Not every site has the space and porous soil needed to support a septic system. A standard anaerobic (septic) system settles out solids but does little else to treat wastewater. Typically, a large drain field is also necessary to distribute wastewater over a wide area to reduce groundwater pollution. Aerobic treatment systems use a mechanism to inject air into a tank, encouraging decomposition that produces a higher quality effluent. In some cases, the size of the drain field can be reduced to allow development of a site that otherwise could not support a conventional septic system.
The main application for an aerobic treatment system is to retrofit a failed septic system. Other applications include poor soil, high groundwater/bedrock, little available land for a drain field, high proportion of organic matter or the need for high-quality effluent for environmentally sensitive sites.
There are a variety of designs for aerobic systems, but they do have some common features. These include pretreatment to reduce the amount of clogging solids, an aeration process, settling for suspended growth systems, and final treatment/disinfection. The most common kind of aerobic system is "suspended growth." Air is forced into an aeration compartment in which sewage-digesting bacteria are freely suspended in the liquid/air mixture. The other method is "attached growth," in which a surface is provided for bacteria to attach themselves. The surface is alternately exposed to the liquid and air. Because of the possibility of disruption under a sudden heavy waste load, some systems do not allow continuous flow, but restrict it through various devices such as pretreatment tanks, surge chambers, and baffles.
|
|
|
|
|
|
|
Aerobic treatment of wastewater does not require a large drain field or special soil conditions, so this system presents a means of treating wastewater where existing systems have failed.
|
|
|
|
|
|
|
A variety of aerobic treatment systems are available from different manufacturers. The demand for these systems has increased in certain areas as the need for improved wastewater treatment increases.
In addition to regulatory restrictions, aerobic systems have typically cost between $3,200-5,000 to install. The systems have regular maintenance demands that some municipalities require professional maintenance contracts to cover.
The electrical cost to operate an aerobic system is about the same as that required of a well pump – about 2% of monthly consumption or approximately $4 per month. The cost of a maintenance contract will run in the $50-75 range.
Aerobic systems are not accepted in all areas; check with the local health department. Some jurisdictions do not allow for a reduction in drain field area for aerobic systems, but some jurisdictions do allow for a reduction or elimination of a reserve area, with pretreatment. In other areas, aerobic systems are being allowed on an experimental or trial basis, or to repair failed systems. Approval usually comes from the state or local health department. NSF/ANSI 40 - 2000 Standard for Residential Wastewater Treatment Systems details requirements for approval of the system. Aerobic units are required to include two years of manufacturer maintenance service and renewal options, and to have alarms to alert the homeowner of malfunctions.
Anne Arundel County, MD
Aerobic units can be installed above or below ground. They may require electrical connections, additional excavation, installation of pretreatment or final treatment components, and access for maintenance.
Not Applicable
Aerobic systems can cost two to three times that of a standard septic tank, although this varies based on design, location, size, and installation and maintenance requirements. The aeration mechanism requires electricity and routine maintenance.
Savings can be realized if the drain field can be reduced or if its life can be prolonged. There is the potential for space-saving and less groundwater pollution than ordinary septic systems. Aerobic systems may allow a site to be developed that could not otherwise support a conventional septic system.
In addition to regulatory restrictions, aerobic systems have both a higher first cost and other maintenance demands than septic systems. Treatment diminishes if these systems are neglected or turned off, if harmful chemicals are poured down the drain, or if the ambient temperature is too low. The aerobic process reduces the effluent's ammonia content, but may increase its nitrate content. However, re-circulation back to an anaerobic environment (septic tank) can reduce total nitrogen output.
|