Aerobic Septic Treatment

Aerobic Septic Tank- Tiny Modular Home Builders in Maryland David M. Gustafson; Extension Educator, Department of Biosystems and Agricultural Engineering
James L. Anderson; Professor, Department of Soil, Water, and Climate
Sara Heger Christopherson; Extension Educator, Department of Biosystems and Agricultural Engineering

An aerobic treatment unit (ATU) pretreats wastewater by adding air to break down organic matter, reduce pathogens, and transform nutrients. Compared to conventional septic tanks, ATUs break down organic matter more efficiently, achieve quicker decomposition of organic solids, and reduce the concentration of pathogens in the wastewater.

More than 20 brands of ATUs are available, but efficiency varies widely. A properly operating ATU should produce high-quality effluent with less than 30 mg/liter BOD (biochemical oxygen demand, a measure of the organic matter), 25 mg/liter TSS (total suspended solids), and 10,000 cfu/mL fecal coliform bacteria, an indicator of pathogens and viruses.

Aerobic Treatment Application

Since wastewater leaves an ATU as high-quality effluent, the soil in the trench or mound soil treatment system may be better able to accept it, and the system should last longer. Because ATUs produce cleaner wastewater, they are useful in sites with “disturbed” (compacted, cut, or filled) soil, and in environmentally sensitive areas such as those near lakes in shallow bedrock areas, aquifer recharge areas, and wellhead protection areas. Pretreatment may allow a reduction in the three-foot separation requirement between the soil treatment system and the limiting soil layer. Researchers are testing this hypothesis.

ATU systems may also be successfully retrofitted into drainfields that have failed because of excessive organic loading from lack of maintenance.

How do aerobic treatment units work?

By bubbling compressed air through liquid effluent in a tank, ATUs create a highly oxygenated (aerobic) environment for bacteria, which uses the organic matter as an energy source. In another stage bacteria and solids settle out of the wastewater and the cleaner effluent is distributed to a soil treatment system.

ATUs are more complicated than septic tanks. In a septic tank, solids are constantly separating from liquid. As individual bacterial cells grow, they sink to the bottom, along with less decomposed solids, to form a layer of sludge. Floating materials, such as fats and toilet paper, form a scum layer at the top of the tank.

In an ATU, the bubbler agitates the water so solids cannot settle out, and floating materials stay mixed in the liquid. Well-designed ATUs allow time and space for settling, while providing oxygen to the bacteria and mixing the bacteria and its food source (sewage). Any settled bacteria must be returned to the aerobic portion of the tank for mixing and treatment.

There are three basic ATU operation styles: suspended growth, fixed-film reactor, and sequencing batch reactor. All three types usually have a septic tank (sometimes called a trash tank) ahead of them that removes the large solids and provides some protection to the ATU.


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