Rhodococci display a diverse range of metabolic capabilities and they are a ubiquitous feature of many environments. They are able to degrade short-chain, long-chain, and halogenated hydrocarbons, and numerous aromatic compounds, including halogenated and other substituted aromatics, heteroaromatics, hydroaromatics, and polycyclic aromatic hydrocarbons. They possess a wide variety of pathways for degrading and modifying aromatic compounds, including dioxygenase and monooxygenase ring attack, and cleavage of catechol by both ortho- and meta-routes, and some strains possess a modified 3-oxoadipate pathway. Biotransformations catalyzed by rhodococci include steroid modification, enantioselective synthesis, and the transformation of nitriles to amides and acids. Tolerance of rhodococci to starvation, their frequent lack of catabolite repression, and their environmental persistence make them excellent candidates for bioremediation treatments. Some strains can produce poly(3-hydroxyalkanoate)s, others can accumulate cesium, and still others are the source of useful enzymes such as phenylalanine dehydrogenase and endoglycosidases. Other actual or potential applications of rhodococci include desulfurization of coal, bioleaching, use of their surfactants in enhancement of oil recovery and as industrial dispersants, and the construction of biosensors.

Biotransformations catalyzed by the genus Rhodococcus.

This paper presents fundamental and theoretical aspects of biological waste gas treatment technologies as well as examples of applications to di†erent compounds. The three most widely used technologies are described, namely bio- -ltration, bioscrubbing and trickling bio-ltration, focusing more extensively on bio-ltration which is the most studied and most extensively used process. A description of the di†erent technologies from technological and economic points of view, including an analysis of models used in waste gas biotreatment is given. Results presented in the literature concerning the removal of aliphatic, aromatic and mixtures of contaminants are reviewed. Carrier materials, inocula selected and alternatives proposed for regulating moisture content, pH values or for con- trolling pressure drop are considered. New technologies and reactor design studied at laboratory-scale are mentioned. 1998 SCI ( J. Chem. T echnol. Biotechnol. 72, 303E319 (1998)

Review: Waste gas biotreatment technology

This paper gives an overview of present biological techniques for the treatment of off-gases and the techniques that are being developed at the moment. The characteristics, advantages, disadvantages, costs and application area are discussed and compared. Biological off-gas treatment is based on the absorption of volatile contaminants in an aqueous phase or biofilm followed by oxidation by the action of microorganisms. Biofilters, bioscrubbers and biotrickling filters are used for elimination of odour and bioconvertable volatile organic and inorganic compounds and are enjoying increasing popularity. This popularity is a result of the low investment and operational costs involved compared to physico-chemical techniques and the elimination efficiencies that can be obtained. The operational envelop is still extending to higher concentrations and gas flow rates (exceeding 200,000 m3 h−1) and a broader spectrum of degradable compounds. Research and development on the use of membranes and the addition of activated carbon or a second liquid phase to the biological systems may lead to a more efficient elimination of hydrophobic compounds and buffering of fluctuating loads. Shorter adaptation periods can be obtained by inoculation with specialized microorganisms. Improved design and operation are made possible by the growing insights in the kinetics and microbiology and supported by the development of models describing biological off-gas treatment. In conclusion, biotechniques are efficient and cost effective in treating off-gases with concentrations of biodegradable contaminants up to 1–5 g/m3. They could play a justified and important role in air pollution control in the coming years.

Biotechniques for air pollution control

A literature study was conducted to compare the feasibility of biofilters and biotrickling filters for the treatment of complex odorous waste air containing hydrogen sulfide (H2S), organic reduced sulfur compounds, and chlorinated and nonchlorinated volatile organic compounds (VOCs). About 40 pilot-plant studies and full-scale applications at wastewater treatment plants and other facilities were reviewed. Reactor design and pollutant removal efficiencies were summarized in tables for easy reference and for a perspective on the current state of the art, and to allow comparison between different projects. The survey indicated that both biofilters and biotrickling filters are capable of combining a high H2S and odor removal efficiency with VOC removal. Apart from odor abatement, biological treatment therefore holds promise for reducing the overall toxicity and potential carcinogenicity of VOC-containing odorous waste air from wastewater treatment plants and other facilities. VOC removal efficiencies were in general lower than those of H2S and odor, although concentrations of individual VOC species were relatively low. This indicates that for effective treatment of VOC-containing odorous waste air, the design and operation should emphasize VOC removal as the rate-limiting parameter. © 2005 American Institute of Chemical Engineers Environ Prog, 2005

