air pollution prevention and control (bioreactors and bioenergy) || innovative bioreactors and...

10Innovative Bioreactors and Two-Stage

Systems

Eldon R. Rene, Marıa C. Veiga and Christian Kennes

Department of Chemical Engineering, University of La Coruna, Spain

10.1 Introduction

For a given set of conditions, biological waste gas cleaning systems have demonstrated their ability tohandle a wide variety of volatile pollutants in industrial facilities as a result of some important advantagescompared to physico-chemical techniques [1–3]. For successful operation, the bioreactor should be ableto maintain adequate moisture, pH and the required nutrient conditions that favour metabolic activity.However, bioreactors are prone to operational problems like filter bed clogging in the case of packed-bedreactors, channelling, pressure drop, mass transfer issues for sparingly soluble pollutants, oxygen deficit,pH and temperature fluctuations, substrate toxicity, and transient-state operations, among others. This hasspurred the search for alternative bioreactor configurations in order to avoid or minimize these operationalproblems. Some new bioreactor configurations have been shown to be able to overcome some of thelimitations of conventional biological waste gas treatment systems. Innovative bioreactor design remainsa topic of interest among researchers and engineers. A novel bioreactor can be defined as a new reactorconfiguration that has unique and improved performance characteristics compared to existing bioreactordesigns, and that will play a major role in decontaminating the pollutants present in waste gases in aneconomic and eco-friendly manner [4]. In the first part of this chapter, we review the operational detailsand performance of innovative bioreactor configurations that have been reported in the literature. In thesecond part, we briefly present an overview of the operation and performance of two-stage reactors forwaste gas treatment.

Air Pollution Prevention and Control: Bioreactors and Bioenergy, First Edition. Edited by Christian Kennes and Marıa C. Veiga.c© 2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd.

222 Air Pollution Prevention and Control

10.2 Innovative bioreactor configurations

10.2.1 Planted biofilter

Phytoremediation is a natural process that uses green plants and relevant microorganisms associated with theplant to remove or stabilize contaminants present in soils, sediments or water. Although phytoremediationwas first tested and put into use to remove pollutants from soil, recently such technique has found numerousother applications, including the removal of volatile pollutants from air. The plant species are selected basedon factors such as: (i) ability to remove the contaminants; (ii) deep root structure; (iii) growth rate; (iv)water absorption potential; (v) adaptation to local climates; and (vi) ease of planting and maintenance. Thepolluted air is passed through a planted soil or directly over the plants, wherein the pollutants are adsorbedand degraded or assimilated by the microorganisms and/or by the plants. The microorganisms containedin the planted clod of soil sustain a rich and diverse rhizosphere microflora that can degrade pollutants byusing them as a substrate or energy source. The efficiency of phytoremediation for waste gas treatmenthas not been fully demonstrated yet [5]. Recently, Rondeau et al . [6] proposed a planted biofilter thatexploits the advantages of both bacteria and plants for waste gas treatment, wherein the polluted air ispassed through a porous packed bed in which the plant grows (Figure 10.1).

Two biofilter columns, packed with Falienor (coconut fibres (coir) and perlite), were tested for theirability to remove low concentrations of toluene, ethylbenzene and xylene (TEX) vapours (600 µg m−3), atan empty bed residence time (EBRT) of 14 s. The authors planted the first biofilter column with Hederahelix , while the second column was a non-planted biofilter where the plants were removed before placingthe clod of soil in the filtering material. The biofilters were maintained at room temperature, under standardhorticultural fluorescent tubes alternating 12 h of light and 12 h of dark. In order to maintain the desiredC : N : P ratio of 100 : 5 : 1, a liquid nutrient medium was regularly delivered after 78 days of operation,while the nutrient medium was not added to the biofilter before that. The authors observed high removal

Contaminated air

Hedera helix

Mound of soil

Treated air

Filter medium

Air

Humidifier

Figure 10.1 Schematic of a planted biofilter (on the right). Adapted from [6] under the guidelines of the STMagreement. Copyright (2012) John Wiley and Sons.

Innovative Bioreactors and Two-Stage Systems 223

efficiencies of 75–80% in both biofilters during the first three days of operation, presumably due to theadsorption of the gas-phase pollutants onto the packing material. However, after that, the removal efficiencydropped significantly, by 30% for the non-planted biofilter and by 15% for the planted biofilter. As theplants consumed nitrogen compounds for their growth, during the first 78 days, the removal efficiencyof the non-planted biofilter was higher than in the planted biofilter. The addition of nutrients showedan increase in the removal efficiency, ranging from 10% up to 70–80% in both biofilters. Furthermore,denitrifying enzyme activity (DEA) was measured in the mound of soil, at the end of the experimentalrun, and the corresponding DEA was estimated to be 2.7 and 2.0 µg of N–N2O per gram of filter material,respectively, for the non-planted and the planted biofilter. Based on the denitrification activity, and thegood conditions for synthesis and the maintenance of the denitrifying enzyme pool in the mound of soil,the authors suggested that NOx emissions could also be treated in the planted biofilter.

10.2.2 Rotatory-switching biofilter

The rotatory-switching biofilter (RSB) is an improvised version of the switch-feed multi-column (SFMC)biofilter. The SFMC consists of multiple filter beds in series, whose sequence can be periodically changedto sustain a proper level of microbial activity and even biomass accumulation across all the beds [7, 8].Although the mode of operation of this biological system resembles a biotrickling filter, that is, witha continuously moving or trickling water phase, the difference in the SFMC is the mode in which thenutrient medium is fed to the system. In an SFMC, the entire filter bed is immersed in liquid containingthe culture and minerals, ensuring complete watering and homogeneous nutrient supply to the entire beddepth. The advantages of this bioreactor configuration can be summarized as follows: (i) good control ofbiomass accumulation; (ii) homogenization of packing material; (iii) avoids nutrient limitations; (iv) filterbed washing, when required, can be done without stopping the gas flow; (v) homogenous pollutant loadingto all filter beds; and (vi) excellent performance characteristics for the removal of high volatile organiccompound (VOC) loads.

The schematic of an RSB is shown in Figure 10.2 [8]. The biofilter is divided into four segments offilter beds. Each segment has one-quarter of the cross-sectional area of the column, and the same height asthat of the column, reaching a total working volume of 39 L in this example [8]. The filter bed is packedwith V-shaped polyvinyl formal (PVF). The column is fitted with a rotor system that periodically rotatesthe column. The biofilter rests on a stainless-steel base that is attached to the rotor assembly.

