Physical and chemical technologies have been successful in treating different classes of gases. The major benefit associated to these technologies is that they have been established over decades in areas of both design and operations. Nevertheless, they have key shortcomings. These include the use of chemicals (chemical absorbers), which can be expensive, high costs of energy (thermal destruction), exceptional operational safety procedures (chemical absorbers), production of waste products (chemical absorbers, activated carbon adsorbers and thermal destruction) and products of incomplete combustion (thermal destruction). All theses shortcomings incur additional cost in operation of the processes.
3. BIOTECHNOLOGY FOR GAS TREATMENT.
The use of biotechnology in gas treatment/purification, especially in waste gas treatment systems, has recently gained attention as an alternative to get rid of some drawbacks associated with the use of conventional physical-chemical techniques. This technology involves the use of microbial cultures and exploits the ability of these microorganisms to degrade (biodegradation) compounds. Microorganisms are noted to be important in geochemical and biogeochemical cycles by mineralizing biopolymers (polymers produced by living organisms) and xenobiotic [1] compounds. Although the underlying biodegradation mechanism of such materials is not completely understood, Brauer (1986) stated a simple transformation process as:
Contaminated gas + O2 ï‚®via bacteriaï‚® more bacterial cells + CO2 + H2O. (3)
For instance, on provision of O2, bacteria oxidizes ionic sulphide species to non-odourus sulphur species.
Biological gas treatment is considered safe, as it is operated at ambient temperatures without the necessity of storage and handling of chemicals and is often applied because of its low operational costs. It is significantly applied in the treatment of waste gases (from either wastewaters or exhaust gases). Waste gas treatment (odour treatment) using biological methods only gained support in Europe in the 1990's and by 1994, it accounted for 78% of odour treatment in Germany (Frechen, 1994).
According to Kurt Kirchner et al (1987), the methods used for biological treatment of exhaust gases can be generally grouped into dry and wet processes. In the dry process, the gas streams are passed through biological filters (with embedded microorganisms), usually made up of soil or compost. Here, the comtaminants are sorbed and then degraded biologically(Kurt Kirchner, 1987). In the wet process absorbers are used. The contaminants passed through an absorber, using bacterial cultures as the absorbent. The contaminants diffuse into the liquid phase where they are degraded by microorganisms (Kurt Kirchner, 1987).
Although biological methods have long been known to be cost-effective, however they have not been generally accepted in practice. According to Kurt Kirchner et al (1987), the reasons frequently mentioned are; Long adaptation periods of the biomass (in particular with large exhaust gas flow discontinuities) or low space-specific purification capacities (space velocities) [2] . Although biological gas treatment is most commonly used for wastewater gas treatment facilities, it has been progressively more useful in treating industrial emissions that contain a mixture of VOCs.(Moe, 2004)
3.1 Biological Reactors (Bioreactors)
The use of biotechnology in waste gas purification technology currently includes the use of bioreactors namely; biofilters, biotrickling filters, bioscrubbers, and membrane bioreactors. Nevertheless, activated sludge (a liquid-based) system can be used as an alternative to the ones mentioned above. These technologies have a similar method of operation (WAWERU, 2005). The contaminated gas stream is passed through the bioreactor where the contaminants are move from the gas phase into the liquid phase.
3.1.1 Microorganisms Used in biological reactors
Microorganisms (such as bacteria or fungi) used bioreactors for gas treatment grow in the liquid phase of the process and are responsible for the biodegradation. They usually grow as a mixture of diverse organisms. This mixture different microorganism is known as microbial community and depends on a number of interactions.
Bacteria are the conventionally used microorganisms but recently, reports have shown that fungi can absorb and degrade contaminants (e.g. H2S and hexane) and also hydrophobic organic compounds, at higher rates. According to van Groenestijn, (2005), the use of fungi has the potential of increasing elimination capacities of waste gas bioreactor systems up to 5-10 times greater than the conventional compost systems. Kennes and Veiga (2004) report that many different fungi can be potentially used in biofiltration; some of these are listed on table 9. A large number of naturally occurring bacteria and fungi can degrade hydrocarbons. The most commonly found genera of bacteria and fungi that are capable of degrading hydrocarbons include; (1) bacteria: Pseudomonas, Arthrobacter, Alcaligenes, Corynebacterium, Flavobacterium, Achromobacter, Micrococcus, Nocardia, and Mycobacterium, and (2) fungi: Thricoderma, Penicillium, Aspergillus and Mortierella (Miller, 1990). Table 9 and 10 lists microorganisms and compounds on which they used for, in biological waste gas treatment.
