Septic System Types

The Septic Owners Manual, Septic System Types

(Chapter 4) Septic System Types

Gravity Drain Fields

As implied, gravity drain fields work by allowing the effluent from the septic tank into a series of trenches by gravity. In this case, the drain field must be below the outlet level of the septic tank.

A conventional gravity system consists of a network perforated 4 inch diameter pipe laid in approximately 3 foot wide gravel-filled trenches. The bottoms of the trenches lie on natural undisturbed soil. The bottom of the trench typically needs to be 3 feet above any restrictive layer, such as a hardpan, or the seasonal high 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 drain lines filters effluent as it passes through the soil pores. 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 drain field 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 drain field area that is needed. The specific size of the system is generally regulated by the State which has jurisdiction.

Cesspool

A cesspool is simply a pit in the ground, with the walls lined by stone or metal with holes in it and covered with a lid. They used to be much more common and can still be found in some older homes. There are many problems associated with failing cesspools. The original type of sewage system, often still in use in older homes. They were simply a single hole in the ground loosely blocked up with locally available materials - stone, brick, block, or railroad ties - and capped either with ties covered with a layer of old steel roofing or a cast-in-place concrete lid with a cleanout hole near the center. All household wastewater entered and the liquid portion was absorbed into the ground. When the soil plugged, a new cesspool was added. Wiser installers placed an elbow, or better still, a tee in the outlet pipe from the first cesspool, creating a baffle to hold back floating grease and scum.

Pressure Distribution Drain fields

Pressure distribution systems are usually installed when the site conditions do not allow the drain field to be installed at a lower elevation than the septic tank outlet. Pressure distribution systems always have a pump and therefore they dose the entire drain field with effluent. The effluent is absorbed into the drain field until the pump tank accumulates enough effluent from the household for another dose.

Drywell or Seepage Pit

Constructed identically to a cesspool and differs only in that the clarified effluent from a septic tank or the wastewater from a washing machine or other grey water may enter. Modern drywells are often pre-cast perforated rings surrounded by crushed stone to increase the absorption area. Drywells can also be used to return storm water to the ground or to relocate basement drainage water to another location above the water table. Depending on the geographical location, these systems may be 6 - 8 feet deep and 6 - 8 feet in diameter or 30 - 40 feet deep and 3 - 4 feet in diameter.

Mound Systems

A mound system can be used when a site has inadequate soil depth. A mound is a drain field 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. Drainage around the mound site is critical if the system is to function properly. On sloping sites the down slope area below the mound must remain protected.

Gravity

Consists of a septic tank which flows effluent by gravity to a distribution box (D-box); one or more connections to this D-box drain into trenches typically filled with gravel. Only a small part of the drain field is used at any one time. These systems require the least amount of maintenance required of any system type but requires the greatest soil depth in order to provide adequate treatment of the effluent.

Holding Tank

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.

Drain Field, Drywell / Pit, Mound System or Cesspool.

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