222
Water for Energy and Fuel Production
stage can be advantageous because the ideal pH required for hydrolysis (5.5–6.5) is
different from that required for methanation (6.8–7.2) [5,126,128]. This technology
is mainly applied to MSW, industrial solid wastes and solid manure and seldom to
energy crops. The control of operation and process parameters for the two-stage fermentation system is generally difficult. Furthermore, if the hydrolysis stage does not
work properly, methane and hydrogen can escape in the environment [5,129].
8.7.5 novel digeSTer TeChnology
The University of California at Davis developed a new anaerobic digester technology
called anaerobic-phased solids (APS) digester for biogasification of organic waste
solids that are normally difficult to process using conventional anaerobic digesters.
A variety of feedstock including crop residues, animal manures, feed processing
residuals, paper sludge, and MSW can be processed by APS digesters. The digester
has been used to generate power for the University of California. The first commercial APS digester was built in Boynton Beach, Florida, to process 80 tons/day horse
stable wastes. The possible benefits of this plant are renewable energy generation,
odor control, pathogen and insect control, truck traffic reduction, and production of
high-quality soil amendment.
The APS digester combines the favorable features of both batch and continuous
operations in one system. Solids to be digested are handled in batches while biogas
production is continuous. This allows the solids to be loaded and unloaded without
disrupting an anaerobic environment for bacteria. The typical APS digester system
consists of four hydrolysis reactors and one biogasification reactor. Liquid is recirculated intermittently between each hydrolysis reactor and the biogasification reactor.
The solids are housed in the hydrolysis reactor, whereas the bacteria (methanogens)
are housed in the biogasification reactor. The solids are broken down and liquefied in soluble compounds, which are mainly organic acids, and transferred to the
biogasification reactor to generate biogas. The four hydrolysis reactors are operated
in different time schedules so that biogasification reactor is constantly fed with the
dissolved organic acid. High bacteria concentration in the biogasification reactor is
maintained to get the optimum performance. More details on the APS digester are
given by Zhang [68].
8.8 simUlatiOn, mOdelinG, sCale-UP, and COntrOl
OF FermentatiOn PrOCess
Angelidaki et al. [39,40] and Gavala et al. [130] gave a systematic assessment of complex kinetic models for organic waste digestion. They described the degradation by a
simple first-order reaction that can be applied knowing the yield of substrate and the
specific reaction rate [5]. Their kinetic models also depended on the nature of feedstock and the temperature range of the digestion process. The kinetic of biogas production from energy crops and manure was reported extensively by Mahnert [131].
Several kinetic models were developed for low-temperature (35°C–42°C) mesophilic
conditions as well as high-temperature (45°C–60°C) thermophilic conditions by
Andara and Esteban [132], Linke [133], and Biswas et al. [134].
Water for Energy and Fuel Production
stage can be advantageous because the ideal pH required for hydrolysis (5.5–6.5) is
different from that required for methanation (6.8–7.2) [5,126,128]. This technology
is mainly applied to MSW, industrial solid wastes and solid manure and seldom to
energy crops. The control of operation and process parameters for the two-stage fermentation system is generally difficult. Furthermore, if the hydrolysis stage does not
work properly, methane and hydrogen can escape in the environment [5,129].
8.7.5 novel digeSTer TeChnology
The University of California at Davis developed a new anaerobic digester technology
called anaerobic-phased solids (APS) digester for biogasification of organic waste
solids that are normally difficult to process using conventional anaerobic digesters.
A variety of feedstock including crop residues, animal manures, feed processing
residuals, paper sludge, and MSW can be processed by APS digesters. The digester
has been used to generate power for the University of California. The first commercial APS digester was built in Boynton Beach, Florida, to process 80 tons/day horse
stable wastes. The possible benefits of this plant are renewable energy generation,
odor control, pathogen and insect control, truck traffic reduction, and production of
high-quality soil amendment.
The APS digester combines the favorable features of both batch and continuous
operations in one system. Solids to be digested are handled in batches while biogas
production is continuous. This allows the solids to be loaded and unloaded without
disrupting an anaerobic environment for bacteria. The typical APS digester system
consists of four hydrolysis reactors and one biogasification reactor. Liquid is recirculated intermittently between each hydrolysis reactor and the biogasification reactor.
The solids are housed in the hydrolysis reactor, whereas the bacteria (methanogens)
are housed in the biogasification reactor. The solids are broken down and liquefied in soluble compounds, which are mainly organic acids, and transferred to the
biogasification reactor to generate biogas. The four hydrolysis reactors are operated
in different time schedules so that biogasification reactor is constantly fed with the
dissolved organic acid. High bacteria concentration in the biogasification reactor is
maintained to get the optimum performance. More details on the APS digester are
given by Zhang [68].
8.8 simUlatiOn, mOdelinG, sCale-UP, and COntrOl
OF FermentatiOn PrOCess
Angelidaki et al. [39,40] and Gavala et al. [130] gave a systematic assessment of complex kinetic models for organic waste digestion. They described the degradation by a
simple first-order reaction that can be applied knowing the yield of substrate and the
specific reaction rate [5]. Their kinetic models also depended on the nature of feedstock and the temperature range of the digestion process. The kinetic of biogas production from energy crops and manure was reported extensively by Mahnert [131].
Several kinetic models were developed for low-temperature (35°C–42°C) mesophilic
conditions as well as high-temperature (45°C–60°C) thermophilic conditions by
Andara and Esteban [132], Linke [133], and Biswas et al. [134].