In an experimental study, a waste gas stream containing toluene (247–322 ppmv) entered the first seg-ment (S1) of the filter bed via an airflow pipe that penetrates the filter bed of this segment [8]. Thewaste gas then flowed upwards and passed through the bed of this segment. Once it reached the topsection of the first segment, it entered the second segment (S2), and then the third segment (S3), respec-tively, before being vented to the atmosphere. Segment four (S4), located in the nutrient-supply zone,was bypassed, as it was fed daily the nutrient medium. Nutrient addition to this segment (S4) was doneby feeding nutrient medium from a nutrient tank, for 30 min, using a pump. Once nutrient was addedto this segment of the filter bed, the stainless-steel plate placed on the filter bed was lifted up, thecolumn was rotated 90 degrees in the clockwise direction and the plate was placed again on the col-umn to complete the switching. This switching operation requires only a few seconds, after which thesegment of the biofilter located in the nutrient-supply zone and the segment located in the inlet zonemove to the low-concentration segment and the nutrient-supply zone, respectively. The segment reach-ing the nutrient-supply zone now receives the nutrient medium. This procedure was repeated twice forall the segments before toluene removal experiments were performed. The same procedure was alsoused to inoculate all four segments of the filter bed, by mixing 3 L activated sludge with 17 L of nutri-ent medium. A total of seven experimental runs were performed in order to evaluate the performance


Polluted air Treated air

S1

S2

Packing

S2

S3

S4

Packingmaterial

Direction of rotation

Nutrient tank

Rotor system

Figure 10.2 Schematic of a rotatory-switching biofilter. Adapted with permission from [8] Copyright (2012)Taylor and Francis.

of the RSB. The EBRT was varied from 26 to 52 s, while the gas-phase toluene concentrations werevaried from 247 to 322 ppmv corresponding to toluene inlet loading rates (ILRs) ranging from 70 to142 g m−3 h−1. The authors reported a maximum elimination capacity

(ECmax

)of 75 g m−3 h−1 at an

EBRT of 52 s. The dominant toluene-degrading bacteria were later identified as Burkholderia cepacia andMicrobacterium sp.

10.2.3 Tubular biofilter

The tubular biofilter (TBF) is quite similar to typical biotrickling filters. It was originally developed toavoid excessive accumulation of biomass in biofilters [9, 10]. It was hypothesized that a thin tubular layerof polyurethane sponge is ideal for biofilters when the waste gas stream contains low concentrations ofpollutants and they could penetrate only a small distance into the filter bed when the dominant removalmechanism is of surface or shallow type [10]. A schematic of a TBF is illustrated in Figure 10.3 [9]. TheTBF is a biofilter with a tubular medium bed configuration through which the waste gases flow in a radialdirection. It consists of a cylindrical outer canister, tubular filter bed medium packed with polyurethanefoam, nutrient medium distributor and adequate tubing for inlet and outlet gases. The waste gas entersinside the canister section, bypasses the nutrient medium distributor, and flows through the tubular filterbed in a radial direction. The microorganisms are attached to the porous polyurethane foam packing bed,and, after treatment, the exhaust gas is discharged from the TBF through a port located at the bottom ofthe reactor.


Waste gas

Waste gas flow direction

Tubular Biofilter

Cylindrical canister

Treated air

Nutrient tank

Figure 10.3 Schematic of a tubular biofilter. Adapted with permission from [9] Copyright (2011) EnvironmentalEngineering Group, University of La Coruna.

The start-up time for the TBF was reported as 35 and 52 days for handling ethylbenzene and toluene,respectively [9, 10]. The performance of TBFs for these two pollutants can be summarized as follows.

1. For ethylbenzene removal: At an EBRT of 15 s and at ILRs of 9.7, 19.3, 29.0 and 38.6 g m−3 h−1, thecorresponding removal efficiencies were 90, 81, 71 and 66%, respectively.

2. For toluene removal: At an EBRT of 15 s and at ILRs of 18.7, 37.3, 74.6 and 149.3 g m−3 h−1, thecorresponding removal efficiencies were 99, 84.5, 72.0 and 52.2%, respectively.

During long-term performance evaluation (∼150 days), the authors observed no excessive biomass accu-mulation in the filter material. It was also suggested that excess biomass within the sponges of the TBFcould be removed easily by periodic squeezing of the polyurethane packing in a nutrient medium, similarlyto what was previously suggested for randomly packed polyurethane biofilters [11].

10.2.4 Fluidized-bed bioreactor

A fluidized bed refers to a filter bed in which the particles are not in continuous contact with each otherdue to the flow of fluid up through them [12]. The schematic of a typical gas–solid fluidized-bed bioreactoris shown in Figure 10.4. A humidified stream of polluted air enters the fluidized-bed section with the helpof a nozzle-type distributor/sparger placed near the tapering section of the bioreactor. It is recommendedto pre-humidify the waste gas because the fluidizing air tends to dry the fluidized bed materials. However,


Treated airIntermittent nutrient addition

Disengagement zone

Particles + biomass Polluted air

Humidifiedpolluted air

Air

Drain pipe

Humidifier

Figure 10.4 Schematic of a fluidized-bed bioreactor.

an excessive increase in water or bed moisture content would lead to de-fluidization of the bed. Anoptimal moisture content of the fluidized particles would yield a good fluidization regime, and also helpin maintaining microbial growth [13, 14]. A perforated distribution plate is placed just above the spargerfor better distribution of the waste gas. For biofilm development, it is important to select an appropriateparticle type that can easily be fluidized.

According to Clarke et al . [15], the fluidized state is influenced by particle properties such as size,size distribution, density and shape. Besides, the fluidization state can also affect the mass and heattransfer characteristics, and the pollutant removal characteristics in the bioreactor. Settling velocity isanother important parameter affecting the performance of the bioreactor in three-phase (gas–solid–liquid)fluidized-bed bioreactors. Particles with a settling velocity less than the superficial gas velocity are washedout, while those with larger settling velocities are retained in the bed [12]. In gas–solid fluidized-bedbioreactors, small particles in the size range of 0.15–0.3 mm have been used as the solid phase. Typicalexamples include the following: sand, carbon, fly ash, anthracite, glass and calcined clay, among others.These fine particles can be easily fluidized by the upward flow of waste gas entering the reactor. Poorfluidization behaviour could lead to gas channelling and poor waste gas treatment. In some cases, highlyporous, fabricated media can also be used, which allows biomass formation within the porous internalstructure. The surface shear due to the waste gas flow is much lower in fluidized-bed bioreactors thanthe values frequently encountered in continuous stirred-tank bioreactors. This is particularly advantageousfor the growth of filamentous fungi, as their aerial mycelia continue to grow as a biofilm on the surfaceof the particles.


There are only a few laboratory-scale studies that have reported the removal of gas-phase pollutantsin gas–solid fluidized-bed bioreactors. The results from some of the recent literature on this bioreactorconfiguration are briefly summarized here after.

Clarke et al . [16] tested a gas–solid fluidized bed bioreactor for the removal of ethanol from polluted air,in a specially designed bioreactor packed with moist sawdust particles and glass spheres. The bioreactor,depending on the applied superficial gas velocity, was designed to be operated in either packed or fluidizedmode. When the bioreactor was switched to fluidized mode, the sawdust and glass sphere mixture wasmaintained in a bubbling/slugging regime. The authors reported an ECmax of 75 g per cubic meter of sawdustper hour when the bioreactor was operated in fluidized mode, and 225 g m−3 h−1 when it was operated as apacked bed. The high EC value achieved during packed-bed mode of operation was attributed to the highbiomass growth. The low EC value in the case of the fluidized-bed bioreactor was speculated to be dueto the inability of the microbial cells to attach to the rigid sawdust particles during fluidization. Besides,as a performance improvement strategy for the fluidized-bed bioreactor, it has been suggested to recyclethe non-treated air from the exit to the inlet, resulting in an increase in the concentration of ethanol to thebottom of the bed.

