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About this chapter: Chapter 6 covers the design of 'conventional' soil absorption systems. These systems are conventional in the sense that nonpotable water from the outlet of a septic tank flows by gravity into a piping network for distributing the water in an excavated area nearby. The piping is backfilled with the finished grade blending into adjacent grade level.

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

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 (18 925 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.
A pressure distribution system shall be permitted in place of a 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).
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).

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.

TABLE 603.1


RATE (minutes
required for water
to fall 1 inch)
PITS (square feet
per bedroom)
(square feet per
10 to less than 10165205
210 to less than 30250315
330 to less than 45300375
445 to 60330415

For SI: 1 minute per inch = 2.4 s/mm, 1 square foot = 0.0929 m2.

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:

(Equation 6-1)


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.

TABLE 604.1(1)


(minutes required for
water to fall 1 inch)
(square feet
per unit)
BEDS (square
feet per unit)
10 to less than 10110140
210 to less than 30165205
330 to less than 45220250
445 to 60220280

For SI: 1 minute per inch = 2.4 s/mm, 1 square foot = 0.0929 m2.

TABLE 604.1(2)


Apartment building1 per bedroom1.5
Assembly hall—no kitchen1 per person0.02
Auto washer (service buildings, etc.)1 per machine6.0
Bar and cocktail lounge1 per patron space0.2
Beauty salon1 per station2.4
Bowling center1 per bowling lane2.5
Bowling center with bar1 per bowling lane4.5
Camp, day and night1 per person0.45
Camp, day use only1 per person0.2
Campground and camping resort1 per camping space0.9
Campground and sanitary dump station1 per camping space0.085
Car wash1 per car1.0
Catch basin—garages, motor-fuel-dispensing facility, etc.1 per basin2.0
Catch basin—truck wash1 per truck5.0
Church—no kitchen1 per person0.04
Church—with kitchen1 per person0.09
Condominium1 per bedroom1.5
Dance hall1 per person0.06
Dining hall—kitchen and toilet1 per meal served0.2
Dining hall—kitchen and toilet waste with dishwasher or food waste grinder or both1 per meal served0.25
Dining hall—kitchen only1 per meal served0.06
Drive-in restaurant, inside seating1 per seat0.3
Drive-in restaurant, without inside seating1 per car space0.3
Drive-in theater1 per car space0.1
Employees—in all buildings1 per person0.4
Floor drain1 per drain1.0
Hospital1 per bed space2.0
Hotel or motel and tourist rooming house1 per room0.9
Labor camp—central bathhouse1 per employee0.25
Medical office buildings, clinics and dental offices
Doctors, nurses and medical staff
Office personnel
1 per person
1 per person
1 per person
Mobile home park1 per mobile home site3.0
Motor-fuel-dispensing facility1 per car served0.15
Nursing or group homes1 per bed space1.0
Outdoor sports facility—toilet waste only1 per person0.35
Park—showers and toilets1 per acre8.0
Park—toilet waste only1 per acre4.0
Restaurant—dishwasher or food waste grinder or both1 per seating space0.15
Restaurant—kitchen and toilet1 per seating space0.6
Restaurant—kitchen waste only1 per seating space0.18
Restaurant—toilet waste only1 per seating space0.42
Restaurant—(24-hour) kitchen and toilet1 per seating space1.2
Restaurant—(24-hour) with dishwasher or food waste grinder or both1 per seating space1.5
Retail store1 per customer0.03
School—meals and showers1 per classroom8.0
School—meals served or showers1 per classroom6.7
School—no meals, no showers1 per classroom5.0
Self-service laundry—toilet waste only1 per machine1.0
Showers—public1 per shower0.3
Swimming pool bathhouse1 per person0.2
Seepage trench excavations shall be 1 foot to 5 feet (305 mm to 1524 mm) wide. Trench excavations shall be spaced not less than 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 not greater than 100 feet (30 480 mm) long, except as otherwise approved.
Seepage bed excavations shall be not less than 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 not greater than 5 feet (1524 mm) and not less than 3 feet (914 mm) apart, and not greater than 3 feet (914 mm) and not less than 1 foot (305 mm) from the sidewall or headwall.

A seepage pit shall have not less than an 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 1/2 inch to 21/2 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 of the 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 not less than 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.

TABLE 605.3



For SI: 1 foot = 304.8 mm.

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. 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.
Not less than 6 inches (152 mm) of aggregate ranging in size from 1/2 inch to 21/2 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 not less than 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. Not less than 18 inches (457 mm) of soil backfill shall be provided above the covering.
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.

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 not less than 25 feet (7620 mm) from any window, door or air intake of any building used for human occupancy. Not more than four distribution pipelines shall be served by one common 4-inch (102 mm) observation pipe where 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.

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