The worldwide yeast extract business is large and may increase with the interest in
natural products and nutrients. The wastes from the yeast extract industry have
traditionally been used as animal food or soil injection for agricultural benefit. This
process is now being challenged and is experiencing increasing costs therefore
alternative options for disposal routes are being considered. The poor biodegradability
of activated sludge cell walls is well known and it has been suggested that the rigid,
double layered yeast cells will be even more recalcitrant. Previous work reviewed in the
thesis, had indicated yeast cell walls are some of the most refractory natural microbes in
comparison, for example with activated sludge. The thesis revisits the issue of solids
hydrolysis and in particular with a waste containing yeast cell walls as model solids.
The literature review discusses previous work on the treatment of yeast containing
wastes, including reactor designs and potential pre-treatments. It covers the work done
on the fundamental characteristics of solids which might affect biodegradation rates,
e.g. particle size, cross linking, rigidity and viscosity. Laboratory experiments were conducted and the results analysed from batch
biodegradability testing and continuous simulation trials comparing anaerobic reactors.
These were CSTR, Filters and UASB the latter noted for its vulnerability to solids.
Laboratory work is also reported on the potential for ultrasonic, thermal and
acclimatization to improve degradation rates. Utilisation of ultrasonic pre-treatment at
20,000 KHz, increased soluble organic carbon from between 14 to 120% dependant
upon power and exposure time period. The results also showed that continuous recycle
at low power produced the best results with increased gas yield and organic conversion
from a lower solids retention.
Results are also reported from onsite pilot trials using a 25m3 UAF digester and an
analysis of previously unreported full scale yeast processing plants in the UK. These
results confirmed that solids (cell) degradation rates were low. In conclusion the thesis suggests the degradability of the yeast cells are linked to their
unique cell walls. Anaerobic digestion does give organic conversion albeit with long
HRT's. The use of ultrasonics as a pre-treatment process enhances this conversion and
improves gas yield.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.