In-Situ Liquid Storage Capacity Measurement of Subsurface Wastewater Absorption System Products

Article excerpt


In the United States, onsite wastewater treatment systems serve 25 percent of existing homes and 37 percent of new residential construction. Conventional systems consist of a septic tank that discharges to a subsurface wastewater infiltration system (SWIS) constructed with gravel-filled trenches. Over the past 30 years, gravelless products have been replacing gravel in the trenches. These alternative products include molded plastic chambers, precast concrete galleys, multiple bundled pipes, plastic and geo-textile composites, and pipe and expanded polystyrene (EPS) bundles. All these manufactured products provide temporary storage for septic-tank effluent and a permeable conduit to soil infiltration surfaces, thus eliminating the need for gravel and pipe installation within an excavated trench.


The proper functioning of either a conventional gravel-filled trench SWIS or an alternative gravelless SWIS requires a certain quantity of liquid storage capacity to accommodate peak flows. When septic-tank effluent discharges into the SWIS at a rate that exceeds the rate of infiltration into the soil, effluent is temporarily stored within the SWIS, providing time for infiltration to occur. Storage is essential because household water usage may periodically be elevated or the soil within and surrounding the trench may get saturated (Kropf, Laak, & Healey, 1977). In both of these situations, the SWIS provides a factor of safety against system failure and is integral to the proper functioning of the system. When the SWIS fills to capacity with wastewater and the wastewater cannot infiltrate into the soil, the water may either pond at the ground surface or back up into the plumbing system.

A multitude of factors determine the long-term successful operation of a SWIS. The type of soil in which the SWIS is installed is of primary importance. Other factors include the rate at which septic-tank effluent is discharged, the nature and quantity of the septic-tank effluent (as a result of homeowner use or abuse), local topography, and vegetation. With the advent of gravelless SWIS technologies came reductions in the total length of the trench. Sizing reductions are typically determined as some fraction of the trench length or basal area of a conventional gravel system. The reduction in liquid storage capacity must also be considered with SWIS sizing reductions. If, compared with the length of a gravel trench SWIS, the length of a gravelless SWIS is reduced by 25 percent, the percentage reduction in liquid storage capacity can be, and often is, much greater. Reduction in liquid storage volume depends, almost exclusively on the in-situ porosity of the product.

Some states have pursued regulation of liquid storage capacity for SWISs that employ gravelless products. For example, in Virginia, "a gravelless system that uses plastic or other types of media must allow for the storage capacity that is substantially equivalent to that available in a gravel system" (Virginia Department of Health, 1999, page 4). In Washington, the state guidance document says, "The total measured volume of any installed gravelless system must be equivalent to or greater than the void volume provided by a conventional gravel-filled trench system" (Washington State Department of Health, 1999, p. 8). Other states that have addressed liquid storage capacity as part of their onsite wastewater system regulations include Arkansas, Georgia, Louisiana, Mississippi, Missouri, Oklahoma, South Carolina, and Tennessee. The U.S. Environmental Protection Agency (U.S. EPA) Onsite Wastewater Treatment System Manual (U.S. EPA, 2002) also addresses the importance of liquid storage within a SWIS. Unfortunately, among all of these regulations, no guidelines or protocols exist for quantifying liquid storage capacity.

The intent of this work is not to define a necessary liquid storage capacity for a particular product type. …