Title 124 - NEBRASKA DEPARTMENT OF ENVIRONMENTAL Quality
Chapter 14 - DESIGN AND CONSTRUCTION OF SOIL ABSORPTION SYSTEM
001 The bottom of trenches and beds shall be at least four feet above the seasonal high groundwater table. If a water table is not present then the bottom of the trench shall be at least four feet above the bedrock or other barrier layer.
002 A soil absorption system shall not be installed in fill, except when the fill material is sand, or when the bottom 12 inches or more of the trench or bed is located in undisturbed native soil below the fill. When constructing a system in sand fill, sufficient time shall be allowed after placement of the fill, or sufficient compaction effort applied to the fill to prevent settlement after the system is installed.
003 When installing a trench or bed in soil that has a percolation rate faster than 5 minutes per inch, a twelve-inch thick loamy sand soil liner with a percolation rate of 15 to 20 minutes per inch shall be installed in the trench or bed. The liner shall cover the bottom of the trench or bed and extend up the sidewalls a minimum of nine (9) inches for filter material absorption systems, to the top of the slotted sidewalls in gravelless chamber systems, or to the top of the pipe in gravelless pipe systems. The soil absorption area is then sized on this soil liner’s percolation rate.
004 Trenches and beds shall not be more than 100 feet in length unless it is installed using an instrument to insure that the trench is level, then trenches can be up to 150 feet in length for gravity systems. Pressure systems are not restricted in length when an instrument is used to insure that the system is level.
005 The bottom of the trench or bed excavation shall be level.
006 The bottom and sides of the soil absorption system to the top of the filter material shall be excavated in such a manner as to leave the soil in a natural, unsmeared, and uncompacted condition. Excavation shall be made only when the soil moisture content is at or less than the plastic limit.
007 When the percolation rate is slower than 10 minutes per inch, excavation equipment or other vehicles shall not be driven on the soil absorption area.
008 The distribution pipes shall be laid level or on a uniform slope away from the distribution device of no more than four inches per 100 feet.
009 Distribution pipes in beds shall be uniformly spaced no more than 5 feet apart and not more than 30 inches from the side walls of the beds.
010 The trenches or beds shall be backfilled and crowned above finished grade to allow for settling. The top six inches of soil shall have the same texture and density as the adjacent soil.
011 The minimum depth of cover over the distribution pipes shall be at least eight inches. The maximum depth of cover over the distribution pipes shall be no more than 36 inches.
012 No parking area, driveway, or impermeable surface or cover shall be installed, created, or located by the owner, or anyone acting for the owner, over or within five (5) feet horizontally of the soil absorption system.
013 Gravity Distribution:
013.01 When a soil absorption system is located in sloping ground from 0 to 3%, septic tank effluent may be distributed to the soil absorption system by either a distribution box, drop box or a header pipe.
013.02 A soil absorption systems in sloping ground with greater than 3% slope shall use a drop box or pressure distribution.
013.03 The bottom of each trench shall be level.
013.04 The absorption trenches shall follow the ground surface contours so that variations in trench depth are minimized.
013.05 When ground slope is less than 10%, there shall be a minimum of 4 feet of undisturbed soil between adjacent trenches and between the septic tanks and the nearest trench. When ground slope is 10% to 20%, there shall be a minimum of 6 feet of undisturbed earth between adjacent trenches and between the septic tanks and the nearest trench. When the slope exceeds 20%, there shall be a minimum of 10 feet of undisturbed soil between adjacent trenches and between the septic tanks and the nearest trench.
014 Distribution box: Septic tank effluent may be distributed by a distribution box. The distribution box may be built as an integral part of the septic tank or may be a separate unit set on solid ground and anchored in the drainfield. When a distribution box is used the following criteria shall be observed.
014.01 The distribution box shall be set level and arranged so that effluent is evenly distributed to each distribution line.
014.02 Each distribution line shall connect individually to the distribution box.
014.03 The pipe connecting the distribution box to the distribution line shall be of a tight joint construction laid on undisturbed earth or properly bedded throughout its length.
014.04 Distribution boxes shall be constructed of a durable watertight, non-corrosive material. They shall be designed to accommodate the necessary distribution lines.
014.05 Distribution boxes shall be provided with a minimum 12-inch diameter opening which will serve as a ready access for inspection, cleaning, and general maintenance.
014.06 The inverts of all outlets shall be at the same elevation as measured from a liquid surface in the bottom of the box.
014.07 The inlet invert shall be at least one inch above the outlet inverts.
014.08 The outlet inverts shall be at least four inches above the distribution box floor.
014.09 When septic tank effluent is delivered to the distribution box by pump, either a baffle wall shall be installed in the distribution box or the pump discharge shall be directed against a wall or side of the box on which there is no outlet. The baffle shall be secured to the box and shall extend at least one inch above the crown of the inlet flow line.
015 Header pipe: If a header pipe is used the following criteria shall be observed.
015.01 Header pipe shall have a minimum diameter of 4 inches.
015.02 When a header pipe is used, there shall be an equal number of distribution lines spaced evenly on both sides of the junction of the leader to the header.
015.03 The header pipe shall be laid level with direct watertight connections to each drainfield line and the septic tank outlet pipe. The header pipe shall be encased in filter material.
016 Drop box (see Figures 14.1 and 14.2): When drop boxes are used the following criteria shall be followed:
016.01 The drop box shall be watertight and constructed of durable materials not subject to excessive corrosion or decay.
016.02 The invert of the inlet pipe shall be at least one inch higher than the invert of the outlet pipe to the next trench.
016.03 The invert of the outlet pipe to the next trench shall be at least two inches higher than the invert of the outlet pipe of the trench in which the box is located.
016.04 When septic tank effluent is delivered to the drop box by a pump, the pump discharge shall be directed against a wall or side of the box on which there is no outlet.
016.05 The drop box shall have a removable cover either flush or above finished grade or covered by more than six inches of soil.
Figure 14.1 Drop Box
Figure 14.2 Drop Box On A Slope Profile View | 
File Size 38 KB |
017 Filter Material Absorption Systems
017.01 The trenches shall not be less than 18 inches nor more than 36 inches wide for pipe laterals and no more than 5 feet wide for chambers. Any trench wider than 36 inches for pipes and 5 feet for chambers shall be considered a bed and have the absorption area reduced by the appropriate factor in Section 019.03. See Figures 14.3 and 14.4.
017.02 There shall be a layer of at least six but not more than 24 inches of filter material in the bottom of the trenches and beds. See Figures 14.3 and 14.4.
017.03 Distribution pipes, gravity distribution.
017.03A Distribution pipes used in trenches or beds for gravity flow distribution shall be at least four inches in diameter and constructed of sound and durable material not subject to corrosion or decay or to loss of strength under continuously wet conditions. When open joint tile is used, the tile sections shall be spaced not less than 1/4 inch nor more than 1/2 inch apart.
017.03B Perforated pipe used for wastewater distribution pipes shall have one or more rows of holes of no less than one-half inch in diameter and no more than 3/4 inch in diameter spaced no more than 36 inches apart. Holes shall be spaced
to prevent failure of pipe due to loads. Distribution pipes shall have a load bearing capacity of more than 1000 pounds per linear foot.
017.03C Half moon concrete or plastic tile may be used for wastewater distribution and shall be placed in trenches resting on concrete blocks suitably placed before filter material is added unless specifically designed to be self supporting on the gravel bed with an appropriate supporting foot. See Figure 14.4.
017.04 The filter material shall completely encase the disposal pipes and chambers to a depth of at least two inches. See Figures 14.3 and 14.4.
017.05 The filter material shall be covered with untreated building paper or a two-inch layer of hay or straw or similar, approved permeable materials. See Figures 14.3 and 14.4.
Figure 14.3 Filter material with 4 inch pipe
Figure 14.4 Filter material and chambers | 
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018 Non Filter Material Absorption Systems
018.01 Gravelless pipes and chambers may be used for effluent treatment under the following conditions.
018.01A Pipes and chambers shall be of durable, non-degradable construction (Figures 14.5, 14.6, 14.7) specifically designed for installation without filter material.
018.01B Pipes and chambers shall be able to meet load requirements of 1000 lbs per linear foot.
018.02 Non Filter Material Effective Trench Bottom Area Square Footage
018.02A Systems using pipe with filter fabric specifically designed for gravelless use shall use 75% of the outside perimeter of the pipe, up to a 12 inch diameter pipe, as the effective width of the trench. A construction permit as outlined in Chapter 3 is required when using 12 inch or larger diameter pipe.
018.02B Systems using chambers with at least six inches of slotted sidewall, specifically designed for gravelless use may use up to 1.50 times the bottom width of the chamber, measured as the distance between the inside edges of the base flanges, as the effective width of the trench. (See Fig. 14.7) The effective width of the trench shall not exceed five feet for design purposes.
Figure 14.5 Gravelless Pipe with Filter Fabric
Figure 14.6 Gravelless Chamber Profile
Figure 14.7 Gravelless Chamber End View | 
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019 Absorption Trench Sizing:
019.01 The required square footage for an absorption trench for a dwelling shall be determined by Table 14.1 when a percolation test was performed.st was performed.
TABLE 14.1
Square Feet of Drain Field Trench Required for Single Family Dwelling
Perc Rate | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
in minutes | Bedroom | Bedroom | Bedroom | Bedroom | Bedroom | Bedroom | Bedroom | Bedroom | Bedroom |
per inch | 200 gpd | 300 gpd | 400 gpd | 500 gpd | 600 gpd | 700 gpd | 800 gpd | 900 gpd | 1000 gpd |
<5 | Systems must be designed with a 12 inch loamy sand liner that would have a percolation rate of 15 to 20 minutes per inch and shall be designed at the 11-20 minute per inch level |
5-10 | 165 | 330 | 495 | 660 | 825 | 990 | 1155 | 1320 | 1485 |
11-20 | 210 | 420 | 630 | 840 | 1050 | 1260 | 1470 | 1680 | 1890 |
21-30 | 250 | 500 | 750 | 1000 | 1250 | 1500 | 1750 | 2000 | 2250 |
31-40 | 275 | 550 | 825 | 1100 | 1375 | 1650 | 1925 | 2200 | 2475 |
41-50 | 330 | 660 | 990 | 1320 | 1650 | 1980 | 2310 | 2640 | 2970 |
51-60 | 350 | 700 | 1050 | 1400 | 1750 | 2100 | 2450 | 2800 | 3150 |
>60 | Systems must be designed by a professional engineer. Construction Permit Needed |
019.02 The required square footage for establishments shall be determined by the following equation: The daily design flow divided by (Five divided by the square root of the percolation rate). 
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019.03 Absorption area for a bed shall be calculated by determining the required square footage for a trench and multiplying the area by the factor from Table 14.2.
TABLE 14.2
Absorption Bed Multiplication Factor
Width of Bed in feet | Factor |
>3 to 10 | 1.25 |
>10 to 15 | 1.33 |
>15 to 20 | 1.50 |
>20 | Unacceptable |
020 Dosing.
020.01 Dosing is recommended for all systems and shall be provided when the design wastewater flow requires more than 500 linear feet of distribution line. When the design wastewater flow requires more than 1,000 linear feet of distribution line, the absorption field shall be divided into two equal portions and each half dosed alternately, not more than four times per day.
020.02 Dosing may be accomplished by either pumps or siphons. Each side of the system shall be dosed not more than four times per day. The volume of each dose shall be the greater of the daily wastewater volume divided by the daily dosing frequency, or an amount equal to approximately 3/4 of the internal volume of the distribution lines being dosed (approximately 0.5 gallons or 1.89 L per linear foot of 4-inch pipe.)
Enabling Legislation: Neb. Rev. Stat. §81-1505(8).
Legal Citation: Title 124, Ch. 14, Nebraska Department of Environmental Quality
Nebraska Department of Environmental Quality
1200 "N" Street, Suite 400
P.O. Box 98922
Lincoln, Nebraska 68509
(402) 471-2186