I would agree that the bolting looks suspect - is the sewage of a saline nature? If so, check that the bolts are compatible. It wouldn't be the first time that bolts of an inappropriate grade had been fitted following a repair and they get severely weakened by corrosion, (galvanics, etc), in a...
Gents,
Although 316 is used pretty extensively in the pump industry it is still a relatively poor performer cf. to Duplex steel, particularly where corrosion / erosion / abrasion combined wear mechanisms exist. If you can afford the up front costs and downtime is an issue, go for Duplex.
