MinireviewMembrane bioreactors for waste gas treatment
Introduction
The application of membranes has been proposed for a variety of purposes in waste management, as there are the separation of solids, biomass retention, aeration of bioreactors, and extraction of pollutants from wastewater. These processes were recently reviewed by Brindle and Stephenson (1996). In this review, however, we will focus exclusively on the removal of pollutants from air and the subsequent biodegradation in membrane reactors.
In bioreactors for waste gas treatment, organic pollutants diffuse into the liquid phase, where micro-organisms degrade the pollutants into harmless products like CO2, H2O and minerals. Membrane bioreactors were designed as an alternative for conventional types of bioreactors for waste gas treatment, like the biofilter. In a biofilter waste gas is blown through a bed of compost or soil in which micro-organisms consume the gaseous organic pollutants. In such a biofilter no separate water phase is present. An advantage of the membrane bioreactor over the biofilter is the presence of a discrete water phase allowing optimal humidification of the biomass and removal of degradation products, thus avoiding inactivation of the biomass.
In a membrane bioreactor for waste gas treatment the membrane serves as the interface between the gas phase and the liquid phase (Fig. 1). The gas–liquid interface that can thus be created e.g. in hollow fibre reactors, is larger than in other types of gas–liquid contactors (Yang and Cussler, 1986). Moreover, membrane reactors do not contain moving parts, are easy to scale up, and the flows of gas and liquid can be varied independently, without the problems of flooding, loading, or foaming commonly encountered in bubble columns (Sirkar, 1992). Disadvantages of membrane bioreactors are the high construction costs and the fact that their long-term operational stability still has to be demonstrated.
In this review we describe a new type of gas–liquid contactor, in which the excellent mass transfer properties of membrane devices are combined with the clean technology of biological air purification. Based on a critical evaluation, the potential application niches will be defined.
Section snippets
Theory
Biological waste gas treatment can be described as an extraction of the gas phase with water, followed by consumption of the biodegradable components. The flux of a volatile component over the membrane in such a gas–liquid extractor is described by Eq. (1)(Prasad and Sirkar, 1992).in which J=flux through the membrane (mol s−1), Kl=overall mass transfer coefficient based on concentrations in the liquid phase (m s−1), A=membrane surface area (m2), Cg, Cl=concentrations in the gas
Gas–liquid contactors
Both microporous and dense membranes have been used for a variety of processes that involve gas–liquid contact. Microporous material is generally applied in hollow fibres, although spiral-wound and plate-and-frame modules have also been used (Sirkar, 1992, Wickramasinghe et al., 1992). Microporous membranes can be applied as gas–liquid contactors when selective action of the membrane is not required. Volatile components diffuse through this material depending on their diffusion coefficient in
Applications in biological waste gas treatment
In addition to the relatively new process of membrane-based gas absorption (Sirkar, 1992), membrane contactors have recently been tested for biological treatment of gas streams. In such a process the pollutants diffuse through the membrane and are degraded by the microbial population present in the liquid phase (Fig. 1). An overview of publications on this subject is shown in Table 1.
In general, the biomass is supplied with carbon and oxygen from the gas phase, while water and mineral nutrients
Comparison with conventional bioreactors for waste gas treatment
Membrane bioreactors were designed as an alternative for conventional types of bioreactors for waste gas treatment, like the biofilter, the trickle bed reactor and the bioscrubber.
A biofilter usually consists of a bed of compost through which waste gas is blown. Micro-organisms present in the compost degrade the organic pollutants in the gas. To prevent dehydration of the biofilter, the waste gas has to be prehumidified. Treatment of gases containing chlorinated pollutants, sulphur compounds or
Membrane resistance
The resistance of both silicone membranes and microporous hydrophobic materials for various volatiles are shown in Table 2. For silicone membrane material the data were taken from various sources in literature and for microporous membrane the value of km/Dg was used as determined by Reij et al. (1995)for microporous polypropylene with a porosity of 70–75%. For both types of membrane a thickness of 100 μm was assumed.
The resistance depends largely on the gas–liquid partition coefficient (m) of
Microbial growth in membrane bioreactors
Irrespective of the membrane resistance, the driving force for mass transfer depends on the concentration to which the pollutant is reduced in the liquid phase. Therefore, the removal rate in a membrane bioreactor depends largely on the activity of the microbial population.
As can be seen in Table 1, in most studies biofilm formation was observed or was even essential (Reiser et al., 1994). Both mixed cultures and pure cultures formed biofilms. The hydrophobic nature of both microporous and
Outlook and conclusions
All studies presented in this review are laboratory-scale studies. By their modular nature, membrane modules are relatively easy to scale up (Karoor and Sirkar, 1993), but before full-scale membrane reactors can be applied in waste gas treatment, the long-term performance should be tested extensively.
The effects of biomass on the membrane material in the long run have not sufficiently been tested. During prolonged operation microbial polysaccharides might absorb to the membrane material,
Acknowledgements
The authors would like to thank Dr A.C. van Aelst of the Department of Plant Cytology and Morphology, Wageningen Agricultural University for performing the scanning electron microscopy. The financial support by Stork Engineers and Contractors B.V. is gratefully acknowledged.
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