Literature Review of Air Pollution Control Biofilters and Biotrickling Filters for Odor and Volatile Organic Compound Removal

Biological methods for odor treatment – A review

This paper presents the results of basic investigations on reactions and process engineering in the biological purification of exhaust air in a trickle-bed reactor. The biocatalysts used were pollutant-specific bacterial monocultures, which were immobilized on various carriers. By using different pollutants (e.g. acetone, propionaldehyde, naphthalene and toluence, crude gas concentrations: 5–35 ppm), the effect of the water solubility of the gaseous substances on separation efficiency was studied. Furthermore, a combination of monocultures was used for degradation of a mixture of pollutants. The results show that, with suitable combinations of bacteria, pollutants and carriers, conversions of more than 80% at a space velocity of about 1000-1 can be achieved by this method.

Biological purification of exhaust air using fixed bacterial monocultures

This paper reports experiments on the purification of exhaust air containing organic pollutants by a new biological process using a trickle-bed reactor. Pollutant-specific microorganisms in high concentration were fixed to a suitable bed. The absorption and conversion of propionaldehyde as a model pollutant was measured by systematic variation of the gas and liquid flow rates in the reaction system. At a space velocity of 1000 h−1, it was possible to achieve conversion rates of between 68 and 96%, depending on the trickling density. The degradation capacity of the biological trickle bed is over 500 g propionaldehyde/m3 of reactor per hour. By using a tube bundle (honeycomb tube), it was possible to ensure continuous operation of the reactor with reduced conversion and pressure loss.

https://link.springer.com/content/pdf/10.1007/BF00172732.pdf

Purification of exhaust air containing organic pollutants in a trickle-bed bioreactor.

This paper presents results of experiments on the influence of O2 and substrate (pollutant) concentration on the overall reaction rate of a trickle-bed reactor used for biological waste gas purification. The biocatalyst was a pollutant-specific bacterial monoculture fixed on porous glass carriers. The conversion of acetone and propionaldehyde, as model pollutants that are easily soluble in water, was measured. Under constant hydrodynamic conditions (gas and liquid flow rates) the inlet pollutant concentration was varied. The O2 partial pressure in the model gas was increased to investigate the influence of O2 supply on pollutant conversion. At higher pollutant concentrations (>117 mg acetone.m-3 gas and > 150 mg propionaldehyde.m-3 gas) higher concentrations of dissolved O2 led to a significant rise in the maximum degradation capacity of the reactor. This maximum reaction rate was independent of the pollutant mass flow. It seems that the diffusion of O2 in the biofilm is rate-determining. The reaction rate at lower inlet concentrations was not affected by the improved O2 supply. Here the external mass transfer through the liquid film limits the reaction rate and the maximum separation efficiency of about 80% at a residence time of 1.2s (space velocity 3000h-1) is achieved.

Exhaust gas purification using biocatalysts (fixed bacteria monocultures) — the influence of biofilm diffusion rate (O2) on the overall reaction rate

The paper describes some reaction engineering fundamentals of the separation of organic air pollutants (volatile organic compounds) from waste gases using fixed-bacteria monocultures (biocatalysts) in a trickle-bed reactor. In particular the influence of pollutant concentration and oxygen concentration are investigated. The separation efficiency of certain substances such as acetone and isopropanol depends strongly on the oxygen concentration. The results obtained can be described by a mathematical model based on the diffusion of oxygen into the biofilm (diffusion regime of the catalyst). The non-stationary operation of the reactor – interruption of the oxygen stream and strong fluctuation in the exhaust gas stream – showed that other components such as propionaldehyde and n-propanol could be eliminated for a certain time without oxygen. Propionic acid is formed.

K. Kirchner

Papers

Biological purification of exhaust air using fixed bacterial monocultures

Purification of exhaust air containing organic pollutants in a trickle-bed bioreactor.

Exhaust gas purification using biocatalysts (fixed bacteria monocultures) — the influence of biofilm diffusion rate (O2) on the overall reaction rate

Removal of organic air pollutants from exhaust gases in the trickle-bed bioreactor. Effect of oxygen