Biofiltration of Volatile Organic Compounds (VOCs) – An Overview

Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 1(8), 83-92, Nov. (2011) Res.J.Chem.Sci. International Science Congress Association 83 Review Paper Biofiltration of Volatile Organic Compounds (VOCs) – An Overview Thakur Prabhat Kumar1, Rahul1, Mathur Anil Kumar2 and Balomajumder Chandrajit1 1Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee -247667, INDIA 2Uttar Pradesh Pollution Control Board, Agra, INDIA Available online at: www.isca.in (Received 20th August 2011, revised 01st September 2011, accepted 17th September 2011) Abstract Volatile organic compounds excreted to the environment are highly susceptible to ecological and health hazards. Many conventional methods have been developed for the waste air treatment in the recent past but biological waste air treatment processes have acquired high approval due to its cost effectiveness and environment friendly technologies. This review presents an overview of biofiltration technologies for the control of VOCs and odours, functioning mechanism and its operational parameters. Key words: VOCs, Gas biofiltration, Biofilter, Biodegradation. Introduction Over the past few decades enormous quantities of industrial pollutants have been released into the environment. Due to high releases of wide variety of pollutants there has been increase in number of environment related problems1. These xenobiotic compounds are usually removed slowly and tend to accumulate in the environment. Due to the high degree of toxicity, their accumulation can cause severe environmental problems2. With increasing public concern about deteriorating environment air quality, stringent regulations are being enforced to control air pollutants. In spite of the fact there are numerous technologies for control of volatile organic compounds (VOCs) emission, all are not applicable everywhere. Table 1 compares the various available VOC control technologies. All technologies have its own applicability depending upon the source, type and concentration of the VOC3. The conventional methods such as thermal incineration, adsorption, absorption, condensation and some recent techniques such as membrane separation, electronic coagulation are very effective at reducing emission of VOCs from various industrial operations4, 5, 6. But they generate undesirable byproducts7. These are energy intensive and may not be cost-effective for treating high flow air streams contaminated with low concentrations of pollutants. Biological treatment is an attractive alternative for low concentration gas streams because of its low energy consumption, relatively moderate operating costs and minimal by-products generation. The most successful removal in gas-phase bioreactors occurs for low molecular weight and highly soluble organic compounds with simple bond structures. Compounds with complex bond structures generally require more energy to degrade which is not always available to the microbes. Hence, little or no biodegradation of these types of compounds occurs, as microorganisms degrade those compounds that are readily available and easier to degrade. Organic compounds such as alcohols, aldehydes, ketones, and some simple aromatics demonstrate excellent biodegradability table-2. Some compounds that show moderate to slow degradation include phenols, chlorinated hydrocarbons, polyaromatic hydrocarbons, and highly halogenated hydrocarbons. Rate of biodegradation for inorganic compounds such as hydrogen sulphide and ammonia is also good. Certain anthropogenic compounds may not be biodegradable at all because microorganisms do not possess the necessary enzymes to break the bond structure of the compound effectively8, 9. In biodegradation, the contaminants are sorbed from a gas to an aqueous phase where microbial attack occurs10, 11, 12. Through oxidative and occasionally reductive reactions, the contaminants are converted to carbon dioxide, water vapour, and organic biomass13, 14. These air pollutants may be either organic or inorganic vapours and are used as energy and sometimes as a carbon source for maintenance and growth by the microorganism populations. In general, natural occurring microbes are used for biological treatment. These microbial populations may be dominated by one particular microbial species or may interact with numerous species to attack a particular type of contaminant synergistically. Microbes within these biological treatment systems are also engaged in many of the same ecological relationships that are typical to macro organisms. Such relationships are necessary to provide an important balance within the system. In this study, an attempt has been made to provide an overview of biofiltration technologies used for the control of VOCs and odours, functioning mechanism and its important operational parameters. Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 1(7), 83-92, Nov. (2011) Res.J.Chem.Sci International Science Congress Association 84 Table-1 Current technologies for air pollution control Methods (Conventional and upcoming) Technology involved Operational characteristics Advantages Limitations Gas flow (m3 h-1) Temperatu re °C VOC (gm-3) Adsorption Activated carbons, zeolites 5-50000 <55 < 10 Proven and efficient Adsorbant is too specific and can saturate fast; Risk of pollutant reemission Incineration Thermal oxidation >10000 371 2- 90 Efficient Not cost effective, incomplete mineralization and release of secondary pollutants. Catalytic oxidation Thermal catalysts (Pt, Al, ceramics) >10000 149 2-90 Efficient, conserves energy Catalyst deactivation and its disposal, formation of by- product Absorption Washing gas with contaminated water 100- 60000 Normal 8-50 Possible recovery of VOC Not suitable for low concentrations, generates wastewater Condensation Liquefaction by cooling or compression 100- 10000 Ambient >60 Possible recovery of VOC Further treatment is required, Applicable in high concentrations only Filtration Air passed through fibrous material coated with viscous materials 100- 10000 10-41 >60 Efficient for particle removal, compact and commonly used Unable to remove gases, fouling, particle reemission can occur due to microbial growth. Electrostatic precipitator with Ionization Electric field is generated to trap charged particles - - - Efficiently removes particles and are compact Generates hazardous by- products Ozonation Strong oxidizing agent - - - Removes fumes and gaseous pollutants Generates unhealthy ozone and degradation products. Photolysis UV radiations to oxidize air pollutants and kill pathogens - Normal - Removes fumes and gaseous pollutants Release of toxic photoproducts, UV exposure may be hazardous and energy consuming. Photo catalysis High energy UV radiation used along with a photocatalyst - - - Energy intensive popular method suitable for broad range of organic pollutants Exposure to UV radiation may be harmful Membrane separation Separation through semi permeable membranes 5-100 Ambient >50 Recommended for highly loaded streams Membrane fouling and high pressure is needed Enzymatic oxidation Use of enzymes for treatment of air pollutants - 35-55 - Promising Requirement of new enzymes periodically Phytoremediati on Use of plants and microbes for the removal of contaminants - - - Cost effective, pollution free and complete mineralization occurs Large as compared to other technologies Microbial abatement Air passed through a packed bed colonized by attached microbes as biotrickling filters or microbial cultures in bioscrubbers, 200-1500 - <5 Cost effective, more efficient, eco-friendly, Need for control of biological parameters Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 1(7), 83-92, Nov. (2011) Res.J.Chem.Sci International Science Congress Association 85 Table-2 Biodegradability of typical indoor VOCs Substance Biodegradability Henry’s law constants Hb (atm m3 mol-1) References Acetaldehyde (Ethanal;CH3CHO) Good 5.88 x10-5 7.69 x10−5 5.88 x10−5 Zhou and Mopper(1990) Sander (1999) US EPA (1982) Benzene (C6H6) Moderate 6.25 x10−3 5.55 x10−3 4.76 x10−3 Staudinger and oberts(1996) US EPA(1982) Sander (1999) Formaldehyde (Methanal; HCHO) Good 3.33 x10−7 3.23 x10−7 3.13 x10−7 Sander (1999) Zhou and Mopper(1990) Staudinger and Roberts(1996) Naphthalene (C10H8) Low 4.76 x10 −4 4.76 x10−4 Sander (1999) US EPA (1982) Tetrachlorethylene (Tetrachloroethene; C2Cl4) Low 2.78 x10−2 1.69 x10−2 1.56 x10−2 US EPA (1982) Staudinger and Roberts(1996) Sander (1999) Toluene (Methylbenzene; C6H5CH3) Moderate 6.67 x10−3 6.67 x10−3 US EPA (1982) Staudinger and Roberts(1996) Trichlorethylene (Trichloroethene; C2HCl3) Low 9.09 x10−3 1.12 x10−2 1.00 x10−2 Sander (1999) US EPA (1982) Staudinger and Roberts(1996) Table-3 Comparison of bioreactors for VOC and odour control Bioreactor Application Advantages Disadvantages Biofilter • Removal of odour and low VOCs concentrations • Target VOC concentration is less than 1 g m-3 • Low initial investment and subsequently operating cost is minimized • Degrades a wide range of components • Easy to operate and maintain • No unnecessary waste streams are produced • Low pressure drop • Less treatment efficiency at high concentrations of pollutants • Extremely large size of bioreactor challenges space constraints • Close control of operating conditions is required • Packing has a limited life • Clogging of the medium due to particulate medium