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How Septic Systems Work

Septic systems (also known as onsite sewage disposal systems or individual sewage disposal systems) are the primary method for treating and disposing sewage in rural areas where sewer systems are not available or too expensive to install. Septic systems are designed to provide partial treatment of the sewage, with disposal to the soil in such a manner that the sewage stays under the ground and is further treated by soil organisms so that contaminants do not reach groundwater or streams.

A septic system typically consists of a septic tank and a leaching device. The tank is usually 1000-2000 gallons in size and is designed to trap solids and grease and provide initial, primary treatment of the sewage. Treatment in the septic tank is anaerobic (without oxygen) and produces a fairly raw effluent that is still very high in bacteria and pathogens, dissolved solids and organics, ammonia, and organic nitrogen. The sewage then typically flows by gravity to the leaching device where the sewage soaks into the soil and most of the treatment takes place. Good treatment is primarily a biological process and it occurs most rapidly in upper soil layers that are rich in soil organisms (bugs) with plenty of oxygen to provide aerobic treatment.

Leaching devices typically consist of perforated pipe set along the top of one or more gravel-filled trenches (leachfields). The sides and bottom of the trench below the level of the perforated pipe provide the absorption area for the effluent to soak into the soil. The total amount of trench and absorption area needed is determined by the expected amount of sewage flow into the system and capabilities of the soil to absorb water. A sandy soil requires less absorption area than a clay soil. The effective depth of a trench is the depth of the trench below the distribution pipe. Because the pipe is covered with soil and typically laid 1-2 feet below the soil surface the total depth of the trench is usually 1-2 feet greater than the effective depth.

Besides the basic tank and trench leaching device, an onsite sewage disposal system may include other components:

  • A pump chamber and pump may be used to send the sewage to a higher, more suitable disposal area on the property. (Pump systems include extensive electrical controls, alarms, and excess storage capacity to ensure proper timing of pumping and safeguards in the event of power failures, pump breakdowns, or system overloading.)
  • An effluent filter may be set in the tank to help ensure that solids don't escape from the tank to clog the leaching device.
  • Inspection Risers are vertical pipes which extend from the bottom of the leaching device to above the ground surface which can be opened and inspected to determine the level of the effluent in the trench.
  • A distribution box or flow divider ensures that the sewage is evenly distributed to all parts of the leaching system. If this is not installed properly, one part of the system can be overloaded and fail, while other parts remain dry.
  • A diversion valve is used in older systems to cut off the flow of sewage to part of a leachfield, potentially allowing it to rest or recover while the other part of the leachfield is being used.
  • Pressure distribution systems may be used in conjunction with pumps to deliver sewage under pressure evenly to all parts of the leaching device.
  • Chamber leaching devices may be placed in trenches instead of gravel and perforated pipes. An example of this are the Infiltrator devices.
  • Enhanced treatment units may be used in place of or in addition to the septic tank to provide a much higher level of effluent treatment before the sewage is discharged to the soil. Examples of these are sand filters, or proprietary devices such as Multiflo units, Microfast Systems, Clearwater Systems.
  • Mounded Bed Systems or At-grade systems are engineered leaching devices which provide for sewage disposal right at, or above the natural ground surface for use in areas where there are high groundwater levels.
  • Seepage pits are leaching devices that consist of a circular pit 3-4 feet in diameter drilled 20-40 feet into the ground.
  • Greywater Sumps are small leaching devices designed to dispose of sewage from clothes washers, sinks, showers, or other sources that do not contain toilet waste (blackwater).
  • A haulaway system is used where standards for in ground sewage disposal cannot be met. Sewage is contained in a holding tank for regular removal by a septic tank pumper.