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