u/Lagger-meister

We have a very heavy phorid fly infestation in our 1150 gallon in ground grease interceptor. This unit is located outside of the kitchen, below a slab. We have had it cleaned out, but the flies persist. The best we've been able to do is caulk the lids shut so they don't emerge and fly into the back door of the kitchen. Sanitation is quite good inside the building, and we have been cleaning and adding enzyme to floor sinks, drains, etc. we have been managing the small percentage of flies that get in through exterior doors with multiple glue board UV light traps. But how to get rid of the flies in the interceptor? I'm considering fogging with a mycoinsecticide after our FOG removal contractor cleans it this summer. Anyone have experience with that? Any other ideas? Obviously there are not many products that are labeled to put into a sanitary sewer.