In another similar study, Clarke et al . [15] used peat granules (Sauter mean diameter of 680 µm), andinoculated the fluidized-bed bioreactor with Hansenula anomala, for the removal of gas-phase ethanol.Water was added to the peat particles in order to obtain a moisture content of 40% (dry basis). Besides,nutrient medium was also added in batches, 50–125 mL once every two days, to the peat particles. Theauthors reported an ECmax ranging between 1150 and 1520 g m−3 h−1 when fluidized-bed bioremediationexperiments were carried out at superficial gas velocities of 0.5, 0.75 and 1 m s−1. According to the authors,at the lowest velocity (0.5 m s−1), the bed showed good bubbling fluidization, while at the highest velocity(1 m s−1) there were more large bubbles (slugs) present at greater depths in the bed. Thus, as gas velocityincreased, the size and amount of slugs increased, which appeared to reduce the ethanol removal efficiency.

Delebarre et al . [14] used sawdust obtained from pinewood as the fluidization particles, and mixedthose particles with activated sludge from a wastewater treatment plant to obtain activated sawdust, whichwas later used for gas-phase ethanol and toluene removal in a fluidized-bed bioreactor. For ethanol, thefluidized-bed bioreactor showed high removal efficiencies (>85%) for ILRs varying between 200 and750 g m−3 h−1. However, for higher loads (750–1250 g m−3 h−1), the removal efficiency values decreasedonly slightly, and stabilized around 80%. The fluidized-bed bioreactor was then acclimated with toluene forabout 30 days at a constant ILR of 26 g m−3 h−1. When the load was increased from 50 to 206 g m−3 h−1,the removal efficiency dropped significantly, from >70% to 20%.

10.2.5 Airlift and bubble column bioreactors

Airlift reactors offer some advantages. They have been used extensively for wastewater treatment. However,their application to handle waste gases was initiated only in the 1990s [17, 18]. The advantages ofthis reactor configuration can be summarized as follows: (i) complete mixing of the components within thereactor; (ii) simple mechanical design; (iii) low shear rate; (iv) absence of mechanical agitators; (v) easyto scale-up; and (vi) low power consumption for agitation and oxygenation [19]. The schematic of a con-centric, internal-loop airlift bioreactor is shown in Figure 10.5. The waste gas is introduced into the airliftbioreactor through a sparger located at the central section, for the purpose of supplying the pollutant andoxygen to the microorganisms and to provide adequate mixing. The inner draft tube improves circulationand oxygen transfer and equalizes shear forces in the reactor. The hydrostatic pressure difference dueto gas sparging in the middle section causes a circulating motion in different parts within the reactor.


Treated airNutrient addition

RiserDown comer

Biocatalyst or immobilized beads with biomass

Internal tubeBubbles

Sparger

Polluted airLiquid outlet

Figure 10.5 Schematic of an airlift bioreactor.

The direction of waste gas movement is upwards in the light section (riser section) and downwards in theheavier section (downcomer section), resulting in a well-controlled fluid circulation pattern. Adequateheadspace is provided above the riser and downcomer to allow the treated gas to be vented out.

In some conventional airlift bioreactors, the inner tube serves as the downcomer, and the annularspace between the two tubes serves as the riser. The biomass is usually dispersed in the liquid mediumor immobilized onto a suitable polymer matrix, such as polyvinyl alcohol (PVA)–sodium alginate gels[20, 21].

Vergara-Fernandez et al . [20] reported that an increase in biomass concentration will allow for greaterelimination capacities. This was explained by the fact that a greater use of the carbon source favours masstransfer in the medium, which also increases the pollutant solubility in the medium, produced by changesin the liquid phase generated by microbial growth. For instance, when the average biomass concentrationwas 3700 g m−3, the ECmax was 203 g m−3 h−1, and at a maximum biomass concentration of 8000 g m−3,the ECmax was 310 g m−3 h−1. The same authors also reported that the gas-phase toluene RE decreasedfrom 100 to 40% when the gas flow rate was increased from 0.024 to 0.132 m−3 h−1.

Namgung et al . [21] tested an airlift bioreactor, having suspended yeast-immobilized polymer media,for the biodegradation of a mixture of gas-phase toluene and methyl ethyl ketone (MEK). The yeast strainCandida tropicalis , and a mixture of sodium alginate, polyethylene glycol (PEG) and powdered activatedcarbon were used to formulate the PEG–alginate–carbon–yeast medium. Entrapment of microbes within


a polymer matrix would also minimize the impact received from unexpected shock loads and help tomaintain the microbial activity during long-term operations. In that study, the EBRT was varied from 15to 60 s and the ECmax was reported to reach 70.4 g m−3 h−1 and 56.4 g m−3 h−1, for toluene and MEK,respectively.

Jianping et al . [22] evaluated the performance of an airlift bioreactor for treating a contaminated airstream containing a mixture of ethyl acetate and ethanol. Activated charcoal, with an average diameterof 0.2 mm, was mixed with activated sludge for biofilm formation on the solid phase. The bioreactor wasoperated at a dilution rate of 0.09 L h−1. The ECmax was found to be 504 and 685 g m−3 h−1 for ethylacetate and ethanol in the mixture, respectively.

Nikakhtari and Hill [23] proposed an improvised version of the conventional airlift bioreactor by incor-porating a small packed bed near the riser section. The bioremediation of phenol-polluted air was studiedin that reactor using a pure strain of Pseudomonas putida . The authors have also reported a three-stepprocedure, using 0.2% polyethylenimine, to develop the biofilm on the packed bed (stainless-steel meshpacking). The bioreactor achieved steady state in less than 6 h, and was able to handle phenol loads of33.12 g m−3 h with 100% RE.

Rocha-Rios et al . [24] used a methanotrophic consortium enriched from an activated sludge to inoculatean internal loop airlift bioreactor for methane biodegradation under the following test conditions: EBRT,7.3 min; inlet concentration, 20 gm−3; dilution rate, 0.05 day −1; steady-state biomass concentration, 3 gL−1;and ILR, 171 g m−3 h−1. The authors reported that the ECmax of this airlift bioreactor increased from∼11.6 g m−3 h−1 to ∼17 g m−3 h−1 when the gas recirculation rate was increased from 0 to 1 vvm (gasvolume flow per unit of liquid volume per minute (volume per volume per minute)), suggesting that theECmax increased due to an enhancement in KLaO2

when the gas recirculation rates were increased.Bubble column bioreactors are suspended growth bioreactors that have similar operational features and

advantages as an airlift bioreactor, except for the absence of the riser and downcomer sections. In thisbioreactor configuration, the addition of granular activated carbon (GAC) particles within the bioreactor,as a second solid phase, has been shown to have more beneficial effects during high pollutant loadings tothe bioreactor. Under such conditions, the GAC particles can adsorb the pollutants and slowly deliver thetarget compound to the microorganisms [25].

Ahmed et al . [25] investigated the biodegradation of toluene in a bubble column bioreactor usingthe yeast Candida tropicalis , and monitored the performance of the bioreactor in the absence and thepresence of GAC particles. The bubble column bioreactor was operated under two different conditions inthe absence of GAC: (i) residence time (RT) of 1 min and toluene concentration of 119 ppmv; and (ii) RT of2 min and toluene concentration of 306 ppmv, respectively. However, in the presence of 30 g GAC (2.3%w/v), the toluene concentrations were gradually increased from 72 to 251 ppmv, in three steps, at a constantRT of 1 min. The fourth step involved the addition of 30 g fresh GAC to the bioreactor in order to estimatethe effect of GAC adsorptive capacity on the bioreactor performance. The addition of GAC not only allowedthe bubble column bioreactor to achieve high ECmax values (172 g m−3 h−1), but also helped the bioreactorto withstand a high ILR of toluene (291 g m−3 h−1) in comparison to the ILRs fed to the bioreactor in theabsence of GAC (<60 g m−3 h−1). The authors suggested that the bubble column bioreactor with GACparticles would help to overcome bioreactor operational problems such as microbial activity loss underdynamic loading conditions, and biomass accumulation over an extended operational period.

