Biofilter for the deodourization of industrial emissions
- A Gangagni Rao, Bharat Gandu, Kranti Kuruti, CSIR-IICT, Hyderabad
Odour is one of the major problems of industrial emissions and it could range from offensive to noxious. Biological methods are quite effective in addressing this problem compared to physico-chemical techniques as described in the article.
Gaseous emissions from various industries pose problem to human and environmental health. Stringent environmental legislations enforced by government agencies, have led polluting industries to adopt effective air pollution treatment processes to comply with these regulations. Industrial waste gases are traditionally being treated by physico-chemical methods like adsorption, scrubbing, condensation, etc. Biological waste gas treatment represents a new treatment alternative. The suitability and performance of biological methods for the treatment of a wide range of organic and inorganic compounds has been proven at pilot level and ac-cordingly their implementation and use at industrial scale is currently growing exponentially compared to physico-chemical technologies. Biological methods are the most cost-effective and sustainable technologies as the contaminants are degraded into innocuous or less contaminating products unlike in physico-chemical methods where the contaminant is simply transferred from one phase to another. This article reviews the biological methods of for the treatment of emissions causing noxious odour.
Dr A. Gangagni Rao is Chief Scientist at CSIRIndian Institute of Chemical Technology (IICT), Hyderabad. He has about 28 years of research experience in the field of biological waste management (anaerobic digestion) and biological gas purification. The technologies developed by him are commercially proven in the field and working successfully. He is retained as advisory consultant by reputed companies and he has won several prestigious awards. He has 50 research publications and 4 patents to his credit.
Dr Bharath Gandu has obtained his Doctoral degree under the guidance of Dr A Gangagni Rao. Presently carrying out his post-doctoral studies in Israel and expertise in the areas of biological gas purification, anaerobic digestion and bioelectrochemical cells.
Kranti Kuruti is pursuing Doctoral studies in Engineering sciences (AcSIR) under the guidance of Dr A Gangagni Rao. His exper-tise is in the areas of biogas, bioethanol, biological gas purification, and volatile fatty acid generation from various organic substrates.
Malodourous gases (volatile organic compounds and volatile inorganic compounds (VOC & VIC)) emitted from various industries pose problem to human and environmental health and this affects the image of company also. However, this problem has not received sufficient attention till recently. Gaseous emissions having volatile organic compounds (VOCs) and volatile inorganic compounds (VICs) that cause odour problem is encountered in various industrial sectors such as refineries, latex processing, pharmaceuticals sectors, tanneries, waste treatment plants, poultry farms, fish processing facilities etc. 1,2]. Therefore, gaseous emission control is very
[ much essential not only due to problems to public, but also from the VOCs and VICs removal point of view. Industrial waste gases have traditionally been treated by physicochemical techniques such as absorption, adsorption, condensation, thermal, catalytic incineration and membrane separation. Advanced oxidation processes are most popular among techniques. Biological waste gas treatments represent a new treatment alternative. Four major bioreactor designs are: biofilter, bio trickling filter, bio scrubber and membrane bioreactors. Amongst these biofilter, biotrickling and bioscrubber technologies have largely been accepted by industry, but membrane bioreactors are still in developmental stage.
The most common non-biological treatment technologies are absorption, adsorption, oxidation and thermal methods. These can be used as standalone processes or in combination with bioprocesses[ 3]. The physical methods involve transfer of waste gas from one phase to another phase such as transfer to a solid or liquid media. Following is the brief outline of the above processes.
Generally masking is done to control the bad odour. Masking involves addition of pleasant smell compounds to overcome undesirable odour 4]. This
[ method may be applicable where area of bad odour spreading is small and the concentration is low. In fact, masking of the odorous components is a temporary solution only even for small area. Hence, masking method is unsuitable for the purification of the waste gases emanating industrial sectors where the quantity is high.
