Removal of Hydrophobic Volatile Organic Compounds in an Integrated Process Coupling Absorption and Biodegradation -- Selection of an Organic Liquid Phase

International audienceSince usual processes involve water as ab- sorbent, they appear not always really efficient for the treatment of hydrophobic volatile organic compound (VOC). Recently, absorption and biodegradation cou- pling in a two-phase partitioning bioreactor (TPPB) proved to be a promising technology for hydrophobic compound treatment. The choice of the organic phase, the non-aqueous phase liquid (NAPL) is based on various parameters involved in both steps of the pro- cess, hydrophobic VOC absorption in a gas - liquid contactor, and biodegradation in the TPPB. VOC sol- ubility and diffusivity in the selected NAPL, as well as NAPL viscosity, seems to be the main parameters during the absorption step, while biocompatibility, namely the absence of toxic effect of the NAPL to- wards microorganisms, non-biodegradability and VOC partition coefficient between NAPL and water were revealed as the key factors during the biodegra- dation step. The screening of the various NAPL avail- able in the literature highlighted two families of compounds matching the required conditions for the proposed integrated process, silicone oils and ionic liquids

Equivalent Absorption Capacity (EAC) concept applied to the absorption of hydrophobic VOCs in a water/PDMS mixture

International audienceAbsorption of hydrophobic Volatile Organic Compounds (VOCs) in multiphase system gas water PolyDiMethylSiloxane (PDMS, i.e. silicone oils) was considered. Absorption experiments were carried out in a countercurrent gas-liquid absorber filled with Raschig rings as packing. Three absorbing liquids, water, PDMS and a mixture of water/PDMS (90/10 v/v) were used to transfer toluene and dimethyl disulfide (DMDS) selected as VOC targets. Moreover, the Equivalent Absorption Capacity (EAC) concept previously developed to characterize the mixtures of water/PDMS was applied to the experimental data obtained at three different gas flow rates (18, 25 and 32 m3 h-1). Experimental measurements showed that absorption efficiencies (E) were low for water (around 2-8% for toluene and around 12-25% for DMDS) and higher for PDMS (from 88% to 98% according to the operating conditions). For the water/PDMS mixture, it was shown that the PDMS addition increased significantly the absorption of pollutants (E values in the range 25 to 65% according to the operating conditions). Besides, it was emphasized that the EAC concept describes satisfactorily the absorption behavior of the water/PDMS mixture. Finally, results confirmed that pure PDMS has to be used rather than a (90/10 v/v) water/PDMS mixture for hydrophobic VOC absorption. Using pure PDMS as absorbing liquid (dynamic viscosity of 5 mPa s), high absorption efficiencies (up to 98%) were obtained for L’/G’ value around 5 (L’/G’ corresponding to the ratio between the specific flow of the liquid and the specific flow of the gas). Under such conditions, pressure drops (ΔP) in the packed column and overall mass transfer coefficients (KLa) were around 1000 Pa m-1 (G’ = 1.06 kg m-2 h-1) and 5 10-3 s-1, respectively

Hydrophobic VOC absorption in two-phase partitioning bioreactors; influence of silicone oil volume fraction on absorber diameter.

International audienceA methodology to determine the diameter of an absorber contacting a gas phase and two immiscible liquid phases (water/silicone oil mixture) is presented. The methodology is applied to a countercurrent gas/liquid randomly packed column for the absorption of three VOCs (toluene, dimethyl sulfide, or dimethyl disulfide). Whatever the silicone oil volume fraction, Eckert's generalized pressure drop correlation was used. The results present the change in the column diameter through the change in the dimensionless ratio D(ϕ)/D(ϕ=1) versus the silicone oil volume fraction for the same operating conditions. For toluene and dimethyl disulfide, characterized by Hvoc,silicone oil values equal to 2.3 and 3.4 Pa m3 mol−1, respectively, it is highlighted that it is unwise to use water/silicone oil mixtures for mass transfer. In these cases, the contact between the polluted air and pure silicone oil requires roughly the same amount of silicone oil as for a (90/10 v/v) water/silicone oil mixture, but enables a 2-fold decrease in the column diameter. For dimethyl sulfide, which is characterized by a larger partition coefficient value (Hvoc,silicone oil=17.7 Pa m3 mol−1), the mass transfer operation should not be considered because large amounts of silicone oil are required (whatever the silicone oil volume fraction), which is not acceptable from an economic point of view. The feasibility of using a bioscrubber for the treatment of hydrophobic pollutants depends mainly on the partition coefficient Hvoc,silicone oil. VOC absorption in TPPB should therefore be restricted to pollutants characterized by a Hvoc,silicone oil value of around 3 to 4 Pa m3 mol−1 or less. In this case, absorption can be efficiently carried out in a biphasic air/silicone oil system

Volumetric mass transfer coefficients characterising VOC absorption in water/silicone oil mixtures

International audienceThe physical absorption of three Volatile Organic Compounds (dimethyldisulphide (DMDS), dimethylsulphide (DMS) and toluene) in "water/silicone oil" systems at a constant flow rate for mixtures of different compositions (f = 0, 5, 10, 15, 20 and 100%) was investigated using a dynamic absorption method. The results indicate that silicone oil addition leads to a dramatic decrease in KLa which can be related to the change in the partition coefficient (Hmix). They confirm the results obtained for styrene absorption using another measurement technique [15]. The interpretation of the results using dimensionless ratios KLa(f)/KLa(f=0%) and KLa(f)/KLa(f=100%) versus f also confirms the importance of the partition coefficient ratio mR = Hwater/Hoil in the KLa change. Moreover, the results obtained for toluene absorption in "air/water/silicone oil" systems (f = 10, 15 and 20%) suggest that the mass transfer pathway is in the order gas→water→oil for these operating conditions

