system is six inches.

3. The minimum soil cover, after settling, shall be six inches as measured from the finished ground surface to the uppermost limit of the dispersal media (or gravelless material) utilized in the absorption area. The finished sideslopes cannot exceed 1:4 (rise:run). The soil cover, with amendments as needed, shall be of a quality, character, and fertility suitable to establish a vegetative cover that is uniform and sufficiently mature to survive and inhibit erosion.

4. Vertical separation to limiting features as found in 12VAC5-613-80 shall be maintained under the entire infiltrative surface of the basal area.

5. Designs supported by division approved manufacturer design manuals may deviate from pressure dosing but require dosing to a gravity distribution system at a minimum.

Statutory Authority

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

Historical Notes

Derived from VR355-34-02 § 4.31, eff. February 5, 1986; amended, eff. May 11, 1988; Virginia Register Volume 16, Issue 16, eff. July 1, 2000; Volume 39, Issue 4, eff. November 24, 2022; Errata, 39:6 VA.R. 985 November 7, 2022.

12VAC5-610-965. Sand-on-sand.

A. Sand-on-sand is a process of modifying a soil absorption system site using fill material which is similar in texture to the original, naturally occurring material. Filling is accomplished in a manner which allows for adequate treatment and disposal of effluent, protection from frost and traffic damage, and does not promote the creation of restrictive horizons. Sand-on-sand systems are considered Type II systems and are used to overcome limitations encountered with certain high water table soils. A detailed description of the siting criteria used for sand-on-sand systems is found in 12VAC5-610-597.

B. Site preparation. All surface vegetation and organic matter, including but not limited to grass, shrubs, trees, leaf litter, branches, limbs, and tree stumps shall be removed. Special consideration shall be given to site preparation to avoid soil compaction and other damage which may create discontinuities and restrictive horizons when the fill material is placed on the site.

C. Fill material. Fill material may be added to such a site and used for residential onsite wastewater disposal for flows up to 1,000 G.P.D. on sites meeting the criteria established in 12VAC5-610-597 provided that:

1. The fill material used is the same texture as the underlying material;

2. The coarse fragment content of the fill material is neither greater than 10% by volume nor noncarbonaceous and inorganic material greater than two mm in size;

3. The fill material is placed in such a manner as to prevent the formation of any restrictive horizons;

4. The fill material covers an area 10 feet greater in all horizontal dimensions than the soil absorption system;

5. The sides of the filled area are sloped at not greater than 1:10 (rise:run);

6. The fill material provides at least six inches of cover over all portions of the system; and

7. The fill material is stabilized to prevent surface erosion.

D. Application rate. All sand-on-sand systems shall be designed on a 30 mpi loading rate and shall use a method of pressure distribution which achieves unsaturated flow conditions.

E. Fill depth. All sand-on-sand systems utilizing gravel trench designs shall have a minimum of 25 inches of fill over the original site.

F. System placement. The drainfield trenches shall be placed so that there is six inches of fill, or more, beneath the trench bottom and six inches of fill over the trenches.

G. The remaining design and construction criteria for sand-on-sand systems are the same as for the design and construction of a similar system in original undisturbed soils.

Statutory Authority

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

Historical Notes

Derived from Virginia Register Volume 16, Issue 16, eff. July 1, 2000.

12VAC5-610-966. Pads.

A. A pad is an absorption area wider than three feet but not longer than 100 feet with a level infiltrative surface where the bottom of the pad meets the original soil. The minimum standoff to a limiting feature in accordance with 12VAC5-613-80 is to be met under the entire infiltrative surface.

B. The minimum effluent quality dispersed to a pad is TL-2 effluent and pad bottom loading rates shall not exceed the values for pads noted in Table 5.5 of 12VAC5-610-950.

C. The longest dimension of the basal area of the pad, its length, shall be oriented parallel to the natural surface topographic contours. Minor deviations from surface contours are acceptable as long as the bottom of the pad is level (the entire bottom surface of the pad is at the same elevation, not to exceed 10% of the depth of the pad from the ground surface or plus or minus two inches, whichever is less), and intersects a similar soil horizon across its surface.

D. Pads and trenches may be used together in a single system when the respective pad or trench subsystems follow the respective design criteria found in this chapter and are separated by a minimum of six feet between the sidewall of the pad and the trench. When multiple pads are used on a site, the pads must be separated by the width of the pad as measured perpendicular to the natural surface topographic contour.

E. Pads shall be limited to sites with slopes of 10% or less (less than or equal to 10 feet of rise for every 100 feet of run).

F. All pads must be dosed. Pad systems over 1,000 gallons per day must be pressure dosed. When pads are installed at less than 12 inches from the ground surface, timed dosing shall be used to disperse the effluent.

G. The minimum absorption area for single family residential dwellings shall be 400 square feet.

H. Pad construction.

1. Gravel pads shall have a minimum installation depth of 12 inches, unless in Texture Group I or II soils where the installation depth can be reduced to eight inches. On sloping sites, the minimum installation depth is measured on the downhill side of the pad infiltrative surface. The construction of the pad's gravity percolation line and gravel bedding shall follow 12VAC5-610-930 E with the exception that the bottom of the pad is level and not sloping. Piping shall have a maximum center to center spacing of nine feet.

2. Gravel pads utilizing low pressure distribution shall follow 12VAC5-610-940 for construction and dosing cycle (volume). Gravel pads using low pressure distribution shall have a minimum installation depth of 12 inches, unless in Texture Group I or II soils where the installation depth can be reduced to eight inches. On sloping sites, the minimum installation depth is measured on the downhill side of the pad infiltrative surface. Piping shall have a maximum center to center spacing of nine feet.

3. Pads utilizing gravelless material as found in 12VAC5-610-930 F shall follow 12VAC5-610-930 F and manufacturer instructions on minimum depth of installation, but in no case shall a pad be installed at less than eight inches from the original soil surface. Gravelless material shall have a maximum center to center spacing of nine feet. On sloping sites, the minimum installation depth is measured on the downhill side of the pad infiltrative surface.

4. Designs supported by a division approved manufacturer's design manual may deviate from the maximum slope, depth of installation, separation distance between pads, and timed dosing when the dispersal area is constructed in accordance with the approved manual.

I. The minimum soil cover, after settling, shall be six inches as measured from the finished ground surface to the uppermost limit of the dispersal media (or gravelless material) utilized in the absorption area. If the cover is mounded above grade, the finished sideslope cannot exceed 1:4 (rise:run). The soil cover, with amendments as needed, shall be of a quality, character, and fertility suitable to establish a vegetative cover that is uniform and sufficiently mature to survive and inhibit erosion.

Statutory Authority

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

Historical Notes

Derived from Virginia Register Volume 39, Issue 4, eff. November 24, 2022.