Michèle Heitz

Biofiltration of Air: A Review

Abstract In this paper we present a review of the existing air pollution control technologies (APCT), when used essentially for the elimination of volatile organic compounds (VOC). The biotechnologies referred to, bioscrubbers, biotrickling filters and biofilters, are also described. A more detailed review of biofiltration is proposed, presenting the most recent and latest developments achieved in the field of bioprocessing. In particular, the influence of the filter bed, the polluted air flowrates, the pollutants, the pressure drop, bed moisture content, temperature, nutrients, pH and the microorganisms are reviewed. Models of biofiltration are also presented.

Fractionation of Populus tremuloides at the pilot plant scale : optimization of steam pretreatment conditions using the STAKE II technology

Fractionation of a prototype hardwood, Populus tremuloides, was optimized in a 4 t/h pilot plant STAKE II unit located in Sherbrooke, Quebec. The aim was to find operational zones which maximized the recoveries of hemicelluloses (pentosans), lignin and cellulose. A phenomenological approach, based on the definition of a severity parameter R = t∗exp[T − 100)/14·75] which combines time, t (min), and temperature, T (°C), to express the severity of a given pretreatment, was developed. Performance curves, using the R0 parameter, have been experimentally determined. These indicated that the maximum recovery of pentosans is 65% of the potential at log10Ro = 3·8. Under these conditions, lignin recovery by caustic extraction was about 80% of total lignin. Cellulose was completely recovered at this severity but needed to be bleached to achieve its natural coloration. At severities both greater and smaller than log10Ro = 3·8 pentosans recovery decreased. The cellulose derived from the steam fractionation process was rapidly depolymerized as severity increased, whilst its enzymatic digestibility was close to theoretical at severities beyond log10Ro = 3·2. The results obtained suggest that steam explosion processes are fundamentally hydrolytic in nature.

Treatment of air polluted with high concentrations of toluene and xylene in a pilot-scale biofilter

Air biofiltration is now under active consideration for the removal of the volatile organic compounds from air polluted streams. In order to investigate the performance of this newly developed technology, a biofiltration pilot unit was operated for a continuous period of 8 months. The biofilter column was packed with commercially conditioned peat. At start-up, the filter bed was inoculated with four species of microorganisms. The resulting biofilter was fed with air contaminated with toluene, xylene or a mixture of toluene and xylene. The maximum elimination capacities attained were 165 g m−3 h−1 for toluene, 66 g m−3 h−1 for xylene and 115 g m−3 h−1 for the mixture of toluene and xylene. These specific performances exceed the values published in the technical and commercial literature for similar processes. Xylene isomers were degraded in decreasing order of reactivity, m-xylene, p-xylene, o-xylene. In the case of air polluted with a toluene and xylene mixture, it was noticed that the metabolism of toluene biodegradation was inhibited by the presence of xylene. Characterization of the biofilm microbial populations after several weeks of operation showed that the dominant strains among the isolated culturable strains from the biofilm, even if different from the initially inoculated strains, had at least one physiological property favoring degradation of aromatic organic rings. The performance of the biofilter was found to be dependent on the temperature of the filter media and the pressure drop through the bed. Finally, a steady state mathematical model was tested in order to theoretically describe the experimental results. This model is used to illustrate the operating diffusion and reaction regimes at steady state for the case of each pollutant.

Biofiltration of air contaminated with toluene on a compost-based bed

Many studies have focused on problems created by emissions to the atmosphere of gaseous effluents containing volatile organic compounds (VOCs). Over the more recent decades, such studies have led to the development of various bioreactors such as the bioscrubber, the biotrickling filter and the biofilter. This paper presents the results of a study on the biofiltration of airborne toluene, the biofilter employed being operated at the laboratory-scale for a continuous period of 3 months. The focus of this particular study has been the development of a new compost-based filter-bed material, which consists of an association between matured compost and a proprietary organic binder that is intended to prolong the period of the beds efficient operations. No inoculum was added to the filter-bed material. During the experimental program, the performance of two different bed irrigation solutions was examined, the most effective nutrient supply solution then being used, along with toluene input levels varying from 0.6–2.6 g/m3, and toluene polluted air flow rates ranging from 0.4–1 m3/h, equivalent to empty bed residence times of 65–165 s. The results of this program have demonstrated removal efficiencies approaching 95%, while maximum elimination capacities of 55 g/m3 h, for an inlet load of 65 g/m3 h, have been achieved, supporting the view that the compost-based filter material tested in this work functions as a promising biofilter medium in this application. Finally, in order to present the biofilter performance observed under the best operating conditions, a simplified representation based on Ottengrafs model has been developed from the experimental results and is included here.

Trace gas compound emissions from Municipal Landfill Sanitary Sites

Abstract The literature on certain aspects of trace gas compounds emitted from Municipal Landill Sanitary Sites is reviewed. Aspects covered are the formation, nature and origin of such compounds, as well as the problems caused by them. Risks posed to human health and the environment by even low concentrations of these compounds are examined and methods to reduce and control them discussed.

