How to remove struvite from anaerobic digesters
Struvite is magnesium ammonium phosphate. If you are using magnesium hydroxide for pH adjustment, I'd go back to lime (calcium hydroxide).
Otherwise, your digesters are seeing a lot of ammonia and phosphate that's glomming onto the available magnesium.
Figure out a way to reduce your ammonia and/or your phosphate feed and you can stop the problem.