What is the main difference between PRV meant for sewege application and the one used for clean water application...is it only in body and internal parts material of construction, or there is something else?
I'd ASSume that the sewage-rated device would have some ability to shear/clear fibers and other debris from the valve seat, to re-establish sealing after opening.
Using a more or less normal PRV, protected by a bursting disc, is an option, but this would require full check and service if relieving. Also regular inspection of the disc and total device, to avoid leaks and fastening occuring over time.
One must also differ between air relieving devices, letting air out when filling the pipeline, and air in by emptying giving vacuum in the pipeline contra PRV's, bursting discs (or, seldom, explosion panels).