Conventional Treatment Systems

The term ‘conventional’ refers to the standard gravity flow design, which consists of a septic tank, a distribution box, and a drainfield. Conventional systems function very well on sites with few constraints as they rely on the native soils to treat bacteria, dilute other waste that is not treated, such as nitrogen, disperse wastewater generated in the home or business. For low-density sites with sufficient space for setbacks and dilution, good soil conditions, and deep water table, conventional systems are the least expensive to build and maintain, and are remarkably efficient. Reliance on native soils for treatment of wastewater can be a severe limitation in areas where soil conditions are not ideal for the infiltration and treatment of wastewater, or where the land area available is insufficient to handle large water loads. Primary treatment (settling of solids and fats, oils, and greases) of wastewater that takes place in an onsite system is the exact same step found in municipal sewer treatment plants, but done on a much smaller scale.

The Septic Tank

The main purpose of the septic tank is to temporarily retain and store wastewater to allow solids, and fats, oils, and greases (FOG) to separate before the wastewater flows to the drainfield. The tank is generally designed to provide a 2-day retention time for wastewater settling to occur. Heavy solids sink to the bottom forming a layer of ‘sludge’, while light solids and FOG form a floating ‘scum layer’.

The minimum tank size for a single-family home is typically 1,000 gallons. Various types of tanks are approved for use in Rhode Island – tanks can vary in size, configuration, and material. Tanks that were installed prior to current RI state standards (adopted in 1970) could vary from steel drums to concrete cylinders. Tanks are most commonly constructed from concrete, though fiberglass tanks are regularly used on lots with limited access to heavy equipment.

The interior of tanks may also vary, generally containing either one or two compartments. Single-compartment tanks have a divider at the outlet end to keep floating FOG from leaving the tank and entering the drainfield. The tank shown here is a typical two-compartment tank, split by a divider called a baffle; the first compartment contains 2/3 of the tank volume, the second contains 1/3. Two compartment tanks are typically larger than single-compartment tanks, with a capacity of 1,500 gallons. Wastewater flows from the first to the second compartment through a gap in the baffle at a height that should correspond with the ‘clear zone,’ or the most clarified water in the tank. Additional separation of solids and FOG occurs in the second compartment. The two-compartment configuration, combined with a larger storage volume for settling and storing waste, ensures that the greatest possible amount of separation of solids and FOG takes place before the wastewater leaves the tank to be distributed to the drainfield. Because of this additional protection for the drainfield, some communities such as the town of Charlestown RI, require a 1,500 gallon, two-compartment tank for new construction.

The Distribution Box

The distribution box lies between the septic tank and the drainfield, and serves to evenly distribute the wastewater evenly amongst the drainfield lines. Ensuring even distribution of the wastewater is crucial in maintaining the longevity of the drainfield. If only a small portion of the drainfield is actually used due to poor distribution, localized saturation of a drainfield line may occur. Organic matter and solids may build up faster than they are broken down by soil microbes clogging soil pores, causing the drainfield to fail prematurely. A typical D-box is pre-cast with multiple holes, as seen here, to allow for use in various system configurations. For example, the drainfield lines may be located in various positions relative to the incoming pipe; on either side of the D-box, both on one side, etc. The D-box shown here has three pipes; one delivering wastewater from the primary treatment tank, and two to distribute effluent to the two drainfield lines or trenches.

The Drainfield

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.

Trench Drainfield

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
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
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.
Deep Galleys
Deep galleys are 4’x4’x4’ units, and may be installed as deep as 10 feet below ground, depending on the site characteristics and system design.

Eljen In-drains

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.

Information contained in this resource center was generated, in part, due to work prepared by URI Cooperative Extension for the National Decentralized Water Resources Capacity Development Project as well as work done under the Block Island and Green Hill Pond Watershed National Decentralized Wastewater Demonstration Project, funded by the US EPA.