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

compartment 1 and 2, the increase was not sufficient to support the claim that phase separation
occurred in the pilot-scale ABR where acidogenic reactions dominated in the first compartment, and
methanogenic processes dominated in later compartments. It was clear that hydrolytic and acidogenic
processes occurred in all compartments. However, spatial separation of compartments allowed regions
characterised by slightly different pH values to develop, and this may have improved overall digestion
rates and process stability (Section 6.1.1).
7.1.1.4 Solids accumulation rate vs. feed flow rate
Solids were observed to accumulate faster at an A-HRT of 22 h than at an A-HRT of 42 h. The solids
accumulation rate per kg COD applied was also higher in the 22 h A-HRT period. Mass balance
calculations indicated that approximately 30% of influent COD was removed as CH4 at the higher
flow rate, while 60% was removed as CH4 at the lower flow rate. In addition, the free and saline
ammonia concentration increase between inflow and outflow was greater at the lower flow rate. These
data indicate that better digestion, termed extent of treatment of influent COD occurred at lower flow
rates. A study using scanning electron microscopy confirmed this conclusion in that it observed low
micro-organism concentrations, poor microbial diversity and few acetoclastic methanogens at higher
flow rates, and conversely, good granulation, good microbial diversity, and many methanogens,
including acetoclastic morphotypes at the lower flow rate. It was concluded that at the higher flow
rate, the washout rate of micro-organisms was of similar magnitude to their generation rate, and thus
diverse and stable microbial communities failed to establish (Section 6.1.2). It was concluded that
liquid upflow velocity was an important factor for ensuring microbial stability in an ABR treating
sewage, sludge accumulation rates, and thus ultimately the required desludging interval.
It was further concluded that, since microbial respiration rates were limited by low pH values and low
substrate concentrations, the critical upflow velocity is not a global value, but system specific and
dependent on prevailing pH conditions determined by alkalinity concentration and organic strength of
the wastewater to be treated.
7.1.1.5 Sludge production rates and condition of accumulated solids
It was estimated that sludge accumulated at a rate of 0.43 kg dry solids/ kg applied COD at an A-HRT
of 22 h (Phase III) and at 0.11 kg dry solids/ kg applied COD at an A-HRT of 42 h (Phase IV). These
values corresponded to desludging intervals of 105 d and 405 d of uninterrupted operation respectively
(Section 6.2.1).
It was concluded that at the higher flow rate, accumulated solids contained significant amounts of
undegraded particulate organic material since this was not recovered in the effluent. It was concluded
that the main function of ABR was a solids accumulator at high flow rates. However, at low flow
rates, approximately 80% of incoming COD was removed, while it was calculated that approximately
60% of inflow COD was converted to CH4. Therefore, the ABR behaved as a solids digester at these
flow rates, and it is expected that the residual biodegradability of the accumulated sludge was far
lower at the lower flow rate than at the higher flow rate. This proposal was supported by the nitrogen
balance, which indicated that little of the influent TKN was retained as sludge (Section 6.2.2). The
residual biodegradability of the accumulated sludge will have an impact on how removed sludge may
be disposed of.
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