Virginia Administrative Code › Title 12 - HEALTH › Agency 5 - DEPARTMENT OF HEALTH › Chapter 610 - SEWAGE HANDLING AND DISPOSAL REGULATIONS › Part V - Design and Construction Criteria › Article 5 - Subsurface Soil Absorption System › Section 12VAC5-610-930 - Gravity distribution

Current through Register Vol. 40, No. 24, July 15, 2024
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.