78
5.2.1.2.2 Hard-Plastic Digester Types
The most common processes used to produce hard-plastic biodigesters are rotation
and injection molding. Injection molding machines are more expensive, but the process allows for more intricate shapes, can utilize more types of materials, and is
much faster, resulting in lower production costs. Materials used include hard PVC,
ABS, HDPE, LLDPE, and modified plastics.
5.2.1.2.3 Soft-Plastic Digester Types
The archetype of soft-plastic digester types is the Taiwanese bag digester, which
was developed in the 1960s. It is a popular model especially in Central and South
America (Gunnerson et al. 1986). The digester is made of a cylindrical plastic tank
(tube) nested horizontally on a hardened layer of masonry, concrete, sand, or mud
(Gunnerson et al. 1986). A great advantage of the bag digester is the simple design
and low material costs. However, these low-cost materials often break down easily
and often render the effective lifespan of the digester to less than 2 years. In the last
decade, SP digester designs emerged that use higher-quality materials, such as geomembranes and HDPE instead of polyethylene. Such digesters are durable and
should have a lifespan of from 6 to 10 years. Lower-quality soft-plastic digesters
made from simple PE sheets are also available. These digesters are cheap, but are
easily damaged and require frequent repairing and, as such, are not actively
promoted.
5.2.2 Medium- and Large-Scale Plants
Two-stage AD is widely used for medium- and large-scale biogas plants with the
two digester tanks in series. The advantage of having two stages for AD is the separation of the methanogenesis stage from the acidogenesis stage because acids produced in the acidogenesis stage can inhibit the methanogens. The first tank is
optimized for hydrolysis and acidogenesis and the second tank is optimized for
acetogenesis and methanogenesis. An example of the two-stage AD power plant is
presented in Sect. 5.3.2.2.
Medium- and large-scale biogas plants are used on commercial farms and often
produce more biogas than the farming households can use. Consequently, biogas is
turned into electricity or supplied to gas grids or fuel stations after being upgraded
to a desired purity. Most of the large-scale biogas plants are constructed in Germany,
USA, UK, and China. Many of the these plants are generating biogas from landfill,
wastewater, fuel crops, and to a lesser extent from livestock manure.
N. V. C. Ngan et al.
5.2.1.2.2 Hard-Plastic Digester Types
The most common processes used to produce hard-plastic biodigesters are rotation
and injection molding. Injection molding machines are more expensive, but the process allows for more intricate shapes, can utilize more types of materials, and is
much faster, resulting in lower production costs. Materials used include hard PVC,
ABS, HDPE, LLDPE, and modified plastics.
5.2.1.2.3 Soft-Plastic Digester Types
The archetype of soft-plastic digester types is the Taiwanese bag digester, which
was developed in the 1960s. It is a popular model especially in Central and South
America (Gunnerson et al. 1986). The digester is made of a cylindrical plastic tank
(tube) nested horizontally on a hardened layer of masonry, concrete, sand, or mud
(Gunnerson et al. 1986). A great advantage of the bag digester is the simple design
and low material costs. However, these low-cost materials often break down easily
and often render the effective lifespan of the digester to less than 2 years. In the last
decade, SP digester designs emerged that use higher-quality materials, such as geomembranes and HDPE instead of polyethylene. Such digesters are durable and
should have a lifespan of from 6 to 10 years. Lower-quality soft-plastic digesters
made from simple PE sheets are also available. These digesters are cheap, but are
easily damaged and require frequent repairing and, as such, are not actively
promoted.
5.2.2 Medium- and Large-Scale Plants
Two-stage AD is widely used for medium- and large-scale biogas plants with the
two digester tanks in series. The advantage of having two stages for AD is the separation of the methanogenesis stage from the acidogenesis stage because acids produced in the acidogenesis stage can inhibit the methanogens. The first tank is
optimized for hydrolysis and acidogenesis and the second tank is optimized for
acetogenesis and methanogenesis. An example of the two-stage AD power plant is
presented in Sect. 5.3.2.2.
Medium- and large-scale biogas plants are used on commercial farms and often
produce more biogas than the farming households can use. Consequently, biogas is
turned into electricity or supplied to gas grids or fuel stations after being upgraded
to a desired purity. Most of the large-scale biogas plants are constructed in Germany,
USA, UK, and China. Many of the these plants are generating biogas from landfill,
wastewater, fuel crops, and to a lesser extent from livestock manure.
N. V. C. Ngan et al.