Advances in Biogas Technology - Biotechnology in China: Biofuels and Bioenergy - page 122

2 Biogas Fermentation Processes
2.1 Anaerobic Digesters
An anaerobic digester (AD), also known as a methane fermentation reactor, is a
central device in a biogas system. Relatively mature reactor configurations, such as
anaerobic filter (AF), up-flow anaerobic sludge bed (UASB), continuous stirred-
tank reactor (CSTR), and up-flow blanket filter (UBF), have been widely used to
treat urban sewage and wastewaters from alcohol, starch, and organic acid fer-
mentation industries and at the same time to recover methane. Most biogas
facilities for combined treatment of agricultural solid waste and livestock manure
are still on household scales. Many obstacles remain in the pretreatment of bio-
mass, further treatment of residue and slurry, and promotion of their effective use.
In addition, challenges in scale-up, economy, environmental protection, and
recycling issues are yet to be addressed [ 11 ].
Statistics for 2002 showed that the major reactor types adopted for biogas
projects in China were UASB, CSTR, internal circulation (IC), and expanded
granular sludge bed (EGSB) [ 12 ]. Among the existing large- and medium-sized
methane projects, there were 200 sites of UASB and 128 sites of CSTR, which in
all accounted for more than 80% of the total market in China.
Due to numerous advantages, such as needing no mechanical stirring, being
applicable to wastewater of various organic loads, easy operation, small footprint,
and high performance in granular sludge cultivation, UASB reactors have been
widely used in China, accounting for 49.26% of the total market. UASBs enable
reduced biomass washout, and thus usually have high fermentation efficiency.
However, they are not suitable for wastewaters that contain high suspended solid
(SS) content of more than 3,500 mg/L. In such cases, CSTRs could be a more
desirable choice, as they demonstrate superior capability for treatment of high
SS-containing and high temperature wastewaters. For this reason, CSTRs are more
frequently adopted in alcohol wastewater treatment processes.
In China, the largest anaerobic fermentation tank group of CSTRs is located in
Taichang Xintai Alcohol Company, Jiangsu Province. This facility was first built
and put into operation in 1999, with a total volume of 13,200 m 3 . It is capable of
processing 2,000 m 3 /d of cassava wastewater and producing 40,000 m 3 of biogas.
The largest single-tank anaerobic fermenter is in Nanyang alcohol plant of Henan
province, which has a volume of 5,000 m 3 . The largest UASB anaerobic project is
in Xuzhou Fangting winery, with a tank group total volume of 10,000 m 3 .
In China, the maximum single-tank volume of UASB has reached 4,000 m 3 .
In recent years, ICs and EGSBs are gaining increasing popularity in China due
to their high efficiency and small footprint. These reactors are especially suitable
for wastewater with low SS and low organic concentration, such as paper mill
wastewater, beer wastewater, and starch wastewater.
The development of anaerobic reactors has evolved from the traditional civil
engineering model to the equipment model in China. The content of engineering