1 Introduction

Biological control of air pollution has many operational and cost advantages over the conventional physico-chemical methods in most of the chemical industries.Footnote 1 Biofiltration have been used for almost 100 years for waste treatment and especially in treating highly concentrated effluents (Metcalf and Eddy 2003). It is an air pollution control technology (APCT) frequently used for treating odour and volatile organic compounds (VOC’s) from waste air streams. It is a cost-effective approach to volatile organic compound (e.g. toluene) removal for large air flows (>1,000 m3 h−1 and mostly low concentrations <1,000 ppm) (Devinny et al. 1999). In biofiltration, polluted air is blown through a porous media, typically a mixture of compost, soil or wood chips that supports a population of microbes. Under optimum conditions, these microorganisms convert the absorbed biodegradable contaminants mostly into carbon dioxide, salt and water (Deshusses and Johnson 2000). Moreover, in biofiltration the microbial biomass is static/immobilized to the bedding material and the treated fluid is mobile/flows through the filter (Girard et al. 2009). The biological degradation process in a biofiltration can be written as follows,

$$ {\text{Organic}}\,{\text{Pollutant}} + {\text{O}}_{2} \xrightarrow{{{\text{microbes}}}}{\text{CO}}_{2} + {\text{salt}} + {\text{H}}_{2} {\text{O}} + {\text{heat}} + {\text{biomass}} $$
(1)

A suitable packing material should provide minimal pressure drop, minimal tendency for compaction, neutral pH, good water holding capacity, pore volume greater than 80 %, particle diameter of greater than 4 mm and total organic matter content of more than 55 % (Oh and Choi 2000). The parameters which are used to express the performance of the biofilters are pollutant loading capacity (L), elimination capacity (EC) and removal efficiency (RE). These are expressed in Eqs. (2)–(4) (Kennes and Veiga 2001),

$$ {\text{L}} = \frac{{{\text{C}}_{\text{g,in}} \times {\text{Q}}}}{\text{V}}({\text{g}}\,{\text{m}}^{ - 3} \,{\text{h}}^{ - 1} ) $$
(2)
$$ {\text{EC}} = \frac{{ ( {\text{C}}_{\text{g,in}} - {\text{C}}_{\text{g,out}} ) \times {\text{Q}}}}{\text{V}}({\text{g}}\,{\text{m}}^{ - 3} \,{\text{h}}^{ - 1} ) $$
(3)
$$ {\text{RE}} = \frac{{ ( {\text{C}}_{\text{g,in}} - {\text{C}}_{\text{g,out}} ) \times 1 0 0}}{{{\text{C}}_{\text{g,in}} }}(\% ) $$
(4)

2 History of biofiltration

Biofiltration is considered as one of the less energy utilizing technologies in treating the air pollutants. Though it has been employed widely in odour treatment and VOC removal for the past 100 years in industrial scale, it has been naturally occurring in soil for millions of years. Germans were the first to get a patent for this technology during 1941 (Leson and Winer 1991). Between the years 1960 and 1990, there was a huge development in the field of biofiltration. In 1963, biofilter was used effectively for treating odour from waste water treatment plants in California (Pomeroy 1982). During 1977, the first soil biofilter was designed for organic waste gas removal in Germany (Bohn and Bohn 1986). During 1987, it was discovered that odour removal through biofiltration was due to biodegradation and not by sorption. They also studied the removal efficiency (RE) of soil bed biofilter filled with different media (Carlson and Leiser 1966). Most of the biofiltration research was carried out in European countries until late 1980’s. After 1980’s many biofilters were installed in and huge number of research articles pertaining to biofiltration were published in journals and conference proceedings (Leson and Winer 1991). Figure 1 compares the different APCT technologies available so far in treating the industrial air pollutants.

Fig. 1
figure 1

Comparison of different APCTs (Devinny et al. 1999). J.S. Copyright 2012 Reproduced with permission of TAYLOR & FRANCIS GROUP LLC-BOOKS in the format Journal via Copyright Clearance Center (Order detail ID: 62054904)

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3 Biofiltration operating parameters

Although biofiltration is a simple process, it depends on many factors which are considered to be most critical in the operation of biofilters. They include temperature, pH, pressure drop, moisture content, bed porosity, packing materials, air flow rate, nutrient requirement, oxygen requirement, inlet pollutant concentration, maintenance, residence time, microorganisms and acclimation time. These are all the most important physical, chemical and biological parameters influencing the biofiltration process and are described in detail in the following sections. Figure 2 shows the operation of a typical biofilter used to treat a polluted air stream at an industrial scale.

Fig. 2
figure 2

Schematic view of a conventional below ground open biofiltration system (Devinny et al. 1999). J.S. Copyright 2012 Reproduced with permission of TAYLOR & FRANCIS GROUP LLC-BOOKS in the format Journal via Copyright Clearance Center (Order detail ID: 62054904)

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3.1 Temperature

Biofilters are normally operated at ambient temperature. Most researchers have reported no significant changes in the pollutant degradation rate with temperatures between 20 and 30 °C (Diks and Ottengraf 1991). However, cooling is mostly needed to avoid microbial death above 40 °C (Leson and Winer 1991) unless the microbes are thermophilic in nature. There are a few reports which suggests that changing the operating temperature would increase the removal efficiency considerablyFootnote 2 (Sorial et al. 1994). Since the biodegradation reaction which takes place in a biofiltration system is exothermic, the changes in the biofilter bed temperature are also a consequence of the microbial activity (Delhomenie and Heitz 2005). Moreover, it was proved that the quantity of energy released by the biological reaction can reach maximum of 50 kcal h−1, which means that the temperature gradients within the filter bed of the order of 2–4 °C and even may reach 10 °C sometimes for higher VOC inlet concentrations (Hwang et al. 2002). A study on toluene degradation rates at different operating temperatures showed that maximum toluene degradation rates were obtained at between 30 and 35 °C (Park et al. 2002). This optimum temperature was also recommended for the removal of BTEX (Lee et al. 2002).

3.2 pH

pH has a similar effect on the biofiltration than temperature. Beyond the optimum range of pH, microbial activity is severely affected in biofiltration as most of the microbes in biofilters are neutrophilic in nature (Delhomenie and Heitz 2005). The by-products of microbial degradation in a biofilter are mostly organic acids (e.g. acetic acid). Oxidation of halogenated organics and reduced sulphur compounds (e.g. H2S) may produce inorganic acid by-products. Moreover, pollutants that have heteroatoms are also converted into acid products, which tend to reduce pH (Christen et al. 2002). Accumulation of these acids may reduce the pH of the bed media below the active pH rangeFootnote 3 for the microbial degradation. A drop in pH may also lead to excess carbon dioxide and intermediate production (Ottengraf and Vandenoever 1983). In order to overcome this problem, buffering materials like calcium carbonate, limestone etc., are usually added to the bed (e.g. biofilters treating ammonia vapour). However, biofilters using acidophilic bacteria for degrading hydrogen sulphide may tolerate a lower pH (van Groenestijin and Hesselink 1994). A study carried out on pH during BTEX degradation showed that maximum degradation was observed at pH between 7.5 and 8.0. However, for alkylbenzene degradation, it was reported between 3.5 and 7.0 (Lee et al. 2002).

3.3 Pressure drop

Large pressure drop across the biofilter can result in air channelling in the bed. This will also increase the blower power requirement. Increase in the moisture and decrease in the bed pore size may also lead to an increase in pressure drop. Accumulation of biomass may also contribute to the increase in pressure drop (Farmer et al. 1995). Overall biofilter dimensions also influence the pressure drop in biofilter bed. Usually, the biofilter bed volume ranges between 10 and 3,000 m3 (Delhomenie and Heitz 2005). For a typical biofilter the pressure drop ranges between 1 and 10 hPa. Several methods have been developed to prevent filter bed clogging and thereby pressure drop due to excess biomass accumulation. These methods are generally helpful in nutrient control and the introduction of biomass predators in the biofilter bed on top of pressure drop control strategy (Delhoménie et al. 2003; Woertz et al. 2002).

