- Biofuels from bacterial fermentation - Nature by dan freedigitalphotos.net
Biofuel production has developed within existing industrial infrastructure. Industrial applications and technology are mature although there are barriers.
Bacterial fermentation
Bacterial fermentation producing methane requires first hydrolysis of polysaccharides, proteins and fats into oligosaccharides and sugars, fatty acids and glycerol and then fermentation into acetic, propionic and butyric acid, carbon dioxide and hydrogen, alcohols and other minor compounds by acidogenesis. Then acetic acid and carbon dioxide (acetogenesis) are produced before the methanogenesis decomposition (Antoni, Zverlov and Schwarz 2007).
The microbiological processes are well-known (Madigan, Martinko, Dunlap and Clark 2009) and the latter stages of acetagenesis and methanogenesis have barriers to overcome if the significant roll-out is to become reality.
Rittman (2008) cites from his monograph with McCarty on E
nvironmental Biotechnology that methane CH4 is a product of two slow growing groups of
Archaea methanogens which, under strictly anaerobic conditions, have two reactions: One group that oxidises hydrogen and respires carbon dioxide:
CO2 + 4H2 →CH4 + 2H2O.The other group that ferments acetic acid to methane and carbon dioxide:
CH3COOH →CH4 + CO2.
In 2007, methanogenesis was cited as offering “up to 70% (
v/v) CH4 methane and 30% carbon dioxide CO2” (Antoni
et al. 2007 p.27). This capacity of returning methane needs greater efficiency. Currently some operations are optimising conditions of the hydrolysis stage to enhance methanogenesis. Tackling the limitations of the acetagenesis stage is less straightforward. Bacteria are the means to channel electrons towards producing CH4 (Rittmann 2008) but a long generation time is restrictive in operations (Antoni
et al. 2007).
Nature and size of operations
In Germany and Austria, methane has been produced as a gaseous biofuel in what Antoni and colleagues term the “traditional farm biogas plant” (p.27). At this scale of operation, a farm plant operates as a single or two stage process at around 37oC with an uncontrolled secondary fermentation in large storage tanks. However, measures to optimise conditions for acetogenesis and methanogenesis (Antoni
et al. 2007) and a scaling up of operations, the potential of methane production from bacterial fermentation as a source of fuel will remain capped. Utilising excess heat from dried and desulfurised biogas has been overlooked generally in the traditional farm plants (Antoni
et al. 2007). Larger biogas plants, often adjacent to other processing plants such as composting or power plants, are the commercial bridge to the adoption of the biorefinery model and scalability towards a higher level of production.
The Biorefinery Model Approach
Applied microbiological research suggests that larger biogas plants adopting two separated stages of processing, calibrated to optimal conditions hydrolysis and acidogenesis, acetogenesis and then methanogenesis is the only way to overcome limitations (Antoni
et al. 2007).
Current large industrial biogas plants run a 2 stage process with biogas fermentation tanks run as liquid fermenters, either set up as liquid fermentation or containing more than 12% (
w/v) dry mass for dry fermentation. Microbiological optimal conditions of temperature, humidity/water, pH specific for the hydrolytic and the methanogenic bacteria (Madigan MT, Martinko JM, Dunlap PV, Clark DP.2009) has informed this two-stage process, termed by Antoni and colleagues (2008) as “the biorefinery model approach”.
Prospects in reducing generation time of bacteria are good as thermophilic processes are more common (Antoni e
t al. 2007). Methods of capturing, cleaning and using CH4 as it is released as a gas out of the water are improving (Rittmann 2008).
References
Antoni D, Zverlov VV, Schwarz WH. Biofuels from microbes.
Applied Microbiology and Biotechnology. 2007; 77:23-35
Madigan MT, Martinko JM, Dunlap PV, Clark DP.
Brock Biology of Microorganisms. 12th ed. San Francisco (USA): Pearson Benjamin Cummings; 2009.
Rittmann B. Opportunities for Renewable Bioenergy using Microorganisms.
Biotechnology and Bioengineering. 2008; 100 (2): 203-212.
