A aerobic treatment unit treats wastewater using natural processes that require oxygen. Simply put, the bacteria that thrive in oxygen-rich environments tackle, break down, and digest the wastewater inside of the unit while suspended within the liquid. A residential aerobic treatment unit will also treat the wastewater in several stages. For example, in some aerobic treatment systems, the wastewater goes through pretreatment before entering the actual unit, and it eventually requires additional disinfection before being released into the environment.
To work properly, a aerobic treatment system requires regular inspections and maintenance because the mechanical aerators need to be oiled, and on occasion, the filters, vanes, and seals may require replacement. Most of the time, homeowners do not possess the expertise to assess, repair, and install these components.
How Aerobic Treatment Units Work
Aerobic treatment units (ATUs) force compressed air through the liquid effluent in the tank to create a highly oxygenated (aerobic) environment for bacteria. Bacteria that thrive in oxygen-rich environments work to break down and digest the wastewater inside the aerobic treatment unit. Aerobic units come in a variety of sizes and shapes. They can be made of concrete, fiberglass or polyurethane.
Some ATUs have a step before the actual aeration begins. This is step is called pretreatment. Pretreatment helps to reduce the amount of solids and other materials that the bacteria cannot degrade that go into the system. These solids include grease, oil, toilet paper, and other substances that get put down a drain or flushed into the system. This stage may take place in a separate tank or in a compartment of the ATU itself.
The aeration chamber is where the bacteria decompose the sewage. The ATU consists of a pump, pipes, and diffusers. The pump compresses the air that flows into the chamber. The pipes carry the air from the aerator to the diffusers. The diffuser forces air into the water in the form of bubbles which introduces oxygen into the sewage. There are two main types of ATUs: suspended growth units and attached growth units. Suspended units are the most common type of ATU and work by mixing air with the sewage to feed the free floating bacteria without the use of any type of medium. In attached growth units the bacteria are attached to some type of medium located inside the ATU itself.
The settling chamber is a calm area which allows settling to occur with the use of gravity. This is the area where the clear, treated water is separated from the bacteria that have been treating the sewage and other solids before they leave the system and travel to the drainfield.
After all the treatment is complete the effluent is discharged into unsaturated soil and allowed to filter through the sand and continue further treatment. The effluent from aerobic treatment units can be discharged into a variety of different drainfield systems. There are gravel systems, chamber systems, pipe systems, and Dripline systems. All these different materials have different requirements, sizes and capacities. There are gravity feed systems and pump systems. The type of drainfield material you have present depends on what you, the builder or the engineer wanted installed.