Types of septic systems
The common types of septic systems are gravity, pressure distribution, sand filter, and mound systems
GRAVITY DRAINFIELDS
Download the Homeowner's Manual: Gravity System
As the name implies, gravity drainfields work by letting gravity drain 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.
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
Download the Homeowner's Manual: Pressure Distribution System
Pressure distribution systems are usually installed when there is less than optimal soil depth available for complete treatment of effluent by a gravity system. Pressure distribution systems always have a pump and, therefore, 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.
A pressure distribution system
AEROBIC TREATMENT UNIT
Consists of a watertight tank with an aeration chamber where sewage and microorganisms come in contact with each other in the presence of dissolved oxygen. Blowers, compressors, or air pumps supply the air. The treated sewage is then pumped into a pressure distribution system for final treatment and disposal. To meet the highest treatment standards, a disinfection unit must be part of the device to reduce the bacteriological counts. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to ensure continued performance standards be met over time.
SAND FILTER SYSTEMS
Download the Homeowner's Manual: Sand Filter System and Sand Filter/Mound System
When there is minimal soil available for treatment, a sand filter system is sometimes used to make up for the lack of soil. Septic tank sewage is pumped through pipes in controlled doses to ensure uniform distribution. Sewage is treated as it trickles down through layers of sand in a sand filter. The sewage is then discharged into a pressurized drainfield through a second pump chamber that is commonly located within the sand filter.
Intermittent Sand Filter Anatomy
Sand filter on property
MOUND SYSTEMS
Download the Homeowner's Manual: Mound System and Sand Filter/Mound System
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 made from specific sand materials. Within the sand fill is a gravel bed with a network of pressurized pipes. Septic tank sewage is pumped through the pipes in controlled doses to ensure uniform distribution throughout the bed. Treatment of sewage occurs as it moves through the sand and into the natural soil.
Mound system on property
Mound system anatomy
Sub-surface mound system anatomy
OTHER TYPES OF SEPTIC SYSTEMS
A treatment-based system consisting of pressurized lines lying in sand-filled trenches. The sizing of drainfield laterals is equal to a standard system. They are used in situations where the soil is deep but very porous, thus lacking treatment capability. This occurs in areas where soils are very gravelly. They can be used as an alternative to a mound or sand filter system. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to ensure continued performance standards be met over time.
Consists of different layers of sand and gravel placed in a watertight box built into the soil. Sewage is pumped into the bottom of the filter and allowed to wick itself up through the sand and over the rim of the box into the soil. Several boxes or pods may be used to accommodate varying site conditions and number of bedrooms. A splitter and a timing device are used to ensure even flow to all pods. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to ensure continued performance standards be met over time.
Consists of a self-contained toilet with a chamber and venting system. The chamber contains sawdust or some other composting media, which then combines with the waste material to form compost over time. There is usually some method to turn the pile to ensure an even mixture and complete composting. Once composting is complete, the residue is removed manually from the chamber. These systems require a separate grey water discharge and disposal system. Composting toilets are most commonly used where water availability is limited. The long-term proper operation of composting toilets depends on regular maintenance by the owner.
Self-contained watertight sewage tank with a high water alarm. These must be routinely pumped to prevent overflows or back-ups into the house. They are most commonly used as a temporary measure to allow continued occupancy of a house until a more permanent fix can be arranged. Some properties use this system when sewers are not available, and no suitable site exists for an on-site sewage system. These require posting a bond for potential spills and being on contract with an approved sludge hauler. Oversight is needed to prevent sewage overflows.