Rule 15-18-77-5.4.5 - Underground Absorption1. The size of the subsurface sewage disposal system shall be determined by soil texture [See Table II].2. Soils with excessively rapid permeability rates, gravel and coarse sand, shall be considered unsuitable for subsurface disposal unless the native soil is replaced with a suitably thick (greater than two feet) layer of loamy sand or sand textured soil.3. Soils with excessively slow permeability rates, silty clay and clay, shall be considered unsuitable for conventional subsurface disposal.4. Subsurface disposal systems shall be placed no deeper than 36 inches below the surface.5. Conventional subsurface disposal systems shall have a minimum 12 inches of soil backfill [Figure 1][Figure 2]. 6. The minimum distance between absorption trench sidewalls shall be six feet.7. Aggregate -type absorption trenches shall be a minimum of 24 inches and a maximum of 36 inches in width.8. Trenches shall not be excavated when the soil is wet enough to smear or compact easily.9. The bottom of the trenches or bed and the distribution lines shall have a grade from level to no greater than two inches fall per 100 feet.10. There shall be a minimum of three feet of undisturbed soil between the excavation for the septic tank or treatment plant and the beginning of the absorption trench, bed or effluent line.11. Media for the disposal fields shall extend from at least two inches above the top of the perforated field line pipe to at least six inches below the bottom of the perforated field line pipe a minimum of 12 inches total [Figure 1].12. Stone media for the disposal fields shall consist of crushed rock, gravel or other suitable material, as approved by the Mississippi Department of Health, Division of Onsite Wastewater, varying in size from- to 2- inches. The material shall be free from dust, sand, clay, or excessive fines.13. Tire chips shall be allowed for use as coarse aggregate in onsite wastewater treatment and disposal system drainfields and may substitute for stone aggregate on a one-for-one basis, volumetrically when the following physical properties are met: a. Tire chips are to be a normal two (2) inches in size and may range from one-half (1/2) inch to a maximum of four (4) inches in any direction.b. Exposed wire may protrude no more than one-half (1/2) inch from the sides of the chip. No more than (10%) by weight shall exceed this standard.c. No more than (10%) by weight shall pass through a one-half (1/2) inch screen.d. At least 80% of the bead wire must be removed from the tires to be chipped.e. Fines of less than 2 mm in size are prohibited. Fines in this context is defined as particles or substances which can settle to the bottom of the absorption trench and contribute to the clogging or blocking of infiltrative surfaces (dirt, dust, grit, crumb rubber and similar substances).14. The media for the disposal fields shall be covered with untreated building paper, heavy craft paper, a layer of straw at least two inches thick, or other acceptable material, as approved by the Mississippi Department of Health, Division of Onsite Wastewater.15. Soil material excavated from trenches shall be used in backfilling and should be left mounded over the trenches until initial settling has taken place.16. When a change in elevation of the disposal trench is required, a connecting lateral or crossover must be used. At the point where a crossover line leaves a lateral, the trench for the crossover line shall be dug no deeper than the top of the aggregate in the preceding trench so that an undisturbed block of earth will remain in place for the full depth of the aggregate. Crossover lines shall be laid on undisturbed earth. The invert of the crossover must be at least four inches lower than the invert of the septic tank effluent line.17. Standard manufactured fittings compatible with the pipe shall be used to connect all pipes within the effluent disposal field.15 Miss. Code. R. 18-77-5.4.5