SIFT-MS analysis of the removal of dimethyl sulphide, n-hexane and toluene from waste air by a two phase partitioning bioreactor
Introduction
Air pollution is probably one of the most serious problems for the future of our planet. Volatile organic compounds (VOCs), largely emitted in the atmosphere, are key pollutants due to their ozone depletion potential, global warming potential, toxicity and carcinogenicity. In order to reduce these VOC emissions, biological gas treatment techniques such as bio(trickling)filtration, bioscrubbing and membrane biofiltration [1], [2], [3], [4] have been studied and used as alternatives for the traditional physical–chemical techniques. In these biotechniques the Henry’s law constant is a very important characteristic. This dimensionless Henry’s law constant of a compound indicates the concentration of this compound in the gas phase proportional to the concentration of this compound in the aqueous phase ((g m−3)gas/(g m−3)liquid). This constant directly affects the performance of a bioreactor, since the transport of the VOC from the gas phase into the water could be rate limiting [5]. The hydrophobic compounds (lower Henry’s law constants) enter the biofilm much more easily, than hydrophobic compounds (higher Henry’s law constants) [6], so the more conventional biotechniques (i.e. biofiltration, biotrickling filtration and bioscrubbing) are acceptable to treat hydrophilic compounds, but are often limited when dealing with more hydrophobic compounds, e.g. hexane [7]. As the hydrophobicity often limits the pollutant to transfer from the gas to the aqueous phase and industrial emissions often contain a mixture of hydrophilic and hydrophobic compounds, a solution to this problem needs to be found. Two phase partitioning bioreactors (TPPR) which are based on the use of two immiscible liquid phases, i.e., a non-aqueous phase (NAP) and an aqueous phase, is therefore a good alternative [8], [9]. The NAP enhances the transfer of the more hydrophobic compounds to the micro organisms, while the aqueous phase supports the biological activity by supplying the nutrients [10].
Recent studies already reported promising results [11], [12], [13], [14], [15], [16] for the reduction of hydrophobic compounds in a TPPR, but most of these studies are based on the treatment of one single compound. As industrial emissions often contain a complex mixture of compounds, it is therefore necessary to investigate how a TPPR will perform if being fed with a mixture of compounds, more in detail a mixture containing compounds with different Henry’s law constants.
In a first part of this study GC measurements of a preadapted TPPR fed with hexane as single compound are compared with analysis performed by a Selected Ion Flow Tube Mass Spectrometer (SIFT-MS) in order to compare both measuring techniques.
In a second part of this experimental work, SIFT-MS is used to determine the performance of a preadapted TPPR when fed with a mixture of DMS, toluene and hexane. These compounds were selected because of their presence in different waste gases of industrial sources, but mainly because of their difference in hydrophobicity. DMS, which can often be found in the emissions of kraft pulping [17], is known to have a high solubility in water (low Henry’s law constant, 0.087) [18], while hexane has a very high Henry’s law constant of 54 [19]. Toluene has a Henry’s law constant of 0.26 [19] which is about 10 times higher than the one of DMS and about 100 times lower than the one of hexane. Hexane and toluene are both extensively used as solvents in polymer industries, especially during the production of adhesives [20].
Finally the SIFT-MS was used in order to determine the net residence time (NRT) of hexane in the TPPR.
The objective of this laboratory study was first of all to examine the feasibility of SIFT-MS as analytical technique to measure the performance of a preadapted TPPR. Advantages of the SIFT-MS approach include the ability to measure several VOCs simultaneous online, less than 250 ms for one analysis, and the sensitivity to low ppbv levels. Secondly to determine the performance of a preadapted TPPR when being fed with a mixture of compounds with different Henry’s law constants and finally to evaluate the NRT as indicator for the aeration and air distribution within a TPPR.
Section snippets
Experimental set-up
An overview of the reactor set-up can be found in Fig. 1. A 1 L glass reactor was filled with 0.5 L of 25/75 vol/vol silicone oil (47 V 20 Rhodorsil; VWR)/water emulsion. The water phase used in the reactor set-up was obtained from the activated sludge aeration tank of an industrial waste water treatment plant (Ossemeersen WWTP, Ghent), which was inoculated with a mixed microbial culture. Before adding the emulsion to the actual TPPR set-up the emulsion was first preadapted during 1 month in a
Hexane removal in a TPPR
Hexane biodegradation performance of the TPPR was evaluated in terms of elimination capacity (EC) in function of the inlet load (IL), see Fig. 2. In a first experiment (EXP 1), the performance was determined by using a GC as analytical technique. To ensure that the biomass in the TPPR was adapted to the new inlet conditions before every analysis, the inlet conditions were only changed once a day after the previous measurement. When feeding the TPPR only with hexane, an EC of 138 g m−3 h−1 can be
Conclusions
The results obtained during this research illustrate that SIFT-MS is a suitable measuring technique to analyse online the performance of preadapted TPPR in a short time period. Using the SIFT-MS it was possible to measure the NRT of a compound in the TPPR, which can be an indicator of the aeration in the bioreactor. A decrease in the NRT can indicate that part of the biomass needs to be purged in order to keep the aeration in the bottle optimal. When feeding only hexane to the TPPR an EC of 138 g
Acknowledgements
The authors acknowledge the support of Ghent University under the GOA-project: Ugent BOF10-GOA010.
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