The second in a series of articles reproduced from the National Small Flows Clearinghouse publication, Pipeline.
Morgantown, W.V., April 10, 2002 -- In areas where sewer systems are unavailable or impractical, onsite wastewater treatment units, such as septic tanks, sand filters, and home aerobic treatment units pretreat the wastewater that individual homes and businesses generate.
Raw sewage flows from a building into a septic tank or other pretreatment unit, where solids, oils and greases separate from the rest of the wastewater. Some onsite systems, such as sand filter systems, incorporate additional preliminary treatment steps.
In areas where it is permitted, after sufficient pretreatment, the effluent can be disinfected and discharged to the ground surface or to a surface water source. In the vast majority of installations, however, the effluent receives final treatment and dispersal underground via a subsurface drainfield system.
Conventional Drainfield Design
Drainfields usually are constructed as a series of level trenches or beds lined with gravel or coarse sand and buried one to three feet below ground surface. Perforated pipes or drain tiles run through the trenches to distribute the wastewater over the gravel media before it enters the soil. The gravel helps to disperse the effluent, to support the sidewalls of the drainfield trenches, to prop up the pipe or tiles so they don't lie directly on the soil, and to provide a storage area during times of peak wastewater flows.
Additional gravel is placed over the pipe, and the trench or bed is covered with a semipermeable barrier, such as geotextile fabric, so the finer backfill material doesn't filter in. The septic tank effluent is treated as the wastewater effluent slowly trickles from the pipes, through the gravel and down through the soil.
How Treatment Occurs
As the wastewater percolates or moves down through the soil, a variety of complex physical, biological, and chemical processes combine to provide treatment. Particles in the wastewater are filtered by, adhere to, or chemically bond or react with the soil. Bacteria and other organisms in the soil consume the organic matter in the wastewater and perform most of the treatment. Although some treatment also may occur in the gravel layer, most of the work is accomplished in the soil.
As a drainfield matures, organisms in the wastewater and soil multiply and form a dark layer called the biomat on or near the infiltrative surface. The biomat is a miniature ecological system. If oxygen is present, organisms such as worms and parasites, feed on the bacteria as well as material in the wastewater. The biomat is also where most pathogen removal occurs. When the drainfield system is in balance, these organisms prevent the biomat from becoming so thick that it clogs the system completely, but rather allow the wastewater to flow through the soil below at a slow but steady rate.
The biomat also aids the treatment process in medium and coarse soils by maintaining unsaturated conditions in the soil layers below the drainfield and above the groundwater.
The Drainfield Advantage
According to the U.S. Environmental Protection Agency, where site conditions are suitable, subsurface soil absorption is usually the best way to disperse wastewater to the environment because of its simplicity and low cost.
Potential Drawbacks of the Gravel-Lined Drainfields
Gravel is traditionally used in soil absorption system construction, because it is relatively inexpensive and readily available in most areas, not necessarily because it outperforms other materials as a treatment media. In fact, there may be a few drawbacks to gravel-lined systems.
Although many contractors recognize the importance of only using washed gravel in drainfield systems, dust of "fines" can remain in the gravel or can be created when it is installed in the drainfield trenches. These fines may clog the infiltrative surface.
Another potential problem with gravel-lined trenches is that the soil layer can be compacted from the weight of the gravel and the machinery used to transport and install it. Wastewater may have difficulty percolating through the compacted soil. In addition, as the gravel settles against the soil, some say that it may "mask" or "shadow" (block) a significant percentage of the soil surface area that could otherwise contribute to biomat formation and treatment.
Pipeline magazine is published by the National Small Flows Clearinghouse, West Virginia University, Tel: 800-624-8301.
Coming next: More on Gravelless Systems.
