001 The bottom of trenches and beds shall be at least four feet above the seasonal high groundwater table 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 five minutes per inch, a 12-inch thick loamy sand soil liner designed to provide a percolation rate of 15 to 20 minutes per inch shall be installed in the bottom and sides of the trench or bed. The loamy sand soil liner shall cover the bottom of the trench or bed and extend up the sidewalls of the trench a minimum of nine inches for a soil absorption system that uses filter material. For a soil absorption system that does not use filter material, the liner shall cover the bottom of the trench and extend up the sidewalls at least to the top of the slotted sidewalls on a gravelless chamber or to the top of the pipe on gravelless pipe. The loamy sand liner shall be constructed to provide a percolation rate of 15 to 20 minutes per inch. The soil absorption area shall be sized based on the soil liner percolation rate.
004 The bottom of the trench or bed excavation shall be level. A trench or bed more than 100 feet in length shall be installed using an instrument to insure that the trench or bed is level.
005 A trench or bed for a gravity distribution system shall not exceed 150 feet in length. A trench or bed with pressure distribution is not restricted in length when an instrument is used to insure that the trench or bed is level. Dosing (see 020 and 021 below) shall be provided when the distribution system has more than 500 linear feet of distribution line.
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 absorption trenches shall follow the ground surface contours so that variations in trench depth are minimized.
009 When ground slope is less than 10 percent, there shall be a minimum of four feet of undisturbed soil between adjacent trenches and between the septic tanks and the nearest trench. When ground slope is 10 to 20 percent, there shall be a minimum of six feet of undisturbed earth between adjacent trenches and between the septic tanks and the nearest trench. When the slope exceeds 20 percent, there shall be a minimum of 10 feet of undisturbed soil between adjacent trenches and between the septic tanks and the nearest trench.
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 A soil absorption system shall not be installed in an area that has an impermeable surface or where the soil has been compacted excessively by vehicle traffic or parking. 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 feet horizontally an existing soil absorption system or reserve area.
013 Gravity Distribution Piping and Devices
013.01 When a soil absorption system is located in ground that slopes three percent or less, septic tank effluent may be conveyed by gravity through piping to the soil absorption system through one of the following distribution devices: a distribution box, drop box or a header pipe.
013.02 A soil absorption system in ground with greater than three percent slope shall use either gravity distribution through a drop box or pressure distribution.
013.03 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.
013.04 Distribution pipes in a seepage bed shall be uniformly spaced no more than five feet apart and not more than 30 inches from the side walls of the bed.
014 When a distribution box is used it can be an integral part of the septic tank or a separate unit set on solid ground and anchored in the drainfield. When a distribution box is used the following criteria shall be followed.
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 secured opening which will serve as 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 two 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 When a header pipe is used the following criteria shall be followed.
015.01 Header pipe shall have a minimum diameter of four inches.
015.02 The header pipe shall be spaced evenly on both sides of the junction of the leader pipe to the header with an equal number of distribution lines of the same length on each side.
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 When a drop box is 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 or siphon, the pump or siphon 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. If the cover is located at or above finished grade it shall be secured to help prevent unwanted access.
017 Soil Absorption Systems Where Filter Material Is Used
017.01 The trenches shall not be less than 18 inches nor more than 36 inches wide for pipe laterals and no more than five feet wide for chambers. For soil absorption systems where filter material is used, any trench wider than 36 inches for pipes and five feet for chambers shall be considered a bed and have the required square footage of the soil absorption area or drainfield trench increased by the appropriate absorption bed multiplication factor from Table 14.1.
017.02 There shall be a layer of at least six but not more than 24 vertical inches of filter material in the bottom of the trenches and beds below the distribution piping or chamber extending uniformly to the width of the trench.
017.03 Distribution pipes and gravity distribution
017.04 The filter material shall completely encase the distribution pipes, chambers, or synthetic aggregate bundles to a depth of at least two inches extending uniformly to the width of the trench.
017.05 The filter material shall be covered with a permeable layer that prevents the movement of fine soil particles into the filter material. Geotextile fabric, or a two-inch layer of hay or straw or similar permeable material shall be used.
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 one-quarter inch nor more than one-half 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 three-quarter 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 1,000 pounds per linear foot.
017.03C Half moon concrete chambers 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 filter material.
017.03D Plastic chambers meeting the requirements for gravelless chambers installed without filter material may be used for effluent distribution in a trench or bed with filter material and may be installed directly on the trench bottom. For such installations, the width for calculating the trench bottom area shall be the width of the filter material covered trench.
017.03E Bundled expanded polystyrene synthetic aggregate contained in high-strength polyethylene netting in cylinders 9 to 12 inches in diameter specifically designed for use without filter material and meeting the requirements for bundles installed without filter material may be used for effluent distribution in a trench or bed with filter material and may be installed directly on the trench bottom. For such installations, the width for calculating the trench bottom area shall be the width of the filter material covered trench.
