Membrane bioreactor for waste gas treatment

SummaryThis thesis describes the design and testing of a membrane bioreactor (MBR) for removal of organic pollutants from air. In such a bioreactor for biological gas treatment pollutants are degraded by micro-organisms. The membrane bioreactor is an alternative to other types of bioreactors for waste gas treatment, such as compost biofilters and bioscrubbers. Propene was used as a model pollutant to study the membrane bioreactor.A membrane bioreactor for waste gas treatment consists of a gas and a liquid compartment, separated by a membrane. Gaseous pollutants diffuse through the membrane and are consumed by microorganisms present in the liquid phase. The organisms are supplied with water and inorganic nutrients via this liquid phase. Various membrane bioreactors described in the literature are reviewed in Chapter 2. In the work presented in this thesis, microporous hydrophobic material was selected because of its low mass transfer resistance and the availability of both sheets and fibres. For the removal of propene from air the mass transfer resistance of this type of membrane was found to be negligible (Chapter 3).The propene-degrading bacterium Xanthobacter Py2 was shown to form biofilms in membrane bioreactors. Continuous propene removal by biofilms of Xanthobacter Py2 was demonstrated in both flat sheet reactors and hollow-fibre reactors. In both configurations the biofilms are situated on the membrane in the liquid phase. Propene consumption rates could be described quite accurately with the computer programme BIOSIM, that describes simultaneous diffusion and reaction in a biolayer (Chapter 3).During continuous operation of hollow-fibre reactors at inlet concentrations of 0.5 to 6 gram propene per m 3, the propene conversion decreased after several weeks (Chapter 4). Clogging of the fibres by excess biomass formation and acidification due to ammonium oxidation, were identified as possible causes. However, when both clogging and ammonium oxidation were prevented, the propene conversion still decreased in time.Apparently other factors than clogging and nitrification affect the long-term performance of biofilms of Xanthobacter Py2, growing In an MBR. These factors might be Identified with new methods for biofilm analysis, which allow the localization of activity within the biofilm.According to the Dutch emission standards, hydrocarbons such as propene, in offgas have to be reduced to less than 150 mg m -3. In Chapter 5, two propenedegrading strains were compared for their ability to degrade such low concentrations of propene and the faster growing strain, Xanthobacter Py2, was selected. At a concentration of 300 to 600 mg m -3in the gas phase, a 20 days startup period was required for biofilm formation. Once the biofilm had been established, the amount of active biomass adapted to the amount of propene available Within several days. Propene could be removed continuously from air at a concentration of 15 to 50 mg m -3in the gas phase without supplying other organic nutrients to the microbial population (Chapter 5).Besides the removal of poorly water soluble pollutants like propene, the membrane bioreactor is also suitable for the removal of pollutants that result in acidification, such as chlorinated hydrocarbons. Therefore, in Chapter 6 the biodegradation of trichloroethene (TCE) by Xanthobacter Py2 was tested during growth on propene in a stirred vessel. The aerobic biodegradation of TCE is difficult because of toxic intermediates that are formed. With Xanthobacter Py2 continuous cometabolic degradation of TCE was shown to be feasible with concentrations up to 206 μM in the liquid phase. The amount of TCE that could be degraded, depended on the TCE concentration and ranged from 0.03 to 0.34 grams of TCE per gram of biomass.Membrane bioreactors for gas-liquid contact have several potential applications. They are suitable for the removal of poorly soluble pollutants from air because of their large gas-liquid interface and small mass transfer resistance. Especially if biodegradation of a poorly soluble pollutant results in acidification, the membrane bioreactor might be a unique tool, since the acidic product can be removed via the liquid phase. Other applications might be the removal of highly chlorinated hydrocarbons from air by an aerobic or a combined anaerobic/aerobic: process, as was recently suggested in literature. Membrane bioreactors may also be useful tools in biofilm research, because of easy handling and processing of biofilm samples, excellent oxygen transfer properties and the possibility to apply counter gradients.