Table 9. Biodegradation and growth of filamentous fungi on some volatile aliphatics and phenolic pollutants.
Compounds Organisms
n-Alkanes
C2-C4-Alkanes Acremonium sp.
C6-C19-Alkanes Cladosporium resinae , ATCC 22711
n-Butane Graphium sp. ATCC 58400
C1-C9-Alkanes Scedosporium sp.
n-Hexadecane Trichosporon veenhuisii CBS 7136
Phenols
Phenol Aspergillus japonicus
Phenol Penicillium spp.
Phenol Trichosporon cutaneum
Phenol Trichosporon guehoae CBS
m-Cresol 8521
Phenol Trichosporon veenhuisii
m-Cresol CBS 7136
p-Cresol Aspergillus fumigatus ATCC 28282
o-Cresol Penicillium frequentans ATCC 96048a
(growth on glucose and phenol)
o, m, and p-Cresol Trichosporon cutaneum KUY-6A
Others
Formaldehyde Gliocladium deliquesecens
Methanol
Formaldehyde Paecilomyces variotii
Methanol
Diethyl ether Graphium sp. ATCC 58400
Methyl tert-butyl ether Graphium sp. ATCC 58400a
(growth on n-alkanes)
Source: Kennes and Veiga, (2004)
Table 10. Examples of microorganisms degrading H2S and dimethyl disulfide.
Compounds Microorganism Other characteristic
Hydrogen sulphide Thiobacillus sp. CH11 Autotrophic
Thiobacillus thioparus TK-m Autotrophic
Thiobacillus thiooxidans KS1 Autotrophic
Hyphomicrobium sp. I55 Methylotrophic
Xanthomonas sp. DY44 Heterotrophic
Unidentified basidiomycete Fungus
Cephalosporium sp. Fungus
Dimethyl disulphide Thiobacillus sp. Autotrophic
Hyphomicrobium sp. I55 Methylotrophic
Pseudomonas acidovorans DMR11 Heterotrophic
Unidentified basidiomycete Fungus
Cephalosporium sp. Fungus
Source: Kennes, and Veiga, (2004)
These microorganisms are normally prearranged in biofilms (thin layers). In most situations, according to WAWERU, (2005) the contaminants in the air (such as toluene, methane, ethanol, aldehydes) serve as a source of energy and carbon for growth and maintenance of the microorganisms.
Microorganisms also require essential nutrients and growth factors in order to function and produce new cells. Theses include nitrogen, phosphorus, sulfur, vitamins, and trace elements (WAWERU, 2005). Most often these nutrients and growth factors are absent from the waste gas and have to be provided.
3.1.2 Bioreactor packing media
Selection of a packing media for a bioreactor is generally based on the ability to support microbial growth, but this may also vary depending on the reactor types. According to Shareefdeen and Singh, (2005) the performance of a bioreator for odour or VOC control depends on the nature of the media where adhesion of microorganism takes place. This results to the formation of a biofilm due to contaminants degradation. Also cost, chemical reactivity, large surface area and pressure drop are essential factors to consider in selecting a bioreactor medium. These packing medias will be discussed alongside types of bioreactors in Section 3.2 below.
3.1.3 Operational factors
The three principal factors that ascertain the total effectiveness of a bioreactor are moisture, temperature and pH. Other factors that have some influence include nutrients, particulate material in the emission stream, direction of airflow, type of contaminant(s) and available oxygen (Boswell, 2002).
3.1.3.1 Moisture
The moisture level of the biomass (media containing microorganisms) is very essential to sustain the microorganisms alive. An extreme degree of dryness may result to the death of the organisms. It also causes a rapid passage of a great portion of the contaminated gas stream through the system, resulting in an incomplete treatment.