Thermal or Catalytic incineration
Thermal incineration aided by catalysts is very fast, takes less than a second. It ensures 99% destruction of virtually all organic compounds 5]. Such systems are
[ designed to handle a capacity of 1000 to 500,000 cfm (cubic feet per minute) and VOCs concentration ranges from 100 to 2000 ppmV. But it consumes large quantity of fuel and is therefore an expensive process. Since the operating temperature is 710oC to 980oC incineration produces NOx which should be captured and treated before dispensing, thus adding to the expenditure. Halogenated compounds are converted to their acidic counterpart and it may necessitate the use of expensive corrosion resistant materials of construction and use of additional acid gas controls such as scrubbing as follow up treatment. In addition, there are concerns regarding the formation of dioxins when chloro organic contaminants are incinerated. Catalysts and heat recovery methods can reduce fuel costs but it needs greater capital and maintenance costs. The method of catalytic combustion can only be used with well-defined waste gases, since poisoning of catalyst is likely to take place by certain compounds.
It is one of the emerging purification techniques for both wastewater and waste gases due to its versatility and high effectiveness at low temperatures. Oxidation processes are either chemical or photo catalytic. This mechanism primarily depends on the characteristics of irradiation, photo catalyst and concentration of the oxidants 6]. Recent studies have shown greater removal
[ of VOCs by a combined O /TiO /UV process, as excess
3 2 ozone molecules could scavenge hydroxyl radicals produced from the excitation of TiO by UV radiation
8]. The reaction in several instances is quite fast and [7, removal efficiency often exceeds 90%. Chemical oxidation is ineffective for hydrocarbons 10].
Absorption or Scrubbing
Absorption or scrubbing is a diffusion mass transfer operation by which soluble gaseous pollutants are removed by direct dissolution in an absorbent liquid. Absorption or scrubbing is one of the most frequently used technologies for controlling the concentration of VOCs and VICs (odorous compounds) before they are discharged into the atmosphere 11]. It involves the
[ transfer of the pollutant from the gas phase to the liquid phase across the interface in response to a concentration gradient with the concentration decreasing in the direction of mass transfer. A key variable of this process is the selection of a suitable liquid absorbent.
Scrubbing with water: VOCs and VICs from air stream can be removed by scrubbing with water using sieve plate column, spray chamber etc. Counter cur-
rent operation is most common in packed scrubbers for waste gas purification. Treatment of contaminated water by biological or chemical methods before disposal is required that adds to the capital and operating cost of the integrated process. The limitation of the process is that it is applicable for waste gas containing water soluble compounds only 12]. [
Scrubbing with solvents: The VOCs and VICs from gas stream can be scrubbed with suitable solvents (Ex: Hydrogen peroxide, sodium hypochlorite, etc.) and the solvent can be regenerated appropriately. The costs involved in regeneration are expensive. The major drawback of this technology is the necessity to dissolve the gaseous pollutants in an aqueous phase. This is critical, as residence time of the gas phase in the absorption column is short. Scrubbing is therefore of interest for gaseous compounds with a Henry’s Constant (or) parti-tion coefficient of less than 0.01. This is of major importance since most of the target odours causing compounds are volatile and poorly soluble in most of the solvents and water 13]. [
In a typical membrane separator 10], the waste gas [ stream is fed to an array of membrane modules, where organic solvents preferentially permeate the membrane. The organics in the permeate stream are then condensed and removed as liquid for recycle or recovery. The purified gas stream is removed as the residue. Transport through the membrane is induced by maintaining the higher vapor pressure on the permeate (downstream) side of the membrane and lower vapor pressure on the feed (upstream) side. In some cases, a vacuum pump is required on the permeate side to maintain this driving force. A compound permeates the membrane at a rate determined by its permeability in the membrane material and partial pressure (driving force). In some systems, the feed stream is compressed on the feed side of the membrane to provide the pressure drop for the membrane and to allow operation of the solvent condenser at a higher temperature.
