Environmental Engineering Reference
In-Depth Information
Biogas
Weir
Effluent
Baffles
Gas bubble
Biomass granule
Sludge blanket
Influent
FIGure 32.12 (see color insert)
Scheme of UASB.
90 days. Sugars dissolved in the liquid waste stream can be converted into gas quickly in the liquid
phase, which can exit the system in less than a day.
32.4.2.1 Wastewater characteristics
Wastewater that contains substances that can adversely affect sludge granulation, cause foaming,
or cause scum formation is of concern. Wastewaters with higher concentration of proteins and/or
fats tend to create more and more such problems. The fraction of particulate versus soluble COD
is important in determining the design loadings for UASB reactors and the applicability of the
process. As the fraction of solids in the wastewater increases, the ability to form a dense granulated
sludge decreases. At a certain solids concentration (greater than 6 g TSS/L) anaerobic digestion and
anaerobic contact processes may be more appropriate.
32.4.2.2 volumetric loading
Typical COD loadings as a function of the wastewater strength, fraction of particulate COD in
the wastewater, and TSS concentration are summarized in Table 32.9. Removal efficiencies of up
to 95% for COD have been achieved at COD loadings ranging from 12 to 90 kg
COD
m
-3
/day on a
variety of wastes at 30° to 38°C with UASB reactors. Values for hydraulic retention time (HRT) for
high-strength wastewater have been as low as 4 to 8 h at these loadings. Where less than 90% COD
removal and higher-effluent TSS concentrations are acceptable, higher upflow velocities can be
used that will develop a more dense granulated sludge by flushing out other solids. Thus, the higher
volumetric COD loadings are shown for these conditions.
Recommended loadings as a function of temperature for wastewaters with mainly soluble COD
are presented in Table 32.10. These loadings apply to the sludge blanket volume, and a reactor
effectiveness factor of 8.8 to 0.9 is used to determine the reactor liquid below the gas collector. The
higher loading recommendation for the wastewater containing mainly volatile fatty acids (VFA) is
based on the potential of obtaining a more dense granulated sludge.
A typical HRT is shown in Table 32.11.