Biotrickling filter • Low / medium VOC concentrations • Target VOC concentration is less than 0.5 g m-3 • Less operating and capital constraints • Less relation time / high volume through put • Capability to treat acid degradation product of VOCs • Accumulation of excess biomass in the filter bed • Requirement of design for fluctuating concentration • Complexity in construct and operation • Secondary waste stream Membrane bioreactor • Medium/High VOC concentrations • Target VOC concentration is less than 10 g m-3 • No moving parts • Process easy to scale up • Flow of gas and liquid can be varied independently, without the problems of flooding, loading, or foaming • High construction costs • Long-term operational stability (needs investigation) • Possible clogging of the liquid channels due the formation of excess biomass Bioscrubber • Low/medium VOC concentrations • Target VOC concentration less than 5 g m-3 • Able to deal with high flow rates and severe fluctuations • Operational stability and better control of operating parameters • Relatively low pressure drop • Relatively smaller space requirements • Treats only water soluble compounds • Can be complicated to operate and maintain • Extra air supply may be needed • Excess sludge will require to disposal • Generation of liquid waste Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 1(7), 83-92, Nov. (2011) Res.J.Chem.Sci International Science Congress Association 86 Material and Methods Increasing stringent environmental legislation is generating great interest in industry towards the biological waste air treatment technique15,16. All biological technologies rely on two primary fundamental mechanisms-sorption and biodegradation. The biodegradation is done by microorganisms, which are either supported on media or maintained in suspension. Supported microorganisms are immobilized on organic media or inorganic structures, while suspended microorganisms are maintained in a liquid phase such as activated sludge. In all instances, VOCs and odour are biodegraded by microorganisms into carbon dioxide and water. Organic compounds serve as the substrate or source of carbon and energy. These compounds provide the food supply, which allows the microorganism to function and multiply17. Biological waste air treatment technology makes use of several types of bioreactors. There are mainly four types of related biological treatment units: biofilter, biotrickling filter, membrane bioreactor and bioscrubber. A comparison of bioreactors for removal of VOCs and odour has been done table-3. These systems have differences in their complexity, process design, equipment dimensions and working parameters, but all of these operated based on the same principle of biological removal18, 19, 20. Results and Discussion Biofilters (BFs) are reactor in which polluted air stream is passed through a porous packed bed on which a mixed culture of pollutant-degrading organisms is immobilized. Biofiltration uses microorganisms fixed to a porous medium to break down pollutants present in an air stream. The microorganisms grow in a biofilm on the surface of a medium or are suspended in the water phase surrounding the medium particles. The filter-bed medium consists of relatively inert substances like compost, peat, etc. which ensure large surface attachment areas and additional nutrient supply. As air passes through the bed, the contaminants in the air phase sorb into the bio film and onto the filter medium. The contaminants are biodegraded on biofilm21. Biofilters usually incorporate some form of water addition for control of moisture content and addition of nutrients. In general, the gas stream is humidified before entering the bio filter reactor. The overall effectiveness of a biofilter is largely governed by the properties and characteristics of the support medium, which include porosity, degree of compaction, water retention capabilities, and the ability to host microbial populations. Critical biofilter operational and performance parameters include the microbial inoculums, pH, temperature, moisture and nutrient content. Biofiltration is a general term applied to the biodegradation of chemical compounds in gas phase to the carbon dioxide, water and inorganic salts. Biofiltration is the oldest and the simplest method of the four biological technologies for the removal of contaminated components from waste gases19, 20, 22 . A typical biofilter configuration is shown in figure-1. The contaminated off-gas is passed through a preconditioner for particulate removal and humidification (if necessary). The conditioned gas stream is then passed from the bottom of a filter bed of soil, peat, composted organic material (such as wood or