3.4 Moisture content

Microbial activity is hugely dependent on the amount of moisture present in the biofilter bed. Moreover, reduced moisture content may also lead to cracking of biofilter bed (Kampbell et al. 1987). Biofilter researchers have already found the highest performance for a typical biofilter at moisture content between 47 and 60 % dry weight for compost (Ottengraf 1987) and between 60 and 70 % dry weight for peat (Beerli and Rotman 1989). Furthermore, humidity of the pollutant stream entering the biofilter should also be monitored periodically to prevent drying out of the bed (Wang and Govind 1997). Usually around 95 % relative humidity is maintained for the pollutant stream entering the biofilter and to achieve this, the pollutant stream can be prehumidified before entering the biofilter. Sometimes water can be sprayed on to the biofilter bed periodically in addition to the prehumidification. It was determined that, in a biofilter treating high concentration of pollutants, evaporation and stripping can cause water losses up to 70 g of water per day per kg filter bed (Delhomenie and Heitz 2005).

3.5 Bed porosity

In order to maintain an even flow rate of the pollutant gas and to decrease the pressure drop in a biofilter, adequate bed porosity is most essential.Footnote 4 A typical biofilter which uses soil as its bed medium should have the bed porosity in the range of 35–40 % (Leson and Winer 1991). Generally the biofilter bed is mixed with packing materials in order to increase its porosity and to decrease the compaction (Bohn 1992).

3.6 Packing materials

Choosing suitable packing materials for biofiltration operation is very important for the effective operation of biofilters. Factors which need to be considered before selecting a good packing material include (a) type of packing material (b) packing porosity (c) packing moisture capacity (d) packing nutrient content and (e) sorption characteristics of the packing surfaces. In addition, adsorption characteristics of the packing material with the adsorption characteristics of the target chemical should also be studied before selecting a proper packing material in biofiltration. Natural packing materials like soil, compost or peat are often used as packing material in biofiltration as they are inexpensive and moreover, microorganisms can simultaneously able to degrade the packing as well as the VOC of interest (Oh and Choi 2000). However, these types of packing materials tend to settle and compact, which in turn result in increased pressure drop and channelling. In order to improve degradation of hydrophobic VOCs which don’t partition well into the aqueous phase and recalcitrant compounds with microorganisms, Granulated Activated Carbon (GAC) has been used as a packing material in compost biofilters. Mixtures of GAV and compost are reported to be effective for treating certain VOCs (Aizpuru et al. 2003). Inert materials such as ceramic or glass can also able to maintain a rigid structure with large pores which minimize pressure drop build ups in a biofilter (Aizpuru et al. 2005).

3.7 Air flow rate

One of the major advantages of using a biofilter is, it can handle higher inlet gas flow rates in the range of 100–100,000 m3 h−1 when compared with other air pollution control technologies. When the flow rates are too high, the residence time becomes shorter which would lead to an incomplete biodegradation. Furthermore, if the flow rate is more, the water in the biofilter bed would get stripped by the flow which tends to desiccate the biofilter. A typical biofilter requires an airflow rate of 0.01 cfm per square foot of surface area (Leson and Winer 1991).

3.8 Nutrient requirement

Aerobic microorganisms present in the biofilter media require nutrients such as nitrogen, phosphorus, potassium, sulphur and trace elements in addition to oxygen and carbon for their growth. Though the biofilter mediaFootnote 5 have the residual nutrients, extra nutrients are needed for the long-term performance of biofilters (Yang et al. 2002). Since nitrogen is the second most important element in the biomass next to carbon, addition of nitrogen to the biofilter media can increase the performance of a biofilter significantly (Morales et al. 1998). A study of a compost biofilter treating toluene proved that its performance strongly depended on the nitrogen supply and suggested that a stoichiometric mass ratioFootnote 6 of 3.8 assuming that bacteria contained 13 % of their mass as nitrogen and 50 % as carbon (Delhomenie et al. 2001).

3.9 Oxygen requirement

Biofilters are driven by aerobic oxidation and hence require oxygen which is normally supplied with the pollutant stream. A minimum of 100 mol of oxygen per each mole of oxidizable gas should be supplied to those aerobic biofilters (Williams and Miller 1992). In usual practise a supply of additional oxygen to the biofilter is provided using an air feed blower to the upstream of prehumidification.

3.10 Inlet pollutant concentration

Biofilters perform best when treating a pollutant concentration less than 1,000 ppm. Higher inlet pollutant concentrations will lead to substrate inhibition which will inhibit the microbial activity. Moreover, higher inlet concentration will also lead to an insufficient oxygen availability (Ottengraf 1987). Researchers found that 30 ppm of toluene had a removal efficiency of 99 % but when the inlet concentration was doubled, the efficiency decreased to 82 %. Moreover, studies suggest that at lower pollutant (toluene) concentration, the elimination capacity was observed to be lower when compared to a higher pollutant concentration, in a differential biofiltration reactor using compost as a bed media (Beuger and Gostomski 2009).

3.11 Maintenance

Maintenance of a biofiltration system is required periodically and especially during the initiation process. Moreover, periodic sampling of the biofilter bed for the percentage of moisture and nutrient content is recommended (Leson and Winer 1991). Extreme weather can also affect the performance of a biofilter. During heavy rainfall and snow, the biofilter should be monitored for excess water or snow more than twice a day in order to make sure there are no adverse gas flows. Addition of wood bark layer on the biofilter surface may prevent the compaction caused due to heavy rain.

3.12 Empty bed residence time (EBRT)

Both air flow rate and EBRT are parameters that have significant impact on biodegradation performance of a biofilter (Elmrini et al. 2004). Increasing the EBRT will produce higher removal efficiencies. In order to improve the biofiltration performance, the EBRT should always be greater than the time needed for diffusion processes in case of low operating flow rates. Most of the research reports suggest that longer EBRT give rise to better VOC removal efficiencies (Christen et al. 2002; Delhoménie et al. 2002; Martin Jr et al. 2002; Yoon and Park 2002). However, to attain longer EBRT, larger filter bed volumes are required. EBRT value also depends on other operating parameters such as pollutant concentration, biodegradability level and the available bed volumes (Delhomenie and Heitz 2005).

3.13 Microorganisms and acclimation time

Bed media used in most of the biofilters are natural packing materials like soil, peat, compost etc. They are the major source of microbial population. A major advantage in biofiltration is that the viability of microorganisms are maintained for a longer period although the system is not in function for a longer period. This is because of using natural materials as the filter bed. However, if an inert packing material is used in a biofilter then it needs a microbial exposureFootnote 7 before a biofilm develops, as microorganisms are considered as the catalysts for pollutant degradation in biofilters. Choice of microbes is usually done as per the composition of the pollutant. A single microorganism is enough to degrade certain pollutants and for certain group of pollutants, even a consortium of microorganisms is used (Nanda et al. 2012). An acclimation time required by the microorganism for handling a new substrate environment can take a few days to a few weeks in general (Li and Liu 2006; Torkian et al. 2003). This lag phase can be shortened by introducing an inoculumFootnote 8 to the bed media. A typical biofilter usually contains 106–1010 cfu of bacteria and actinomycetes per gram of bed and fungi in the range of 103–106 cfu per gram of bed (Ottengraf 1987). The degrading species present in a biofilter are usually between 1 and 15 % of the total microbial population (Delhomenie et al. 2001; Pedersen et al. 1997). So far much of the biofiltration research has been focussed on bacteria; however, fungi have also been exploited (García-Peña et al. 2001; Spigno et al. 2003). Compost has been reported to use bacteria belonging to group Proteobacteria, Actinobacteria, Bacteroidetes and Firmicutes (Chung 2007). Although restricted information is available on the microbial communities involved in biofiltration, new technologies such as denaturing gradient gel electrophoresis (DGGE), temperature gradient gel electrophoresis (TGGE) and single strand confirmation polymorphism (SSCP) have allowed for a better understanding of microbial population dynamics in open and closed biofilter systems (Chung 2007; Xie et al. 2009). Table 1 shows the list of microorganisms which were reported to degrade different VOCs.

