The latter...it's cemented and the only way to install the new nut is to cut the pipe so you install the new nut or use a split nut like the one linked by
@JamesW. You'll need to find the correct one for your filter as the thread are usually proprietary. If you can't find the correct fitting, you'll need to do some repair work...
You don't have enough room to fit a coupler between the valve and filter union. You can use a Magic Mend repair fitting that slips inside of your existing PVC pipe. Cut the PVC pipe, remove old nut, install new nut, then slip the fitting below into each pipe. You'll need primer and PVC cement...
Why use an additional fitting when you can use a MagicMend Insider Connector? The MagicMend Insider Connector will join two pieces of PVC Schedule 40 pipe together. Should a pipe be cut off flush with concrete or buried underground, a MagicMend will easily solve the problem.Find the INSIDER...
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