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