018 Soil Absorption Systems Where Filter Material Is Not Used
018.01 A gravelless distribution system may be used to distribute effluent for treatment in a soil absorption system under the following conditions.
018.02 Effective Soil Absorption System Trench Bottom Area When Filter Material Is Not Used
018.01A The pipes, chambers, or other conduit shall be of durable, non-degradable construction specifically designed for installation without filter material.
018.01B The pipes, chambers, or other conduit shall be able to meet load requirements of 1,000 pounds per linear foot.
018.02A The effective width of the trench for a system using pipe wrapped with filter fabric, specifically designed for use without filter material, shall be 75 percent of the outside perimeter of the pipe, for up to a 12 inch diameter pipe. A construction permit as outlined in Chapter 3 is required when using pipe larger than 12 inches in diameter. The effective width calculation does not apply when filter material is used in the trench.
018.02B The effective width of the trench for a system using gravelless chambers with at least six inches of slotted sidewall, specifically designed for use without filter material, may be up to 1.5 times the bottom width of the chamber, measured as the distance between the inside edges of the base flanges of the chamber. The effective width of the trench shall not exceed five feet for design purposes. The effective width calculation does not apply when filter material is used in the trench.
018.02C The effective width of the trench for a system using bundled expanded polystyrene synthetic aggregate contained in high-strength polyethylene netting in cylinders 9 to 12 inches in diameter specifically designed for use without filter material may be up to 1.5 times the maximum external width of the synthetic aggregate bundle. The trench may contain multiple bundles but at least one bundle length in the trench must include perforated distribution pipe. The effective width of the trench shall not exceed five feet for design purposes. The effective width calculation does not apply when filter material is used in the trench.
018.02D For soil absorption systems that do not use filter material, any trench wider than 36 inches for pipes and five feet for chambers shall be considered a bed and have the required square footage of the absorption area or drainfield trench increased by the appropriate multiplication factor from Table 14.1.
019 Absorption Trench Sizing
019.01 The required square footage for an absorption trench for a dwelling shall be determined by use of Table 14.2 when a percolation test was performed.
019.02 The required square footage for a non-dwelling facility shall be determined by use of the appropriate wastewater flow rate in Table 14.2 or by use of the following equation: The daily design flow multiplied by (0.20 multiplied by the square root of the percolation rate).
019.03 The required soil absorption area for a bed shall be calculated by determining the required square footage for a trench and multiplying that area by the soil absorption bed multiplication factor from Table 14.1.
020.01 Dosing of the soil absorption system is required when the total length of distribution line is greater than 500 linear feet. Dosing may be accomplished by either pumps or siphons. The discharge from the pump or siphon shall be to a device designed to dissipate the velocity of the discharge and prevent erosion or disruption of the filter material or soil in the soil absorption system.
020.02 When the design wastewater flow requires more than 1,000 linear feet of distribution line, the soil absorption system shall be divided into two equal portions and each half dosed alternately with equal volumes of effluent.
020.03 The system shall be designed so that the soil absorption system, or each half if divided into equal portions, is dosed at least once per day but not more than four times per day based on the design flow.
020.04 The volume of each dose for an undivided system shall be the greater of the volume from one day of design flow (the wastewater daily design flow divided by the daily dosing frequency), or an amount equal to approximately three-fourths of the internal volume of the distribution lines being dosed (approximately 0.5 gallons per linear foot of four-inch pipe) to ensure distribution over the entire system. The volume of each dose for a divided system shall be half the amount determined for an undivided system.
020.05 When a dosing pump or siphon device is installed directly in a tank or tank compartment, the volume of that tank or tank compartment shall not be used to meet, or be considered part of, the minimum septic tank capacity requirement of this Title.
021 Pressure Dosing and Pressure Distribution
021.01 Pressure distribution must be accomplished by use of a pump. Use of a siphon in a pressure distribution system is not permitted.
021.02 A permit is required for the construction of an onsite wastewater treatment system with a pressure dosed soil absorption system, except for a pressure dosed system that is part of a mound system designed and constructed in accordance with this Title by a properly endorsed Master Installer, a professional engineer, or a registered environmental health specialist.
TABLE 14.1 - Soil Absorption Bed Multiplication Factor
Wide of Bed, feet
> 3 to 10
> 10 to 15
> 15 to 20
TABLE 14.2 - Soil Absorption System Requirement, square feet
|Percolation Rate, mpi||200 gpd||300 gpd||400 gpd||500 gpd||600 gpd||700 gpd||800 gpd||900 gpd||1,000 gpd|
Design for a 12 inch loamy sand liner with a percolation rate of 15 to 20 minutes per inch and use the 11-20 mpi design below or obtain construction permit.
5 to 10
10 to 20
20 to 30
30 to 40
40 to 50
50 to 60
|Construction permit required when percolation rate is slower than 60 mpi.|
Enabling Legislation: Neb. Rev. Stat. §81-1505(8).
Legal Citation: Title 124, Ch. 14, Nebraska Department of Environmental Quality.