Conversion Technologies 95
and carbon dioxide [39, 40]. The process is composed of four steps: hydrolysis, acido-
genesis, acetogenesis and methanogenesis. In the first step, large complex substances
are hydrolysed into individual components like simple sugars or amino acids. In the
second step, these molecules are converted into volatile fatty acids, hydrogen and car-
bon dioxide by another group of microorganisms. In the third stage, volatile fatty acids
are converted into acetic acid, carbon dioxide and hydrogen that need to be removed by
methanogens to maintain the conversion in the process called synthropy. The final stage
of anaerobic digestion is methanogenesis. There are two groups of organisms that per-
form two separate methane synthesis reactions. The first group is capable of converting
acetic acid into methane and carbon dioxide. This reaction is performed by anaerobic
archeaMethanothrixandMethanosarcina.
CH 3 COOH↔CO 2 +CH 4
Another group of anaerobic archeaMethanobacteriasp. converts gaseous products
of acidogenesis into methane.
4H 2 +CO 2 ↔CH 4 +2H 2 O
Anaerobic digestion is a very efficient process of biomass decomposition with effi-
ciency approaching 90% [39]. Most important advantage of this process is its high feed-
stock flexibility. Mixed microbial communities are capable of transforming essentially
any feedstock to biomethane. Most widely utilised include municipal and animal waste
and wastewater, food waste, agricultural by-products and dedicated energy crops [39].
Providing that anaerobic conditions are maintained and pH controlled within 6.5–8.0,
the composition of microbial communities will adjust themselves to utilise the available
feedstock and decompose it into methane [40]. The productivities from different feed-
stocks, however, vary, and highest methane productivity is usually achieved for mixtures
of manure with corn silage or fat slurries [39]. The fermentation time largely depends on
the type of substrate and temperature of operation, and hydraulic retention time could
range from several days to few months depending on those conditions. Methane is con-
stantly extracted at the head of the digester and can be used in one of the two routes:
initially cleaned of corrosive contaminants and used gas engine to generate heat and
power; or purified from contaminants like CO 2 and H 2 S and supplied to the gas grid for
household and industrial use.
3.5 Metrics to Assist the Transition Towards Sustainable
Production of Bioenergy and Biomaterials
3.5.1 EROI – Primary Metrics of Energy Carrier Efficiency
Inevitable depletion of fossil fuel resources forces societies to move from more
efficient sources of energy and carbon that can be acquired at minimal costs into
increasingly more difficult ones; those require larger investments and may come at
increased environmental cost. Despite its utility to compare various methods of energy
production, EROI does have its limitations. First, it does not take into consideration
functionality of a particular source of carbon and energy. For example, liquid bio-fuels
such as bio-ethanol, Fischer–Tropsch liquids or bio-oil have much lower EROI than
