This disclosure is directed to an apparatus and method for highly concentrating an already relatively concentrated feed stream in a two-stage freeze-crystallization system. The feed stream is introduced into the first stage freezer-crystallizer without prior dilution, which has heretofore been customary. Highly concentrated output is withdrawn from a first stage wash column. Diluted concentrate is recirculated in the second stage of the system. This relatively simple expedient of diluting in the second stage, rather than in the first, allows the system to treat highly contaminated feed streams directly and produces significant savings in energy for the same throughput.