Anaerobic Digestion of Aqueous Waste for Methane and Hydrogen
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As the interest in using the anaerobic digestion technique to generate biogas and
biomethane increases due to economical and environmental reasons, it is important
to determine the ultimate methane potential for a given solid substance. In fact,
this parameter determines to some extent both design and economic analysis of a
biogas plant. The ultimate methane potential thus identifies the “thermodynamic
limit” for a given substance. Furthermore, to compare the potentials of various substrates, the definition of common units to be used in anaerobic assays is becoming
increasingly important. Angelidaki et al. [39,40,64] presented some guidelines for
biomethane assays of the anaerobic digestion prepared by the specialists group of
the International Water Association. The guidelines include the considerations of
biodegradability, bioactivity, inhibition, and matrices for biostability.
Narra et al. [70] evaluated a model for anaerobic digestion of household
organic waste in high solids concentration (25 wt%) in an urban city in India and
showed with pilot-scale experiments that the biogas production of 209 l/kg of
the total solids is possible in a 30-day incubation period. High solids concentration reduces the water requirement and slurry handling problems. Composting
takes 35 days and yields a quality product that can be used either as manure or
a part of chemical fertilizer. A batch pilot plant was developed. Abderrezaq [63]
evaluated the use of anaerobic digester for the MSW generated in Jordan. They
showed that the digester technology can generate the energy from waste without
generating GHG.
It is difficult to find a suitable and simple control parameter to control the complex
fermentation process. Furthermore, only few parameters can be measured on-line. In
agricultural biogas plants, the methane production is the only continuously measured
parameter. However, complex and variable process dynamics make the interpretation of data difficult [5,135]. Only VFA can serve as an efficient indicator of process
imbalances. Weiland [5,135] proposed that an indicator for process failure is the
propionic acid/acetic acid ratio of >1.
Ahring et al. [136] suggested that if the propionic acid concentration is >1000 mg/l,
the concentration of both butyrate and isobutyrate could be a reliable tool for indication of process failure. Nielsen et al. [137] suggested that propionate is the key
parameter for process control and optimization. VFA analysis by manual sampling
and the subsequent analysis by gas chromatography or high-pressure liquid chromatography is a slow process. On-line measurement is a difficult process [138]. A fast
control of the process stability is possible by determining the ratio of total VFA to
total inorganic carbonate. If this ratio is >0.3, the process is stable.
8.9 PUriFiCatiOn OF BiOGas
Biogas mainly contains methane and carbon dioxide with some impurities of hydrogen
sulfide (with sulfur concentration from 100 to 3000 ppm) and ammonia, and it is generally saturated with water vapor. Before it can be used for heat and electricity generation, sulfur concentration should be reduced to the level below 250 ppm [5]. This will
prevent the excessive corrosion and expensive deterioration of lubrication oil.
H 2 S removal is carried out by biological desulfurization either within digester or
outside digester. For this type of desulfurization, Sulfobacter oxydans bacteria and