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 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
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
SAND FILTER SYSTEMS
When there is minimal soil available for treatment, a sand filter system is sometimes used to make up for the lack of soil. Sand, located in a sand containment vessel between the pump tank and pressurized drainfield, treats effluent before it enters shallow soils. Septic tank effluent is pumped through pipes in controlled doses to insure uniform distribution. As 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 into a gravity flow drainfield. The second pump chamber is commonly located within the sand filter.
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 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
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. Used in situations where the soil is deep but very porous thus lacking treatment capability. This occurs in areas where soils are very gravelly or extremely gravelly in nature. 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 assure continued performance standards be met over time.
Consists of different layers of sand and gravel placed in a watertight box built into the soil. Effluent 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 along with a timing device is used to assure even flow to all pods. This system can be used in situations where a lot has 18 inches of soil and only12 inches if 5 acres or greater. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to assure continued performance standards be met over time.
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 effluent 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. With disinfection the system may be used with as little as 18 inches of soil. Without disinfection the system requires no less than 30 inches of soil. The complexity of this system and the situations in which it is used requires periodic maintenance and proper operation to assure 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 when combines with the waste material to form compost over time. There is usually some method to turn the pile to assure 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. Most commonly used where water availability is an issue. The long-term proper operation of composting toilets depends on regular maintenance by the owner.
Self-contained watertight wastewater tank with a high water alarm. Must be routinely pumped to prevent overflows or back-ups into the house. Most commonly used as a temporary measure to allow continued occupancy of a house until a more permanent fix can be arranged. Some schools use this system when sewers are not available and no suitable site exists for an on-site sewage system. Requires posting a bond for potential spills and being on contract with an approved sludge hauler. Management oversight is needed to prevent sewage overflows.