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Gravel-Less Pipe Leach Fields

Alternative leach field design

Multi-pipe array in a bed of sand.

A shortage of drainrock (gravel) in Texas led to the original development of this alternative leach field design. For standard corrugated gravel-less pipe, no reduction in trench length is allowed relative to a conventional field. However, one innovative design allows a substantial reduction in field length.

See the article "On-Site Sewage Disposal Systems - Overview" for an overview that will help relate this technology to other options and to the overall system.

What might be called "conventional" gravel-less pipe leach fields are of two types. A single-pipe system uses a corrugated pipe from 8" to 12" in diameter wrapped in filter fabric (typically spun-bonded nylon), laid in a trench that is backfilled with native soil. Drain holes are located up from the bottom of the pipe, allowing some sludge to collect at the pipe's bottom without clogging the holes. Effluent can circulate within the corrugations under the filter fabric, allowing it to discharge into the soil from all side of the pipe. For this reason, the effective absorption width of a pipe is typically considered to be its circumference. Dosing probably will extend the life of this type of field. It is possible but probably impractical to suspend a pressure pipe within the pipes for a pressurized effluent application.

A multiple-pipe system uses from 3 to 13 pipes with an outside diameter of 4" to 4-½", not covered with fabric, bundled together in an array. Typically, one pipe at the top-center of the array will discharge effluent into the array, where it can move into and out of the pipes through holes and slots. The principle of its operation, according to the manufacturer, is that it acts as a "trickle filter," in which aerobic bacteria grow on the inside and outside surfaces of the pipes. While in theory this increases the available area per width of trench, local jurisdictions (such as the State of Washington) may not accept this theory of operation.

The "Enviro-Septic" system, while superficially similar to conventional pipes, is much more sophisticated. The piping acts as an underground radiator, cooling warm effluent from the septic tank to ground temperature. Cooling and the internal skimmers cause many of the solids to separate from the effluent and either float or sink inside the pipe. The effluent, with a lower percentage of solids, exits the pipe through the ½" holes and enters the mat of coarse, random, fibers where solids stick to the fibers. Effluent then contacts the outer layer of fabric where any remaining solids are trapped and effluent is wicked away by the surrounding sand. The fabric fibers are then subjected to a wetting and drying action by fluctuating liquid levels inside the pipe according to system loading. The system's multiple layers of fibers and fabric provides for a large bacterial surface constantly stimulated by air and water, capable of digesting large quantities of suspended solids.

Because the filter fabric around the pipes must be kept aerated, single-pipe gravel-less pipe leach fields are not applicable to clayey soils, although this caveat may not apply to the Enviro-Septic system. Because conventional gravel-less pipes are less effective than gravel-filled trenches, most jurisdictions do not allow any downsizing of the field compared with a conventional gravel-filled design (this does not apply to the Enviro-Septic system). The State of Washington calculates the effective width of multiple pipe applications as the diameters of those pipes that touch the bottom of the trench. A study by the University of Minnesota noted that these systems seemed sensitive to proper design and execution.


Environmental Performance

Using a gravel-less pipe leach field eliminates the environmental consequences surrounding the use of tons of gravel including mining, transport, and soil compaction and damage to fauna by equipment.


Easy

Conventional gravel-less pipe systems are available from distributors throughout the United States and Canada, and have been installed and evaluated for over 15 years. The Enviro-Septic system is manufactured and in use in New England, but is expanding nationally.


Not Applicable


Not Applicable


Conventional gravel-less pipe systems are widely accepted, although the sizing requirements will vary from state to state. The Enviro-Septic system is relatively new, and may not yet be accepted in some states except on an experimental basis. Some states regulate on-site wastewater issues at the county level. Some jurisdictions require observation ports.


Not Applicable


Single pipes are laid in a trench, and can form a gentle curve. Multiple pipe systems can be laid side-by-side in a bed array, with the pipes displacing the normal drainrock. Installation time can be reduced compared with a gravel system. The Enviro-Septic system is complete with end adapters.


Not Applicable


Gravel-less pipes eliminate the labor in placing tons of drainrock aggregate. Eliminating the equipment needed to spread the aggregate reduces undesirable compaction of the surrounding soil and disruption to nearby shrubs and trees. The cost varies considerably, but is comparable for conventional gravel-less pipes to a gravel system of above-average cost. The Enviro-Septic system, because it requires a reduced length of trench, may be less costly than a conventional system. Observation ports can be added that allow inspection of the system for bio-mat buildup.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.