How to treat a baby who was delivered at term with more than 18 hours of prom with no othervrisk factors?

Usually no treatment. As long as newborn infant remains asymptomatic, careful watching with minimal blood work will be all needed.
Premature rupture. Your baby will be fine and will not need more care than any other newborn. The concern for infection is more when there are 24 or ore hours of prom.(premature rupture of membranes).