Why Fine Bubble is better then Coarse Bubble? Generally the volume of air here we are taking about in size of bubble diameter, suppose if bigger say 2 inch roughly , the bubble travel time is quickly reach to the upper surface of the liquid medium from the bottom, which deliver might be Diffusers or Jet aerators.
in other words we take into 1″ or 3/4″ dia of bubble the weight of air into the bubble is less, so it takes much higher time to reach the upper surface.
The fine Bubble sizes are less than 3 mm coarse bubbles are 3 to 50 mm
Again you may rise the question, so what?, suppose if bubble travel time vary?
Good, the answer is, our intention as much as possible we should extract the oxygen from bubble content air, if increase the travel time from bottom to top, as much as possible we can get maximum Oxygen (O2) transfer ratio.
Tip: if possible try to move the bubble in tangential direction that result is more better thn normal straight direction.
By volume, dry air contains only 20.95% oxygen, but nitrogen 78.09% and balance are other gases
You can Determine Aeration tank air volume, the purpose of fine bubble diffuser, which is uses in the aeration basin to remove BOD and COD with help more oxygen transfer fine bubble diffusers.
Now, you understand the importance of fine bubbles in aeration tank.