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

• Investigation into COD removal in each compartment shows that the majority of COD was
removed in the first compartment (Figure 4.12). Much of the removed COD was retained
particulate material that could not move beyond compartment 1 due to the baffle construction.
The sludge bed created also acted as a filter that ensured good contact between organic
material (particulate or soluble) and anaerobic micro-organisms. For very low loading rates,
much of the reactor volume was not used for biological activity. This was as much a function
of the lack of sludge present in these compartments as of the apparent efficiency of the first
compartment. When loading rates increased, it was seen that the amount of treatment that
occurred in later compartments increased, as did the sludge load. It was also seen that,
although the contribution to overall COD removal of later compartments was low, they
fulfilled an important buffering function to ensure that later compartments were not subject to
low pH values caused by acidogenesis in the early compartments (Section 4.5.3 below).
As may be expected, the rate of accumulation of sludge in the ABR was directly related to the OLR.
The reactor was tested up to hydraulic and organic loading rates approaching 20 h and
0.9 kg COD/m 3 .d respectively, although precise figures cannot be calculated due to uncertainty in flow
rate data. The loading rate was substantially less than the value of 1.5 kg CODbiodegradable/m 3 /d proposed
by Lettinga (2001) who indicated that 80 % COD removal could be achieved at a HRT as low as 4 h
for anaerobic systems for the pre-treatment of domestic wastewater in tropical climates.
4.5.3 Phase-separation between compartments
A number of authors observed a phase-separation phenomenon during the operation of
compartmentalised anaerobic digesters (Sections 2.5.1.4 and 2.2.3). The following observations were
made in this study:
• In situ pH values may have been lower than those measured because of the lag between
sampling and analysis (Section 4.4.3)
• pH measurements made on samples taken from within compartments show that significant
acidification occurred in the first compartment at longer retention times. It was inferred that
methanogenesis did not proceed at the same rate as acidogenesis, resulting in the accumulation
of VFA and lowering of the pH value. However subsequent analysis indicated that low pH
values may not necessarily have been due to methanogenesis inhibition (Section 6.4)
• Conventional inhibition terms were used to estimate the extent of inhibition of
methanogenesis that may have occurred as a result of low pH values. Significant inhibition of
methanogenesis is expected in compartment 1, although the extent of inhibition decreases in
subsequent compartments.
• pH profiles for the 60 h and 32 h THRT periods were similar, with pH depression observed in
the first compartment. However, in the 20 h T-HRT, low pH values were observed in the first
three compartments. The difference was attributed to different sludge load profiles in the
different operating periods. The pH value at the reactor outflow during the 20 h THRT period
was similar to those observed in the earlier periods.
These results point to a very useful attribute of the ABR in the treatment of domestic wastewater. The
compartmentalisation of the ABR provides a degree of phase separation such that hydrolysis and
acidification of organic material in the early compartments can proceed at pH values that are lower
82