Table 1 Identified VOC degrading microbes (Delhomenie and Heitz 2005)
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4 Few VOCs treated through biofiltration

Control of volatile organic compound (VOC) emissions into the atmosphere from industrial facilities has become more critical following the amendment of 1990 Clean Air Act in United States (Aizpuru et al. 2001). Toluene, benzene, ethylbenzene and xylene are few examples of VOCs commonly used as solvents in the manufacture of paints, cosmetics, gasoline and adhesives. Though other air pollution control technologies like adsorption and incineration can be effective in treating the VOCs, they can generate unwanted by-products and may not be suitable for handling high flow pollutant stream with low concentrations of contaminants. The reliability of biofiltration for the treatment of VOCs has been proven in a very large number of reports as it is more suitable to treat low concentration and high volume of VOCs in a cost effective approach (Mpanias and Baltzis 1998; Zilli et al. 1993). Moreover, biofilters are good at handling pollutants which are poorly soluble in water due to the higher superficial area available for mass transfer.

5 Few Non-VOCs treated through biofiltration

Biofiltration is also used widely in treating complex odorous waste air containing hydrogen sulphide. The removal efficiencies for H2S degradation is generally higher than that of VOC degradation although the concentrations of individual VOC species are lower (Iranpour et al. 2005). Biofilters tend to be used for applications with lower H2S loadings due to the concerns of inhibition of H2S removal and packing deterioration by sulphuric acid production over the long term. However, there are few successful reports for biofilters been operated at low pH and high H2S concentrations (Nicolai and Janni 2000; Yang and Allen 1994). Ammonia is another highly odorous pollutant usually treated through biofilters in most of the food processing and petrochemical refining industries. Many researchers indicated that biofiltration technology is particularly effective in treating large air streams with low ammonia concentration (Baquerizo et al. 2005).

6 Choice of model pollutant in biofiltration research

Selecting a model pollutant for a biofiltration research is always important. Among the volatile organic compounds, toluene is one of the well-studied compounds in both laboratory-scale biofilters and industrial-scale biofilters. Moreover, toluene is one of the widely used solvents in the production of paints, gums, resins and rubber. It is also used widely as a reagent in the production of drugs, dyes and perfumes. In addition, toluene is highly volatile and is poorly solubleFootnote 9 in water. Furthermore, the American Conference of Government Industrial Hygienists has set the following threshold limit values (TLVs) for the concentration of this compound in air: (a) the time weighed average (TWA) is 0.375 g m−3, (b) the short time exposure level is 0.560 g m−3 and (c) the olfactory threshold value is 8.8 × 10−3 g m−3 (Guelfo et al. 1987). Based on these reasons toluene may be used as a model pollutant in any biofiltration research. In addition to toluene, VOCs such as benzene, xylene and styrene may also be used as a model pollutant as few recent reports suggest that these pollutants are effectively degraded using biofiltration. Removal efficiencies higher than 68 % were reported for xylene degradation in a typical lab scale biofilter at a pollutant loading rates lesser than 60 g m−3 h−1 (Rene et al. 2009a). Studies carried out in a compost biofilter for treating xylene vapour has showed an EC of 73 g m−3 h−1 with a removal efficiency of 91 % (Torkian et al. 2003). Removal efficiencies higher than 90 % were achieved for inlet benzene loading rates lesser than 40 g m−3 h−1 in a laboratory scale biofiltration set up with compost as the filter bed (Rene et al. 2009b). Under steady state conditions, average removal efficiency of 84 % at loading rates between 60 and 120 g m−3 h−1 was achieved for styrene in a compost biofilter. However, maximum EC of 81 g m−3 h−1 was obtained at a styrene loading rate of 120 g m−3 h−1 (Bina et al. 2004).

