Biological Techniques

9.3.2.1 Biological Techniques for Carbon Dioxide Removal

Although very little has been reported about biological methods for the removal of carbon dioxide from biogas, recently some research groups have been investigating the possibility to use natural biodegradation processes such as photosynthesis or enzymatic catabolism, in biogas upgrading.

Converti et al. (2009) have reported a laboratory-scale apparatus for biogas production and purification using a two-stage biological system, viz., anaerobic digestion of sewage sludge from a waste water treatment plant followed by carbon dioxide removal by the photosynthetic Cyanobacteria sp., Arthrospira platensis (anaerobic digester+photo-bioreactor). Cyanobacteria sp. as well as some microalgae, can remove simultaneously, organic and inorganic pollutants, some of these chemicals are very hazardous like phosphoric compounds. A. Platensis sp . presents the advantage of mixotrophic metabolism, to remove either carbon dioxide or organic pollutants. Mixed sludge was added daily into the digester by fed-batch pulse feeding mode of operation and retention times were varied in the range of 6.2-50 days (beyond mesophilic digester ranges). The biogas thus produced was transferred into a sulfuric acid solution and fed into the photo-bioreac-tor daily, by liquid displacement. A. Platensis was initially acclimated in a batch system and later cultivated in the photo-bioreactor that was made from a 1.0 L closed glass vessel, illuminated by a 40 W-fluorescent lamp. Temperature was maintained at the microorganism's optimum value of 30°C. Carbon dioxide was the only carbon source available for the growth of cyanobacteria. The results from that study showed almost complete carbon dioxide removal from biogas (99%) and, the removal rate showed a near linear relationship with the microbial growth. Besides, from the photosynthesis step, oxygen was also produced in a range of 10-24%.

A research group in Sweden has studied the use of the enzyme carboanhydrase for carbon dioxide removal from biogas. Carboanhydrase is an enzyme present in human blood that catalyzes the dissolution of carbon dioxide generated during cell metabolism. The dissolved carbon dioxide is transported as a carbonate to the lungs (Eq. 9.9), where the same enzyme catalyzes the reverse reaction where carbon dioxide and water is formed (Petersson and Wellinger 2009).

According to their study, biogas could be purified up to a methane content of 99% (Mattiasson 2005) .

9.3.2.2 Biological Techniques for Hydrogen Sulfide Removal

Bioprocesses for odour control are not a new technology, and bioreactors were already used several decades ago in waste water treatment plants (Pomeroy 1957; Ottengraf and Diks 1992). However, recently they have further been developed and optimized as relatively new alternatives to non-biological processes, for waste-gas treatment (Kennes and Veiga 2001; Kennes et al. 2009a) and several new types of reactors, other than conventional biofilters, have recently been used as well. Biological treatment systems are based on the activity of microorganisms to decontaminate polluted air through a series of phenomenological steps like absorption, adsorption, diffusion and biodegradation (Rene et al. 2009). The pollutants are oxidized to innocuous end-products like carbon dioxide, water and biomass, by a thriving microbial population present in the bioreactor (Kennes and Veiga 2001). For biogas upgrading, the most common biological systems are biofilters, biotrickling filters and bioscrubbers.

Microbial Considerations

Biogas can be desulfurized through the action of microorganisms. The main best performing species are the chemotrophs, namely Thiobacillus sp. and Sulfolobus sp. In oxygen-rich environments, chemotrophs use oxygen as their electron acceptor, and hydrogen sulfide, thiosulfate or elemental sulfur as the electron donor. Under oxygen limited conditions, sulfur is the major end-product, which can be represented by the following equation (Kuenen 1975):

H2S + CO2 + nutrients + O2 ® cells + nutrients + Sand/orSO42- (9.10)

Oxygen is the key parameter to control the level of final oxidation (Rene et al. 2010), therefore in biogas treatment, the main end-product would be elemental sulfur, considering that biogas is anaerobic. Recently, several studies have reported the biological treatment of hydrogen sulfide using sulfur-utilizing chemolithoautotrophic denitrifiers. Among others, two species are well known, Thiobacillus denitrificans and Thiomicrospira denitrificans. These microorganisms can degrade hydrogen sulfide in the absence of oxygen, using nitrate as the electron acceptor. Sulfoxidation by these microorganisms can lead to the formation of elemental sulfur or sulfate, under both aerobic and anaerobic conditions (Beristain Cardoso et al. 2006).

Raw biogas

Fig. 9.8 Schematic of a biofilter for H2S removal. A well-optimized biofilter provides the required ideal habitat for the growth of sulfide-oxidizing bacteria, to the exclusion of competing microbes which can normally predominate in such systems. Microorganisms play a vital role in oxidizing hydrogen sulfide to sulfuric acid

Treated biogas

Fig. 9.8 Schematic of a biofilter for H2S removal. A well-optimized biofilter provides the required ideal habitat for the growth of sulfide-oxidizing bacteria, to the exclusion of competing microbes which can normally predominate in such systems. Microorganisms play a vital role in oxidizing hydrogen sulfide to sulfuric acid

Overview of Bioreactor Configurations

Biofilters

A biofilter consists of a filter bed, with an organic packing like peat or compost or synthetic ones like plastic pall rings (Fig. 9.8). A nutrient solution comprising inorganic salts and trace elements is added periodically to the bioreactor to supply the nutrients required for microbial growth, and to maintain optimal moisture contents in the filter bed (usually 40-60%). The packed bed acts as a carrier medium for the biomass and the nutrient source (Kennes and Thalasso 1998) . The raw biogas is allowed to pass through the filter bed, in an upflow or downflow mode, and hydrogen sulfide present in the biogas is degraded by the microorganisms.