lawn waste), activated carbon, ceramic or plastic packing or other inert or semi-inert media. The media provides a surface for microorganism’s attachment and growth. The bed and air stream are kept moist to encourage microbial activity. Humidification is generally the most influential parameter affecting the sorptive capacity of a biofilter, especially at lower inlet concentration, where Henry’s Law controls mass-transfer rates within the biofilter. Nutrient could be mixed with the packing material either before biofilter installation or after construction18, 19, 20, 22. Figure-1 Schematic diagram of a biofilter unit Blower Discontinuous Water Addition Waste Air Leachate Waste Air Particulate, Temp. and Load Control Nutrients, Buffer (Discontinuous) Water Influent Humidifier Clean Air Biofilter Reactor Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 1(7), 83-92, Nov. (2011) Res.J.Chem.Sci International Science Congress Association 87 Biofilter Operations: The operations of biofilters involve a series of steps beginning with the transfer of the pollutant air to the aqueous phase. Transfer of pollutant from air to aqueous phase. Adsorption onto the medium or absorption into the biofilm Biodegradation of VOCs withn the biofilm The most important physical, chemical and biological parameters affecting the biofiltration process are described below: Biofilm: In the biofiltration system, the pollutants are removed due to the biological degradation rather than physical straining as in the case of normal filters. Biofilm is a group of microorganisms (aerobic, anaerobic, and facultative type bacteria, fungi, algae and protozoa) which attach themselves on the surface of the packing media and forms a biological film or slim layer of a viscous, jelly like structure23. The development of biofilm may take few days or months depending on the microorganisms’ concentration. There are three main biological processes that occur in the biofiltration systems - Attachment of microorganisms, Growth of microorganisms and Decay and detachment of microorganisms. Since the microorganisms are attached to the surface, the supply of organics or substrate (food) to the microorganisms in a biofilm is mainly controlled by the bulk and substrate transport phenomena. The substrate must be transported from the bulk fluid to the biofilm’s outer surface where it is metabolised after diffusion. The factors which influence the rate of substrate utilization within a biofilm are (i) substrate mass transport to the biofilm, (ii) diffusion of the substrate into the biofilm, and (iii) utilization kinetics of the biofilm. Biomass detachment is one of the most important mechanisms that can affect the maintenance of biomass in the biofilter24. Several forces (i.e. electrostatic interaction, covalent bond formation and hydrophobic interactions) are involved in microbial attachment to a surface. The strength of the attachment and the composition of forces are dependent on various environmental conditions viz gas flow rate, pollutant concentration, oxygen supply, nutrient availability, type of microbial species and their surface properties23, 25. Generally a rapid flow rate through the biofilter will hinder the growth of bacterial film resulting in thin film formation. Microorganisms form thinner layers upon smooth surfaces in comparison to those upon porous materials and each treatment system has a typical biofilm thickness. The biofilm thickness usually varies from 10 micro meters to 10,000 micro meters, although an average of 1,000 micro meters or less is usually observed. However, whole of the biofilm is not active. The activity increases with the thickness of the biofilm up to a level termed the ‘active thicknesses’. Above this level, diffusion of nutrients becomes a limiting factor, thus differentiating an ‘active’ biofilm from an ‘inactive’ biofilm25. Biofilter bed: Biofilter bed is the vital part of the biofiltration process as it provides the main support for microbial growth. A list of characteristics that are necessarily needed for an ideal biofilter reactor is established by Bohn. The most anticipated characteristics of the BF bed comprise: Optimum specific surface area for development of microbial biofilm and gas-biofilm mass transfer. High porosity to expedite homogenous distribution of gases. High-quality water retention capacity to preclude bed drying. Manifestation and availability of intrinsic nutrients and Presence of a dense and diverse indigenous microflora. The most habitually used materials in BF beds are peat, soil, compost and wood chips. These materials are generally abundant and economical as well. They satisfy most of the desirable criteria and has their own