On the contrary, the biomass may be drowned in a case of extreme moisture levels, resulting to a reduced treatment level. Also, airflow may be restricted, which causes an increase in backpressure, hence increased power consumption. To avoid these situations, a robust moisture control system has to be used and also the incoming stream must be sufficiently humidified (Boswell, 2002).
3.1.3.2 Temperature
The metabolism of the microbes is controlled by temperature. There are three general temperature classes of aerobic microorganisms: psychrophilic, mesophilic and thermophilic microorganisms. Psychrophiles operate best at temperatures less than 20°C, while mesophiles and thermophiles in the range of 15-40°C, and 60-100°C respectively.
Microbial activity approximately doubles with each 10°C increase in temperature, as long as the organisms remain within their thermal tolerance zone (within the temperature range). When the temperature goes above each species' upper thermal maximum range, the microbial activity starts to decrease and at elevated temperature, the microorganisms will begin to die. The temperature-metabolism correlation can be challenging, due to the presence of thousands of microbial species, with diverse thermal tolerance, contained in a bioreactor.
Sudden changes in temperature can affect the biodegradation of the contaminants (Boswell, 2002). Considering this, temperature control system is essential, notwithstanding the ability of a bioreactor to operate effectively over a wide temperature. This system will be essential to maintain a narrow operating temperature range, which is vital for moisture control.
3.1.3.3 pH
There is a definite optimal pH operating range for each microorganism. According to Boswell, (2002) bioreactors can function at a pH in the range of 2 - 9, but the pH can change regularly (usually decreasing) once operation has commenced. An effective pH system has to be put in place in order to maintain a maximum operational capacity. Acids are produced upon biodegradation of chlorinated or sulphur-containing compounds (such as H2S) therefore requires a control system (irrigation) to neutralize the acids. Consequently, a water removal system must be in place to prevent an extremely low pH in the system. In a situation where two compounds that are biodegraded at different pH (such as H2S and organosulfurs [3] , emitted from pulp and paper mills), a multi-stage bioreactor can be applied. This can be achieved by setting the media bed(s) near the inlet at a low pH and the beds further downstream at a higher pH. The pH of a bioreactor can also be managed by the addition of crushed limestone in the media, adding a buffering solution to the irrigation/humidification water, and by more or less frequent water removal or addition (Boswell, 2002).
3.1.4 Performance parameters
The performance of different biotechnologies used in gas treatment can be evaluated by a set of factors discussed below.
3.1.4.1 Empty bed residence time (EBRT) or True residence Time
According to Datta (2005), this is defined as the ratio of the empty bed filter volume and the airflow rate measured in seconds. It can be used for an easy estimation of the filter size for a given air flow (Datta, 2005).
EBRT = Vf/Q [s] (4)
Where Vf = filter bed volume (m3) and Q = the airflow (m3/s)
True residence time is the actual time air remains in the filter and is defined by equation (5) below. It can be used as an indicator of the time available for mass transfer of the contaminants from the gas phase to the liquid phase via the biofilms (WAWERU, 2005)
ï´ = (Vf ï‚´ ï±)/Q [s] (5)
where ï´ is true residence time (s), and ï± is porosity = volume of void space/volume of packing material.
3.1.4.2 Surface loading rate (BA)
This is defined as the volume of air that is passed through the bioreactor per unit surface area per unit time (WAWERU, 2005).
BA = Q/A [m3 m-2 h-1] (6)
where A = total surface of the packing or filter material in the bioreactor [m2].
3.1.4.3 Mass Loading Rate (BV)
This can be defined as the mass of the contaminant entering the biofilter per unit volume and per unit time (Datta, 2005). This indicates that if flow is constant through the bioreactor packing or filter material, the mass loading along the length of the bed will decline as contaminant is removed (Datta, 2005).
