As {HEAD ~ the liquid} moves in {TAIL ~ the serpentine path} gas is introduced below the surface of the moving liquid at, at least one, but preferably many locations in each zone 
moves in
As { HEAD ~ the liquid } moves in { TAIL ~ the serpentine path } gas is introduced below the surface of the moving liquid at , at least one , but preferably many locations in each zone
OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH