Sludge generation tends to be solids-limited -- electrostatic forces between the oil, water, and solids keep the oil and water from separating. Big solid particles make sludge and small ones make emulsions. Something like ten pounds of sludge and float is generated for every pound of solids that enters the separator. It's hard to do anything about the solids that come in with the crude, but anything the refinery can do to keep solids out of the drain (catalyst and coke fines, dirt getting into drains) will cut back on sludge generation. Chemical surfactants used to clean equipment need to be carefully kept from going to primary separation as well. Sometimes refineries send all wastewater (like boiler and cooling tower blowdown, which contain high solids) to primary separation and if it's possible to pipe them into wastewater treatment after the oil separation step and limit the feed to the separator to oily wastewaters, sludge generation can be dramatically reduced. Limiting turbulence can reduce sludge and float generation as well. Some oil recycle loops (like float) might contain a lot of solids.
Once the sludge is generated, there are several dewatering options that allow for more oil recovery from the sludge. But the best thing to do is to prevent it from being generated in the first place.
Hope this helps.
Kirsten Sinclair Rosselot, P.E.
P.O. Box 8264
Calabasas, CA 91372-8264