Types of Septic Systems

The following are descriptions of the various types of septic systems, according to the U.S. Environmental Protection Agency.

Conventional Septic System - Consists of a settling or septic tank and a soil absorption field. The traditional system accepts both greywater (wastewater from showers, sinks, and laundry) and blackwater (wastewater from toilets).

Intermittent Sand Filter - Used in conjunction with pretreatment methods such as septic tanks and soil absorption fields. An intermittent sand filter receives and treats effluent from the septic tank before it is distributed to the leaching field. The sand filter consists of a bed (open or buried) of granular material 24 to 36 inches deep. The material is usually 0.35 to 1.0 mm in diameter. The bed of granular material is underlain with graded gravel and collector drains. These systems have been shown to be effective for nitrogen removal, however, this process is dependent on temperature. Water loading recommendations for intermittent sand filters are typically between 1 and 5 gallons per day/square foot (gpd/ft2) but may be higher, depending on wastewater characteristics. Primary failure of sand filters is due to clogging, and maintenance is recommended to keep the system performing properly—resting the bed, raking the surface layer, or removing the top surface medium and replacing it with clean medium. In general, the filters should be inspected every 3 to 4 months to ensure that they are operating properly.

Intermittent sand filters are used for small commercial and institutional developments as well as individual homes. The size of the facility is limited by land availability. The filters should be buried in the ground, but they may be constructed above ground in areas of shallow bedrock or high water tables. Covered filters are required in areas with extended periods of subfreezing weather. Excessive, long-term rainfall and runoff may be detrimental to filter performance, requiring measures to divert water away from the system (USEPA, 1980).

Recirculating Sand Filter - A modified intermittent sand filter in which effluent from the filter is recirculated through the septic tank and/or the sand filter before it is discharged to the soil absorption field. The addition of the recirculation loop in the system may enhance removal effectiveness and allow media size to be increased to as much as 1.5 mm in diameter.

Buried or recirculating sand filters can achieve a very high level of treatment of septic tank effluent before discharge to surface water or soil.  Dosed recycling between sand filter and septic tank or similar devices can result in significant levels of nitrification/dentrification, equivalent to between 50 and 75 percent of overall nitrogen removal, depending on the recycling ratio. Regular buried or recirculating sand filters may require as much as 1 square foot of filter per gallon of septic tank effluent.

Mound Systems - An alternative to conventional OSDS and are used on sites where insufficient separation distance or permeation conditions exist. Mound systems are typically designed so the effluent from the septic tank is routed to a dosing tank and then pumped to a soil absorption field that is located in elevated sand fill above the natural soil surface. There is evidence suggesting that pressure dosing provides more uniform distribution of effluent throughout the absorption field and may result in better performance. A major limitation to the use of mounds is slope.

Where adequate area is available for subsurface effluent discharge, and permanent or seasonal high ground water is at least 2 feet below the surface, the elevated sand mound may be used in coastal areas. This system can treat septic tank effluent to a level that usually approaches primary drinking water standards for BOD5, suspended solids, and pathogens by the time the effluent plume passes the property line for single-family dwellings. A mound system will not normally produce significant reductions in levels of total nitrogen discharged, but should achieve high levels of nitrification.

Evapotranspiration (ET) and Evapotranspiration/Absorption (ETA) Systems - Combine the process of evaporation from the surface of a bed and transpiration from plants to dispose of wastewater. The wastewater would require some form of pretreatment such as a septic tank. An ET bed usually consists of a liner, drain field tile, and gravel and sand layers. ET and ETA systems are useful where soils are unsuitable for subsurface disposal, where the climate is favorable for evaporation, and where groundwater protection is essential. In both types of systems, distribution piping is laid in gravel, overlain by sand, and planted with suitable vegetation. Plants can transpire up to 10 times the amount of water evaporated during the daytime. For an ET system to be effective, evaporation should be equal to or greater than the total water input to the system because it requires an impermeable seal around the system. In the United States, this limits use of ET systems to the southwest. The size of the system depends on the quantity of effluent inflow, precipitation, local ET rate, and soil permeability (Otis, undated).

Aerobic Treatment Units - Can be employed on-site. These systems require regular supervision and maintenance to be effective. An aerobic bacteriological farm can digest 90% to 95% of the solid wastes that are deposited into the septic tank. Typically, oxygen is supplied to a septic system when there is a problem with pumping frequency or the overall effectiveness of the system.  The typical design of these systems is such that a small compressor is located near the sewer line as it exits the house, which provides oxygen for the tank.

Disinfection Systems - Chlorination, ozonation, and ultraviolet disinfection are the most common methods of disinfection in the U.S.Chlorine, the most widely used disinfectant in municipal wastewater, destroys organisms by oxidizing cellular material.  Chlorine can be applied as chlorine gas, hypochlorite solutions, and other chlorine compounds in solid or liquid form.

Ozone, an unstable gas generated by an electrical discharge through dry air or pure oxygen, is another oxidizing agent. Ultraviolet radiation, generated by an electrical discharge through mercury vapor, is absorbed into the genetic material of microorganisms and hinders their ability to reproduce.