The aim of the present study was to remove toluene from the gas phase using an industrial-scale low-pressure two-phase scrubber. The bioscrubber used in this study consisted of two scrubber sections to separate toluene from the gas phase and transfer it to the aqueous phase, and a microorganism section that removed toluene from the scrubber effluent. Also, to increase the efficiency of the bioscrubber, two aqueous and organic were used in this study. The results of the study showed that the designed microorganism in the presence of organic phase with a concentration of less than 10% had the highest efficiency and also the efficiency and removal capacity of bioscrubber in the presence of organic phase with a concentration of 10% were higher than other organic phases.
To isolate the microorganisms that degrade VOCs, we should use them in environments where these microorganisms are naturally present. In various studies, to enrich the microbial population in order to decompose volatile organic compounds, they have used the soils around the tanks of oil refineries and gas stations(17), sewage or sludge of municipal and industrial wastewater treatment plants(18). In this study, the source of microorganisms was the return part of the refineries of Tehran refinery. The results of the present study showed that the concentration of toluene in the control sample whose bacterial population was killed by KCN was 60 times higher than the samples containing the bacterial consortium. It can be concluded that toluene is degraded by the microorganisms in the bottle. Color change in the original samples indicated the growth of microorganisms in the sample bottles.
Due to the fact that toluene is a hydrophobic compound, in order to increase the absorption in the liquid phase, various compounds are used, which must have properties, one of which is the lack of effect on microorganisms. One of the compounds used in this study is cutting oil, which is widely used in industry and is cheap and available. As the results showed, adding cutting oil in concentrations less than 10% has no effect on reducing the growth of microorganisms. However, at concentrations of 20 and 30%, the growth of microorganisms is significantly reduced compared to the control sample, which can be due to the use of side bacteria in the composition of this oil, which at concentrations of 20 and 30% causes an effect on microorganisms. Lalanne et al. also showed that cutting oil had no effect on the population of microorganisms(19). A study by Fabian et al. to investigate the growth of bacteria in oil-soluble emulsions found that the use of cutting oil had no effect on the growth of microbial populations in the emulsion. They also showed that with increasing the concentration of organic phase, the microbial population in the logarithmic phase decreases(20). In a study by Bennett et al., 30 different species of bacteria survived at 9 concentrations of cutting oil. Gram-negative bacteria were able to survive for a considerable time, while gram-positive bacteria died very quickly. Darracq et al. also examined the biodegradation of silicone oil and concluded that silicon oil was not degraded by microorganisms and did not observe toxicity on the microorganisms(14).
In recent years, the use of two-phase systems to eliminate VOCs has been developed. In gas-liquid-liquid processes, mass transfer rate is a key parameter for estimating the bioreactor performance(21–23). The mass transfer coefficient depends on the physic-chemical properties of the pollutant and the characteristics of the adsorption medium (viscosity and salt content) as well as the internal properties of the reactor and operating conditions (gas viscosity, liquid viscosity, pH, and temperature). An important aspect of liquid-liquid systems was studied by Nielsen et al(24). They concluded that the use of organic phase increased the oxygen transfer. They found that this increase in oxygen transfer was due to the organic phase having solubility. They also found that the oxygen transfer coefficient in the bio-scrubber containing the organic phase was much lower than that of the aqueous phase bio-scrubber, which was consistent with the results of this study(18). As the results of the present study showed, by adding cutting oil to the liquid phase, the concentration of toluene in the gas phase was significantly reduced. This indicates that the contaminant is trapped in the liquid phase. Also, based on the results, adding cutting oil reduced the mass transfer coefficient (KLa) and increased the concentration of toluene in the liquid phase. Aldrich’s study showed that the maximum oxygen uptake in silicon oil occurred with lower viscosity (Cst 10), and the mass transfer coefficient was directly related to the volume fraction of silicon oil. Cesario et al. observed an increase in toluene mass transfer in a 1: 1 ratio with the organic phase and an increase in oxygen mass transfer in a volume ratio of 10% of FC40 as the solvent. (25)Montes et al. also showed a tendency to absorb silicon oil for oxygen 10 times more than water(26). Karimi et al.'s study showed that the use of 10% by the volume of silicone oil significantly reduced energy consumption compared to concentrations of 20% and 30% in two-phase bioreactors. They also found that the optimum organic phase concentration of silicon oil in terms of oxygen transfer was 10%. They also observed that the use of silicone oil at a concentration of 10% reduced the oxygen transfer coefficient. Jean et al. reported that silicon oil at a concentration of 10% had no effect on the mass transfer coefficient(26).
Various studies have been performed to determine the effect of aeration on oxygen transfer rate(27). In this study, two pumps with constant flow were used in the bioreactor section to increase the amount of dissolved oxygen in the liquid phase. The results showed that the performance of bioscrubber in toluene purification without the presence of organic phase with increasing input concentration did not change the efficiency and removal capacity of toluene. The removal efficiency without organic phase was 20%, which is higher than the study of Lalanne et al., who reported the removal efficiency of toluene without the presence of organic phase at 11%, and this could be because the L/G ratio in this study was 3.6(28). It was much greater than the study of Lalanne et al. Malhautier et al. also used an industrial-scale scrubber to remove a mixture of VOCs including chlorinated, aromatic, and oxygenated compounds. In water, the removal efficiency was between 80% and 85% and for hydrophobic compounds such as toluene, the removal efficiency was reported to be 35%. Compared to the results of this study, toluene was on average 17% lower(19).
Due to the fact that the use of organic phase improves the performance of two-phase bioscrubber. Few studies have used cutting oil as an organic phase. One of the most important characteristics of cutting oil that can be used on an industrial scale is its low cost and availability. In the first part of this study, the results showed that cutting oil in concentrations above 10% could reduce the microbial population, which can reduce biodegradation in two-phase systems. The results of bioscrubber performance without the presence of organic phase showed that the efficiency and removal capacity of toluene were very low, which could be due to the hydrophobicity of the toluene compound. Cutting oil was used to increase toluene absorption in the bioreactor. Cutting increases the removal efficiency from 22% without the use of organic phase to 55% in the presence of 5% cutting oil. While Lalanne et al. obtained the removal efficiency of aromatic compounds in the bioscrubber, the results showed that the removal efficiency when using cutting oil increased from 12–36%(28). The removal efficiency in the absence of organic phase and using the organic phase was lower than that of this study, which may be due to the fact that the L/G ratio in this study was higher than the study of Lalanne et al. on of chlorinated compounds on microorganisms. In the study of Kan et al.’s examination of bio-removal of toluene and trichloroethylene in bioreactor emulsion, the results showed that the optimal concentration of oocyte alcohol as a phase was 3–5% and the increase of the organic phase was more. This amount has no effect on increasing the efficiency and removal capacity(29).