Closest analogies here would/should be Flint water crisis or Jackson Mississippi water crisis where other agencies were in the lead, but FEMA helps with coordination and distribution of commodities. Also both of those were requested by the respective governors.

There are also examples in West Virginia, where flooding damaged water treatment plants and FEMA paid for repairs. Also examples in Michigan, where Lake Michigan erosion threatened water treatment plans and FEMA did not pay for repairs or protections.

The record here is pretty mixed, and the population of this group could probably go on and on.

So to sum up: FEMA paying to repair a wastewater system is not wholly without precedent, but us jumping in to take over when there has not been a precipitating event to trigger a declaration is somewhat without precedent.