Integrated process for hydrophobic VOC treatment--solvent choice

International audienceA process coupling absorption of a gaseous pollutant in an organic phase and biodegradation was considered to treat hydrophobic volatile organic compounds (VOC). The purpose of this work was to choose the best solvent for the absorption of some VOC (dimethylsulfide, dimethyldisulfide, and toluene) and to examine solvent biodegradability as well as VOC biodegradation by activated sludge. Some experiments were carried out on some selected solvents leading to select di-2-ethylhexyl-adipate (DEHA) and poly-di-methyl-siloxane (PDMS) for their high absorption capacity. Biodegradation experiments showed that toluene and DMDS can be removed in solvent on water emulsions, while DMS removal by biodegradation remains to confirm owing to its high volatility. However, experiments showed DEHA biodegradation, contrarily to PDMS which was therefore selected for subsequent experiments

Kinetics of toluene and sulfur compounds removal by means of an integrated process involving the coupling of absorption and biodegradation

International audienceBACKGROUND: Scrubbing using an organic solution instead of an aqueous solution could be a useful way to improve the removal of hydrophobic compounds. Absorption of toluene, dimethyldisulfide (DMDS) and dimethylsulfide (DMS) in an organic solution (di-2-ethylhexyladipate--DEHA), followed by biodegradation by activated sludge was considered, with particular attention to kinetic aspects. DEHA was selected for its relevance in terms of absorption capacity and absorption velocity of the selected volatile organic compounds (VOCs). After the biodegradation step and owing to its cost, recycling of the VOC-free solvent should be considered. RESULTS: Enhancement of VOC mass transfer from the organic to the aqueous phase due to bacterial activity was highlighted and the main driving force was found to be biosurfactant production rather than biodegradation reaction. However, the mass transfer rate between the two phases was shown to be lower than VOC biodegradation rate; hence, significant biodegradation of DMDS and toluene was recorded in a few days during batch experiments, 0.10 and 0.09 mmol respectively. Toluene showed higher biodegradation rates (about 0.05 and 0.10 mg h−1 for DMDS and toluene), leading to higher growth rates. Contrarily, owing to its high volatility, important DMS losses were observed. CONCLUSION: The relevance of the proposed integrated process was shown for hydrophobic VOC removal, at least for toluene and DMDS. Unfortunately, the absorbent phase was also degraded, proved by detection of by-products during analyses of the aqueous phase headspace. The comparison of DEHA with other solvents or solid polymers available for multiphase bioreactor applications may be a reliable option to continue this work

Activated sludge acclimation for hydrophobic VOC removal in a Two-Phase Partitioning Reactor.

International audienceThe effect of activated sludge acclimation on the biodegradation of toluene and dimethyldisulphide (DMDS) in the presence of a nonaqueous phase liquid, polydimethylsiloxane (PDMS), in a two-phase partitioning bioreactor was characterized. The influence of the presence of PDMS, at a ratio of 25% (v/v), and acclimation of activated sludge on two hydrophobic VOC biodegradation was studied. Activated sludge were acclimated to each VOC and in the presence of the non-aqueous phase liquid, namely in the emulsion of PDMS in water. Using acclimated cells, 97.9% and 108.7% improvement of the mean biodegradation rates were recorded for toluene and DMDS,respectively, if compared to the values recorded in the absence of acclimation. While and in agreement with the lower solubility in water of DMDS if compared to toluene, a most significant effect of PDMS addition on the rate of DMDS removal was recorded, 87.0% and 153.6% for toluene and DMDS, respectively. In addition and if both biomass acclimation and PDMS addition were considered, overall improvements of the removal rates were 204% and 338% for toluene and DMDS

Equivalent absorption capacity concept applied to the VOCs absorption in a countercurrent packed-bed column using water/silicone oil mixtures

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Absorption and Biodegradation of Hydrophobic Volatile Organic Compounds in Ionic Liquids

International audienceFour room temperature ionic liquids, [BMIM][PF6], [BMIM][NTf2], [PEGMIM][PF6], and Aliquat, were investigated regarding their use in a two-phase partition bioreactor dedicated to remove two hydrophobic VOC, dimethyldisulfide and toluene. Aliquat and [PEGMIM][PF6] cannot be further considered, owing to the toxicity of the former shown during glucose uptake inhibition tests and the water solubility of the latter. The partition coefficients of [BMIM][PF6] and [BMIM][NTf2] were found comparable to those recorded for typical liquid solvents used in multiphase bioreactors. They were also non-biodegradable, showed during long-term biodegradability tests. After 1 day of lag time, similar glucose biodegradation rates were recorded in the presence of 5 % [BMIM][PF6] or [BMIM][NTf2], if compared to controls deprived of ionic liquid. However, a clear inhibitory effect of the ionic liquids was observed during VOC biodegradation experiments. This phenomenon was significantly minimized after acclimation of activated sludge to VOC, since nearly similar consumption rates of toluene were recorded in the control deprived of IL and in the presence of 5 % bmimPF6, 0.49 and 0.48 g m−3 h−1, respectively. These promising results showed that more complex acclimation strategies, including microbial acclimation to both ionic liquids and VOC, will have to be considered