The aerobic treatment unit (ATU) is a tank that has air added to it by mechanical means so that aerobic bacteria will reproduce and consume the biodegradable waste. There are many suppliers of ATUs and there are many suppliers of the air pumps that are used to supply the air but they all perform the same function and are usually certified by the National Sanitation Foundation (NSF). The ATU is rated for a certain wastewater flow such as 500 gallons per day (GPD) and it is certified by NSF for this rating. NSF certifies using a certain standard which is referred to as NSF 40.
The air is added by an air compressor and air stone or air diffuser. Here are some examples of some common air diffusers.
Here is an example of a before and after picture of a Pro Flo air diffuser that has been used and needed to be replaced.
The best way to tell when a diffuser has reached its end of life is by taking an air pressure reading when the air pump is running. Here is an air pressure reading after the air diffuser was replaced.