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

methanogenic (Akunna and Clark, 2000; Uyanik et al., 2001b; Baloch and Akunna, 2003; Sallis and
Uyanik, 2003; She et al., 2006).
Baloch and Akunna (2003) seeded an ABR with granular sludge and fed it with a synthetic glucosebased
wastewater at OLRs from 1.25 to 20 kg COD/m 3 /d. They observed floating and breaking up of
granular sludge in the early compartments of the reactor, while methanogenic sludge in later
compartments retained their structure. These authors reported that a white sticky mass formed in the
sludge of the early compartments. This proved to be bacteria of the Enterobacteriaceae genus that can
use glucose as a sole carbon source and are tolerant to low levels of dissolved oxygen.
Bell (2002) investigated microbial population characteristics through fluorescent in-situ hybridisation
(FISH) analysis of samples drawn from different compartments of an 8-compartment ABR fed a
soluble sucrose/ protein feed. Eubacteria were found to predominate in early compartments, while
Archaea dominated in later compartments. There was also a distinct shift between H2, CO2 and
formate utilising methanogenic Archaea in the early compartments and acetoclastic Archaea in the
later compartments. Micro-organisms from the genus Methanosarcina were only occasionally
observed in the first compartment. This genus is usually outcompeted by acetoclastic methanogens
from the genus Methanosaeta and can only predominate at high acetate concentration (Speece, 1996).
2.5.1.5 Response to hydraulic and organic shock loads
The ABR has exhibited superior resilience to hydraulic and organic shock loads compared to other
reactor configurations (Barber and Stuckey, 1999). Grobicki (1989) simulated a hydraulic shock by
decreasing the hydraulic retention time from 20 h to 1 h, for a period of 3 h in an ABR operating at an
OLR of 4.8 kg COD/m³ on a soluble synthetic carbohydrate / sucrose / protein feed. The reactor
returned to its previous COD removal efficiency of in excess of 95 % within 24 h of resuming normal
operating conditions. Less than 15 % of the active biomass was lost. In a similar experiment, the OLR
was increased to 20 kg COD/m³ and, under these conditions a COD removal efficiency of 72 % was
still achieved.
Nachaiyasit and Stuckey (1997b; c) investigated the effect of hydraulic and organic shock loads on a
10-ℓ 8 compartment ABR fed a synthetic sucrose/protein wastewater. Baseline conditions for these
experiments were a hydraulic retention time of 20 h and an OLR of 4.8 kg COD/m 3 .d (Feed
concentration = 4 g COD/ℓ). When the feed concentration was doubled to 8 g COD/ℓ at constant
hydraulic retention time, increases in compartment VFA, dissolved H2 and COD concentrations and
decreases in measured compartment pH values were observed in the early compartments, while no
significant changes were observed in the last few compartments or outflow, and COD removal
efficiency did not change. A further increase of feed concentration to 15 g COD/ℓ resulted in increases
in VFA and COD concentration in all compartments and the outflow, but only saw significant
increases in H2 concentrations in the early compartments. Measured pH values actually increased as a
result of increased alkalinity production at the higher OLR, indicating that the reactor was not at risk
of going sour. These results clearly showed that the compartmentalised design of the ABR resulted in
the overall process anaerobic digestion of the feed COD being staged to allow development of an
acidic zone (and higher dissolved H2 concentrations) in the early compartments and a neutral zone in
later compartments. As a result, methanogenesis in later compartments was protected from high
dissolved H2 and low pH incidents due to increased organic load, thereby enhancing the stability of the
reactor (Nachaiyasit and Stuckey, 1997b).
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