A study of the impact of particle size and adsorption phenomena in a compost-based biological filter

Abstract The removal of toluene by biofiltration has been investigated. The first objective was to study the impact of the filter bed pellet size on the biodegradation performance. Three different pellet sizes (5.0, 10 and 20 mm diameter) were evaluated with an air flowrate of 1.0 m 3 h −1 , toluene inlet concentrations between 1.8 and 3.9 g m −3 and with variable nitrogen concentrations in the irrigation solutions (0.0– 7.0 g of N l −1 ). It was shown that pellet size and specific surface area were major limiting factors for the biodegradation process. Thus, for the optimal N-concentration (2.0– 3.0 g of N l −1 ), the maximum elimination capacities, decreasing with pellet size, but increasing with specific surface area, were achieved: 180 g m −3 h −1 (5.0 mm , 590 m −1 ) , 90 g m −3 h −1 (10 mm , 280 m −1 ) , and 45 g m −3 h −1 (20 mm , 120 m −1 ) . The second objective was the determination of the adsorption isotherm (22°C) for toluene on the wet filter bed. Toluene inlet concentrations between 0.0 and 3.2 g m −3 were introduced in a bench-scale biofilter. The experimental adsorption isotherm was modeled by either a Langmuir model (K 1 =344.8 μg g −1 of moist bed, K 2 =16.7 g m −3 ) or a Freundlich approach ( k f =19.4 μg g −1 of moist bed, n =0.92).

Effect of inlet mass loading, water and total bacteria count on methanol elimination using upward flow and downward flow biofilters

An upward flow biofilter and a downward flow biofilter using compost for removing methanol from air were investigated to compare the biofilter performance and to realize the advantages of using downward flow biofilters for accessibility to water make-up. Both the upward flow and downward flow columns showed similar performance in terms of elimination capacity (EC) versus inlet mass loading (IC). The maximum elimination capacity (EC) from these two biofilters was approximately 101 g m−3 h−1 with an optimum methanol loading rate at inlet (IC) of 169 g m−3 h−1 (7.5 g m−3 of methanol with superficial velocity of 7.6 m h−1). The effect of water movement within the bed on elimination capacity was monitored. In addition, it was found that when the water content in the compost was below 35% by weight, microbial activity was impaired. Once the compost media had dried, it became hydrophobic and could be rewetted only with great difficulty. Total bacteria count was performed on compost samples during the entire operation. The relationship between elimination capacity and total bacteria count was reported. Similar trends were shown by the variations of elimination capacity and total bacteria count with methanol loading: both initially increase, go through a plateau, then decrease with loading. © 2000 Society of Chemical Industry

A high performance biofilter for VOC emission control

Biofiltration is a cleaning technique for waste air contaminated with some organic compounds. The advantages of the conventional biofilter over other biological systems are a high-superficial area best suited for the treatment of some compounds with poor water solubility, ease of operation, and low operating costs. It has crucial disadvantages, however; for example, it is not suitable to treat waste gases with high VOC concentrations and it has poor control of reaction conditions. To improve on these problems and to build a high-performance biofilter, three structured peat media and two trickling systems have been introduced in this study. The influences of media size and composition have been investigated experimentally. Peat bead blended with 30% (w/w) certain mineral material with a good binding capacity has advantages over other packing materials, for example, suitable size to prevent blockage due to microbial growth, strong buffering capacity to neutralize acidic substances in the system, and a pH range of 7.0-7.2 suitable for the growth of bacteria. Dropwise trickling system offers an effective measure to easily control the moisture content of the bed and the reaction conditions (pH, nutrient) and to partially remove excess biomass produced during the metabolic processes of microorganisms. The influence of nutrient supplementation has also been investigated in this study, which has revealed that the biological system was in a condition of nutrient limitation instead of carbon limitation. The biofilters built in our laboratory were used to treat waste gas contaminated with toluene in a concentration range of 1 to 3.2 g/m3 and at the specific gas flow rate of 24 to120 m3/m2.hr. Under the conditions employed, a high elimination capacity (135 g/m3.hr) was obtained in the biofilter packed with peat beads (blended with 30% of the mineral material), and no blockage problem was observed in an experimental period of 2-3 months.

Treatment of Air Polluted with Xylenes using a Biofilter Reactor

Biofiltration of air polluted by VOCs is now beingrecognized by the industrial and research communitiesto be an effective and viable alternative for theclassical environmental technologies. While a numberof biological aspects of the biofiltration process arewell understood, the effect of certain engineeringparameters such as temperature, pressure drop,bacterial count, etc., remained ambiguous especiallywhen several isomers have to be removedsimultaneously. In this paper are reported theresults of purification of air containing vapors ofxylenes in a laboratory-scale biofilter reactor. Thelatter is a packed bed of peat balls particlesspecifically designed and produced for this purpose. Three types of micro-organisms strains werescrutinizingly selected and immobilized on thefiltering material. Xylenes entering the biofilter ata relatively high inlet load (110 g m-3 h-1)are removed with an elimination capacity of60 g m-3 h-1 (at steady state). Theexperimental results obtained on the reduction ofxylenes were satisfactorily represented by thegeneralized Ottengrafs model.

Catalytic Mechanism of Cu2+ and Fe3+ in Alkaline O2 Oxidation of Lignin

Abstract The catalytic mechanisms of Cu2+ and Fe3+ in the alkaline oxidation of lignin are discussed. Best aldehyde yields were obtained by using Cu2+ and Fe3+ as catalysts. Without these two ions present or in the absence of one or other of them, yields were lower. It is clear that Cu2+ and Fe3+ significantly influence the reactions. Cu2+, acting as the electron acceptor, accelerates the formation of the phenoxy radical, thus speeding up the radical reaction. In order to interpret the effect of Fe3+ on oxidative degradation of lignin and formation of aldehydes, formation of an intermediate, O2-Fe3+-lignin complex and its catalytic mechanism have been proposed.