Septic systems used to treat household and other wastewater typically make use of bacteria to help process waste into a final product for disposal called effluent. Anaerobic systems involve bacteria that do not require oxygen to process waste. Oxygen is, however, required in an aerobic system. It typically must be pumped into the system to ensure the bacteria are able to process the waste. Aerobic systems have some disadvantages in comparison to anaerobic systems, but they are more appropriate for use under certain circumstances.
In an aerobic system, bacteria are used to pre-treat wastewater for safe disposal. Such bacteria make use of the oxygen and waste pumped into a holding tank as energy sources for their survival and growth. This results in decomposition of the waste and a reduction in the concentration of harmful microorganisms in the final liquid waste product. There are three basic types of aerobic treatment system (ATS): a suspended-growth system, a sequencing batch reactor, and a fixed-film reactor.
Bacteria float throughout the main tank in a suspended-growth system. Air is pumped throughout the liquid waste. Treated solid waste settles out in a secondary tank and the bacteria are cycled back to the main tank. The treated liquid waste is then piped out of the ATS. Maintaining a proper balance of waste, wastewater, and bacteria is important to prevent clogging of a suspended-growth system.
A sequencing batch reactor is similar to a suspended-growth system in that bacteria float freely throughout the tank. Decomposition and settling take place in just one tank, however. Air is supplied to this single tank only during the decomposition phase. It is turned off during the settling phase to allow solids to settle to the bottom of the tank. Effluent is then pumped out of the tank, thereby completing the cycle.
Whereas bacteria float freely in these two types of systems, the bacteria in a fixed-film reactor are attached to a particular surface. Decomposition takes place in one tank, and settling takes place in a second tank. Air is supplied only to the area of the decomposition tank with the surface to which the bacteria are attached. Recirculation of the bacteria back to the decomposition tank is unnecessary because the bacteria always remain attached to the surface on which they are growing.
An aerobic system of any of these types is typically more expensive than a traditional anaerobic system both in terms of initial cost and maintenance. Such systems typically have designs that are more complex and involve more moving parts than traditional systems. They require electricity for the system circulating air during the decomposition phase, so they entail ongoing operational costs as well.
Despite the higher costs associated with such a system, an aerobic system may be the most appropriate or possibly even the only option under certain circumstances. Production of higher-quality effluent makes such a system well suited to sensitive environmental areas. Space constraints may also necessitate an aerobic system where a large drainage field would not be possible.
Some locations do not permit the use of an aerobic system due to government codes and regulations. Others allow the use of an aerobic system but require the owner to have a contract with a professional to ensure proper maintenance of the system. Alarms may be required in some areas to alert the owner in the event of a system malfunction. Codes and regulations for installation and maintenance of an aerobic system are typically administered by state or local agencies such as a health department.