One disadvantage is the drop in pH down to extremely low values, often < 3, during the conversion of hydrogen sulfide into sulfur or sulfate, which then needs periodic adjustment of media pH, by the addition of NaOH (Jin et al. 2005).

Bioscrubber

Fig. 9.9 A typical schematic ofbiotrickling filter. In practice, pilot-scale or full-scale biotrickling filters are periodically exposed to many perturbations as a result of fluctuation or discontinuous shock loads. Industrial experiences have shown that H2S concentrations can be reduced from >2,000 ppm to below 3 ppm

Nutrient solution

Fig. 9.9 A typical schematic ofbiotrickling filter. In practice, pilot-scale or full-scale biotrickling filters are periodically exposed to many perturbations as a result of fluctuation or discontinuous shock loads. Industrial experiences have shown that H2S concentrations can be reduced from >2,000 ppm to below 3 ppm

Biotrickling Filters

The main differences between conventional biofilters and biotrickling filters, is that in the later, the aqueous phase is continuously trickled over the packing and the bed is always made of inert materials, (Kennes et al. 2009b). A schematic of the biotrickling filter is shown in Fig. 9.9. Biotrickling filters are more complex than biofilters, yet they have been used in field situations to remove hydrogen sulfide from biogas. This is due to its better performance in handling acidic compounds like hydrogen sulfide, and its easiness in controlling the different physico-chemical operational parameters, viz., pH, temperature and others.

Profactor Produktionsforschungs GmbH® recently designed a biotrickling filter, seeded with aerobic bacterial consortia, to remove hydrogen sulfide from biogas. The system did not pose any operational problem and was found to be effective during long-term operation, bringing down hydrogen sulfide concentrations from about 2,000 ppm to below 3 ppm (Trogisch and Baaske 2004; Ahrer et al. 2006).

Soreanu et al. (2008) developed a pilot-scale biotrickling filter for hydrogen sulfide removal from biogas, using Thiobacillus denitrificans as the predominant biocatalyst. Biogas produced from a pilot anaerobic digester was continuously fed to an anoxic biotrickling filter. Nitrate-rich waste water from a pilot-scale sequencing batch reactor was used as nutrient medium and the same effluent, rich in microorganisms, acted as an inoculum. The system was tested for around 5 months, operating with hydrogen sulfide concentrations between 1,000 and 4,000 ppmv, achieving removal efficiencies greater than 99%.

Another biotrickling filter inoculated also with Thiobacillus denitrificans as the dominant genus, was operated by Bailón (2007), for hydrogen sulfide removal from biogas. Aerobic Thiobacillus sp. were selected rather than anaerobic bacteria, due to several reasons, as low nutritional requirements, high hydrogen sulfide affinity or more economic value of the air compared to nitrate. Due to safety reasons, the inlet stream used in lab-scale assays was a mixture of nitrogen (65%), carbon dioxide (35%) and traces of hydrogen sulfide, instead biogas. The biotrickling filter consisted of three packed beds, stacked one above the other, and oxygen enriched nutrient medium was supplied to these beds by bubbling air through a bubble column that contained the nutrient medium. Reported removal efficiences were >99% at 1,000 ppm hydrogen sulfide concentrations. Even when working at 2,000 ppm, the removal never dropped below 93.5%.

Bioscrubbers

The schematic of a bioscrubber is shown in Fig. 9.10 . Removal of hydrogen sulfide in a bioscrubber is achieved in two stages. The first-stage involves physical or chemical absorption of the pollutant in a liquid stream (normally water), followed by the biological treatment of this liquid in a separate bioreac-tor unit. Aerobic bioscrubbers have been reported in the literature to yield higher hydrogen sulfide removals than other biological processes. For anaerobic gases like biogas, similar systems can perform successfully taking into account the limitations of oxygen supply. Besides, compared to pure absorption-based techniques, like liquid redox systems (iron cheleates); bioscrubbers may serve as a good eco-friendly alternative (van Groenestijn 2001). As described previously, chemical absorption based on iron chelated solutions needs one step only to regenerate the liquid absorbent, while in the case of bioscrubbers; regeneration can easily be achieved by the activity of microorganisms like Acidithiobacillus ferrooxidans.

Mesa et al. (2002) used a two-stage bioscrubber to remove hydrogen sulfide from biogas. In the first step, the biogas was absorbed in a ferric sulphate solution, producing ferrous sulphate and elemental sulfur. The ferrous sulphate produced was regenerated by biodegradation using a low-pH tolerant bacterium, Acidithiobacillus ferrooxidans, in the next stage bioreactor. By working at such low-pH values, the reactor required no addition of any chelating agent.

Fig. 9.10 Schematic of a bioscrubbers (absorption tower+bioreactor). It consists of two units, the absorption column and the bioreactor. The process involves in recirculating a biologically active, nutrient-rich scrubbing solution over a packed tower, while polluted air is forced upward through the media bed. Biodegradation of pollutants in liquid-phase occurs in the bioreactor unit

Fig. 9.10 Schematic of a bioscrubbers (absorption tower+bioreactor). It consists of two units, the absorption column and the bioreactor. The process involves in recirculating a biologically active, nutrient-rich scrubbing solution over a packed tower, while polluted air is forced upward through the media bed. Biodegradation of pollutants in liquid-phase occurs in the bioreactor unit

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