Design and performance of an inorganic MF/polymeric UF hybrid system for the treatment of a difficult waste stream containing both colloidal and micron sized particles

Wastewater streams containing colloids, used oils and suspended solids are difficult to treat using membrane technologies. Bilge water accumulating onboard a ship is a typical example of such a wastewater. This paper will discuss the merits of using a back-flushed, small tube, inorganic MF membrane followed by a hollow fiber polymeric UF membrane in treating this particular wastewater. The hybrid system yields 3-8 times the flux obtained using a single hollow fiber UF membrane. The modeling of permeate fluxes and the type of wastewaters where this hybrid would be most useful will be presented. Wastewaters accumulating in ship's bilges contain used lubricating oils, saltwater, and detergents. They must first be treated before being discharged overboard. These waters are currently being treated by settling (parallel plate separators) and coalescence. These methods easily meet older less stringent discharge requirements but are in many cases unable to meet new stricter discharge limits. This is due in part to the presence of emulsified and free oils in bilge water. The discharge limit for oil and grease in North American coastal waters, within 12 nautical miles from land, is 15 mg/L. The discharge limit is also 15 mg/L in international waters (International Maritime Organization). In Canada, a discharge limit of 0 mg/L is set for arctic waters. In some jurisdictions, the 15 mg/L limit is expected to be reduced to 5 mg/L.