10.2.6 Monolith bioreactor

A monolith bioreactor (Figure 10.6) is a reactor in which the biomass is attached on a structured packingwith uniform parallel channels separated by thin walls and usually made of plastic, ceramic or other inertmaterials [4, 27]. The channel geometries can be different, that is, square, triangular and sinusoidal. The


Polluted air Trickling media

Polluted air

Distributor

Channel with microorganisms

Treated air

Pump

Treated air

Nutrient tank

Figure 10.6 Schematic of a monolith bioreactor.

operating mode depends on the size and geometry of the straight parallel channels. In large channels,the fluid trickles downwards along the channel walls and the gas travels co-currently or counter-currentlythrough the channel in the core. In smaller channels, the dominant flow pattern is a segmented slug flowor bubble train flow of elongated bubbles and slugs, commonly referred to as Taylor flow in the literature[26–28].

Theoretically, the gas and liquid would ideally follow near-plug-flow conditions as they travel withuniform velocities as separated packages within a monolith channel. The wall is wetted by a thin liquidfilm, essentially a trickling nutritive medium, while the gas-phase pollutant is easily transported through thefilm, allowing higher mass transfer rates. The biofilm layer remains at the wall when the liquid slug passesby. Inside the liquid slug itself, a recirculation pattern is observed. This recirculation enhances transfer ofgas from the caps of the bubble to the biocatalyst, with higher removal by the attached biomass.

Commercially available monolith supports can be tailored to meet the needs of a given end-user. Theadvantages of this reactor configuration for waste gas treatment can be stated as follows: (i) relativelyinexpensive, light weight and inert materials of construction can be used; (ii) better liquid distribution atlow liquid trickling rates; (iii) high external and specific surface area; (iv) short diffusion path within thechannels; (v) high interfacial mass transfer rates; and (vi) relatively easy to scale-up [4, 27–29].

So far, there are only a few reports available in the literature on the removal of gas-phase pollutantsin monolith bioreactors. Jin et al . [28] used a yeast-dominant enriched culture that could tolerate lowpH (pH 2.0) to inoculate a monolith reactor for treating methanol-polluted air. More than 80% removal,for methanol loads greater than 250 g m−3 h−1 with an ECmax of 234 g m−3 h−1, suggests the extent ofmineralization possible for hydrophilic VOCs in such a monolith bioreactor. Similar loads are often nottreatable in biofilters and biotrickling filters at such low pH.


Fang and Govind [30] tested an activated-carbon-coated monolith reactor having porous channel walls,made of porous cordierite to support biomass, under diffusive and convective flow conditions to treatgas-phase toluene at low concentrations. High pollutant removal during the first two weeks of operationwas attributed to adsorption of toluene by activated carbon on the monolith, and the difference in removalefficiencies was a result of the different flow patterns inside the monolith modules. Though the maximumEC values were low in both flow types, the convective flow type gave higher EC (7 g m−3 h−1) than thediffusive flow-type configuration (3 g m−3 h−1).

Rene et al . [29] investigated styrene removal in a monolith bioreactor, inoculated with the fungusSporothrix variecibatus , at different concentrations, ranging between 0.07 and 2.5 g m−3, and at two differ-ent gas flow rates corresponding to EBRTs of 77 and 19 s, respectively. A maximum elimination capacity(ECmax) of 67.4 g m−3 h−1 was observed at an EBRT of 77 s, and at an inlet styrene load of 73.5 g m−3 h−1.The monolith bioreactor was later tested for its ability to handle transient-state operations, that is, styreneshock-loads. Both low to medium and medium to high overload tests were conducted at EBRTs of 77and 19 s. It was reported that a sudden increase in the styrene load from 10 to 85 g m−3 h−1 decreased theremoval efficiency from nearly 100 to 61% during medium shock loads, and during this step increasethe EC increased from an original low value of 10 to 55 g m−3 h−1, and then remained almost constantduring the 6 h medium shock load. During high shock-load tests, when the ILR was increased from 56 to186 g m−3 h−1, the removal efficiency dropped suddenly, to nearly 30%, and then remained almost con-stant at that value during the shock-load period of 36 h. Furthermore, the EC values remained basicallyunchanged during the shock load compared to the values before the shock load. In both these overloadtests, the response of the monolith bioreactor was fast, and when pre-shock steady-state conditions wererestored, the recovery time of the fungus-inoculated monolith bioreactor to achieve its original performancewas almost instantaneous, i.e. less than 1 h [29].

Channels clogging is an aspect to be considered in a monolith bioreactor for waste gas treatment. It mayappear as a result of biomass growth within the small channels, resulting in an increasing pressure drop[28, 29]. Biomass accumulation and uneven biomass distribution usually occur simultaneously within thechannels of the monolith support and can lead to operational problems such as channelling, high pressuredrop and consequently reduced bioreactor performance. Jin et al . [28] reported that the pressure drop ofa monolith bioreactor was low during the initial 50 days of operation (6 mm H2O per metre of channellength) for methanol ILRs less than 75 g m−3 h−1. However, during the subsequent days of operation athigh methanol loads of 150 g m−3 h−1, there was a rapid increase in biomass growth within the channels,leading to an abrupt increase in pressure drop from 6 to 22 mm H2O. Pressure drop increase can, however,be solved more easily in such systems than in biofilters, by applying high liquid flow rates for a shortperiod of time. This generates shear forces capable of removing part of the biofilm. Pressure drop can thenquickly be brought back to its original low value. It has also been recommended to apply this temporarywashing step periodically during the onset of pressure drop increase, which allows for a long-term stablebioreactor operation [28, 29]. Similar highly structured packing materials have also already been appliedsuccessfully in large-scale bioreactors [31].

10.2.7 Foam emulsion bioreactor

The foam emulsion bioreactor (FEBR) was developed to overcome some of the operational difficultiesexperienced in biofilters and biotrickling filters such as low cell activity of the resting cells, cloggingfrom excessive biomass growth, high pressure drop and process instability [32]. The mechanism of pol-lutant removal in a FEBR is somewhat analogous to that of a two-liquid-phase partitioning bioreactor(TLPB) (Chapter 8). It consists of a first-stage FEBR unit and a second-stage defoamer unit (Figure 10.7)[32]. The first-stage FEBR contains an emulsion of highly active pollutant-degrading microorganisms and


Treated air

Foamed emulsion bed reactor

De-foaming equipment

Foam

Liquid medium

Moving foam

Recycled medium

Waste gas

Cell collection tank

Figure 10.7 Schematic of a foam emulsion bioreactor. Adapted from [32] under the guidelines of the STMagreement. Copyright (2005) John Wiley and Sons.

water-immiscible organic phase, which is made into a foam by passing the waste gas. The amount oforganic phase is low and it uses a biocompatible surfactant for foam production [33]. The air gets treatedwithin the FEBR unit, and after the desired level of treatment is achieved, the foam is continuouslycollapsed in the defoamer unit, and the cells with the emulsion are collected in the cell collection tanklocated downstream of the defoamer unit. The advantages of using a FEBR for waste gas treatment can besummarized as follows [32]: (i) large interfacial area between the gas and liquid phase; (ii) high oxygenand pollutant mass transfer rates; (iii) high partitioning of the pollutants into the organic phase; (iv) rapidbiodegradation can be achieved by a high-density cell culture; and (v) no bed clogging and pressuredrop problems.

