The role of anaerobic digestion in the sustainable treatment of yeast related food industry waste

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.