Not sure about a threshold ...
Published by jeremy dudley, Senior engineer at WRc plc
Not sure about a threshold oil concentration, but treating oily water at refineries has a similar issue. I know of a flowsheet that uses oil plate interceptor, pH adjustment, induced air flotation, activated sludge, and then sand filtration. The gradual acuumulation of oil on the sand filter results in frequent backwashes and a treated water that is much closer to the consent values than would be wanted from a filtered effluent.