Each of the sections is separated by {HEAD ~ a retainer ring} having {TAIL ~ an opening} therein smaller in area than the area of the next preceeding scrubber chamber 
having
Each of the sections is separated by { HEAD ~ a retainer ring } having { TAIL ~ an opening } therein smaller in area than the area of the next preceeding scrubber chamber
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH