Hi, Andrew, just a few considerations/ideas on cold temperatures that might help you...
I have an underground rainwater harvesting tank for 8 years now, it never freezes, even a few years back when we hit -18°C a couple of times, the barrel, nor the pump line froze.
It did freeze in the first winter I lived here, but the pump line was left uncovered at that time, the second winter it was covered with no more than 20cm of soil and it didn't freeze.
Keeping the worm-compost tank under ground and having warm liquids coming in regularly will keep the tank from freezing, covering the lid with 20-50cm of soil will help. You might want to wrap the plastic barrel sticking out with straw as an insulation material, to keep it from cooling down too much from the coldest winter frosts.
The pump line as well should be ideally 50cm underground.
If you use a reed field/basin to work as a green filter, you could design it so you can cover it in winter with arches and create a poly tunnel, to keep the reeds from freezing over. A little bit of sun regularly would do to heat up the tunnel structure. I'm not sure how much water you'd be processing a day, but in worst case you could also add a 1000l/2000l buffer in the coldest of winter so you can wait out the hardest frosts for a week or so.
One thing to consider is since you're close to a stream ground water will probably be relatively high. If your compost/worm tank has too much air, the ground water will undoubtedly push your tank out of the ground, so you will need to keep it well filled.