As the wastewater splashes on the rock, oxygen is dissolved into the water. The microorganisms use this dissolved oxygen to live. As they grow and reproduce, the organics are removed. When the bacteria slough off the rock, they will eventually be removed in the final clarifiers. The aeration basins are the structures for the activated sludge process.
Activated sludge is also an aerobic biological and secondary treatment process.
The microorganisms live on small particles of sludge instead of rock. Because these particles are so much smaller, billions more bacteria can be grown in a smaller space. But they now use oxygen at a much faster rate than can be dissolved naturally into the water. To insure that they have plenty of oxygen, four 2000 horsepower motors blow filtered air through fine diffusers in the bottom of the basins. There are six aeration basins, each with four passes.