As 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 {TAIL ~ each zone} 
As 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 { TAIL ~ each zone }
OTH OTH OTH 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 TAIL TAIL TAIL TAIL TAIL TAIL