The typical gravelless technology used today consists of a corrugated percolation pipe embedded in mesh-wrapped, segmented bundles of polystyrene "peanuts", with a filter fabric on the upper side. Another type of gravelless technology less commonly used, called "chambers", is made up of plastic domed segments. Due to State policies, this material difference translates into a potential for a 25% reduction in the wastewater absorption area (drainfield trenches) as designed. This means that if the design calls for four drain lines, then using only three (75%) could be permitted. As the septic system designer, a soil consultant can either allow or prohibit this reduction on a case-by-case basis.
Up to 40% of the designed absorption area could be reduced in the early 2000's, but the negative results of this practice caused the reduction amount allowed to be dialed back to only 25%. The merits of reducing absorption area to save money now are debatable.
There are some challenging drainfield situations, such as space limitations on very small lots, where the reduction of one drain line may be necessary in order for the entire system footprint to fit on the property, or where using this light-weight material makes easier the transport and installation of drainfield lines on a steep mountainside, instead of traditional gravel and pipe, which requires dump truck access and is more labor intensive. However, the potential implications of reducing the wastewater absorption area for the average septic installation should be contemplated.
This newer gravelless technology has not been in use as long as the traditional gravel and pipe drainfields that, in many cases, have functioned successfully for decades. A key difference between the two technologies centers on difference in the material cost and the amount of labor required for these two types of drainfield technologies. The Virginia Department of Health (VDH) allows absorption area reduction for a gravelless system by up to 25%, which helps to even the cost gap, as the polystyrene bundles also require less time and labor to install (for some installers), but are more expensive up front than gravel. SSC takes pride in formulating drainfield designs that offer the best product possible for the site and soil conditions present on a piece of land. Speak with your septic installer about their experiences with the available drainfield technology options, and about the associated costs.