Aerobic Septic Systems

An aerobic septic system is also called an aerobic treatment system or ATS. An aerobic septic system is a sewage treatment system, which is similar to a septic tank system. This system uses an aerobic process to facilitate digestion. In areas where public sewer systems are unavailable, an aerobic septic system may be the perfect solution.

An aerobic septic system can be of secondary use as an irrigation system, as it produces a high quality secondary water source (which is rich in nutrients). This can be sterilized and used as a surface irrigation system. In contrast to a leach field, an aerobic septic system allows much more flexibility and cuts the size requirements needed for the system.


An aerobic septic system has 4 phases:

  • The Pre-treatment stage removes large solids and any other undesirable substances.
  • The aeration stage is where the aerobic bacteria digests biological wastes.
  • The settling stage allows undigested solids a time to settle. This stage forms the sludge, which must be removed periodically from the system.
  • The disinfecting stage is where chlorine or a similar disinfectant is mixed with the water. This produces the antiseptic output. You can also use a UV disinfection, where the water is exposed to UV light inside of a UV disinfection unit.

The disinfecting stage can be optional. It is only needed where a sterile effluent is required. Calcium hypochlorite tablets are the most commonly used for this process. These are specifically made for waste treatment systems.

There are things that may adversely affect the system, which is a living ecosystem of microbes which digest the waste products in the water. Excessive quantities of bleach or antibiotics for instance can damage the balance and reduce treatment effectiveness. Non-digestible items should also be avoided, as they will build up in the system and require more frequent sludge removal.

Types of aerobic treatment systems

Small scale aerobic systems generally use one of two designs, fixed-film systems, or continuous flow, suspended growth aerobic systems (CFSGAS). The pre-treatment and effluent handling are similar for both types of systems, and the difference lies in the aeration stage.

Fixed film systems

Fixed film systems use a porous medium which provides a bed to support the biomass film that digests the waste material in the wastewater. Designs for fixed film systems vary widely, but fall into two basic categories (though some systems may combine both methods). The first is a system where the media is moved relative to the wastewater, alternately immersing the film and exposing it to air, while the second uses a stationary media, and varies the wastewater flow so the film is alternately submerged and exposed to air. In both cases, the biomass must be exposed to both wastewater and air for the aerobic digestion to occur. The film itself may be made of any suitable porous material, such as formed plastic or peat moss. Simple systems use stationary media, and rely on intermittent, gravity driven wastewater flow to provide periodic exposure to air and wastewater. A common moving media system is the rotating biological contactor (RBC), which uses disks rotating slowly on a horizontal shaft. Approximately 40 percent of the disks are submerged at any given time, and the shaft rotates at a rate of one or two revolutions per minute.

Continuous flow, suspended growth aerobic systems

CFSGAS systems, as the name implies, are designed to handle continuous flow, and do not provide a bed for a bacterial film, relying rather on bacteria suspended in the wastewater. The suspension and aeration are typically provided by an air pump, which pumps air through the aeration chamber, providing a constant stirring of the wastewater in addition to the oxygenation. A medium to promote fixed film bacterial growth may be added to some systems designed to handle higher than normal levels of biomass in the wastewater.

Retrofit or portable aerobic systems

Another increasingly common use of aerobic treatment is for the remediation of failing or failed anaerobic septic systems, by retrofitting an existing system with an aerobic feature. This class of product, known as aerobic remediation, is designed to remediate biologically failed and failing anaerobic distribution systems by significantly reducing the biochemical oxygen demand (BOD5) and total suspended solids (TSS) of the effluent. The reduction of the BOD5 and TSS reverses the developed bio-mat. Further, effluent with high dissolved oxygen and aerobic bacteria flow to the distribution component and digest the bio-mat.Doing so on single tank systems where solids do not have anywhere to settle, or there is no a clarifying area can do damage to the field lines as the solid matter is stirred up in the tank.

Composting toilets

Composting toilets are designed to treat only toilet waste, rather than general residential waste water, and are typically used with water-free toilets rather than the flush toilets associated with the above types of aerobic treatment systems. These systems treat the waste as a moist solid, rather than in liquid suspension, and therefore separate urine from feces during treatment to maintain the correct moisture content in the system. An example of a composting toilet is the clivus multrum (Latin for ‘inclined chamber’), which consists of an inclined chamber that separates urine and feces and a fan to provide positive ventilation and prevent odors from escaping through the toilet. Within the chamber, the urine and feces are independently broken down not only by aerobic bacteria, but also by fungi, arthropods, and earthworms. Treatment times are very long, with a minimum time between removals of solid waste of a year; during treatment the volume of the solid waste is decreased by 90 percent, with most being converted into water vapor and carbon dioxide. Pathogens are eliminated from the waste by the long durations in inhospitable conditions in the treatment chamber.

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