analysis of a pilot-scale anaerobic baffled reactor treating domestic ...

The UASB has a number of significant advantages over earlier single stage anaerobic digesters
(Seghezzo et al., 1998). Particulate components including active biomass granules and undegraded
particulate material are able to settle and be retained in the reactor due to the relatively slow upward
flow of liquid. Thus, good solids retention and high solids loading may be maintained. High rates of
anaerobic digestion can be achieved in the dense sludge bed that collects at the bottom of the reactor.
Particulate material with poor settling characteristics are entrained in the liquid flow and are not
retained in the sludge bed or sludge blanket. Consequently, the system selects for well settling
anaerobic biomass that is not susceptible to wash-out from the system (Hulshoff Pol et al., 1983). The
high sludge load that can be achieved results in successful treatment at high organic loading rate
(OLR). It has been found that agitation of the sludge bed results in attrition of sludge granules,
resulting in poorer sludge settling characteristics (Lettinga et al., 1980). Thus natural agitation as a
result of biogas generation in the sludge bed and the hydrodynamic forces of the incoming wastewater
are often sufficient to ensure adequate contact between sludge and wastewater. Hence the operating
costs of the technology are fairly low. Similarly, the simple design and high treatment rate per unit
volume result in low capital cost for the construction of this type of system (Seghezzo et al., 1998).
However, it has been shown that internal mixing in UASB reactors is not ideal; significant zones of
dead space 1 may be observed in the reactor (Wu and Hickey, 1997). Furthermore, use of a single-stage
UASB does not permit separation of acidogenic and methanogenic processes (Section 2.2.3), an effect
which has been shown to be beneficial in the digestion of certain wastewaters.
UASB technology has been successfully used to treat a wide variety of wastewaters, both domestic
and industrial (Lin and Yang, 1991). Lin and Wang (1991) review many published applications of
UASB technology and reported that UASB technology could successfully treat soluble wastewaters at
OLR values of up to 30 kg COD/m 3 /d, (although OLR values of between 5 and 20 kg COD/m 3 .d were
more common), while lower loading rates were required for partially soluble wastewaters
(0.5 to 5 kg/m 3 .d).
1 Danckwerts (1953) describes dead water as fluid which is trapped in eddies and therefore spends longer than
the average hydraulic retention time inside the reactor. The remainder of the flow passes through more rapidly
than the average hydraulic retention time as a result of the reduced passage for flow (reactor volume less eddy
volume). Dead space is a region of the reactor volume that is not available for liquid flow due, for example, to
the presence of grit, or internal reactor features that have non-negligible volume.
Dead space is often understood to be a stagnant area of fluid that reduces the volume of a reactor for fluid flow
and thus resulting in short circuiting of fluid around the stagnant area. This is used to explain measurements of
mean residence time that are shorter than would be predicted from the empty reactor volume. However, any
volume filled with liquid cannot be completely dead since liquid and soluble components can diffuse into and
out of the volume, resulting in a long tail in the exit concentration curve of a tracer test, the end of which may be
below the detection limits of the tracer, and thus left out of the residence time distribution analysis.
Levenspiel (1999) simplifies the analysis by using dead water to describe a hypothetical inert region of liquid
into and out of which no diffusion occurs. This is as an approximation for the early part of a tracer curve when
there is a significant volume of stagnant water (or eddy volume). However, this approach does not completely
describe the tail of the tracer curve, and may result in an under-recovery of tracer.
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