Drainfields serve to disperse wastewater to the soil environment, where it is expected that pathogens will be killed by beneficial soil microorganisms. The size of the drainfield will depend on the anticipated volume of water to be produced in the home and the soil conditions at the site. In Rhode Island, the drainfield may be one of three basic types: (i)Trenches, (ii) Leaching Chambers, or (iii) Eljen In-drains. In situations where required separation to groundwater cannot be met, modifications to the conventional drainfield, such as mounding or filling may be approved for use.
A trench type drainfield is comprised of two or more parallel trenches containing a perforated PVC pipe surrounded by washed crushed stone. The crushed stone is covered by geotextile fabric to ensure that fine soil particles do not fill voids between the stones, and native soil is used to fill the trench to the ground surface. Wastewater flows from the distribution box into the drainfield PVC pipes, to the crushed stone, and then to the soil environment.
Leaching chambers are bottomless, perforated, concrete boxes surrounded by crushed stone on the sides and bottom. These are generally installed in a series of three or more units, depending on the anticipated volume of wastewater. Liquid effluent flows from the septic tank into the chamber, where it seeps out the side walls and bottom. Flow diffusers and galleys are the two types of leaching chambers commonly used in the state; both are pictured here.
Flow diffusers and galleys are the two types of leaching chambers commonly used in the state; both are pictured here. Flow diffusers are shallow units, typically installed about one to two feet below the ground surface and are 8’x4’x2’ in size.
This style of drainfield was one of the first alternative drainfield technologies used in Rhode Island, but now is considered to be a conventional drainfield option. These are gravelless trenches containing prefabricated units of geotextile fabric and cuspated plastic spacing cores. These units are bordered on the sides and beneath by six inches of specific sand media, and are covered with native soil. A layer of protective fabric is placed on top of the units (prior to covering with soil), to prevent fine soil particles from clogging the filter fabric pores. Picture from the US EPA New England Center for Environmental Industry and Technology’s Virtual Trade Show.
Mounded and Filled Systems
Mounded and filled systems are common solutions to overcoming shallow depth to ground water table, ledge, or other restrictive features, where site conditions allow for the use of conventional onsite wastewater treatment technologies. Adding fill to a site is a means to increasing the distance to such features, as well as addressing other requirements such as cover of components or drainfield fill-perimeter. The onsite treatment system components are installed within the mound of fill. Mounds are often contained within a retaining wall. It should be noted that these systems alter the drainage patterns of a site and the surrounding area.