When {HEAD ~ the freeze front} reaches {TAIL ~ a collection zone}, at least a portion is melted and the resultant water bearing concentrated impurities is collected and removed 
reaches
When { HEAD ~ the freeze front } reaches { TAIL ~ a collection zone } , at least a portion is melted and the resultant water bearing concentrated impurities is collected and removed
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH