Biofuels in Perspective

Biomass Digestion to Methane in Agriculture 183
liquid phase (52 %) can be almost completely digested and that bagasse (48 % of the
organic material) has a 50 % digestion efficiency, and knowing that 1 t of cane (with a
COD of 168 kg in the liquid and 155 t in the solid phase) has a methane producing potential
of (168 + 0.5*155)/4 = 61 kgCH4, with a power generating potential of 5*61 = 307 kWh.
Combustion of the residual bagasse could yield another 77 kWh so that the total electric
power production would be 384 kWh per t of cane or 65*384 = 25 MWh/ha.year or a constant
production of 2,8 kW/ha. Depending on the demand for electric power, possibilities
to generate it by other means and the price that can be paid for the commodity, this may be
an interesting alternative. The perspective of growing plants for electric power generation
is not limited to cane: other plants such as certain types of cactus actually have a higher
production potential of organic material per ha and can be grown in regions with much less
favourable climate conditions and their composition (much less fibre) may allow a higher
digestion efficiency.
10.4 Biogas Production Configurations
In the previous sections it was shown that the biogas production potential for renewable
energy is very significant, although by itself it is insufficient to substitute the main energy
sources in the matrix that is presently used. The reactor configuration to be used to unleash
the potential of energy production as biogas is of great importance, since the investment
in construction of the reactors represents a large fraction of the total production costs. The
selection of the reactor configuration for anaerobic digestion of wastes depends first of all
on its suspended solids content.
10.4.1 Configurations for Wastewater Digestion
For predominantly liquid wastes (with less than, say, 1 g/l of settleable solids) such as
stillage, it is convenient to use one of the so called high rate digestion systems, which are
characterized by two main design characteristics: (1) there is a mechanism for retention
or recirculation of sludge, so that a high mass of active microorganisms is maintained in
the reactor and (2) intense contact between influent organic material and the sludge mass
is effected by appropriate measures of flow direction or mixing of the reactor contents.
The first measure effectively means that the retention time of the solids or sludge age
becomes independent of the liquid retention time. This means that the sludge age rather
than the liquid retention time is the relevant design parameter. Hence the wastes can be
treated at very short liquid retention times as long as the sludge retention mechanism is
able to maintain an adequate sludge mass (compatible with the sludge age) in the reactor.
The most well known high rate anaerobic digester is the UASB reactor (upflow anaerobic
sludge blanket reactor, Lettinga et al., 1980), which is schematically shown in Figure 10.9.
The most characteristic device of the UASB is the phase separator, placed in the upper
section and dividing the reactor in a lower part, the digestion zone, and an upper part, the settling
zone. The waste water is introduced as uniformly as possible over the reactor bottom,
passes through the sludge bed and enters into the settling zone via the apertures between the
phase separator elements and is uniformly discharged at the surface. The biogas produced