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

sampling was not possible since in both cases, the influent channels were well below ground level.
The problem with the feeding system was that the submersible pumps macerated the feed; therefore
the feed to the ABR was not identical to the influent WWTP wastewater (although the chemical
composition was of course not changed). In addition, the pumps used were not well suited to the
pumping of raw sewage and the impellors were regularly entangled with strings and rags, resulting in
low flow and reactor down time.
Gas measurement: It proved impossible to obtain accurate measurements of gas production rates
from the pilot-scale ABR system as a result of the pressure buffering provided by the standing baffle
system. Gas production in the reactor displaced liquid within the reactor as well as in a liquid
displacement gas measuring system. To overcome internal pressure buffering, all compartment gas
production needed to be collected simultaneously to prevent the increased pressure in each
compartment being redistributed to neighbouring compartments. This was not possible with the
available equipment. Without CH4 production data, it was not possible to complete a mass balance to
determine the extent of treatment achieved in the pilot-scale ABR.
6.1.4.2 Missing experimental data
Although the experimental data yielded much information relating to mechanism and rate of domestic
wastewater treatment in the ABR, accurate measurements of the feed and outflow stream
biodegradability were not made. At the time, it was assumed that the feed wastewater was similar to
wastewater in any other facility in the region, and that the residual biodegradability of the outflow
stream would be negligible. However, the absence of these data has two consequences;
• Firstly, since there were no gas flow and composition measurements, it is not possible to
accurately calculate the amount of COD converted during treatment in the ABR, and therefore
how much CH4 was produced or how much particulate biodegradable COD accumulated; and
• Secondly, it is not clear how close the system approached to completely removing degradable
COD from the wastewater, and therefore what the outlet stream characteristics (COD,
alkalinity, free and saline ammonia) would have been if near-complete treatment had been
achieved.
To address these gaps, two exercises were undertaken: (i) Mass balance analysis of all COD data to
determine probable CH4 production rates (Section 6.2), and (ii) Estimation of outflow stream
characteristics from stoichiometric principles (Section 6.4).
6.2 MASS BALANCE TO DETERMINE PROBABLE SLUDGE/METHANE PRODUCTION RATES
In this section, mass balance principles were used to determine the composition and quantity of the
three outputs of the ABR, i.e. the liquid outflow stream, biogas produced and accumulated sludge. The
pilot-scale ABR is considered to be a non-steady-state system with continuous feed, liquid outflow
and biogas streams, but with accumulating solids that are removed discontinuously (Figure 6.2)
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