Investigation regarding the biosolvent yield of industrially relevant biocatalysts, and an economic evaluation for the valorisation of agricultural waste

There are 3.3 million tonnes of agricultural waste a year produced at the farm stage of the food production chain in the UK (1). Waste agricultural product is sent for composting, to landfill, to be spread on fields or sent for anaerobic digestion producing biogas for electricity. There are 650 anaerobic digestors in the UK (2), which produce ~300kWh per tonne of food waste (2). This project is investigating the economic viability of producing bio-solvents from waste agricultural produce by fermentation using S. cerevisiae, S. stipitis, or C. saccharoperbutylacetonicum, or whether it is more economically viable to send the waste for anaerobic digestion. The amount of bio-solvent produced by these microorganisms when supplemented with different carbon sources has been analysed by GC-MS. With the bioethanol yield produced by S. cerevisiae being greater than that of S. stipitis, whilst S. stipitis has a higher biomass yield and can utilise a wider variety of carbon sources than S. cerevisiae. The biosolvent yield achieved by C. saccharoperbutylacetonicum was low in comparison to S. cerevisiae or S. stipitis, as well as being an unreliable and temperamental strain, but does have the advantage over S. cerevisiae and S. stipitis of producing acetone and butanol as well as ethanol. An economic evaluation has been undertaken to determine the economic viability of bioethanol production and how it compares financially to anaerobic digestion. Bioethanol fermentation requires a high upfront investment, due to the large amount of infrastructure, and once the feedstock costs are considered bioethanol produced by fermentation only becomes financially viable once the by-product of ‘Dried Distillers Grains’ is sold as a protein rich animal feed. Whereas anaerobic digestion is simpler in terms of operation and feedstock requirements, and is much more scalable than bioethanol fermentation. The conclusion of this investigation has been that the disposal of agricultural waste by means of anaerobic digestion is preferable to the fermentation of said waste to produce biosolvents. This is due to the high upfront investment because of the large amount of infrastructure required for bioethanol production. As well as the simplicity of anaerobic digestion in terms of operation and feedstock requirements, together with its scalability.