Abstract: A five stage treatment system for treating oily wastewater to fit it for discharge to surface waters comprising a coalescer for separating free oil, an ultrafiltration unit for separating emulsified oil, an activated carbon filter for separating residual light organic compounds, a cation exchanger for removing heavy metal ions, a pH adjusting unit for adjusting the treated water to substantially neutral pH prior to discharge, and a controller for regulating the operation of the system. An optional gravity separation stage and/or a clarifier stage may also be included, if desired. Flow of permeate from the ultrafiltration stage is monitored and control valves admitting feed solution to the ultrafiltration unit or discharging retentate from the ultrafiltration unit are adjusted in response to the measured values to maintain a desired rate of permeate flow.