Aerobic Treatment Units

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.

A suspended-growth tank has a main treatment chamber where bacteria are free-floating and air is bubbled through the liquid. The second chamber where the solids settle out is separated from the main tank by a wall or baffle. The two chambers are connected at the bottom or by a pump, and settled bacteria from the second chamber are brought back into the main treatment chamber. This return and mixing is critical for proper operation. Treated effluent from the second chamber is piped to the soil treatment system (Figure 2). Though simple, the system is likely to have problems with bulking (the formation of chains or colonies of bacteria that don't settle or sink to the bottom as they should). Bulking is caused by changes in wastewater strength or quantity. When too much water/wastewater is added to the system, the bacteria can run out of food or become overloaded. Bulked bacteria remain sus-pended in the liquid and can clog the outflow.

A fixed-film reactor has bacteria growing on a specific surface medium and air is provided to that part of the tank. The bacteria can grow on any surface including fabric, plastic, styrofoam, and gravel. Decomposition is limited to this area, and settling occurs in a second chamber. This design is expensive, but the effluent is of consistently high quality, and bulking is uncommon. There is no need for a return mechanism because the bacteria stay on the film (Figure 3).

In a sequencing batch reactor, aerobic decomposition, settling, and return occur in the same chamber. Air is bubbled through the liquid during the decomposition cycle. The bubbler shuts off, and the wastewater goes through a settling cycle (Figure 4). Once the bubbler turns back on, the tank reenters the decomposition cycle, and settled bacteria mixes back into the aerobic environment. After settling of bacteria and solids, the treated effluent is discharged to the soil treatment system. Bacteria settle out more consistently in this kind of tank, but since it has more moving parts and requires a controller, it has more potential for mechanical and electrical failure.

 ATU