As the liquid moves in the serpentine path gas is introduced below the surface of the moving liquid at, {HEAD ~ 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 the moving liquid at , { HEAD ~ 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 OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL