South Carolina Code of Regulations › Chapter 61 - DEPARTMENT OF HEALTH AND ENVIRONMENTAL CONTROL › Subchapter 61-56 - ONSITE WASTEWATER SYSTEMS › Section 61-56.400 - Appendices of Standards for Permitted Systems › Appendix 61-56.400.O - System Standard 431 - Mounded Fill System

Current through Register Vol. 48, No. 6, June 28, 2024
(1) Site/Permitting Requirements
(a) The texture in the upper eighteen (18) inches of
				  naturally occurring soil must be Class I or Class II.
(b) The absorption bed within the mound shall be sized on the
				  long-term acceptance rate of the most limiting texture in the upper eighteen
				  (18) inches of naturally occurring soil.
(c) The linear footage of the absorption bed shall be
				  determined in accordance with Standard 270.
(d) The absorption bed width shall be a minimum of five (5)
				  feet and a maximum of 10 feet.
(e) Mounded fill systems must not be placed on sites with a
				  slope in excess of three (3) percent.
(f) No part of this system can be installed within one
				  hundred twenty-five (125) feet of the critical area line or tidal waters as
				  determined by the Department; or within one hundred twenty-five (125) feet of
				  the ordinary high water elevation within the banks of non-tidal,
				  environmentally sensitive waters. Because of the long buffer, side slope, fill
				  pad, and taper associated with this system, the one hundred twenty-five (125)
				  foot setback shall be measured from the outer edge of the aggregate bed within
				  the mound.
(g) There shall be a replacement area equivalent to at least
				  fifty (50) percent in size of the original system area held in reserve for
				  system repair. This area shall have a suitable configuration and shall meet the
				  minimum soil and site conditions of this regulation.
(h) This system cannot be considered for facilities with peak
				  flow rates in excess of four hundred eighty (480) gpd. In addition, this system
				  shall not be considered for facilities requiring grease traps.
(i) Effluent discharged to this system must receive a higher
				  degree of treatment than that provided by a conventional septic tank (e.g., two
				  (2) compartment septic tank or two (2) septic tanks in series).
(j) This system may be considered for installation on
				  contiguous lots in new subdivisions approved after the effective date of this
				  standard provided a setback of at least seventy-five (75) feet is maintained
				  between the system and all adjacent property lines. Because of the long buffer,
				  side slope, fill pad, and taper associated with this system, the seventy-five
				  (75) foot setback shall be measured from the outer edge of the aggregate bed
				  within the mound.
(2) Installation Requirements
(a) Site Preparation
(i) If present within eighteen (18) inches of the naturally
				  occurring soil surface, organic material and restrictive horizons must be
				  removed from beneath the mound and replaced with USDA medium sand, washed
				  concrete sand, or an equivalent material approved by the Department. The
				  replacement area must extend five (5) feet in all directions beyond the edges
				  of the aggregate filled absorption bed.
(ii) The naturally occurring soil surface underlying the
				  mound shall be thoroughly tilled and mixed with the imported mound fill
				  material to a depth of six (6) inches.
(b) Mound/Absorption Bed Requirements
(i) Low Pressure Pipe Distribution (LPP) must be utilized to
				  preclude localized hydraulic overloading of the imported fill material and to
				  minimize the impact on the shallow zone of saturation.
(ii) Low Pressure Pipe Distribution (LPP) must be designed
				  and installed in accordance with Department standards or equivalent designs.
				  The size and layout of each distribution system will vary based on the size of
				  the filter and the needed dosing.
(iii) Pump design shall be in accordance with Department
				  standards.
(iv) There must be at least twenty-four (24) inches of medium
				  sand placed between the naturally occurring soil surface and the bottom of the
				  absorption bed. The bottom surface of the absorption bed must be placed at
				  least twenty-four (24) inches above the zone of saturation.
(v) If the slope of the site in the proposed mound area is
				  one (1) percent or less, then the mound shall be placed on a twelve (12) inch
				  fill pad which must extend twenty (20) feet beyond the mound in all directions.
				  If the slope of the site in the proposed mound area is greater than one (1)
				  percent but less than or equal to three (3) percent, then the mound shall be
				  placed on a twelve (12) inch deep fill pad which must extend twenty (20) feet
				  beyond the mound area on the sides of the mound; forty (40) feet beyond the
				  mound area on the downslope side of the mound; with no fill pad required on the
				  upslope side of the mound.
(vi) The mound and fill pad material shall be USDA medium
				  sand, washed concrete sand, or other equivalent material approved by the
				  Department.
(vii) The depth of the fill cap material above the absorption
				  bed shall be nine (9) to fifteen (15) inches of soil texture Class II or Class
				  III. Sod may be substituted for four (4) inches of this portion of the fill cap
				  material.
(viii) The depth of the fill cap material above the mound
				  side-slope, the twelve (12) inch deep fill pad, and the taper shall be at least
				  four (4) inches of soil texture Class II or Class III. Sod may be substituted
				  for this portion of the fill cap material.
(ix) A 1:2 maximum slope is required if the mound side-slope
				  and taper are sodded.
(x) A 1:4 maximum slope is required if the mound side-slope
				  and taper are mulched and seeded.
(3) Final Landscaping and Drainage
(a) The septic tank and mound area shall be backfilled and
				  shaped to promote the runoff of surface water.
(b) Where natural surface drainage does not exist, a swale
				  shall be constructed adjacent to the filled area to divert surface water away
				  from the onsite wastewater system to a positive outfall. The installation of
				  ditches, curtain drains, and rain gutters may be required to intercept and
				  divert water away from the onsite wastewater system location.
(c) A barrier to preclude parking and vehicular traffic over
				  the system area may be required.
(d) Following final landscaping, seeding, or sodding may be
				  required to prevent erosion.
(e) Final approval shall be withheld until all landscaping
				  and drainage improvements have been satisfactorily completed.
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