Onsite sewage disposal systems (OWTS) are the primary method for treating and disposing sewage in rural areas where sewer systems are not available or too expensive to install. OWTS 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.
An OWTS typically consists of a septic tank and a leaching device. Treatment in the septic tank is anaerobic (without oxygen) and produces a 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. 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 (leach fields). 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.
The Basic OWTS Components
Chamber Leaching Devices: These devices may be placed in trenches instead of gravel and perforated pipes. An example of this is the Infiltrator Device.
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.
Diversion Valve: Used in older systems to cut off the flow of sewage to part of a leach field, potentially allowing it to rest or recover while the other part of the leach field is being used.
Effluent Filter: A filter set in the tank to help ensure that solids don't escape from the tank to clog the leaching device.
Enhanced Treatment Units: 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 into the soil. Examples of these are sand filters, or proprietary devices such as Multiflo units, Microsfast Systems, and Clearwater Systems.
Greywater Sumps: Normally small, rock-filled leaching devices designed to dispose of sewage from clothes washers, sinks, showers, or other sources that do not contain toilet waste (blackwater).
Haulaway System: A system 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.
Inspection Risers: 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.
Mounded Bed Systems or At-Grade Systems: 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.
Pump Chamber and Pump: 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.)
Pressure Distribution Systems: Used in conjunction with pumps to deliver sewage under pressure evenly to all parts of the leaching device.
Seepage Pits: Leaching devices that consist of a circular pit 3-4 feet in diameter drilled 20-40 feet into the ground.