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

with increasing hydraulic retention time. These authors found that hydraulic dead space was a function
of number of baffles and flow rate, and that biological deadspace was related to biomass
concentration, gas production rate and flow rate, but that there was no clear correlation between total
dead space and hydraulic retention time.
Nachaiyasit and Stuckey (1997c) also undertook residence time distribution studies on laboratoryscale
ABRs and confirmed the value of 18% dead space for normal operation, and further found that
increasing the amount of reactor biomass by three-fold did not affect the value of dead space
calculated in the residence time distribution studies. Langenhoff (2000) undertook tracer tests to
determine residence time distribution in 10 ℓ 8-compartment ABRs, fed synthetic low strength soluble
and colloidal wastewater. No clear trends were observed in the residence time distribution with
changes in hydraulic retention time, feed type or biomass concentration. In all tests, the dead space did
not exceed 37 % of the total reactor volume. In Langenhoff’s study, no quantitative difference was
observed in the amount of dead space determined from the tracer tests for identical reactors with
different amounts of biomass, confirming the findings of Nachaiyasit and Stuckey (1997c). It was also
concluded that the mixing characteristics of the 8-compartment ABR could be simulated by 8
continuous stirred tank reactors (CSTRs) in series.
Grobicki and Stuckey (1992) found that the number of tanks-in-series (N, the number of CSTRs in
series in a hydrodynamic model that would result in the same exit concentration curve as the reactor
tested in a tracer test) calculated from the tracer tests was similar to the number of compartments in the
ABRs studied. Table 2.6 reproduces the results of these tests for 8-compartment ABRs operated at
different hydraulic retention times and different biomass concentrations. It was concluded that for low
values of hydraulic retention time, the calculated number of tanks-in-series closely approximated the
actual number of compartments of the ABR. It was also found that the amount of back-mixing inferred
from a dispersion model decreased with increasing hydraulic residence time. The authors concluded
that the baffles of the ABR inhibit back-mixing, but that there is a large degree of mixing within each
upflow compartment. However, it was noted that the downflow section of each compartment was
more likely to behave as a plug-flow reactor. The authors indicated that a reasonable approach to
modelling the hydrodynamics of the ABR would be a tanks-in-series model with N (number of tanks)
equal to the number of compartments, and that the effluent solutes concentration from each
compartment should be the same as the average concentration within the compartment. However, low
but significant dead spaces were observed, accounting for between 1 and 22% (mean = 9.8%, standard
deviation = 8.2%) of the working volume of the ABR.
Table 2.6: Selected results of residence time distribution studies on 8 compartment anaerobic
baffled reactors (ABR) from Grobicki and Stuckey (1992)
Run no. Retention time
[h]
Biomass [g/ ℓ] Gas production
[m ℓ /h]
39
Dead space
[%]
Number of
theoretical
tanks - N
12 20 5.58 774 18.58 10.95
13 20 3.49 691 1.21 6.95
14 10 2.04 739 17.38 8.28
15 10 6.56 1485 7.69 7.13
16 5 6.16 1938 5.4 8.22
17 5 8.5 1804 9.55 8.03