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

5.4.1.2 Phase III
In Phase III, (Figure 5.8), although the operating A-HRT were similar during this period to the
Phase II, the COD removing performance of the reactor was significantly better. This was attributed to
more stable flow conditions, and fewer sludge carry-over incidents than were observed in Phase II. A
more concentrated biomass, better suited to compartment conditions was able to develop. This is
corroborated by the higher sludge levels seen in Phase III as compared to Phase II (Section 5.6.1).
5.4.1.3 Phase IV
In Phase IV, even lower outflow COD values were obtained, with a mean outflow COD value of
130 mg/ℓ, and with values regularly dipping below 100 mg/ℓ (Figure 5.8, Phase IV). It was
hypothesised that the longer retention time of Phase IV (42 h) as compared to the Phase III (22 h)
resulted in a greater extent of removal (fraction of biodegradable COD converted to CH4). Mass
balance calculations were undertaken to test this hypothesis (Section 6.2). However, the amount or
load of COD converted was less than in previous periods, because of the lower OLR.
5.4.2 Total and volatile solids
Figure 5.9 shows total and volatile solids measurements from inflow and outflow streams for Phase II
and Phase III, and total solids data only for Phase IV. Suspended solids data (solids retained after
settling, filtration, or centrifugation) measured by municipal staff are also presented. Municipal data
are always lower than the equivalent values of total solids measured by the project team as the latter
measurement includes dissolved solids. In Phase II, average total solids and volatile solids removals of
44% and 52% respectively were obtained. These increased to 53% and 59% respectively in Phase III.
The general improvement in removal rates between Phase II and Phase III was attributed to better flow
control resulting in fewer washout incidents and development of a more stable and concentrated
sludge bed.
No volatile solids measurements were made in Phase IV. An overall total solids removal rate of 48%
was calculated for Phase IV. The average concentration of total solids in the outflow was also
significantly higher in Phase IV than in Phase III (Student’s t-test, P = 0.0002), despite the lower
hydraulic loading rate. The cause of this difference is not known.
5.4.3 Alkalinity
Figure 5.10 shows inflow and outflow alkalinity concentrations for Phases II and III. Only a few data
points are available for Phase IV; averages for Phase IV are reported in Table 5.6.
In both Phase II and Phase III outflow alkalinity values were above inflow alkalinity values indicating
that alkalinity is generated by the partial conversion of COD to bicarbonate and concomitant release of
cations (Section 2.1.6.3).
Speece (1996) recommends that alkalinity concentration in the operation of anaerobic digesters is
maintained at sufficiently high concentrations to provide a reserve alkalinity that is available to
neutralise additional acids produced by fermentation. The reserve alkalinity ensures that the operating
pH value does not drop below a range of between 6.2 and 6.5 since metabolic rates may be adversely
affected below these values. It is shown in Section 5.6.2 that the pilot-scale ABR treating a relatively
dilute (700 mgCOD/ℓ) wastewater was operating with very little reserve alkalinity since pH values are
lower than these recommended targets. It is therefore probable that poor buffering as a result of low
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