| Summary: | Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2005. === The Up-flow Anaerobic Sludge Blanket Reactor (UASB) provides a state-of–the-art solution to
effluent treatment by anaerobic digestion: sludge production is dramatically lower than in other
digestion processes, and energy is gained from the system if the produced biogas is converted to
electricity and/or heat. The UASB is a modified fluidised bed reactor, with the solid state
‘catalyst’ being granulated anaerobic sludge, and the liquid phase the effluent that needs to be
treated. A gas cap is installed to serve as a carbon dioxide and methane collector. This biogas
(carbon dioxide and methane) is produced by the stepwise decomposition of complex
carbohydrates and proteins via a consortium of micro-organisms living in a symbiotic
environment known as a granule. A typical UASB reactor has an organic removal rate of 89-93%
Chemical Oxygen Demand (COD) and operates optimally at loadings of 9.8-11 kg COD/ m3
reactor volume/day. Unfortunately, one major problem hampers the efficiency of this reactor to
such an extent that the unit is only economically viable in exceptional cases; if the reactor is
inoculated with un-granulated anaerobic sludge, start-up times of up to 12 months can be
expected.
The lengthy start-up times motivated the search for an artificial way to cultivate USAB granules.
Early research (done on lab-scale, 400ml vessel volumes) proved that, under a specified set of
environmental conditions, granule growth can occur in an artificial environment. Yet these
laboratory-scale vessels did not facilitate scale-up or the study thereof. This led to the main
problem statement of this research project: namely to design, commission, and optimise benchscale
bioreactors that will generate granulated anaerobic sludge in an incubation period of 20
days. These units should also facilitate in the determining of parameters that will assist in the
design of a scale-up to a UASB granule producing reactor of economically viable size. Two
bench-scale reactors were initially designed specifically to “mimic” the motion found in the
laboratory-scale vessels. The results from these initial reactors proved that granulation cannot
only be enhanced, but granules can actually be cultivated from dispersed anaerobic sludge in a
larger artificial environment over an incubation period of only 20 days.
The results were still far from satisfactory, as the granules produced were irregular in shape and
the yield of usable granules (2.2 kg/m3 reactor volume) insufficient. A third test reactor was
designed to “mimic” roller table movement and baffles were included. These results were much
better and the yield was 4.4 kg/m3 reactor volume at a baffle tipspeed of 0.0055 m/s. The
optimisation was extended further to include the inoculation sludge and the feed medium. A
C:N:P ratio of 10:1:4 proved to yield the best results. Monovalent anions, hydrogen
concentration and a pH-level outside the 6.5 to 7.2 range evidently had an inhibitory effect on the
granulation rate. After the optimisation study the third test unit produced a usable granule yield
of 15.2 kg/m3 reactor volume over the 20-day incubation period.
The incubation period can be separated into 3 distinct phases, namely the acidification,
stabilisation and growth phases. From the mass balance it was found that most of the COD and
nutrients were used for ECP production in the acidification phase. During the stabilisation phase,
the COD and nutrients were mostly used for nucleus formation, and finally in the growth phase
the COD was used for granule growth. To study the effect the internal surface area of the reactor
has on the granulation process, 3 scale-down versions of the third test unit were constructed.
Within the studied range, a yield of usable granules of 40 kg/m2 reactor internal surface area was
obtained.
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