There are only very few reports on FEBRs for waste gas treatment, and these results suggest that anFEBR could be an alternative to conventional biological treatment systems, if high performance is desired[32–34]. Kan and Deshusses [34] tested a FEBR consisting of a toluene-degrading consortium, an organicphase (oleyl alcohol) and a surfactant (DC-100 silicone oil) for the removal of gas-phase toluene. The effectsof gas velocity, toluene inlet concentration, organic-phase fraction and culture density on the performanceof the FEBR were investigated in that study. The following results were reported: (i) the ECmax was240 g m−3 h−1 with 91% removal efficiency when the oleyl alcohol concentration was 3%; (ii) the ECmaxwas 285 g m−3 h−1 when the cell density was increased from a dry weight of 2 to 16 gL−1; (iii) at a constantcell density dry weight of 48 g L−1, the removal efficiency was 90%, with an ECmax of 130 g m−3 h−1 forgas velocities below 1.6 m min−1 (EBRT > 15 s); and (iv) based on stoichiometric information for complete


toluene mineralization, oxygen diffusion is expected to become a limiting factor at toluene concentrationsabove 0.7 g m−3. However, when pure O2 was continuously fed to the FEBR, to overcome oxygen limitingconditions, at high toluene concentrations (2.2 g m−3), an ECmax of 408 g m−3 h−1 was observed by theauthors. For stable operation of the FEBR, foam stability and cell activity are of prime importance. Besides,replacing and replenishing part of the culture would also be required to maintain a high biodegradationactivity and high pollutant removal in this reactor configuration.

For long-term bioreactor stability, it has been suggested to operate the FEBR by replacing 20% of theculture with a concentrated nutrient solution, so as to maintain optimal cell growth and activity and toavoid nutrient limitations [32]. It would also be important to check the stability of the added organic phase,since compounds such as alcohols might be biodegradable. Shahna et al . [33] selected 6% n-hexadecene asthe organic phase and a benzene-degrading consortium to treat gas-phase benzene in a FEBR. The authorsobserved that, at an EBRT less than 10 s, the foam was unstable and could not properly rise up the FEBRunit, and therefore the optimal EBRT for stable foam formation and good bioreactor performance wasconsidered to be in the range of 10 to 15 s. Thus, at an EBRT of 15 s, the ECmax of this FEBR was250 g m−3 h−1 with 93% benzene removal in the presence of the organic phase, while the ECmax reducedto 164 g m−3 h−1 in the absence of the organic phase. Furthermore, the effect of oxygen content on theECmax was found to be more significant with increasing inlet load of benzene to the FEBR: for an ILRof 450 g m−3 h−1, the ECmax values were 199 and 321 g m−3 h−1 for pure air (no external oxygen supply)and for 60% O2 supply, respectively.

Kan and Deshusses [32] also tested the FEBR under toluene starvation conditions. After steady-statecontinuous operation of the FEBR at 0.5 g m−3, at an EBRT of 15 s for 48 h, the supply of gas-phase toluenewas stopped for 48 h, and then restarted at a concentration of 1 g m−3. The biomass concentrations weremaintained constant, at 13 g L−1, before the FEBR was subjected to toluene starvation. It was observed thatthe biomass concentration decreased slightly within the first 24 h, from 13 gL−1 to ∼12 g L−1, but reducedfurther to ∼9 g L−1 between 24 and 48 h during the starvation phase. This was attributed to the switchin process culture from a highly active metabolism (growth phase) to a lower metabolism (maintenancephase). However, the toluene removal efficiencies after 24 and 36 h re-acclimation with gas-phase tolueneconcentrations of 1 g m−3 showed removal efficiencies of 86 and 89%, respectively.

10.2.8 Fibrous bed bioreactor

A fibrous bed bioreactor is a modified version of the biotrickling filter, which has the following advantages:(i) long-term stability due to its unique spiral-wound fibrous bed configuration; (ii) good hydrodynamics;(iii) efficient mass transfer that prevents the formation of microbial clusters; and (iv) low pressure drop.The fibrous bed bioreactor described by Zhou et al . [35] consists of a cylindrical glass column and afibrous bed (Figure 10.8). The fibrous sheet material, made using 100% cotton terry cloth, was fixed toa stainless-steel mesh, wound into a spiral configuration with gaps of 2–4 mm between each turn of thespiral, and then packed inside the column. Glass beads were placed at the bottom of the column, whichacted as a support for the spiral packing, and to ensure proper distribution of the gas-phase pollutant intothe fibrous bed. As mentioned earlier, the operation of this reactor configuration is analogous to that ofa biotrickling filter, operated in counter-current mode, wherein the polluted air enters the reactor fromthe bottom. In order to keep the fibrous bed wet, and to supply the essential nutrients, liquid medium issupplied continuously from a nutrient tank. Biomass attachment to the fibrous bed reactor can be done byfilling the entire working volume of the fibrous bed with the nutrient medium, and then adding the propermicroorganisms and periodically feeding the liquid-phase pollutant to the reactor. After the bioreactor hasachieved a sufficiently high cell density in the fibrous bed, the liquid can be drained off, and the bioreactorcan then be used for treating gas-phase pollutants.


Stainless steel meshLiquid in Air out

Fibrous matrix

Air in Liquid out

Figure 10.8 Schematic of the fibrous sheet material wound in a spiral configuration.

Zhou et al . [35] inoculated a fibrous bed bioreactor with a co-culture of Pseudomonas putida and Pseu-domonas fluorescens . Gas-phase benzene was supplied as the sole carbon source. The EBRT was variedbetween 2 and 12 min, while the inlet benzene concentration was varied between 0.3 and 1.8 g m−3. Accli-mation of this bioreactor was achieved in 7 days (84% removal efficiency) at an ILR of 3 g m−3 h−1 andan EBRT of 8 min. The authors investigated the effect of the EBRT and inlet benzene concentration on theperformance of the bioreactor, and the following results were achieved: (i) at an EBRT of 10 min, for inletconcentrations ranging from 0.3 and 1 g m−3, removal efficiencies were >90%; (ii) at an inlet concentra-tion of 1.8 g m−3 and an EBRT of 12 min, the removal efficiencies were >80%; (iii) in the concentrationrange tested, the removal efficiency generally decreased with an increase in the ILR, achieving an ECmaxof 12 g m−3 h−1 at an ILR of ∼30 g m−3 h−1; (iv) the bed pressure drop was low (750 Pa m−1) at thehighest gas flow rate tested; and (v) the total cell density in the bioreactor was 5.5 g dm−3, of which 93%were immobilized into the fibrous matrix, while the remaining 7% was present in the trickling nutrientmedium.

