This depends on which type of filter you have. Hollow fiber filters and purifiers should not be allowed to freeze. Water expands when frozen, and this can burst the hollow fibers.
Generally, the MiniWorks EX ceramic filter can tolerate being frozen if stored properly. However, if you didn’t pump out the excess water after filtering, leaving the housing full of water, this could cause the housing or ceramic element to crack.
As a rule of thumb, it’s best to prevent any filter from freezing by bringing it inside your tent at night, or even putting it in your sleeping bag if temperatures are going well below freezing. If you think your filter did freeze, perform an integrity check, which will tell you if it is still working properly. It’s a good idea to make sure you either have that process memorized, or bring a hard copy or photo of the integrity test instructions (unique to each filter or purifier) on every trip so you’re never left wondering.
This process varies from filter to filter, but you can find the instructions for each filter in its owner’s manual, on the products web page, under the documents and FAQ tab.
Our instructional videos are hosted on YouTube, and you may be able to find them more quickly by searching directly there. Each brand has its own channel; Platypus MSR Gear use the channels’ search tool to find your product.