Reductions in Size and How To “Take a Reduction Without Taking a Reduction”

The Eljen Geotextile Sand Filter PA DEP listing allows for up to a 40% reduction in absorption area sizing if the perc rate of the soil is 3–60 minutes/inch. This does not imply that you MUST take a reduction in absorption area size or that it must be a 40% reduction. The key words are up to—allowing for any amount of reduction up to 40%.

In cases where a reduction is taken (and even when reductions are not taken), an easy technique to increase the “true” absorption area is to design the system so that the downslope sand toe is at a 3:1 slope as opposed to the minimum 2:1. In effect, this expands the basal area of the Eljen absorption area and largely returns the available area for infiltration nearly to full-size, even though a reduction has been taken in the size of the top surface face of the Eljen absorption area.

In most cases, this extension of the sand toe uses a relatively small amount of additional sand and is undetectable as most berm slopes are 3:1 in order to allow for reasonable mowing activities to take place. See the graphics below:

So even though a reduction in the number of Eljen modules may be possible (and in doing so, reduces cost and does not compromise the quality of treated effluent), the “true” area for absorption of effluent is largely unaffected. This is why we use the phrase above to describe reductions without taking a reduction!