Gravel less effluent disposal septic system - Geotextile Wrapped Perforated Pipe - image courtesy US EPA, originally from National Small Flows Clearinghouse Gravelless Septic Systems - Geotextile-Wrapped Designs
     


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Gravelless or "no-rock" septic system designs:

This article explains design of gravelless or "no gravel" or "no rock" septic drainfield systems using geotextiles and offering installation specifications. Drainfields, also called leach fields, absorption beds, soil absorption systems, and leaching beds, perform the functions of septic effluent treatment and disposal in onsite wastewater treatment systems, conventionally called "septic systems".

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Use of Geotextile-Wrapped Perforated Pipe for Onsite Wastewater Treatment and Disposal

Gravel less effluent disposal septic system - Chamber system - image courtesy US EPA, originally from National Small Flows ClearinghouseGravelless Systems - Gravelless Septic Absorption Systems

There are three typical gravel-less effluent disposal systems in current use and shown in the US EPA sketches below (originally from NSFC) and I describe a fourth variation which is provided by some manufacturers:

This article describes the second type - Geotextile-wrapped designs.

Gravelless septic systems or "no gravel" septic system trenches use plastic or other prefabricated wastewater distribution systems which are buried in soil without the use of surrounding gravel. Typical gravelless septic systems use a plastic chamber, a geotextile-wrapped pipe, or a polystyrene-wrapped pipe to distribute effluent into the soil.

The necessary soil absorption area is provided by the perforated surface of the gravelless septic system components (or by soil at the bottom of a chamber) themselves rather than by the gravel and trench walls of a conventional septic drainfield. These systems can provide an acceptable effluent disposal system for sites with limited space for a drainfield or where gravel is not available or is quite expensive.

Gravel less effluent disposal septic system - Geotextile Wrapped Perforated Pipe - image courtesy US EPA, originally from National Small Flows Clearinghouse 2. Geotextile-wrapped perforated pipe buried in an earthen trench. In this type of system, a large diameter corrugated plastic pipe (eight inches or greater in diameter) surrounded by a hydrophilic geotextile is installed in an excavated trench using only the original soil as backfill.

Other systems utilizing products such as galleys, flow diffusers or leaching chambers can be installed without aggregate backfill.

One linear foot of these products shall be equivalent to one linear foot of conventional (24 inch wide) absorption trench.(1) For an example of this geotextile-wrapped pipe approach to wastewater disposal using 8" or 10" diameter piping, see Crumpler Plastic Pipe's "No-Rock TM Septic-Leachate drainpipe systems" listed at Product Sources below.

CPP informs us that because the square foot equivalent leaching area when this system is used is not a foot-for-foot ratio based on a 36" trench, the 8" & 10" CPP Filter Wrapped systems are typically longer than the traditional septic drainfield trenches. Although the individual line length may be longer, the CPP system may actually require less total square footage of lot space if the on-center line spacing is controlled by the excavated trench width treatment plume & not an "arbitrary CODE stipulation".

According to CPP, this added length achieves superior distribution to the trench sidewall where the bulk of the septic liquid enters the soil for treatment. [One reason for wider trench on-center spacing advanced by some health departments is to assure that the site affords space for future trench line replacement .--DF].

Most sanitary or health department codes size 8" diameter Geotextile-Wrapped Perforated Pipe to equal a 2 Foot wide conventional septic drainfield trench, and similarly size 10" diameter Geotextile-Wrapped Perforated Pipe to be equal to a 2.5 foot wide conventional drainfield trench.

LARGER SKETCH of A CPP Gravel less effluent disposal septic system - Geotextile Wrapped Perforated Pipe - image courtesy Crumpler Plastic Pipe

In determining how much leach field area shall be provided when using Geotextile-Wrapped Perforated Pipe, most state codes (where they have them) will require that the cubic footage that would have been required if a conventional 3-foot wide conventional perforated pipe & gravel leach field system had been installed will be used to determine the size 2 Foot or 2.5 Foot wide gravelless systems as well.

Thus if 100 feet of a conventional gravel system in 3 Foot wide trenches is required for one bedroom (or 300 Cubic Ft), then 150 feet of 8" Geotextile-Wrapped Perforated Pipe will be required or 120 feet of 10" diameter Geotextile-Wrapped Perforated Pipe.

Sketch of geotextile-wrapped pipe in plan view, courtesy of Crumpler Plastic Pipe - CPP - links below.

For OC (On-Center) spacing of parallel lines treating sewage from a 2 bedroom home, a conventional gravel system of 100 foot length would require 1200 square feet of lot surface area if 3 times the actual 'excavated' trench width is used for OC separation. In comparison, for an 8" Geotextile-Wrapped Perforated Pipe system, which can be placed in a 1.5 Ft(18") wide trench, 1,125 square feet of lot surface area would be required.

For a 10" diameter Geotextile-Wrapped Perforated Pipe, which can be placed in a 1.5 Ft (18") wide trench, 900 square feet of lot space is needed.

If a different or as CPP puts it "arbitrary" OC trench width spacing is mandated (ie: 5 ft or 7 Ft OC required regardless of 'excavated' trench width) by a state code, then the lot surface area calculations will of course be different. [Thanks again to Crumpler Plastic Pipe, Inc. for this added detail. Links to this company's website and products are provided below.]

What all this means is that for a given building usage and set of soil conditions, whether or not the Geotextile-Wrapped Perforated Pipe system is going to provide the necessary effluent handling capacity in a smaller total square feet than a conventional drain field would require depends on the on-center spacing requirements set by the local authorities.

Continue reading at GRAVELLESS SEPTIC CHAMBER SYSTEMS or select a topic from the More Reading links shown below.

Or see GRAVELLESS SEPTIC SYSTEMS - home

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NO-ROCK SEPTIC GEOTEXTILE-WRAPPED PIPE at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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