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

Kennedy and Barriault (2005) performed tracer tests on a water-filled 4-compartment ABR and found
that hydrodynamics could be described by 4 or 5 CSTRs in series. These results did not take the
presence of solids in an operational reactor into account.
Skiadas and Lyberatos (1998) performed residence time distribution studies on a periodic anaerobic
baffled reactor – a modified ABR in which four compartments are arranged to flow around an annulus.
This reactor was equipped with many valves allowing flow to be directed in virtually any sequence
through the four compartments. When operated in ABR mode (i.e. flow passed sequentially through
the compartments), the periodic anaerobic baffled reactor was found to have a residence time
distribution equivalent to four well-mixed reactors in series. These results might not be directly
extrapolated to a simple ABR since flow around the annulus is likely to result in a greater degree of
mixing than in a reactor where there is no enforced direction change in the horizontal plane.
Gopala (2007) performed lithium tracer tests on 8 compartment 10 ℓ bench-scale ABRs fed with lowstrength
synthetic wastewater. The data obtained were analysed using a two-phase dispersion model
from which a total deadspace was calculated to lie between 18.5 and 25.5 % for HRTs of 6, 8 and
10 h, and in a second part of the study, between 22.8 and 34.2 % for HRTs of 6, 8, 10, 15 and 20 h. In
the second study, the calculated dead space increased with increasing HRT for HRTs above 8 h.
In conclusion, previous studies have indicated that the baffled design of an ABR results in a residence
time distribution that can be approximated by a number N of completely mixed tanks in series where
N is the sufficiently close to the number of real compartments of the ABR. However, analysis of
UASB reactors (Section 2.3.2) indicated that a model which considers two completely mixed zones
followed by a plug-flow region is required to adequately describe the residence time distribution data.
These studies have all used a completely soluble tracer that may diffuse in and out of biomass flocs.
The residence time properties reported therefore apply only to the soluble phase.
2.5.1.3 Investigations into the effect of compartment number
Wanasen (2003) undertook a laboratory-scale comparison between a conventional septic tank design,
and septic tanks modified with 1 and 2 internal baffles to create a 2 and 3 compartment ABR. These
reactors were fed with a mixture of septage (matured septic tank sludge) and university wastewater
(understood to have a lower organic load than typical domestic wastewater). At a hydraulic retention
time of 48 h, the baffled septic tanks had approximately the same removal efficiencies (in terms of
COD, BOD, TS, and TSS) as the septic tank. However, when operated with a hydraulic retention time
of 24 h, the removal efficiency in the conventional septic tank was reduced by up to two-fold
compared to the baffled reactors. The three-baffled septic tank removal efficiencies were 10 to 15%
higher than observed in the conventional septic tank 1 .
1 Although not directly concluded by the author, these results imply that near maximum stabilisation of
biodegradable material was achieved in all units when operated at a 48 h hydraulic retention time, but that the
extent of stabilisation decreased more rapidly in the unbaffled units with reducing hydraulic retention time than
in the baffled units
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