What is an Aerobic Treatment Unit?

The Aerobic Treatment Unit (ATU) is a wastewater treatment plant whose main task is to treat domestic wastewater. And it is also called a mechanical treatment plant. With proper design, operation and maintenance, ATU can provide a high level of wastewater treatment. They come in a variety of shapes and sizes and are very important for the protection of human health and the environment.

The aerobic therapy unit (ATU) is made up of several processes that work together to deliver high-quality effluent. This includes complete removal of solids (waste), aeration, purification, and sludge return. These processes are usually found in several chambers of a single tank. A set of water tanks can be adjusted so that the wastewater can pass through the aerobic treatment system.

How does ATU work?

There are many types of ATUS, but the most common home ATU uses a process called suspended growth. These devices have a septic tank in which the air is mixed with the wastewater. This creates an environment where bacteria float freely in liquids and grow as solids dissolve (suspended growth).

Many units contain another septic tank where bacteria cannot digest or deposit solids. The two tanks are connected, so these insoluble solids can be returned to the aeration chamber by gravity or by a pump. It is the spinning and blending process that is essential for efficient operation.

How can you properly maintain an aerobic treatment unit?

One of the main design criteria of the ATU is the level of aeration expected for wastewater treatment. Therefore, fan maintenance is very important in this process.
Another important aspect is the accumulation of waste over time. Therefore, the ATU must be pumped regularly to remove sediment. The amount of organic matter entering the ATU is determined by the pumping schedule. The general rule is to pump every 3-5 years.

Any ATU purchase includes access to services in the first two years. You need to make sure these services are accessible from your provider. After that, you can renew the SLA.

Types of aerobic treatment systems

Small-scale aerobic systems typically use one of two designs, the fixed membrane system or the continuous flow aerobic system (CFSGAS). Pre-treatment and wastewater treatment are two similar systems, the difference depends on the aeration phase.

Fixed film system

The fixed aerobic treatment unit uses a porous medium that provides a substrate to support the biomass membrane that dissolves waste in the wastewater. The design of fixed film systems is different, but they fall into two main categories (although some systems may combine these two methods). The former is a system in which the environment moves relative to the wastewater and alternately immerses the membrane and exposes it to the air, while the latter uses a static environment and modifies the flow of wastewater so that the membrane sinks and exposed to air.

In both cases, the biomass must be exposed to wastewater and air for aerobic digestion. The film itself can be made of any suitable porous type such as molded plastic or peat moss. A simple system uses a fixed environment and relies on frequent gravity to bring the wastewater stream into regular contact with the air and wastewater. A typical mobile system is a rotating biological contactor (RBC) that uses a slowly rotating disc on a horizontal axis. At any given time, about 40% of the discs are submerged in water and the shaft rotates at one or two revolutions per minute.

Continuous flow, suspended growth of the aerobic treatment unit

The CFSGAS system, as the name suggests, is designed to treat the flow of fluids and does not provide a bed for bacterial membranes, but relies on bacteria suspended in the effluent.

The suspension and rotation system are usually carried out by an air pump, which allows the passage of air through the aeration chamber and, in addition to oxidation, also provides a continuous wastewater agitator. An environment that promotes the growth of fixed-film bacteria can be added to some systems designed to treat high levels of biomass in wastewater.

Modified or portable aerobic system

Another common application of aerobic treatment unit is the repair of failed or ineffective anaerobic wastewater systems by modifying existing systems with aerobic functions. This type of product is called an aerobic correction, which aims to eliminate inefficient and biologically inefficient anaerobic distribution systems by significantly reducing the need for biochemical oxygen (BOD5) and total suspended solids (TSS) in wastewater. Decreased BOD5 and TSS reverse the development of the biological layer. In addition, the effluent with a high content of dissolved oxygen and aerobic bacteria flows into the distribution phase and dissolves the biological mat.

Advantages and disadvantages of Aerobic Treatment Unit

ATU has many advantages and disadvantages, the following are adapted from the United States.


  1. The level of treatment is higher than septic tanks
  2. Helps protect water resources in areas where the septic tank system is damaged
  3. Replace sites that are unsuitable for septic systems.
  4. It can increase the life of the drainage area.
  5. Reduce ammonia emissions in receiving waters.


  1. The running cost is higher than the septic tank system
  2. Need for electricity
  3. Including mechanical parts that could be damaged
  4. It requires more maintenance than a septic tank system
  5. You can easily tip over under a sudden heavy load or carelessness
  6. Compared to septic tank systems, large amounts of nitrates can be released into groundwater.