10.2.9 Horizontal-flow biofilm reactor

The horizontal-flow biofilm reactor (HFBR) was originally developed to remove organic carbon and nitro-gen from high-strength wastewaters [36]. However, the same bioreactor configuration could presumablyalso be modified to handle waste gases, wherein the operational mode would resemble a co- or counter-current biotrickling filter. In this bioreactor configuration (Figure 10.9), a series of horizontal plastic platesare firmly placed one above the other, within the reactor unit, supported and separated by vertical plasticcone- or pyramid-shaped frustums formed in the sheets during manufacturing. Biofilm formation usuallyoccurs on these plates and on the frustums. However, as biomass does not readily attach to these plates, due


Polluted air

Flow direction ofliquid and gas

Frustums

Nutrient tankLiquid collection

Treated air

Figure 10.9 Schematic of a horizontal-flow biofilm reactor.

to the use of a flat non-porous surface, other flat porous sheets made directly from fibres or polyurethanefoam sheets can be attached to the surface of the plastic plates. The main advantages of this reactorconfiguration can be summarized as follows: (i) no clogging problems and low pressure drop; (ii) goodgas–liquid–biofilm contact due to the series of several horizontal passes of the components within thereactor; (iii) flexible design and the bioreactor can be tailor-made to suit site and transport considerations;and (iv) less operation and maintenance cost compared to several other bioreactor configurations, becauseof the absence of moving parts [37].

10.3 Two-stage systems for waste gas treatment

10.3.1 Adsorption pre-treatment plus bioreactor

The use of an adsorption pre-treatment step ahead of a bioreactor offers the following advantages:(i) depending on the adsorptive capability of the adsorbent, usually GAC, short-term high peak loadscan be reduced or dampened to levels that can be handled by the bioreactor; and (ii) during low pollutantloading conditions, the adsorption/desorption steps prevent the microorganisms from starvation conditionsby releasing the adsorbed pollutants to the bioreactor [38]. Figure 10.10 shows the schematic of a first-stageadsorption column and a second-stage biofilter system for the treatment of volatile organic and inorganiccompounds (VOCs and VICs).


T

SECOND-STAGE BIOFILTERFIRST-STAGE ADSORPTION UNIT

Treated air

Periodic sprinkling of water or nutrients

Residual pollutants

Adsorbent

Polluted air Nutrient tank

Leachate removal

Figure 10.10 Schematic of a two-stage system (adsorption column plus biofilter).

Li and Moe [39] used a similar two-stage system (adsorption plus biofilter) and studied the performanceof the reactors by subjecting them to intermittent gas-phase acetone and toluene loadings. In order tocompare the level of buffering achieved in the first-stage adsorption column and the subsequent performancegain of the second-stage biofilter, the authors operated another biofilter as the control system, under thesame pollutant load, but without the adsorption pre-treatment step. Pollutant loading was done for 8 h atacetone and toluene concentrations of 550 ppmv each, followed by 16 h non-loading each day, in threephases, at different EBRTs. The EBRT for the buffered and unbuffered biofilter and the adsorption columnwere varied between 14.5 and 58 s, 17 and 58 s, and 10 and 2.5 s, respectively, in these phases of operation.During periods of non-loading, uncontaminated air flowed through the reactors at the same rate as duringpollutant loading intervals. It was reported that the buffered biofilter removed >98% of both contaminantsduring the first two phases of operation, that is, EBRTs of 58 and 29 s. However, during the third phase,at an EBRT of 14.5 s, although the removal efficiency of toluene did not decrease, the removal of acetonedecreased to ∼80%. During all three phases, the unbuffered biofilter exhibited lower removal efficienciesfor both pollutants. Besides, the unbuffered biofilter showed diminished performance following the 16 hstarvation period, while such effects were not observed in the buffered biofilter. Evidently, as the unbufferedbiofilter did not have the benefit of load equalization provided by the first-stage adsorption column, thecontaminant concentrations entering the biofilter during all three phases of operation were much higherthan the pollutant loads received by the buffered biofilter.

Although several studies have shown better performance of such two-stage systems, the use of a singleadsorption bed, prior to a biological system, can also pose some operational problems. The adsorption bedwould lose its buffering capacity when it is exhausted, and the bioreactor would experience a starvationperiod before the adsorption column starts to breakthrough, which would eventually alter the long-term


performance of the biological system [40]. Under such conditions, it is beneficial to use a dual fixedadsorption/desorption cycle as the first-stage non-biological treatment step, followed by a bioreactor.

Aly Hassan and Sorial [40] proposed the utilization of dual GAC beds connected in series and operatedin flow-switching mode prior to a biotrickling filter for the treatment of peak gas-phase concentrations ofn-hexane (10–470 ppm) and benzene (30–1410 ppm). The cyclic adsorption/desorption bed systems weredesigned to run on a short-term cycle that depends on the principle of contaminant pressure variation,and were designed to operate in a two-step cycle, that is, feeding (adsorption) and purging (desorption)within cyclic adsorption/desorption beds. The authors reported that the cyclic adsorption/desorption bedsunit successfully achieved its goal of stabilizing erratic loadings even with very sharp peaks in the inletconcentration, it buffered the fluctuating inlet load, and the biofilter had all the time a continuous stablegas flow rate even during the starvation phase where no contaminant was fed to the cyclic beds.

10.3.2 Bioreactor plus adsorption polishing

GAC adsorption can also be used as a polishing post-treatment step following a bioreactor. Kraakman [41]outlined the advantages of combining biological treatment with GAC polishing, which can be summarizedas follows: (i) this combination of reactors can be used in locations that are extremely sensitive to odournuisance and when very high removal efficiencies are required; (ii) this design is helpful in those situationswhere insufficient footprint is available for expanding biological treatment systems in order to reducepollutant concentrations to very low levels; (iii) generation of less waste, which will also be less hazardouscompared to stand-alone GAC filters; (iv) less maintenance costs; and (v) easy to operate.

As reported by Kraakman [41], practical experiences from two locations (foul air from a wastewatertreatment plant located in Los Angeles, USA, and foul air from headworks from a wastewater treatmentplant located in Utrecht, The Netherlands) showed that the two-stage system achieved odour removal effi-ciencies exceeding 99.5% shortly after start-up. The foul air from these locations contained H2S (maximum60–200 ppmv) and reduced organic sulphur compounds (100–1500 ppbv). It was reported that the first-stage biological treatment system removed a major portion of these pollutants, while the non-treated H2S(2–10 ppmv) and reduced organic sulphur compounds (40–1200 ppbv) from the first-stage bioreactor wereremoved in the second-stage GAC polishing step with removal efficiencies >90% for H2S and ∼30–60%for the reduced sulphur compounds. The following recommendations and precautions were suggested forusing adsorption polishing after a biological treatment step: (i) a good engineering of the whole systemis important to consider this option; (ii) when H2S and other odorous compounds are not sufficientlyremoved in the first-stage biological system, the carbon life is most likely to be low, lasting only fewmonths; (iii) all free water after the biological treatment step has to be removed to prevent problems withcarbon adsorption; and (iv) the trade-off between the investment costs of the biological system and theoperation costs for changing the spent carbon is difficult to assess, as this depends on the type of pollutantspresent in the waste gas and the nature of breakthrough achieved for each compound.

