If you are planning on shutting off the heat in your cottage over the winter months you will need to protect your water treatment equipment from damage. The following steps must be taken to ensure your water treatment equipment does not freeze.
Water Softeners, Iron/Sulphur Filters, Tannin Filters or any other type of Backwashable Filter. See my blog post called "How to Winterize a Water Softener". The procedure is the same, as you are basically draining and removing the tank(s) and storing them in a warm location until spring. Just make sure that if you have a FOC or FOB filters that you relieve the air pressure inside before disconnecting from your plumbing.
Ultra Violet Disinfection Systems need to have the bulb and quartz sleeve removed and all the water drained from the unit ideally by blowing it out with a compressor.
Cannister Type Filter Cartridges either as part of an Ultra Violet Disinfection System or on their own need to have the filters removed and all of the water dumped out and water lines blown out.
Reverse Osmosis Drinking Water Systems should be completely removed, including the storage tank, and stored in a warm location over the winter. All filters and the membrane should be removed and ideally discarded and replaced in the spring, or at the very least the filters and membrane should be removed and stored in fresh reverse osmosis water in a container in the fridge over the winter. If the storage water goes murky, replace the water with fresh reverse osmosis water during storage
The rest of the cottage, including the hot water heater and pressure tank also needs to be drained (make sure to shut off the hot water heater before draining).
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