Types of septic systems
The common types of septic systems are gravity, pressure distribution, sand filter, and mound systems
As the name implies gravity drainfields work by letting gravity drain the effluent from the septic tank into a series of trenches. This means that a gravity drainfield area must be below the draining level of the septic tank. If this is not the case then a pump tank is necessary and it is called a pump to gravity system.
In conventional gravity systems the drainfield consists of a network 4 inch diameter perforated pipes laid in gravel-filled trenches (2-3 feet wide) in natural undisturbed soil. The bottom of the trench needs to be 3 feet above any restrictive layer, such as a hardpan, or water table. The soil between the bottom of the trench and the hardpan or water table is used in the final treatment of the septic tank effluent.
The soil below the drainlines filters effluent as it passes through the pore spaces. Chemical and biological processes treat the effluent as it percolates down through the soil. The treatment process cleans the effluent before it reaches the groundwater. This works best when the soil is somewhat dry, permeable, contains adequate amounts of oxygen and there is enough soil depth to complete the cleaning process.
The size of the drainfield depends on the estimated daily wastewater flow and soil conditions. The number of bedrooms and soil type determines the total number of square feet of drainfield area that is needed.
Pressure distribution drainfields
Pressure distribution systems are usually installed when there is less than optimal soil depth available for complete treatment of the effluent by a gravity system. Pressure distribution systems always have a pump and therefore they dose the drainfield with effluent and then let it rest until the pump tank accumulates enough effluent from the household for another dose. In addition a series of pressurized lines from the pump tank to the drainfield make sure the entire drainfield receives effluent at the same time.
For more information, read the brochure Understanding and Caring for Your Septic Tank System (from the Washington State Dept. of Health.) Hard copies are available at the Eastgate Environmental Health office.
A pressure distribution system
Sand filter systems
When there is minimal soil available for treatment, a sand filter system is sometimes used. This consists of a sand containment vessel between the pump tank and the pressurized drainfield. The sand acts to treat the effluent before it enters the shallow soils on site. This effectively makes up for the lack of soil depth. The sand filter itself is a concrete or PVC-lined box filled with a specific sand material. A network of pressurized lines is placed in a gravel-filled bed on top of the sand. The septic tank effluent is pumped through the pipes in controlled doses to insure uniform distribution. As the effluent trickles down through the sand it is treated. A gravel underdrain collects and moves the treated wastewater to either a second pump chamber for discharge into a pressurized drainfield or the filter can sometimes drain into a gravity flow drainfield. The second pump chamber is commonly located within the sand filter.
For more information, read the brochure Understanding and Caring For Your Septic Tank System (from the Washington State Dept. of Health.) Hard copies are available at the Eastgate Environmental Health office.
Intermittent Sand Filter Anatomy
Sand filter on property
Another system that can be used when a site has inadequate soil depth is a mound. A mound is a drainfield raised above the natural soil surface with a specific sand fill material. Within the sand fill is a gravel bed with a network of pressurized pipes. Septic tank effluent is pumped through the pipes in controlled doses to insure uniform distribution throughout the bed. Treatment of the effluent occurs as it moves through the sand and into the natural soil.
Mound system on property
Mound system anatomy
Sub-surface mound system anatomy
Drainage around the mound site is critical if the system is to function properly. On sloping sites the downslope area below the mound must remain protected.