Biological methods play a very important role in the control of VOCs and VICs gases that are emitted by polluting industries. Although several different configurations exist, there are three basic types of biological reactor systems used to treat waste gases: biofilters, bio trickling filters and bioscrubbers 14]. These can be [ grouped into two types. In bioscrubbers micro-organisms are dispersed freely throughout the liquid phase and in biotrickling filters, biofilters and membrane bio- reactors microorganisms are immobilized or attached on a packing/ carrier material/membrane. In bioscrubbers and biotrickling filters the water phase is continuously moving, whereas in biofilters it is stationary. A bioscrubber consists of a scrubber unit and a regeneration unit. In the scrubber (absorption column), water soluble gaseous pollutants are absorbed and partially oxidized in the liquid phase (the culture medium containing the microorganisms), which is distributed from the top of the unit 13]. The contaminated water is sub
[12- sequently transferred into an aerated stirred tank reactor (regeneration unit), like an activated sludge unit, where the contaminants are fully biodegraded. The regenerated suspension is continuously re-circulated to the top of the scrubber section, thereby enhancing efficiency. The polluted air flows through a biologically active bed, where micro-organisms are attached in the form of a biofilm. As the gas diffuses through the packed bed, the pollut-ants are transferred to the biolayer and degraded. To ensure optimal operation of biofilters, the inlet gas usually requires pre-treatment process such as particular removal in order to prevent possible clogging and sludge build up, load equalization in case the waste gas concentration is subject to strong fluctuations, temperature control and humidification.
In biological trickling filters the packed beds consist only of inert materials (glass, ceramics, and plastics) while the liquid phase, containing inorganic nutrients, flows with the contami-nated gaseous stream and is continuously re circulated through the bioreactor. Bioscrubbers and biotrickling filters are applicable mainly to the treatment of waste gases containing good or moderately water-soluble compounds, whereas biofilters, due to the large surface area available for mass transfer, are also suited to treat poorly water soluble compounds. Moreover, due to their high reaction selectivity, biofilters are particularly suitable for treating large volumes of air containing easily degradable pollutants with relatively low concentrations, typically 1,000 ppm. Compared with the other biological systems, biofilters have the widest application because they are easy to operate, simply structured, and imply low installation and operating/maintenance costs. Also, the reliability of biofilter operation is higher than that of bioscrubbers, where the risk exists of washing away the active microorganisms. Moreover, the presence of a large amount of packing material with a buffering capacity diminishes the sensitivity of biofilters to different kinds of fluctuations. Because the major disadvantage is the difficult control of parameters like
pH, temperature and nutrient supply, biofilters may be unsuitable for degrading halogenated compounds (as acid metabolites are produced) and treating gas streams containing high concentrations of VOC’s, unless long residence times or large bed volumes are applied 14]. Biotrickling filters and Biofilters are currently
[ utilized mainly in compost production plants, sewage treatment plant, and agriculture, whereas biofil-ters and bioscrubbers are preferred in industrial applications. The comparative details are shown Table 1. Biofilter
The odour control using biofilter technology is rapidly gaining popularity around the world. The increased use of this technology is a result of new levels of understanding and the cost advantages of the technology over the life of the equipment. Biofiltration is now regarded as a mature technology rather than a new process. Biofiltration is a relatively new odour (VOCs and VICs) control technology. It was first used for the treatment of off gases from wastewater of chemical manufacturing facilities, solid waste processing plants, composting operations etc. The schematic flow diagram of biofilter is shown in Fig. 1.
In the biofilter, the volatile organic or odour laden gases are passed through a biologically active porous media. The decomposition of the pollutants is carried out by microorganisms growing on the solid carrier, which forms the porous media. Soluble compounds in the gas stream partition into a liquid film (biolayer) surrounding the media. The compounds in the liquid film are available for biodegradation by a resident microbial population. The microbial population mobilizes the hydrocarbons mainly to CO and H O.
2 2 Compounds shown to be degraded in a biofilter includes benzene, toluene, hydrogen sulfide, carbon
 disulfide, mercaptans, dimethyl sulfide, dimethyl disulfide, ammonia, methanol, ethanol, propanol, butanol, aldehydes, butyraldehyde, pyridines acetone, styrene, xylene, methylene chloride, di and tri chloromethane, tri and tetra chloroethene, nitrogen oxides, isopentenyl, gasoline derived VOC’s, triethylamine, etc. 18]. Biofiltration in its simplest form involves the
[15- passing of air through a biologically active filter material to be cleaned through biological oxidation processes. The filter material used may have virtually any composition as long as it supports biological activity.
The biological processes in a biofilter system take place in the water component of the filter material 19].
[ All activity occurs in the biolayer or biofilm surrounding the inert support. The heart of the process is the biolayer. The biolayer is the biologically active water layer that exists within the matrix of the filter material.
As the odorous compounds pass through the filter material they are absorbed into the biolayer. The microorganisms present in the system use these odor-
ous compounds as part of their food source for energy production and reproduction. The compounds taken up by the microorganisms are biologically degraded to CO2 and H2 O .
The biolayer has several roles in Biofiltration including:
• Supplying the aqueous environment for bacterial life.
• Supplying the nutrients for biological activity.
• Acting as the water/air interface for transport of the air components to be treated.
• Acting as the recipient of the by-products of reaction.
Different configurations of biofilters are being employed depending upon the application and performance requirements taking into consideration the techno economics. The details are shown Table 2.
Mechanism of Biofilter: Biofilter is a two-phase process consisting of:
1. The transfer of the compounds from the gas phase to water phase (Biofilm phase)
The speed of this process is dependent on the solu- bility and partial pressure of the com-pound and is best estimated using Henry’s law constant for the compound.
2. The oxidation of the absorbed compound by the bacterial species present in the filter.
The kinetics of this is based on the enzymatic capacity of the bacteria to use the compound as a food or energy source. A further complication is the ability of the biolayer to eliminate the byproducts of the reactions in order to prevent end product inhibition.
The principle is like conventional biofilm processes and is shown schematically in the Fig. 2. First, a constituent compound in the gas phase crosses the interface between gas flowing in the pore space and the aqueous film surrounding the solid matter. Then it diffuses to a consortium of acclimatized microorganisms. Finally, the microorganisms obtain energy from oxidation of the compound as a primary substrate or it is co-metabolized via nonspecific enzymes. Simultaneously, there is diffusion and uptake of nutrients such as nitrogen and phosphorous in available forms from the filter media and oxygen from the gas.
A properly designed and operated biofilter continuously maintains concentration gradient and driving diffusive transport in the biofilm 14,21]. The
[ volatile organic compounds present in the waste gas as well as oxygen, are partially dissolved in the liquid phase of the biolayer and are degraded or consumed by aerobic microbial activity. In this way a concentration gradient is created in the biolayer, which maintains a continuous mass flow of the component from the gas to the wet biolayer. The volatile metabolic products like CO diffuses to the
2 gas phase and are transported in the axial flow direction and leave the bed with the exit gas 21]. The
[ organic nutrients are necessary for microbial life. These nutrients are transported by diffusion from the filter media material to the microorganisms. Natural materials such as humus, compost, peat, wood chips, rice husk, coconut coir, pith and other related substances generally contain these nutrients in sufficient quantity. These ma-
terials also possess buffering capacity for neutralizing acidity or alkalinity formed by oxidation. The elementary nutrients are subjected to a recycling during the operation of the biofilter after the dying off the microbes. Mineralisation processes liberate these nutrients. As the efficiency of recycling is less than 100%, the media material will be eventually being exhausted must generally be renewed after several years of operation 21]. Due to the small size of the particles (few
[ mm) and the compounds to be transferred is generally water insoluble, the mass transfer resistance in the gas phase can generally be neglected. During the elimination of VOC, heterotrophic micro-organismsare predominant comparatively autotrophic microorganisms, most often being bacteria or fungi. The bed inoculation depends on both the nature of the filtering materials and the VOC biodegradability level. Many reviews have suggested taking advantage of the ecosystems indigenous to the beds 24]. After an acclimatization
[22- period, the most resistant populations are naturally selected and a microbial hierarchy is established in the bed. In many other cases (materials with low biomass density, recalcitrant VOC, reduction of acclimatization period), researchers inoculate the beds with consortia, extracted from sewage sludge, for example, or strains derived from either commercial sources or isolated from previously operated biofilters. Biofiltration is effective in removing hazardous compounds like acetaldehyde, butadiene, cresols, ethylbenzene, formaldehyde, methanol, styrene with high biodegradability and acetonitrile, benzene, carbon disulphide, hexane, methylene chloride, methyl ethyl ketone, phenol, toluene, xylene with medium biodegradability.
