When a predetermined quantity or loading of the pollutants, nutrients or {HEAD ~ other undesirable species} has precipitated onto {TAIL ~ the algal cell walls}, a portion of the algal turf is harvested 
precipitated onto
When a predetermined quantity or loading of the pollutants , nutrients or { HEAD ~ other undesirable species } has precipitated onto { TAIL ~ the algal cell walls } , a portion of the algal turf is harvested
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH