STUDIES ON THE BIODEGRADATION OF VOLATILE ORGANIC COMPOUNDS IN BIOFILTERS

Abstract

Enormous quantities of volatile organic compounds (VOCs) as pollutants are being released into the environment by various industries. VOCs are a major group of pollutants, which have now become a cause of concern worldwide. Excessive release of toxic VOCs into the environment due to the industrialization has created a great global concern. VOCs emissions originate from a wide variety of sources such as motor vehicle exhaust, motor vehicle fuel evaporative losses, industrial activities, petroleum refining, petroleum storage and dispensing facilities, surface coating and solvent use, domestic wood heaters, biomass burning, environmental tobacco smoke, use of solvents and glues and cleaners in arts and crafts. Dueto their natural abundance and by virtue of their broad range of application in paint solvents and aromatic compounds in all spheres of life in various chemical forms such as ingredients of several compounds in the form of solvent, volatile organic compounds are regarded as an integral part of the modern society. Some aromatic compounds such as benzene, toluene, ethyl benzene, o-xylene (BTEX) and pyridine are important industrial solvents that are frequently encountered in industrial operations and contaminated sites. Methyl ethyl ketone (MEK), toluene, w-butyl acetate and o-xylene (MTBX) are most frequently used in manufacturing of paint and are the potential pollutants. Many BTEX, pyridine and MTBX vapours are emitted into the atmosphere during its manufacture, transportation, use and disposal every year. These are volatile hydrocarbons with toxic properties. Exposure to BTEX, pyridine and MTBX can cause neurological, respiratory, genetic and excretory system damages. These compounds are also mildly toxic by inhalation; its vapours are severe skin and eye irritant and exposure to it can cause depression, gastrointestinal upset, liver and kidney damage, headache, nervousness, dizziness, insomnia, nausea, anorexia, frequent urination, and dermatitis. Exposure to BTEX, pyridine and paint solvent mixture laden air emissions may have 11 severe health implications. Therefore, before the discharge of these compounds into the environment, an appropriate technique should be implemented for their control. There are number of removal technologies available including biological methods to treat VOC polluted air stream. Among the biological waste gas treatment methods, biofiltration has attracted considerable interest in the last few years. It is cheaper, costeffective and very efficient removal process without generating any secondary air pollutants. It is used to eliminate contaminants from air using microorganisms, which are immobilized on the surface of solid support media. This technique has been applied successfully to control a number of air pollutants such as odours, VOCs and hazardous substances. The granulated activated carbon (GAC), pall ring and polyurethane foam are regarded as the effective packing materials used for better control in loading rate. However, their high cost and not easy availability inhibit their usage. Therefore, in the recent years, the search for agro-industrial residues and cheaper packing materials like sugar cane bagasse, coal, corn-cob, stalks (yellow-gram), wood chips, leaves, saw dust, etc., or mixture of these have attracted the attention of several researchers due to their easy and plenty availability for the removal of various pollutants from industrial effluents. Sugar cane bagasse has been found to be a very potent and effective packing material in the removal of BTEX (Mathur et al., 2007), BTX (Kapse et al., 2004), benzene (Zilli et al., 2004; Sene et al., 2002) and ethanol (Christen et al, 2002) from contaminated air streams. Coal has also been used as a packing material for the biodegradation of monochlorobenzene (Mathur et al., 2006), MTBX (Mathur et al., 2007), BTEX (Lu et al., 1999; 2002), mixture of ethyl acetate (EA), toluene (T) and xylene (X) (Lu et al., 2001) from contaminated air stream. Although, many researchers have reviewed the principles of biological waste air treatment and the advantages over chemical and physical techniques, research is still going on in the biological treatment of waste air for the use of effective new packing media, designs, microbial structure analysis and modelling of VOC removal. Furthermore, attempts to increase the degradation rate of VOCs are being made using genetically modified bacteria of a single pure organism. in Most studies on biofiltration, to date, found inthe open literature on the treatment of asingle pollutant to minimize the complexity of the system and to elucidate the effect of basic operating parameters on performance is currently going on. However, when more than one compound is present in a biofilter system, the response ofthe system is more complicated due to possible microbial and substrate interactions. Although, studies on BTEX biodegradation in biofilter have been conducted over the last 10 years, results of various studies indicate that the substrate interaction between BTEX compounds often vary with microbial culture and culture conditions. In the recent years, there has been increasing trend towards more efficient utilization of agro-industrial residues, including sugar cane bagasse as packing material. Further, literature survey reveals that only few researches have been conducted to investigate biofilter by using sugar cane bagasse as a packing material for treating single or mixed of these four compounds. No work has been found in the open literature on multicomponent biodegradation ofBTEX bearing waste gas by using low-cost sugar cane bagasse as packing material. The literature survey reveals that only a few researchers have investigated the treatment ofpaint VOCs vapour bearing waste air streams in the biofilters and biotrickling filters. Particularly, studies on the combine biofiltration of MTBX in a coal based biotrickling filter are relatively scarce in the literature. To the best of our knowledge to date no work has been reported regarding isolation and identification of most dominant bacteria from biofilter and biotrickling filter treating BTEX and MTBX and on bacterial DNA genome extraction from BTEX and on MTBX degrading isolated selected strains. Here, segments of the 16S rRNA genes are amplified via PCR by using bacterial universal primers. In addition, no work has been reported on the biodegradation of pyridine in aqueous medium with S. putrefaciens and B. sphaericus and the biofiltration of pyridine laden air stream in a corn-cob based biotrickling filter inoculated with S. putrefaciens. Biofiltration of VOC using low-cost corn cob as packing material is rare in the literature. Thus, there is a need to undertake studies to bridge the gap in the above areas which may enhance our understanding of biofiltration of BTEX, MTBX and pyridine compounds from the waste air using biofilter and biotrickling filter technology. iv In the first phase of present work, a mixture of VOCs (BTEX) was degraded in a packed bed biofilter (bioreactor I) with various flow rates and BTEX concentrations by using mixed culture to assess the feasibility of the bioreactor under controlled conditions. Performance of the biofilter was also evaluated by the degradation of VOCs during transient periods at the elevated contaminant loads. In the second phase of work, a mixture of paint VOCs (MTBX) was degraded in a biotrickling filter (bioreactor II) using mixed microbial culture. Mixture of various low cost materials were tested in both the bioreactor as the packing materials. In the third phase of the work, a single VOC (pyridine) from pyridine laden air stream was degraded in the corn-cob packed biotrickling filter (bioreactor III) by using a pure strain, isolated from the bioreactors which were operated for a long time. Both steady state loading conditions and uncontrolled transient loading conditions were assessed for all the bioreactors. Treatment efficiencies were evaluated for various operational and environmental parameters. Isolation and identification of the most superior VOCs degrading bacterium from biofilter and biotrickling filter were done in order to prepare a microbial blend by using taxonomical, biochemical and 16S rRNA gene analysis. Once a highly active strain is obtained, it would be applicable in processes of VOCs degradation on an industrial scale. A single component VOC (pyridine) was degraded in a batch reactor by two new types of isolated strains namely S. putrefaciens and B. sphaericus under aerobic condition to find out the best suited strain among S. putrefaciensand B. sphaericus. Bioreactor I, a lab-scale biofilter, was packed with a mixture of compost, sugar cane bagasse and granulated activated carbon (GAC) in the ratio of 55:30:15 by weight and was used for the biofiltration of air stream containing mixture of BTEX. Microbial acclimation was achieved in 30 days by exposing the system to average BTEX inlet concentration of 0.4194 g m"3 at an empty bed residence time (EBRT) of 2.3 min. Bioreactor-I achieved maximum removal efficiency of more than 99% for all four compounds throughout its operation at an EBRT of 2.3 min for an inlet concentration of 0.681 gm"3, which is quite significant than the values reported