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