Biofuels production 181suspension, upflow velocities in the range of 0.6-0.9 m/hr have been used
(Metcalf and Eddy Inc. 2003).
Table 4.15 summarizes the advantages and disadvantages of the UASB
process, while the design and performance data of some UASB reactors in the
U.S. and Netherlands are given in Table 4.16. To minimize the overflow of
granules in the effluent, the design guidelines of the gas-solids separator device
for UASB reactors, listed in Table 4.17, should be followed.
It should be noted from Table 4.16 that the volumetric gas production rates
from the UASB reactors, ranging from 3.7 to 7.5 m^3 /m^3 of reactor volume, are
over 10 times higher than those of the conventional biogas digesters (see Table
4.8). This is probably due to three reasons: a) the UASB reactors were fed with
wastes which contain high concentrations of soluble COD and are readily
biodegradable; b) better mixing and less shorting-circuiting in the UASB
reactors; and c) due to the internal sludge circulation in the UASB reactors,
more active microorganisms in the form of granules are available for anaerobic
biodegradation. However, it is much more expensive to build a UASB reactor
than a conventional biogas digester.
Besides the anaerobic filter and UASB reactor, other anaerobic fixed-film
reactors have been developed such as the fluidized and expanded bed reactors.
Since the fixed-film bacteria stay in the reactor longer or have longer șc than the
dispersed bacteria, they are able to withstand shockloading or receive higher
organic loadings better than the dispersed bacteria. Application of anaerobic
fixed-film reactors in waste treatment and biogas production is current growing.