inthe literature. The results indicate that when the influent BTEX loadings were less than 68 g m" h" in the bioreactor-I, nearly 100% removal could be achieved. Amaximum elimination capacity (EC) of 83.65 gm"3 h"1 ofthe bioreactor-I was obtained at inlet BTEX load of 126.5 gm" h"1 in phase IV. Elimination capacities of BTEX increased with the increase in influent VOC loading, but an opposite trend was observed for the removal efficiency. The performance ofa coal based biotrickling filter (bioreactor II) for the removal of mixture of four compounds namely methyl ethyl ketone (MEK), toluene, n-butyl acetate and o-xylene (MTBX) typically emitted from the paint industry was studied over an experimental period of five months. The results obtained indicate that when the influent MTBX loadings were less than 120 g m"3 h"\ nearly 100% removal could be achieved. Amaximum EC of 184.86 g m"3 h"1 was obtained at a MTBX load of 278.27 g m"3 h"1 with an EBRT of 42.4 s in phase V. By using Wani's method of macrokinetic determination which is based on the simple Monod kinetics, the maximum removal rate rmax and saturation constant Ks ofMTBX were determined. In this study, the rmax values for MTBX were calculated as 0.085, 0.033, 0.16 and 0.024 g m"3 h"1, and Ks values were calculated as 1.785, 0.736,2.305 and 0.679 gm"3, respectively. Furthermore, an attempt was also made to isolate the most profusely grown BTEX degrading strain from bioreactor I. Agram-positive strain (AKM 01) had a high BTEXdegrading activity and was identified as Bacillus sphaericus by taxonomical analysis, biochemical tests and 16SrRNA gene analysis methods. The seven strains were also isolated from the bioreactor II. Among the seven isolates, an AKM 02 strain had a high MTBX degrading activity and was identified as Shewanella putrefaciens by taxonomical analysis, biochemical tests and 16S rRNA gene analysis methods. In addition, studies were also conducted to test pyridine reduction by two new isolates namely S. putrefaciens and B. sphaericus under aerobic condition and find out the best suited strain among S. putrefaciens and B. sphaericus. Out ofthese the two isolated strains, a S. putrefaciens had a high ability of pyridine-degradation and was studied in detail. Comparatively the S. putrefaciens degrades 500 mg L'1 of pyridine completely within 140 h, whereas the B. sphaericus degrades 500 mg L"1 of pyridine only upto 75% and takes long duration of150 h. S. putrefaciens used pyridine as sole carbon and energy VI source better than B. sphaericus. The maximum growth rate and the half saturation coefficient of S. putrefaciens was 0.033 h"1 and 90.74 mg L"1, respectively. For B. sphaericus, the maximum growth rate and the half saturation coefficient was 0.041 h" and 146.28 mg L"1, respectively. Inthe tested biotrickling filter (bioreactor III), S. putrefaciens exhibited that the maximum removal of pyridine is determined to be 100% at less than average inlet concentration of0.653 gm"3 and more than 93% at higher than average inlet concentration of 1.748 g m"3 with an EBRT of 106 s. But when the bioreactor III was operated at nearly same higher average inlet concentration of 1.752 gm"3 with an EBRT of 53 s, the removal was not attained greater than 85%. The EC of the bioreactor III was 102.34 gm"3 h"1 at inlet pyridine load of119.62 gm"3 h"1 with EBRT of53 s in phase VII. It is our view that the information contained herein in the corn-cob packed biotrickling filter inoculated by S. putrefaciens will be useful for the design of the bioreactor for the successful removal of gaseous pyridine. The Sugar cane bagasse, coal and corn-cob are available in abundance as a waste from various industries at almost no cost, except for the handling charges for the collection and transportation of the material. Therefore, the cost of these packing materials is insignificant than that of the activated carbon, pall ring and polyurethane foam. Onthe basis of above results, it is concluded that the biofilter packed with mixture of sugar cane bagasse, compost and GAC and a biotrickling filter packed with coal and activated sludge are the effective packing materials for the biodegradation of BTEX, MTBX and pyridine. After inoculation, microbial acclimatization needed approximately 30 days and the rapid attainment of more than 99.5% removal efficiency in an initial phase indicates that the inoculation procedure used in a biofilter and biotrickling filter can provide rapid startup and for the good growth of microbial population for the degradation of the mixture of BTEX, MTBX and pyridine. This result is quite significant to design the bioreactor for the successful removal of VOCs.