Oxygen transfer can be achieved with fine bubbles. An efficient aeration system produces bubbles up to a maximum of 2 mm in diameter. The porosity (openings) of the aerators, however, should be much smaller in order to produce 2 mm size bubbles. The average is 100 Mu (= microns which is 0.1 mm). Fine air bubbles can coalesce (clump together) into a large air bubble which is unwanted. System Dimensions, pressure loss and velocity all affect the air bubbles.