10.3.3 UV photocatalytic reactor plus bioreactor

The combination of UV pre-treatment and a bioreactor has only recently received attention amongresearchers. UV pre-treatment can be done in different photoreactor configurations; the most commonlyused reactor configuration for waste gas treatment is the annular-type photoreactor [42, 43]. However,the use of UV photoreactors, as stand-alone systems, for waste gas treatment is limited in applicationdue to their tendency to produce water-soluble by-products that are of environmental concern. In someinstances, the end-products formed during photo-oxidation are more toxic than the parent compound


Periodic nutrient additionFIRST-STAGE PHOTOCATALYTIC

REACTOR

SECOND-STAGE BIOFILTER

Cool water inlet

Residual pollutant

Catalyst coating

UV lamp

Polluted air

Water outlet

Treated air

Leachate collection

Figure 10.11 Schematic of a photocatalytic reactor coupled with a biofilter.

itself. For example, phosgene is one of the photo-oxidation products of gas-phase dichloromethane [42].A combined UV photocatalytic oxidation system as a pre-treatment step followed by a biological wastegas treatment system (Figure 10.11) offers the following advantages: (i) rapid oxidation of a wide varietyof recalcitrant compounds to soluble and biodegradable form; (ii) ability to reduce high concentrations ofpollutants to limits that can easily be handled in a biological system; (iii) versatility to handle unexpectedvariations in pollutant loading rate; (iv) no operational difficulties related to pressure drop and clogging;and (v) the photo-oxidation step can be changed to a post-treatment step when bioreactor performance islimited or inhibited [42, 44].

Heterogeneous photocatalysis uses a suitable semiconductor catalyst (mostly TiO2) to generate a pairof a conduction band electron and a valence band hole in the solid oxide lattice upon absorbing a photonwith energy greater than 3.2 eV, and the subsequent charge transfers at the interface initiate various kindsof redox reactions under well-controlled ambient conditions. Semiconductors such as ZnO, WO3, Fe-TiO2and Sr-TiO2 have also been used to carry out photo-induced redox reactions for the degradation of VOCsin the gas phase. In annular-type photoreactors, the catalyst is coated on the inner side of the outer tube,while the UV lamp is placed in the inner tube. The polluted air can be passed to the annular space ofthe photoreactor in upflow or downflow mode. The electrons can reduce an electron acceptor such asmolecular oxygen, forming superoxide ions, and holes can oxidize electron donors, including absorbedwater or hydroxide groups (—OH), to yield hydroxyl radicals (•OH). The chemistry occurring at thesurface of the photo-excited semiconductor catalyst is related to the radical formed from O2, H2O andthe electron-rich gas-phase pollutants. Thus, at the surface of the catalyst, the hydroxyl radicals drive thechemical reaction by oxidizing and progressively breaking the gas-phase molecules into CO2, H2O, andother end-products [43].


The combined use of a UV photocatalytic reactor and a bioreactor has been reported in a few studies andsynergistic effects in pollutant removal have been observed. A few examples for such synergistic effectsin pollutant removal efficiencies are given here.

Wei et al . [45] studied the removal of gas-phase styrene in a bench-scale photocatalytic oxidation unitusing N-doped TiO2/zeolite as the catalyst, combined with a biofilter inoculated with activated sludge.The EBRTs were varied from 40.4 to 121.3 s. Illumination was provided by different light sources, namelya 4 W UV lamp and a 4 W visible lamp, placed in axial position within the photoreactor. In order toascertain the photo-activity, the authors compared the performance of the photoreactor under differentoperating conditions: pure TiO2 + UV, N-doped TiO2 + UV, N-doped TiO2 + visible light, and N-dopedTiO2 + UV + visible light. They found [45] that the N-doped TiO2 with UV and the N-doped TiO2 with UVplus visible light showed higher purification efficiency (>50%), at toluene concentrations of 290 mg m−3,when operated at a gas residence time of 4.5 s. Besides, under well-optimized conditions, toluene removalefficiencies as high as 96.7% could be attained in the integrated system, at an EBRT of 121.3 s in thebiofilter, for inlet concentrations varying between 210 and 500 mg m−3. The authors also reported thatthe intermediate organic products of toluene oxidation, namely benzaldehyde, benzene, benzal methanol,formaldehyde, vinyl methyl ketone and methyl glyoxal, formed in the photocatalytic unit were subsequentlyconverted to CO2 and H2O in the biofilter.

Palau et al . [43] studied the influence of UV pre-treatment on the performance of a biofilter in long-term experiments (300 days) for the removal of gas-phase toluene. A plug-flow annular-type photoreactorfitted with a 36 W UV lamp was used as the first-stage reactor. The lamp had a spectrum centred ata wavelength of 254 nm and the light intensity on the annular space was around 16.2 mW cm−2. Thesecond-stage peat biofilter was inoculated with activated sludge, and acclimated to toluene for 2 months.At toluene inlet loading rates of 50 and 100 g m−3 h−1, the stand-alone biofilter exhibited ECmax values of33 and 73 g m−3 h−1, respectively. However, when the UV photoreactor was combined with the biofilter,the ECmax improved to 45 and 85 g m−3 h−1, under the same loading conditions. Furthermore, the authorsreported that the ECmax achieved in the integrated system was more than three times greater than theECmax achieved in the photocatalytic reactor alone. Concerning the photo-oxidation end-products fromtoluene oxidation, the authors observed small amounts of benzaldehyde from the photoreactor, which waspresumably removed by the microorganisms present in the second-stage biofilter.

The UV pre-treatment step can also be used as a load equalization step during transient operations,especially during shock loads. In order to save operational costs, it is advisable to use the UV pre-treatment step only when absolutely required. The UV lamps can be energized and turned on when thepollutant concentration is high, that is, during unexpected load fluctuations, with the help of online sensors.The UV-operated photocatalytic reactor can then serve as a load equalization system by bringing downthe pollutant loads to levels easily treatable in the bioreactor.

Rene et al . [38] coupled a photocatalytic reactor and a continuous stirred-tank bioreactor (CSTB) in orderto handle a 7 h shock load of dichloromethane (DCM) vapour. The first-stage annular-type photocatalyticreactor, coated on the inner side of the annular space with commercially available TiO2 plus PVA, wasoperated at a gas residence time of 55.2 s, and illuminated with a 50 W UV light source. The second-stage CSTB was inoculated with Hypomicrobium sp. and was operated at a residence time of 120 s. Itwas observed that, when the inlet load was increased from low (70 g m−3 h−1) to exceedingly high load(1056 g m−3 h−1), the photocatalytic reactor was able to eliminate nearly 70% of the load, thereby savingthe second-stage CSTB from severe shock load that would otherwise inhibit bacterial activity, and affectits long-term performance.

In another recent study [42], a stand-alone Hypomicrobium-inoculated biotrickling filter (EBRT of120 s) tested for the removal of gas-phase DCM reached an ECmax of 163 g m−3 h−1 at an inlet loadingrate of 282 g m−3 h−1. Later, short-term shock-loading experiments were conducted by combining the


photoreactor (first stage) and the biotrickling filter (second stage). The gas retention time and EBRT of thephotoreactor and the biotrickling filter were maintained constant, at 55.2 and 120 s, respectively. When theinlet DCM concentration to the first-stage photoreactor was increased from 0.5 to 5.6 g m−3, correspondingto a maximum DCM load of 364 g m−3 h−1, about 55% DCM was removed in the photoreactor, whilethe remaining DCM was completely removed in the biotrickling filter. In this integrated system, anECmax of 268 g m−3 h−1 was achieved with 89% removal. The photo-oxidation products of DCM wereidentified as phosgene, chloromethane, carbon monoxide and carbon tetrachloride using Fourier transforminfrared (FTIR) analysis. These toxic pollutants were removed in the second-stage biotrickling filter bya combination of hydrolysis, adsorption and microbial degradation.