The advantages of biofiltration are that it is very cost effective and efficient method to eliminate odorous contaminants and other VOCs, which are present in low concentration in the waste gas stream. This method offers complete destruction of contaminants rather than transferring them to another media. This method can be used for both organic as well as inorganic compounds 3].
Types of filter material: The filter matrix of a biofilter has been constructed from many materials over the past century 26]. Examples of media include the
[25, following: soil mixtures, compost, bark, coconut coir pith, peat, carbons and mixtures of the above. All of these types of media have been successful to some extent. The biofilter bed material improves in terms of the biofilm integrity and surface area, then the biofilter efficiency increases and accordingly size of the biofilter decreases for similar applications 27]. An effective
[ biofilter medium should have the following characteristics: high specific surface area for development of a microbial biofilm and gas-biofilm mass transfer, high porosity to facilitate homogeneous distribution of gases, a good water retention capacity to avoid bed drying, presence and availability of intrinsic nutrients, and presence of a dense and diverse indigenous microflora.
Biological trickling filters (BTFs) combine pollutant absorption and biodegradation in the same reactor. Pollutant degrading bacteria are naturally immobilized on a packing material which is either a random packing or a three-dimensional structure. In biotrickling filter, the gas is carried through a packed bed, which is continuously irrigated with an aqueous solution containing essential nutrients required by the biological system. Several studies have shown that the choice of a co or counter current configuration for liquid and gaseous phases does not influence the biodeg-radation performance 28]. Microorganisms grow on the packing
[ material as biofilm. The pollutant to be treated is initially absorbed by the aqueous film that surrounds the bi film, and then the biodegradation takes place within the biofilm. The filtering material used in a biotrickling filter has to facilitate the gas and liquid flows through the bed, favour the development of the micro flora, and should resist crushing and compaction. Biotrickling filter packing that best meet these specifications are made from inert materials such as resins, ceramics, polyurethane foam etc. As they are made from inert or synthetic material, biotrickling filters need to be inoculated with suitable microbial culture 29]. The use of ac
tivated sludge as initial microbial inoculums has been extensively reported. The schematic flow diagram of biotrickling filter is shown in Fig.3.
In biotrickling filters, the contact between the microorganisms and the pollutants occurs after the VOC diffusion in the liquid film, the liquid flow rate and the recycling rate are recognized to be critical parameters for BTF operation. Studies are revealed that an increase in the liquid flow rate should result in proportional increase in the active exchange surface for gas liquid mass transfer, and then improve the degradation rate 30]. Some researchers have shown that maintaining [ minimum water and nutrient supply is sufficient to achieve good performance 32]. In addition, as the dis
[31, tribution and the recycling of nutrient solutions add to energy costs, other studies suggest that the optimum recycling and distribution flow rates have to be found experimentally and on a case-by-case basis 33]. BTFs
[ find wide application in VOC and odour treatment. As compared to conventional compost or soil bed biofilters which are generally limited to the elimination of odorous compounds and no chlorinated volatile organic compounds, a wider range of pollutants can potentially be treated in BTFs. This is because, environmental conditions can be better controlled in the BTFs and potentially toxic dead-end metabolites can be purged out of the system. The major drawback of biotrickling filters is the accumulation of excess bio- mass in the filter bed. Some reviews have demonstrated that, in the course of the process, the biofilm thickness can achieve several millimetres
35], which can cause problems that lead to per[34, formance loss 30]: pressure drop increases, bed
[ channelling, and the creation of anaerobic zones. Accumulation re-moved by the back washings with water are the most efficient and certainly the least drastic for the ecosystem . Nevertheless, the biotrickling filter technology is still employed to a lesser extent than biofiltration, which is certainly related to its more consequential operating costs and to the VOC solubility restrictions. Bioscrubber
Bioscrubbers are reactors in which the gaseous pollutants are first absorbed in a free liquid phase prior to biodegradation by either suspended or immobilized microorganisms. The microbial process occurs either in the absorber or in a separate bioreactor after absorption of the pollutants 13]. Bioscrubbing consists of the absorp
[ tion of a pollutant in an aqueous phase, which is then treated biologically in a second stage in a liquid phase bioreactor. The effluent treated in the liquid phase reactor is recalculated to the absorption column. This technology allows for good gas cleaning when the gaseous pollutants are highly water-soluble. If the absorption solution is water then one can say that it is a biological process, but all the compounds of waste air or gas are not soluble in water 23]. Only
[ some compounds in waste gas are soluble in water and some other are partly soluble. Different type of absorption solutions are to be used in these systems. At this stage, if the absorption solution used for scrubbing is other than water, then the process may be called as biochemical method. It is a combination of both chemical and biological methods. Absorption is one of the most frequently used techniques for controlling the concentrations of gaseous pollutants before they are discharged into the atmosphere. It involves the transfer of the pollutant from the gas phase to the liquid phase across the interface in response to a concentration gradient; with concentration decreasing in the direction of mass transfer 12]. The schematic flow diagram of bio
[ scrubber is shown in Fig.4.
