The principal problem inhibiting widespread adoption of nanofiltration/ultrafiltration to treat shipboard wastewater and allow the Navy to meet future overboard discharge limits is fouling associated with available membranes. Existing nanofiltration/ultrafiltration membranes are finely porous and are subject to surface or internal fouling by particulates, resulting in a dramatic decline in the water flux.
This project developed a shipboard wastewater treatment system based on a novel type of fouling-resistant, composite-membrane module. The composite membrane consisted of an ultrathin (0.2-0.5 micrometer), nonporous, highly water-permeable, rubbery, block copolymer layer coated onto a conventional, microporous ultrafiltration or nanofiltration membrane for support. This coating layer provides fouling resistance without significantly reducing the water flux.
Three candidate materials were developed under earlier Office of Naval Research grants. In this project, development of these membranes was completed and a systematic series of new materials was synthesized and characterized. The properties of these new heterophase block copolymer membrane materials were tailored to provide better fouling resistance than conventional membranes, while maintaining or improving the flux/selectivity combinations relative to currently available materials. These materials were based on aromatic polyamide hard blocks with either hydrophilic ether groups as the soft, waterpermeable block or water-soluble aromatic polyamides as the hydrophilic blocks. This research characterized the physical, chemical, and morphological structure of these materials as well as their water permeation, rejection, and fouling properties. Systematic structure/property relations were developed to guide the preparation of a new generation of advanced high performance materials for shipboard wastewater treatment. The best performing membrane materials were selected for Coating Materials Design scaleup to bench-scale and then to industrial-scale membrane modules for evaluation in a pilot-scale system.
Model compound studies were completed, and polymer precursors were prepared. The first generation of poly(ethylene oxide)-b-poly(p-benzamide) was synthesized and two polymer samples were identified. Films for characterization were also prepared. The performance and stability of a module containing Pebax 1074 membrane coated onto a microporous poly(vinylidene fluoride) support was determined to be an effective solution to oil/water emulsion fouling. Various samples demonstrated the porous membrane water production rate to be more than ten times lower than that of a coated membrane. This project was completed in FY 2000.
Novel, low-fouling membranes for graywater and bilgewater treatment offer longer service life and less frequent cleaning. When housed in high performance modules, these membranes provide a compact, reliable, economical shipboard wastewater treatment facility to enable the Navy to meet current and anticipated wastewater purification targets. This technology is widely applicable to Navy and civilian ships and to onshore treatment of waste waters.