Soil Absorption Systems

This chapter outlines the requirements for sizing and installing soil absorption systems, including effluent disposal from treatment tanks and specific sizing criteria for residential and other building types.

601 — General

601.1 Scope. The provisions of this chapter shall govern the sizing and installation of soil absorption systems.

602 — Sizing Soil Absorption Systems

602.1 General. Effluent from septic tanks and other approved treatment tanks shall be disposed of by soil absorption or an approved manner. Sizing shall be in accordance with this chapter for systems with a daily effluent application of 5,000 gallons (18925 L) or less. Two systems of equal size shall be required for systems receiving effluents exceeding 5,000 gallons (18 925 L) per day. Each system shall have a minimum capacity of 75 percent of the area required for a single system. An approved means of alternating waste application shall be provided. A dual system shall be considered as one system.

602.2 Pressure system. A pressure distribution system shall be permitted in place ofa conventional or dosing conventional soil absorption system where a site is suitable for a conventional private sewage disposal system. A pressure distribution system shall be approved as an alternative private sewage disposal system where the site is unsuitable for conventional treatment (for sizing and design criteria, see Chapter 7).

602.3 Method ofdischarge. Flow from the septic or treatment tank to the soil absorption system shall be by gravity or dosing for facilities with a daily effluent application of 1,500 gallons (5678 L) or less. The tank effluent shall be discharged by pumping or an automatic siphon for systems over 1,500 gallons (5678 L).

603 — Residential Sizing

603.1 General. The bottom area for seepage trenches or beds or the sidewall area for seepage pits required for a soil absorption system serving residential property shall be determined from Table 603.1 using soil percolation test data and type of construction.

A table appears here in the adopted code. Refer to the official source for the full table.

604 — Other Building Sizing

604.1 General. The minimum required soil absorption system area for all occupancies, except one- and two-family dwellings, shall be based on building usage, the percolation rate and system design in accordance with Tables 604.1 (1) and 604.1 (2). The minimum soil absorption area shall be calculated by the following equation:

A= Ux CFxAA (Equation 6-1)

where:

A = Minimum system absorption area.

AA = Absorption area from Table 604.1 (1).

CF = Conversion factor from Table 604.1 (2).

U = Number of units.

A table appears here in the adopted code. Refer to the official source for the full table.

605 — Installation of Conventional

605.1 Seepage trench excavations. Seepage trench excavations shall be 1 foot to 5 feet (305 mm to 1524 mm) wide. Trench excavations shall be spaced a minimum of 6 feet (1829 mm) apart. The absorption area of a seepage trench shall be computed by using only the bottom of the trench area. The bottom excavation area of the distribution header shall not be computed as absorption area. Individual seepage trenches shall be a maximum of 100 feet (30 480 mm) long, except as otherwise approved.

605.2 Seepage bed excavations. Seepage bed excavations shall be a minimum of 5 feet (1524 mm) wide and have more than one distribution pipe. The absorption area of a seepage bed shall be computed by using the bottom of the trench area. Distribution piping in a seepage bed shall be uniformly spaced a maximum of 5 feet (1524 mm) and a minimum of 3 feet (914 mm) apart, and a maximum of3 feet (914 mm) and a minimum of 1 foot (305 mm) from the sidewall or headwall.

605.3 Seepage pits. A seepage pit shall have a minimum inside diameter of 5 feet (1524 mm) and shall consist of a chamber walled-up with material, such as perforated precast concrete ring, concrete block, brick or other approved material allowing effluent to percolate into the surrounding soil. The pit bottom shall be left open to the soil. Aggregate of ½ inch to 2½ inches (12.7 mm to 64 mm) in size shall be placed into a 6-inch minimum (152 mm) annular space separating the outside wall ofthe chamber and sidewall excavation. The depth of the annular space shall be measured from the inlet pipe to the bottom of the chamber. Each seepage pit shall be provided with a 24-inch (610 mm) manhole extending to within 56 inches (1422 mm) of the ground surface and a 4-inch-diameter (102 mm) fresh air inlet. Seepage pits shall be located a minimum of 5 feet (1524 mm) apart. Excavation and scarifying shall be in accordance with Section 605.4. The effective area of a seepage pit shall be the vertical wall area of the walled-up chamber for the depth below the inlet for all strata in which the percolation rates are less than 30 minutes per inch (70 s/mm). The 6-inch (152 mm) annular opening outside the vertical wall area is permitted to be included for determining the effective area. Table 605.3, or an approved method, shall be used for determining the effective sidewall area of circular seepage pits.