Bioscrubbers being operated presently use activated sludge derived from wastewater treatment plants as in oculums[ 37,38]. In some cases, bioreactors are directly inoculated with specific degrading strains. The residence time for such bioreactors range between 20 and 40 days and these are operated practically as ac-
tivated sludge processes including recycle of sludge. Part of the treated solution is recycled for absorption of VOCs to the absorption unit. Substantial modifications in bioscrubber design have been done in the recent past to enhance their performance for VOC and odour treatment. Some modified bioscrubbers are sorptive slurry bioscrubber, Anoxic bioscrubber, Two-liquid phase bioscrubber, Airlift bioscrubber and Spray column bioscrubbers. Membrane bioreactors
Membrane bioreactors were designed as alternative to conventional bioreactors for waste gas treatment. The membrane bioreactor allows the selective permeation of the pollutant, which is not allowed in any of the reactors discussed previously. The concentration difference between the gas phase and the biofilm phase provides the driving force for diffusion across the membrane. The driving force depends strongly on the air water partition coefficient of the diffusing volatile component. For components with a high partition coefficient the driving force for mass transfer is small 39]. The schematic flow diagram of membrane bioreac
[ tor is shown in Fig. 5.
In a membrane bioreactor, the membrane serves as the interface between the gas phase and the liquid phase (Fig. 5)[ 39]. The gas–liquid interface thus created (e.g. in hollow fibre reactors) is larger than in other types of gas–liquid contactors 40]. Two types of mem
[ brane materials have been used to prevent mixing of the gas and liquid phases and simultaneous transfer of volatile components. These types are hydrophobic micro porous membrane and dense membrane. All studies carried out on membrane reactors are laboratory scale experiments. To the best of our knowledge, no reports are available on pilot plant investigations or fullscale applications of membrane reactors in biological waste gas treatment. Membrane modules appear relatively easy to scale up given their modular nature 41];
[ however, an extensive long-term performance testing is necessary before they can be applied on full scale.
Biological purification of industrial gaseous emissions
As already mentioned, odour elimination was the initial aim of waste gas treatment. Previously, biological processes for the removal of malodorous compounds were widely used in only a few developed countries, but due to its advantages, recently, it is spreading to developing countries also. The most extensively studied compounds are sulphur and nitrogen containing compounds. The removal of odours from waste water treatment plants was first installed in 1923 and the ear-liest patent was probably obtained in 1934 42]. It was reported that elimination capacities
[ vary from few grams to more than 200 g/m3/h for several VICs with removal efficiencies often above 90%[ 43]. VOC emissions comprise a wider range of possible contaminating compounds than the VICs. Many VOCs are released from industrial activities as well as from the treatment of solid or liquid wastes and in soil remediation also. VOCs include halogenated and nonhalogenated aliphatic and aromatic pollutants. The
Fig. 2: Pollutant Penetration and Degradation mechanism in Bio filter Ci — Initial Concentration of Pollutant Co — Outlet Concentration of Pollutant