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

Nevertheless, Section 6.4 provides an analysis of alkalinity supplementation requirements for dilute
domestic wastewater.
The low alkalinity in eThekwini waters relative to the rest of South Africa mean that these results and
conclusions apply to a poor case (if not worst case) scenario, since in many other regions, the
alkalinity concentration in the inflow will be substantially higher than observed in this study. Under
these conditions, higher pH buffering capacity and therefore higher pH values may be expected, with
improved degradations rates and therefore higher biomass growth rates.
6.1.1.3 Phase Separation
One of the claimed advantages of the baffled design of an ABR is the spatial separation of acidogenic
and methanogenic stages in different compartments of the ABR (Section 2.5.1.4); it was hypothesised
that a baffled reactor design would be more beneficial in sewage treatment than a single stage system
because of this effect. However, in the general anaerobic digestion literature, several authors suggest
that the advantages of phase separation in the digestion of particulate material are uncertain (Hanaki et
al., 1987; Leitão et al., 2006). This point was also made by Barber and Stuckey in their review of ABR
applications with reference to an unpublished study by Hassouna and Stuckey.
In the present study lower pH values were seen in the first compartment during certain phases of
operation, and in Phase IV, when stable anaerobic digestion was known to have established,
acetoclastic methanogen species Methanosaeta was observed in compartments 2, 3 and 4, but not in
the first compartment. These results indicate that partial phase separation did occur in that
methanogenesis was limited in the first compartment. However, hydrolytic and acidogenic processes
were not limited to the first compartment; hence true phase separation, i.e. where methanogenesis is
negligible in the first phase, and dominant in the second or later stages (Leitão et al., 2006) did not
occur (Section 5.8.4).
From the results of the pilot-scale ABR study, the following understanding of phase separation was
developed for the ABR treating domestic wastewater:
• The extremely high load of biodegradable organic material accumulated in the first
compartment led to the establishment of a large number of anaerobic micro-organisms which
degrade the particulate organics. The high organic load and relatively low pH buffering
capacity of the wastewater and the reactor liquors resulted in pH depression in compartment 1,
where the rate of acidification exceeded the rate at which acids were removed through
methanogenesis. Consequently VFA exited compartment 1 with other soluble components.
• The higher flow of VFA to compartment 2 but overall lower organic load in compartment 2
compared to compartment 1 resulted in the kinetic selection of different microbial
communities in the two compartments. Acetoclastic methanogens made up a larger fraction of
all active micro-organisms in compartment 2, while the fraction of hydrolytic and acidogenic
bacteria were proportionately fewer than in compartment 1 (Section 5.7.2.2). However, the pH
values were not always significantly higher in these compartments; therefore true phase
separation had not occurred.
• Subsequent compartments had similar microbial consortia to one another since the
biodegradable substrates in these compartments would be similar, i.e. particulate components
washed through by pulsing flow or soluble components that had not made contact with micro-
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