A table appears here in the adopted code. Refer to the official source for the full table.

605.4 Excavation and construction. The bottom of a trench or bed excavation shall be level. Seepage trenches or beds shall not be excavated where the soil is so wet that such material rolled between the hands forms a soil wire. All smeared or compacted soil surfaces in the sidewalls or bottom of seepage trench or bed excavations shall be scarified to the depth of smearing or compaction and the loose material removed. Where rain falls on an open excavation, the soil shall be left until sufficiently dry so a soil wire will not form when soil from the excavation bottom is rolled between the hands. The bottom area shall then be scarified and loose material removed.

605.5 Aggregate and backfill. A minimum of 6 inches (152 mm) of aggregate ranging in size from ½ inch to 2½ inches (12.7 mm to 64 mm) shall be laid into the trench or bed below the distribution pipe elevation. The aggregate shall be evenly distributed a minimum of 2 inches (51 mm) over the top of the distribution pipe. The aggregate shall be covered with approved synthetic materials or 9 inches (229 mm) of uncompacted marsh hay or straw. Building paper shall not be used to cover the aggregate. A minimum of 18 inches (457 mm) of soil backfill shall be provided above the covering.

605.6 Distribution piping. Distribution piping for gravity systems shall be not less than 4 inches (102 mm) in diameter. The distribution header (PVC) shall be solid-wall pipe. The top of the distribution pipe shall be not less than 8 inches (203 mm) below the original surface in continuous straight or curved lines. The slope of the distribution pipes shall be 2 inches to 4 inches (51 mm to 102 mm) per 100 feet (30 480 mm). Effluent shall be distributed to all distribution pipes. Distribution of effluent to seepage trenches on sloping sites shall be accomplished by using a drop box design or other approved methods. Where dosing is required, the siphon or pump shall discharge a dose of minimum capacity equal to 75 percent of the combined volume of the distribution piping in the absorption system.

605.7 Observation pipes. Observation pipes shall be provided. Such pipes shall be not less than 4 inches (102 mm) in diameter, not less than 12 inches (305 mm) above final grade and shall terminate with an approved vent cap.

The bottom 12 inches (305 mm) of the observation pipe shall be perforated and extend to the bottom of the aggregate. Observation pipes shall be located at least 25 feet (7620 mm) from any window, door or air intake of any building used for human occupancy. A maximum of four distribution pipelines shall be served by one common 4-inch (102 mm) observation pipe when interconnected by a common header pipe (see Appendix-A, Figure A-4).

Exception: Where approved and where the location of the observation pipe is permanently recorded, the observation pipe shall be not more than 2 inches (51 mm) below the finished grade.

605.8 Winter installation. Soil absorption systems shall not be installed during periods of adverse weather conditions unless the installation is approved. A soil absorption system shall not be installed where the soil at the system elevation is frozen. Snow cover shall be removed from the soil absorption area before excavation begins. Snow shall not be placed in a manner that will cause water to pond on the soil absorption system area during snow melt. Excavated soil to be used as backfill shall be protected from freezing. Excavated soil that freezes solid shall not be used as backfill. The first 12 inches (305 mm) of backfill shall be loose, unfrozen soil. Inspection of systems installed during winter conditions shall include inspection of the trench or bed excavation prior to the placement of gravel and inspection of backfill material at the time of placement.

605.9 Evaporation. Soil absorption systems shall not be covered or paved over by material that inhibits the evaporation of the effluent.

A table appears here in the adopted code. Refer to the official source for the full table.

Frequently asked questions

What is the purpose of Chapter 6 of the IPC?

Chapter 6 governs the sizing and installation of soil absorption systems, which dispose of effluent from septic and other approved treatment tanks.

How is the required size of a soil absorption system determined for a home?

For residential properties, the required bottom area for seepage trenches/beds or sidewall area for seepage pits is determined using Table 603.1, based on soil percolation data and construction type.

What are the dimensional requirements for seepage trench excavations?

Seepage trench excavations must be 1 to 5 feet (305 mm to 1524 mm) wide, spaced a minimum of 6 feet (1829 mm) apart, with the absorption area computed using only the trench bottom.