Biofiltration of BTEX waste gases
Strauss, Johannes Mattheus
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A comparison of biofilter performance under different temperature conditions is of particular importance for the application and commercialization of biofiltration technology due to the fact that many waste gas streams are at elevated temperatures. The efficacy of higher temperature biofiltration reactors, therefore, has important practical and cost implications as it directly impacts on the need and cost for gas cooling prior to biofiltration treatment. In this study the performance of toluene degrading mesophilic (25°C) and thermophilic (50°C) composted pine bark biofilters were evaluated. The effect of oxygen concentrations on reactor performance was also evaluated, and comparisons made for both temperature conditions. Toluene, as part of the benzene, toluene, ethylbenzene and xylene (BTEX) group, are important solvents and constitutes a large percentage of petroleum. These compounds, mainly due to their solubility as well as being confirmed or suspected carcinogens, have been classified as environmental priority pollutants by the US Environmental Protection Agency with increasingly stringent regulations to avoid their release. Investigations were performed at loading rates ranging from 9 to 54 g m'3 h-', retention times of 0.25 to 3.9 minutes, and at various bed heights. Comparison of the performance using empirical models indicated that higher removal efficiencies would be obtained under thermophilic conditions, although a slightly longer retention time was required to obtain the same efficiency. Under thermophilic conditions toluene removal efficiencies exceeding 90% were obtained when the reactor was subjected to retention times in excess of 0.6 minutes (36 seconds) and loading rates below 54 g m-3 h-1. Under mesophilic conditions similar efficiencies would be obtained with a retention time of 0.32 minutes (19 seconds) and loading rates below 42 g m-3 h-1. The influence of oxygen at a single loading rate and retention time indicated reduced performance at oxygen concentrations below 5% for both operating temperatures. A previously developed diffusion reaction model was further applied to this comprehensive dataset and through a process of subset model parameter optimization and parameter sensitivity analyses, both reactor condition performances were simulated with a high degree of accuracy at steady state Conditions. Simulated results further emphasized that higher elimination rates of toluene could be obtained at thermophilic temperatures. BTEX substrate interactions, using this toluene-acclimatized biofilter consortium, were further investigated at a single loading rate of 18 g m-3 h- 1 and retention times ranging from 0.5 to 3 minutes. The mesophilic results obtained were modelled using Michaelis-Menten kinetics and an explicit finite difference scheme to generate Vm and Km parameters, of which the ratio can be used as an indication of the catalytic efficiency in order to quantify substrate interactions occurring within the biofilter. Toluene was found to enhance the catalytic efficiency for p-xylene, while catabolism of all other compounds was inhibited competitively by the presence of toluene. All BTEX compounds could be degraded by the microbial consortium even in the absence of toluene. The catalytic efficiency of the reactor for the compounds was in the order: ethylbenzene > benzene > o-xylene> m-xylene > p-xylene. The catalytic efficiency of the microbial consortium for toluene was reduced by the presence of all other BTEX compounds, with the greatest inhibitory effect caused by the presence of benzene, while o-xylene and p-xylene caused the least inhibitory effect. This BTEX substrate interaction study was further extended to include the thermophilic conditions at a similar loading to that of the mesophilic study, in order to compare results from both temperature conditions. Overall toluene degradation rates under mesophilic conditions were found to be superior to degradation rates of individual BEX compounds. With the exception of p-xylene, higher removal efficiencies were achieved for individual BEX compounds compared to toluene under thermophilic conditions. Overall BEX compound degradation under mesophilic conditions was ranked as ethylbenzene > benzene > o-xylene > m-xylene > p-xylene. Under thermophilic conditions overall BEX compound degradation was ranked as benzene > o-xylene > ethylbenzene > m-xylene > p-xylene. With the exception of o-xylene, the presence of toluene in paired mixtures with BEX compounds resulted in enhanced removal efficiencies of BEX compounds, both under mesophilic and thermophilic conditions. A substrate interaction index was calculated to compare removal efficiencies at a retention time of 0.83 minutes (50 seconds). A reduction in toluene removal efficiency (negative interaction) in the presence of individual BEX compounds was observed under mesophilic conditions, while enhanced toluene removal efficiency was achieved in the presence of other BEX compounds, with the exception of p-xylene under thermophilic conditions. This study illustrated the potential of biofiltration as an emerging technology, especially at elevated temperatures, but emphasized the complexity of interactions that might occur between individual compounds that could influence the performance of the reactors when treating mixed pollutant gas streams.