Comparison of the removal of ethanethiol in twin-biotrickling filters inoculated with strain RG-1 and B350 mixed microorganisms

https://doi.org/10.1016/j.jhazmat.2010.07.035Get rights and content

Abstract

This study aims to compare the biological degradation performance of ethanethiol using strain RG-1 and B350 commercial mixed microorganisms, which were inoculated and immobilized on ceramic particles in twin-biotrickling filter columns. The parameters affecting the removal efficiency, such as empty bed residence time (EBRT) and inlet concentration, were investigated in detail. When EBRT ranged from 332 to 66 s at a fixed inlet concentration of 1.05 mg L−1, the total removal efficiencies for RG-1 and B350 both decreased from 100% to 70.90% and 47.20%, respectively. The maximum elimination capacities for RG-1 and B350 were 38.36 (removal efficiency = 89.20%) and 25.82 g m−3 h−1 (removal efficiency = 57.10%), respectively, at an EBRT of 83 s. The variation of the inlet concentration at a fixed EBRT of 110 s did not change the removal efficiencies which remained at 100% for RG-1 and B350 at concentrations of less than 1.05 and 0.64 mg L−1, respectively. The maximum elimination capacities were 39.93 (removal efficiency = 60.30%) and 30.34 g m−3 h−1 (removal efficiency = 46.20%) for RG-1 and B350, respectively, at an inlet concentration of 2.03 mg L−1. Sulfate was the main metabolic product of sulfur in ethanethiol. Based the results, strain RG-1 would be a better choice than strain B350 for the biodegradation of ethanethiol.

Introduction

Ethanethiol is a toxic organic pollutant. It is a colorless liquid with a low odor threshold of 0.7 μg L−1 and has a flammable vapor and a gas density that is heavier than air. It can emit noxious odors into the air and lead to chronic harmful effects on the kidneys, heart, lungs, and the nervous system of human beings. Thus, the maximum allowable concentration for ethanethiol is strictly regulated and should not exceed 10.0 mg L−1 according to the Occupational Safety & Health Administration [1]. Ethanethiol can be emitted both naturally and anthropologically [2], [3]. The industrial applications of ethanethiol in isethionate and phorate pesticides, as well as its product, can also lead to high local atmospheric concentrations during chemical reactions [1]. Therefore, the removal of odorous organic pollutants, such as ethanethiol, from various waste gases is important in the field of environmental engineering.

Various technologies have been developed to purify waste gas containing volatile organic compounds (VOCs) and volatile organic sulfur compounds (VOSCs) [4], [5]. In most cases, conventional physical and chemical technologies are often unsatisfactory for the treatment of organic gases. The main reason is that organics may be transferred from the gas to other phases and still not be fully destroyed [5], [6]. Comparatively, biological treatment has been validated as a promising technology for the removal of VOCs as well as VOSCs from waste gases because of its low investment and operating costs as well as small energy requirements [7], [8], [9], [10], [11]. Additionally, with biological treatment under optimal conditions, the biodegradable contaminants can be converted to harmless end-products without the accumulation of intermediates or dead-end metabolites [12]. A number of VOSCs, such as methanethiol [13], [14], dimethyl disulfide [15], [16], diethyl disulfide [17], and dimethyl sulfide [18] have been found to be biologically degradable. However, ethanethiol cannot be effectively degraded in anaerobic conditions [19]. Even under aerobic conditions, only one paper been studied by using an aerobic biotrickling filter inoculated microorganism to purify the waste gas containing ethanethiol [20]. No report has been published concerning the biodegradation of ethanethiol as a sole target in biotrickling filters.

Among the biological waste gas treatment technologies, biotrickling filter has attracted considerable interests in the past few years [21] because of their superiority over other biological treatment technologies, such as biofilter and bioscrubber, in terms of mineralization efficiency, especially for highly concentrated acidifying pollutants in waste gas streams. These pollutants include sulfur-, chlorine-, or nitrogen-containing organic pollutants [22]. These pollutants are initially adsorbed onto the carrier material by a sorption process and subsequently degraded by biofilms immobilized on the surface of the carrier material. Thus, microorganisms, which are the catalysts for biodegradation of organics, are expected to be the most important factor of the bioreactor. For example, toluene-degrading bacterium Pseudomonas putida [23], P. putida (MTCC 102) [24], trichloroethene treating bacterium Dehalococcoides sp. [25] and monochlorobenzene-oxidizing microorganism Acinetobacter calcoaceticus [26], and so on have been isolated for VOCs treatment. However, compared with the microorganisms for treatment VOCs, the species isolated for biodegradation VOSCs are very little [5]. Further, most studies have focused on the selection of carrier materials and optimization of process parameters to improve the removal efficiency of VOSCs [27], [28]. Few studies have focused on the microbiological aspects, such as the selection and optimization of biological strains. The commercial mixed microorganism culture, B350, containing 28 species of microorganisms, cellulase, amylase, and hydrolase, has been proven to exhibit high removal capacities for treating phenol and oil-filled wastewater containing aromatic compounds [29], [30], [31]. In addition, a newly isolated Lysinibacillus sphaericus strain, RG-1, capable of utilizing ethanethiol as the sole carbon and energy source was isolated from activated sludge in our laboratory [32], yet no comparative study of the two biological strains has been reported.

In this study, single, newly identified strain RG-1 and B350 were compared as biological strains for twin-biotrickling filters in the removal of ethanethiol from a synthetic waste gas. The influence of inlet concentration and empty bed residence time (EBRT) on removal efficiencies and elimination capacities of ethanethiol were also studied in detail. The pressure drop (Δp) across the biotrickling filter and the pH values of the re-circulating liquid were also investigated to clarify the superiority of the biotrickling filter.

Section snippets

Microorganisms and culture medium

The mixed microorganism culture, B350, was purchased from Bio-System Co. USA. The strain RG-1 was isolated from activated sludge, which has been identified as Lysinibacillus sphaericus [32]. Ethanethiol (99+%, Acros, Belgium) was selected as the representative odorous organic pollutant in the synthetic waste gas. All other reagents were of analytical grade and obtained from Guangzhou Chemical Reagent Co., Inc. China. The microorganisms were grown in mineral salt medium with ethanethiol as the

Bioreactor start-up

During the start-up period, about 0.10 mg L−1 gaseous ethanethiol was first introduced to the columns of the biotrickling filter. The inlet concentration was increased gradually to 0.64 mg L−1 at a fixed EBRT of 332 s. The performance of the RG-1 and B350 from 1 to 38 days is shown in Fig. 1a and b, respectively. Analysis of the removal efficiencies during the whole start-up period demonstrated that the removal efficiencies were very low during the first 9 days. However, 100% of the removal

Conclusion

This paper focused on a comparison of the performance of biotrickling filters inoculated with RG-1 and B350 for the removal of ethanethiol vapors. Results showed that removal efficiencies and elimination capacities strongly depended on the inlet concentration and EBRT. The RG-1 strain showed better performance than B350. When the EBRT ranged from 332 to 66 s at a fixed inlet concentration of 1.05 mg L−1, the removal efficiency of ethanethiol decreased from 100% to 70.90% for RG-1 and 47.20% for

Acknowledgements

This is contribution No. IS-1224 from GIGCAS. This work was financially supported by the Science and Technology Project of Guangdong Province, China (2007A032301002, 2009B030400001, 2009A030902003 and 2009B091300023).

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