Depending on the contaminants, it's more likely a few degrees below 0C but you point still stands. Fish are removed as contaminant but minerals and pollution likely is not.

And normal water takes quite a bit of heat extraction to actually freeze if at 0C, maybe the device does not even extract enough. But you want to be on the safe side of course since clogging up your heat exchanger with ice (which expands) is not great.

(edit: and as noted in other reply pressure is a thing)

> Fish are removed as contaminant

Why do I have a feeling this is one of those "green" ideas that has some horrible environmental consequence. One that could have been solved with a way simpler technology for far less money in exchange for a bit of efficiency loss.

You also don't want to create an iceberg at the point where you dump the water back in the river.