I find it strange that ships mix water runoff, antifreeze, hydraulic oil, engine oil seepage etc etc into a huge big mixed bilge tank and then try to separate it with an oil separator.
It's more like the bilge is a central place where any fluid that leaks out of whatever system normally handles it winds up. On small vessels you just have a straight bilge pump that dumps out the side. At cruise ship scale you can afford to stick an oil/water separator filter on it.