Biomedical Engineering Reference
In-Depth Information
the technology also has significant disadvantages, such as the large reactor volume,
difficult separation of biogas and residue, and so on. [
103
]. In dry anaerobic fer-
mentation, the TS content is 20-30 %. Compared with wet anaerobic fermentation,
dry anaerobic fermentation has the following advantages: (1) utilization of a variety
of sources of solid organic waste, such as municipal garbage, crop straw, manure,
and so on; (2) low operating costs and high capacity per volume unit; (3) saving of
water; (4) relatively easy follow-up treatment; (5) stable running process, no scum
or precipitation; and (6) relatively low odor emissions [
104
].
Although dry anaerobic fermentation has such advantages, its application is
still a problem. The reasons are as follows: On one hand, mass transfer is not
efficient. Accumulation of intermediate metabolites causes feedback inhibition, and
low moisture content influences diffusion of cells or enzyme, further affecting the
contact of cells or enzyme with substrate. Thus, the substrate conversion rate and gas
production rate are reduced. To overcome these difficulties, methods of increasing
the quantity of inoculation and strengthening mass transfer can be used to achieve
the rapid startup and stability of the system, but the running cost would be increased.
On the other hand, the structure, composition, and particle size of substrates are
not uniform; thus, it is difficult to control the system, and continuous operation is
unstable [
104
]. Based on these technical difficulties, studies are being carried out
around the world.
Study on dry fermentation of household biogas was started in the 1980s in
China. Although some achievements have been made, the scale is still small. Ye
and Wei began to study an automatic discharging dry fermentation device and the
corresponding semicontinuous process in 1986 and passed identification chaired by
the Ministry of Agriculture in August 1988 [
105
]. Ma et al. [
106
] developed a dry
fermentation tank that can maintain constant voltage. Its gas production can satisfy
lighting (May to November) and cooking (June to October) for a family of four. Gan
[
107
] studied dry anaerobic fermentation stirring reactors utilizing livestock manure
and designed the horizontal stirring reactor. The influence of operating parameters
was studied through tests, and the optimal operating parameters have been obtained.
The study of dry fermentation abroad began earlier and is more advanced than in
China. At present, biogas dry fermentation technology has been mature abroad. For
example, the garbage dry fermentation system, airbag dry fermentation system, wet
and dry combined fermentation system, leachate storage bucket dry fermentation
system, and other large-scale biogas dry fermentation systems have already been
put into production.
6.4.4
Problems in Biogas Fermentation
(1) Low temperature
As previously mentioned, biogas fermentation is affected significantly by temper-
ature. The temperature changes according to season. Especially in winter, the low
temperature leads to low or even no gas production. For example, in northern China,
