The use of backflushed coalescing microfiltration as a pretreatment for the ultrafiltration of bilge water

Ships produce a large amount of oily wastewaters such as bilge water which needs to be treated prior to being discharged. Bilge water is a difficult wastewater to treat as it contains seawater, particulates, used oils and detergents. Ultrafiltration (UF) is the method of choice to treat this wastewater. The pretreatment of this oily wastewater, prior to ultrafiltration, is desirable as used oils and particulates can block the feed channels of UF spiral and hollow fiber modules. A cascaded membrane system consisting of a coalescing backflushed microfiltration membrane used as a pretreatment and an ultrafiltration membrane were studied. Two microfiltration membranes were studied; single tube carbon membrane and multilumen ceramic membranes. The optimal backflushing cycle was determined for the single tube carbon membrane. The particle size distribution of bilge water and UF membrane pore size were related to membrane performance. Experimental results showed that membranes with a pore size below 0.2 microns can be used directly to treat bilge water. Microfiltration membranes responded well to backflushing with flux enhancements of up to 8 times compared to the case of cross-flow alone. Backflushing of MF membranes offered flux improvements for single tube carbon membranes but not for multilumen ceramic membranes. The clearance of the support structure with respect to particulates was found to be important. The single carbon tube had a shorter path compared to the longer path of the multilumen ceramic tube. The work outlines the need for microfiltration membranes offering good particulate clearance to be used in backflushing coalescence applications. The MF/UF hybrid membrane system was found to be very effective in this application, producing permeate with oil and grease content well below the allowable discharge limit for coastal waters.