220
Water for Energy and Fuel Production
8.6.3 PreTreATmenT
Muller et al. [5,114] showed that thermal, chemical, mechanical, or enzymatic
processes as pretreatment can alter the degradation rate. Particle size reduction
accelerates the biogas production rate but not the methane yield [5,115]. This is
often achieved by the use of a crushing device or an ultrasonic treatment of the feed
stream [5]. Also, thermal pressure hydrolysis at 230°C and 20–30 atm splits polymers into short-chain compounds, which can give better biogas yields with reduced
retention time in the digesters [116,117].
The addition of an enzyme to the feed can have a mixed effect. While it can
increase the biogas yield up to about 20% [5] by the acceleration of decomposition
of polysaccharides, if added in excess, the protease of anaerobic microorganisms
can degrade the enzyme, thus limiting its effectiveness [5,118]. In general, the
enzyme reduces the viscosity of the substrate mixture and increases the degradation rates by avoiding the formation of floating layers [5]. For wheat grass, the
addition of an enzyme improved the biogas production, but at the end of the digestion period, no significant improvement of methane yield or degradation rate was
observed [119].
8.7 tyPes OF FermentatiOn and assOCiated
diGester COnFiGUratiOns
The nature of digester configuration depend on the method of fermentation. Four
types of fermentation and associated digester configurations have been most widely
used: wet fermentation, dry fermentation, batch fermentation, and two-stage fermentation. Solid concentration plays an important role in the choice of the fermentation.
Wet fermentation is used for low solids concentration (<10%), dry fermentation is
used for an intermediate concentration (between 15 and 35 wt%), and batch fermentation is used for solids concentration as high as 70 wt%. More details on the different types of digester are briefly described in Sections 8.7.1 through 8.7.4.
8.7.1 WeT FermenTATion
In the wet fermentation process, the solids concentration is <10% and generally carried out in a vertical, continuous stirred slurry fermenter [5]. Low slurry concentration allows the stirring at lower power cost. The digested material is spread on the
fields for the fertilization. For energy crops, the feed must be mixed with recycled
process water or liquid manure to make the slurry pumpable. In the agriculture sector, wet fermentation is the preferred mode of operation.
Often the fermenter is covered with gas-tight, single- or double-membrane roof
to store the gas before utilization. To achieve the uniform temperature and good
contact between microbes and feedstock, good mixing of the slurry is very important. While mixing can be provided by different types of stirrer (mechanical,
hydraulic, or pneumatic), mechanical stirrers are often used. In order to obtain
maximum mixing in the reactor, the number, size, direction, and depth of the stirrer paddles depend on the nature of feedstocks, solids concentration, and height
Water for Energy and Fuel Production
8.6.3 PreTreATmenT
Muller et al. [5,114] showed that thermal, chemical, mechanical, or enzymatic
processes as pretreatment can alter the degradation rate. Particle size reduction
accelerates the biogas production rate but not the methane yield [5,115]. This is
often achieved by the use of a crushing device or an ultrasonic treatment of the feed
stream [5]. Also, thermal pressure hydrolysis at 230°C and 20–30 atm splits polymers into short-chain compounds, which can give better biogas yields with reduced
retention time in the digesters [116,117].
The addition of an enzyme to the feed can have a mixed effect. While it can
increase the biogas yield up to about 20% [5] by the acceleration of decomposition
of polysaccharides, if added in excess, the protease of anaerobic microorganisms
can degrade the enzyme, thus limiting its effectiveness [5,118]. In general, the
enzyme reduces the viscosity of the substrate mixture and increases the degradation rates by avoiding the formation of floating layers [5]. For wheat grass, the
addition of an enzyme improved the biogas production, but at the end of the digestion period, no significant improvement of methane yield or degradation rate was
observed [119].
8.7 tyPes OF FermentatiOn and assOCiated
diGester COnFiGUratiOns
The nature of digester configuration depend on the method of fermentation. Four
types of fermentation and associated digester configurations have been most widely
used: wet fermentation, dry fermentation, batch fermentation, and two-stage fermentation. Solid concentration plays an important role in the choice of the fermentation.
Wet fermentation is used for low solids concentration (<10%), dry fermentation is
used for an intermediate concentration (between 15 and 35 wt%), and batch fermentation is used for solids concentration as high as 70 wt%. More details on the different types of digester are briefly described in Sections 8.7.1 through 8.7.4.
8.7.1 WeT FermenTATion
In the wet fermentation process, the solids concentration is <10% and generally carried out in a vertical, continuous stirred slurry fermenter [5]. Low slurry concentration allows the stirring at lower power cost. The digested material is spread on the
fields for the fertilization. For energy crops, the feed must be mixed with recycled
process water or liquid manure to make the slurry pumpable. In the agriculture sector, wet fermentation is the preferred mode of operation.
Often the fermenter is covered with gas-tight, single- or double-membrane roof
to store the gas before utilization. To achieve the uniform temperature and good
contact between microbes and feedstock, good mixing of the slurry is very important. While mixing can be provided by different types of stirrer (mechanical,
hydraulic, or pneumatic), mechanical stirrers are often used. In order to obtain
maximum mixing in the reactor, the number, size, direction, and depth of the stirrer paddles depend on the nature of feedstocks, solids concentration, and height