ATU septic system / Drain field

 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. Pretreatment Some ATUs have a step before the actual aeration begins. This is pretreatment and it 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 the drain or flushed into the system. This stage may take place in a separate tank or in a compartment of the ATU itself. Aerobic Treatment Unit 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. Settling Chamber 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. 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.

How Aerobic Treatment Units (ATUs) Work

Aerobic Treatment Units (ATUs) are advanced onsite wastewater systems designed to treat sewage more efficiently than conventional septic systems. They operate by introducing compressed air into the wastewater, creating an oxygen-rich (aerobic) environment where bacteria can thrive and break down organic material.

Some ATUs include a pretreatment stage before aeration begins. This step reduces the amount of solids—such as grease, oils, and toilet paper—that bacteria in the main treatment chamber cannot easily degrade. Pretreatment may occur in a separate tank or a compartment within the ATU itself. This helps improve the overall efficiency and longevity of the system.

The aeration chamber is the core of the ATU. Here, a pump compresses air and pushes it through pipes into diffusers, which release the air in fine bubbles throughout the wastewater. This oxygenates the water and supports the growth of aerobic bacteria, which digest the organic matter in the sewage.

There are two main types of aerobic treatment units:

• Suspended Growth Units: These systems keep bacteria suspended in the wastewater, mixing air and sewage together without a physical medium.
   
• Attached Growth Units: In these units, bacteria grow on a fixed medium inside the tank. The air and sewage flow over this surface, allowing bacteria to digest the waste.
   

ATUs are available in a variety of sizes and configurations, and they may be constructed from concrete, fiberglass, or polyurethane, depending on the application and installation needs.

After aeration, the treated wastewater moves to a settling chamber, where gravity allows remaining solids and bacteria to settle out. This step separates clear, treated water from the residual sludge before it exits the system.

The final step is the discharge of treated effluent into a drainfield. The effluent is released into unsaturated soil, where it undergoes further natural treatment as it filters through layers of sand and soil.

Several types of drainfields can be used with ATUs:

• Gravel systems
   
• Chamber systems
   
• Pipe systems
   
• Dripline systems
   

Drainfields may be gravity-fed or use pump systems, depending on the site and system design. The choice of drainfield type is typically determined by the property owner, builder, or system designer based on soil conditions and regulatory requirements.