The activated sludge process requires certain types of microorganisms to break down organic matter in wastewater. As any other living organism, the microorganisms need energy to reproduce. When breaking down organic matter or oxidizing ammonium (NH4+) to Nitrite (NO2-) energy in the form of ATP for microorganism reproduction is released. However, both conversions need oxygen. Consequently, as more oxygen is available as higher is the number of microorganisms and as better is the treatment efficiency of the activated sludge process.
Wastewater equipment manufacturers develop therefore different products for additional oxygen supply. Most commonly used are surface aerators and subsurface aeration equipment.
Examples for surface aerators are rotors, floating aerators, agitators, fountains and others. They all have in common that they extract water from the first 1 to 2 feet of the water body and utilize air-water contact to transfer oxygen. By disrupting the water surface small water droplets are created and propelled into the air. As small water droplets have a large surface area the oxygen transfer is better compared to oxygen transfer on the water body surface.
Examples for subsurface aerators are fine and coarse bubble diffusers. A compressor on shore pumps air through a hose which is connected to underwater bubble diffuser units. The bubble diffuser units have normally a plastic or stainless-steel corpus and perforated membranes on the outside. Air which is pumped to the diffusers is released through the membranes into the water. The membrane perforation of fine bubble strip diffusers is microscopic small and released bubbles have a size of about 1 mm.
Compared to surface aerators, fine bubble diffusers have in general a better oxygen transfer rate. Fine bubbles have a large surface area and they raise vey slowly which means that their contact time with water is much longer compared to propelled water droplets.
Also, the power consumption of the compressors is less compared to rotors and agitators which makes fine bubble diffusers very energy efficient.
However, on the downside, bubble diffusers have a higher risk of clogging and they are more difficult to maintain because they are submerged.
Finally bubble diffusers provide oxygen but they don’t provide good air-water mixing. Therefore, in most cases they need additional air water mixing equipment such as submersible mixers.