Until alternate coating materials and depainting operations are proven effective, treatment of fugitive volatile organic compound (VOC) releases during application and removal of Department of Defense aircraft coatings is necessary to maintain compliance with the Clean Air Act Amendments of 1990. Currently available VOC emission control technologies are costly because of the high volumetric flow rates and low VOC concentrations associated with ventilation of paint spray booths.
The objective of this project is to develop a membrane-supported extraction and biotreatment (MBT) system. MBT uses microporous hollow fiber membrane modules in a two-step process designed to transfer VOCs from a contaminated air stream through a stripping fluid and to a degradative biofilm where the compounds are mineralized effectively.
The research was conducted in two phases. In Phase I, the process streams were characterized separately at bench scale for both the extraction and the biotreatment processes to show the technical feasibility using simulated streams containing organic constituents of aircraft coatings. In Phase II, larger modules will be developed and assembled into a pilot-scale MBT system. This pilot MBT system will be evaluated using emissions from a small paint booth to determine the long-term performance of microbes and hardware and to identify effective scale-up parameters.
Separation module development and evaluation have resulted in the identification of a module design and operating conditions that result in VOC removal efficiencies greater than 81 percent and as high as 95 percent in some instances. The optimum results thus far resulted from a radial, parallel flow module. These results were used to develop a crossflow module aimed at improving air-side contact efficiency and pressure drop. Both of these modules were coated with a thin (~ 1 micron) layer of plasma-polymerized silicone rubber. Gas residence times in the range of 0.2 to 1.2 seconds are used for these experiments. Several module construction issues will be resolved in the next module version that will be used in pilot-scale testing.
The biotreatment module has been characterized extensively using m-xylene, toluene, and methyl ethyl ketone (MEK) as model compounds and two organisms, X-1 and M-1, which were isolated from contaminated soil and enriched on m-xylene and MEK, respectively. Shake flask studies indicate that consortia of these organisms degrade m-xylene and toluene extremely rapidly. However, degradation of MEK is inhibited significantly in the presence of either aromatic compound.
Biofilms containing one or both of these organisms have been shown to accelerate the transfer of VOCs out of the stripping fluid. Furthermore, the biofilm activity was not inhibited by long-term exposure to the initial stripping fluid, octanol. Degradation of the aromatic compounds investigated was achieved. These compounds were not observed in the aqueous phase above the biofilm. They were consumed concurrently in some mixed substrate experiments but sequentially metabolized in mixtures with MEK.
The proposed system concentrates the VOCs to reduce the size and cost of control equipment and completely destroys the VOCs without generating a secondary waste stream. This system can store captured VOCs for later biotreatment processing, avoiding upset and startup conditions that have proven difficult for other biotreatment systems. Full-time offline processing enables effective treatment with a compact system. These advantages make it a viable option for a broad range of spray booth sizes.