7 Major demerit of biofiltration

Although biofiltration is a simple and environmental friendly technology, several challenges need to be overcome. Specifically, the degradation rate is low in traditional biofilters (in other words lower EC) contributing to the large size of a biofilter. Table 2 compares the footprints of widely used APCTs. The second most significant disadvantage in using a biofilter is, the acclimation period for the microbial population may take weeks or even months, especially for VOC treatment.

Table 2 Comparison of the size of different APCTs (Devinny et al. 1999; Menasveta et al. 2001; Theodore 2008)
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8 Potential future research in biofiltration

Firstly, we believe that one of the potential researches for the future in the field of biofiltration is to increase its biodegradation efficiency by incorporating the metabolic uncouplers in biofiltration. These metabolic uncouplers are the chemical species which can inhibit the production of ATP by preventing the oxidative phosphorylation reaction. Thus in the presence of an uncoupler ATP production cannot take place (Nicholls 1982). Metabolic uncouplers like 3,3′,4′,5-tetrachlorosalicylanilide (TSA), dinitro-phenol (DNP) etc., have been used to reduce the sludge growth in activated sludge cultures (Lewis et al. 1994). In growth systems like trickle bed reactors, the substrate is utilized both for the growth of the cell and for the maintenance and whereas in non-growth systems like traditional biofiltration reactors it is utilized only for maintenance (Fig. 3). Moreover, the addition of metabolic uncouplers to the growth system decreases the biomass growth whereas in non-growth systems it is expected to increase the specific substrate uptake rate since the maintenance increases. Since no research work has been done to study the effect of metabolic uncouplers in biofiltration, we strongly believe that this idea will be a novel one in increasing the efficiency of the biofilter for the future.

Fig. 3
figure 3

Comparison of substrate utilization in a) growth and b) non-growth systems

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Secondly, we believe that incorporation of synthetic packing materials in biofiltration may provide improved rigidity, porosity as well as good biofilm support. These synthetic packing materials vary in their adsorbent properties toward specific VOCs and water vapour. Perlite granules, poly urethane foam cubes, pellet activated carbon, sugarcane bagasse are potential packing materials which may be used in biofilters for improving the performance (Prenafeta-Boldú et al. 2008; Sene et al. 2002). Commercial activated carbon has been reported as 60 % more suitable to pack a biofilter with intermittent loads than the rest of packing media and coconut fiber is the better selection in the biofiltration of inlet air with low relative humidity. In addition, studies shows that watering of these packing materials notably diminished their adsorption capacity of a hydrophobic compound such as toluene (VOC), which has important implications in the design of buffering systems for load equalization (Dorado et al. 2010). Hence incorporation of these packing materials in future biofilter research may improve the performance of a biofilter to a great extent.

Thirdly, we believe that either a fungal biofiltration system or a synthetic microbial biofiltration system (specific to different VOCs) may help to attain higher elimination rates compared to the bacterial systems. In the recent years, fungus Paecilomyces variotii and Scedosporium apiospermum finds more interest in biofiltration research for treating VOCs effectively. Maximum elimination capacity of 245 g m−3 h−1 is reported recently in fungal biofiltration system involving these two organisms for treating toluene (VOC). With the advancement in recombinant DNA technology, a new microbial strain may also be developed which can be used solely for degrading a particular pollutant or mixed pollutants in a pure culture biofiltration system. This will certainly increase the removal efficiency and EC of a biofilter in the near future.

9 Conclusion

This paper reviewed one of the APCTs, Biofiltration. The major physical, chemical and biological factors which need to considered for designing a biofilter was also discussed in detail. Furthermore, the article emphasised on the history, and also emphases on the potential future researches in increasing the efficacy of biofiltration. Though the review focus is primarily on VOCs, most of the terminologies used and ideas suggested for future biofiltration research may also be used for improving the removal efficiency and EC of biofilters used for treating different industrial pollutants. This article will be most handy for emerging biofilter researchers.