10.3.4 Bioreactor plus bioreactor

Different bioreactor configurations (biotrickling filter + biofilter, biofilter + biofilter, biotrickling fil-ter + biotrickling filter, biofilter + CSTB, among other possible combinations) can also be used in series,as two-stage systems, in the following situations: (i) when the waste gas contains mixtures of pollutantswith different biodegradation rates and it is desirable to remove some pollutants with high priority in onestage; (ii) when it is desirable to maintain different microbial species individually in different bioreactors,each targeting specific pollutants from the waste gas; and (iii) when the waste gas contains mixtures ofVOCs (hydrophobic and hydrophilic) and VICs. One of the most commonly reported problems duringthe co-treatment of VICs, such as H2S, and VOCs in bioreactors is that the pH of the biofilm would dropwhen H2S is converted to sulphuric acid, which may partly inhibit the biological activity in a bioreactor,and the subsequent degradation steps. The generation of acidic metabolites could also adversely affectsome characteristics of the packing material, resulting in channelling in some specific areas and filterbed compaction. Under such conditions, and for industrial situations like emissions from the pulp andpaper industries (sulphur compounds, and other volatile organic and inorganic pollutants) and wastewatertreatment plants (H2S and other gas-phase VOCs and VICs), two-stage biological systems have proven tobe beneficial.

Chitwood et al . [46] evaluated the feasibility of using a two-stage biofilter for the treatment of H2S, toxicair pollutants and smog precursors. The first-stage acid-gas biofilter (AGB) packed with lava rock containedacidophilic autotrophic bacteria to remove H2S, while the second-stage wood-chip biofilter removed othertoxic air pollutants that included methanol, acetone, methylene chloride, chloroform, toluene, xylene, ethylbenzene, methyl t-butyl ether (MTBE) and 2-methylbutane. However, they observed that the first-stageAGB removed acetone and methanol completely, while other VOCs were intermittently removed dependingon the concentrations, in addition to 99.6% removal of H2S at an inlet loading rate of 0.057 g m−3 h−1.

Sercu et al . [47] tested two biotrickling filters connected in series for the removal of H2S and dimethylsulphide (DMS) from polluted air. The rationale for using a two-stage bioreactor configuration is the factthat H2S is preferentially degraded over DMS or other reduced sulphur compounds when present in a gas-phase mixture, and the degradation of DMS can be realized with high removal efficiencies at neutral pH,while at low pH values, their removal efficiencies would decrease. Thus, the authors operated the first-stagebiotrickling filter (the ABF) without pH control and inoculated with Acidithiobacillus thiooxidans for theremoval of H2S, while the second-stage biotrickling filter (the HBF) was inoculated with HyphomicrobiumVS and operated at neutral pH for the removal of DMS. This two-stage biotrickling filter was found to beefficient in removing high loads of H2S and DMS. The ECmax of the first-stage ABF was 83 g m−3 h−1

with >99% removal efficiency, while the second-stage HBF showed an ECmax of 58 g m−3 h−1 with 89%removal efficiency.

Rene et al . [48] tested a first-stage biotrickling filter inoculated with autotrophic hydrogen sulphidedegraders and an acid-tolerant yeast (Candida boidinii ), connected in series to a second-stage biofilter that


was inoculated with the fungus Ophiostoma stenoceras , for the removal of H2S, methanol and α-pinenevapours. H2S and methanol were removed in the first-stage biotrickling filter with an ECmax of 45 and894 g m−3 h−1, respectively, while α-pinene was removed predominantly in the second-stage fungal biofilterwith an ECmax of 138 g m−3 h−1. The original idea was to remove H2S in the first reactor, with mediumacidification, and both VOCs in the second reactor, at constant pH. However, methanol degraders presentin the biotrickling filter appeared to be acidophiles, and methanol was easily removed in the first stagetogether with H2S.

In recent work from our group (unpublished data), a hybrid reactor, combining a biofilter and a biotrick-ling section in one single column, was tested for the removal of a methanol, pinene and H2S mixture(Figure 10.12). The microbial populations in the biofilter and biotrickling filter sections were different,and each of them had a different operating pH in order to favour optimal conditions for microbial activity.The initial performance of this hybrid bioreactor was not good because of the low fungal growth in thebiofilter section due to the only moderate tolerance of the fungus to acidification.

Yeom and Yoo [49] also tested a hybrid bioreactor consisting of a bubble column bioreactor section anda biofilter section (Figure 10.13) for the removal of gas-phase benzene. Sodium alginate beads containingimmobilized cells of Alcaligenes xylosoxidans Y234 were used in both sections. Experiments were con-ducted at a constant inlet benzene concentration of 45 ppm, while the residence time was varied between15 and 60 min to give different loading rates of benzene to the hybrid bioreactor. The removal of benzene

Treated airPeriodic nutrient or water addition

BIOFILTER SECTION

Nutrient recycleSprinkler

BIOTRICKLING FILTER SECTION

Polluted air

Nutrient collection tank

Figure 10.12 Schematic of a hybrid bioreactor (biotrickling filter plus biofilter).


Treated air

Nutrient medium

Biofilter

Influent stream

Effluent stream

Inmobilized cells

Air bubbles

Polluted air

Figure 10.13 Schematic of a hybrid bioreactor (bubble column bioreactor plus biofilter).

in the bubble column bioreactor includes both the removal by suspended immobilized cells and strippingby air injection. Thus, in this hybrid bioreactor, the stripped benzene in the bubble column bioreactor wascompletely removed in the biofilter, and the removal efficiency of the hybrid bioreactor was equal to thatof the bubble column bioreactor. The hybrid bioreactor showed removal efficiencies ranging from 65 to100%, while the share of the biofilter section to remove benzene varied between 15 and 72%, dependingon the residence time and the benzene load.

One of the major advantages of this bioreactor configuration is the fact that the pollutant load could beshared between the two sections, and the fluctuation of load on the hybrid bioreactor could be absorbed bychanging the distribution of benzene between the biofilter and the bubble column reactors. These hybridbioreactors and some two-stage bioreactor configurations are still in the developmental stage, and theyrequire further optimization and pilot-scale testing before being put into commercial use.

10.4 Conclusions

Novel bioreactor configurations for waste gas treatment are being developed in order to try to broaden theapplication range compared to conventional bioreactors and/or in order to solve the operational problems


encountered in the latter, such as, for example, clogging phenomena in packed-bed bioreactors. At thelaboratory scale, most of the novel bioreactor configurations mentioned in this chapter have proven tobe effective for handling gas-phase volatile pollutants – usually single pollutants – often without posingmajor operational problems. Future research directions should target process optimization and the scale-upof these innovative bioreactor configurations, and test their ability to handle complex gas-phase mixtures,at low gas residence times (<15–20 s). The advantages of combining bioprocesses with physico-chemicaltechniques such as adsorption and UV photocatalysis were also reviewed. Adsorption pre-treatment canbe operated in cyclic modes, that is, adsorption and desorption, reducing potential problems related toshock loads and starvation conditions. Similarly, photocatalytic reactors, used as the pre-treatment step,can effectively withstand short-term shock loads and reduce pollutant loads to levels acceptable in abiological system.

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