BV = Q ï‚´ Cg - in / V [g m-3 h-1] (7)
where Cg-in = concentration of the pollutant in the inlet gas stream [g m-3]
3.1.4.4 Volumetric Loading Rate (vs)
This is the amount of contaminated gas passed through the reactor per unit reactor volume (WAWERU, 2005).
vS = Q/V [m3 m-3 h-1] (8)
3.1.4.5 Elimination Capacity (EC)
This gives the amount of pollutant removed per volume bioreactor per unit time (Datta, 2005). An overall elimination capacity is defined by the equation below
EC = Q ï‚´ (Cg - in - Cg - out) / V [g m-3 h-1] (9)
where Cg-out =concentration of the pollutant in effluent waste gas [g m-3].
3.1.4.6 Removal Efficiency (RE)
This is the fraction of the pollutant removed in the bioreactor expressed as a percentage (Datta, 2005). It is defined as;
RE = [(Cg - in - Cg - out) / Cg - in ]ï‚´ 100 [%] (10)
3.2 Gas treatment Bioreactors
There are fundamental differences between the types of reactors mentioned in section 4.1 above. According to WAWERU (2005), "these differences range from the way microorganisms are organized (i.e., immobilized or dispersed) to the state of the aqueous phase in the reactor (i.e., mobile or stationary)". The state of the aqueous phase considerably influences the mass transfer characteristics of the system (WAWERU, 2005). A description of each of the types of bioreactors for gas purification currently in use is given below
3.2.1 Biofilters
A biofilter simply refers to a reactor that converts a gas-phase chemical compound to products such as CO2 and H2O. The use of biofiltration in the treatment of contaminated air (gas) streams dates back to the early (Ottengraf and van den Oever, 1983; Ottengraf and Diks, 1990; Bohn, 1992; Bohn and Bohn; 1988).
Biofilters can be classified as; (1) biofilters (bulk media) and, (2) trickling biofilters. The differentiating factor is that the bulk media biofilters have adequate water holding capacity to maintain bioactivity using humidified air while trickling biofilters require a continuous flow of water to maintain the moisture content of the biofilms (Govind R., 2005). The types of media used in biolfilters can be grouped into; (1) fine or irregular particulates, such as soil, peat (partially decayed vegetation), compost or a mixture of these materials (2) randomly packed pellets, and (3) structured materials (such a monoliths, discussed in section 4.2.1.6) with definite passage size and shape. The materials used in making the media can be either sorbent (e.g. activated carbon) or non-sorbent (e.g. metal, ceramic) materials.
3.2.1.1 Bulk media biofilters
In a biofilter the contaminated air stream is passed through a bed of naturally occurring microorganisms immobilised on a media bed. The media used in biofilters include; peat, soil, compost and sand, but soil and compost most commonly used(Burgess, 2001). The influent air stream is humidified, before it is passed through the media bed. This helps to maintain the microbial activity and provide nutrients (nitrogen and potassium) to the microorganisms, while some of the contaminants in the stream provide a source of carbon and energy. As the influent is passed through the bed, the compounds (contaminants) are transferred to a biofilm, where biodegradation occurs.
There are two types of bulk media biofilters; (1) Open soil (or compost) biofilter and, (2) Closed aerated biofilter. The open soil biofilter has been conventionally used and, as the name implies, is a ditch or gutter filled with gravel, containing a perforated pipe through which the gas stream is pumped (Burgess, 2001) (Figure 14). Closed aerated systems have been favoured because of the need to act in accordance with emission monitoring legislations. The system is simply a closed vessel, consisting of a humidifier, a bed of inoculated media, through which contaminated air is passed (Figure 15).
Figure 1. Open soil or compost biofilter. (Burgess, 2001).
Figure 2. Closed aerated biofilter (Burgess, 2001)
3.2.1.1.1 Factors affecting biofilter performance.
There are certain factors affecting the performance of a biofilter and these important factors to be considered in design and operation of biofilter systems. They include; packing media, moisture, temperature, oxygen content, pH, nutrients, pressure drop, depth of media, waste gas pre-treatment and maintenance. These are summarized on Table ??, while the advantages and disadvantages of biofilters are shown on Table ?? below.
Table 11. Influential factors in design and operation of media-based biofilters
Factor
Target value
Influence
