A major source of overboard discharge from ships is oily wastewater (bilgewater), which collects in most machinery spaces and is generated in volumes too large for long-term storage. Ceramic oily waste membrane systems have been developed by Naval Surface Warfare Center, Carderock Division (NSWCCD), as a secondary treatment of the existing parallel-plate separator (OWS) effluent.
The objective of this demonstration was to improve fouling resistance and process reliability of these ceramic membrane systems using a nonporous polymer coating, thereby reducing life-cycle costs. A secondary objective was to improve acquisition cost of membranes through the use of polymeric spiral-wound membranes in lieu of ceramic.
The oily waste membrane system developed by NSWCCD utilizes ultrafiltration membranes to filter small-droplet and emulsified oil that has passed through a primary treatment system. The pore sizes of the membranes are scaled to allow passage of small water molecules but not allow passage of very large molecules and particles, such as oil droplets and other contaminants. Clean water passes through the pores in the membrane in a radial direction and is discharged overboard (permeate). Oil droplets emulsified in water and particulates that are too large to pass through ultrafiltration membranes are retained and concentrated in the membrane system. The concentrate is periodically discharged from the system and stored for disposal ashore.
Membrane Technology and Research, Inc. (MTR), Menlo Park, CA, under a previous SERDP project WP-1108 developed the application of a nonporous polymer coating called Pebax® to membranes used for bilgewater and graywater treatment. It consists of polyamide-polyether copolymer blocks. It is hydrophilic and oil phobic (that is, it attracts water and repels oil), and provides superior strength, water flux, and high resistance to internal and external fouling. It also has excellent resistance to a wide range of contaminants in wastewater including toluene, ethanol, and various detergents. The polymer is stable in the pH range of 2-12 and can withstand temperatures up to 70°C. The first phase of tests in the demonstration included the coating and testing of commercially available ceramic ultrafiltration modules with the fouling-resistant polymer. Over time, oils and other contaminants build up in the permeating pores, reducing the amount of permeable membrane area. With the nonporous polymer coating, oils and contaminants are unable to plug the membrane pores, and therefore extend the life of the membrane.
This study demonstrated that coated ceramic membranes have at least doubled the life of uncoated membranes in the laboratory. Oil separation performance is equal, and the coating proved resistant to all contaminants. Spiral-wound polymeric membranes, however, could not pass the required amount of flux after the coating was applied, and so were deemed unsuitable for this application.
Full-scale coated membranes were then installed on the USS James E. Williams (DDG 95). Resistance data was sent by e-mail from the ship to NSWCCD for the first 21 hours. Then operational conditions did not permit e-mail contact until the 54-hour point. At that point, the membrane system broke down due to an electrical failure and no further run time was accomplished during the deployment, so long-term membrane lifetime at sea has not yet been determined. It is recommended that the membranes continue to be monitored until sufficient data is obtained.
The purpose of this demonstration was to validate the polymer coated ceramic membrane and polymeric membrane performance under the operating parameters required by the shipboard Oily Waste Membrane System (OWMS). Currently, the military performance specifications MIL-PRF-32097 for OWMS membrane modules have been finalized. The performance specification may be updated based on the degree of the demonstration’s success and cost benefits, and will provide DoD with a means for evaluating open competition for future implementation. It is anticipated membrane fouling and costs may be reduced by as much as 50%. This technology is applicable to all Navy ship classes. An additional benefit is the potential to remove additional constituents of concern from bilgewater discharge (bilgewater regulations may be expanded to address other constituents of concern such as metals, pesticides, etc.). It is possible that the nonporous membranes will produce cleaner effluent for overboard discharge. Additionally, other wastewater applications can be investigated for treatment including: gas turbine water wash, vehicle wash-down, and advanced base applications.