Optimization of the production of bioethanol from duckweed (Lemna minor)

This project has investigated the production of bioethanol from duckweed (Lemna minor) biomass. The project includes four main sections: firstly, analysis of the chemical characteristics of duckweed, particularly the polysaccharides of the cell wall; secondly, exploration of suitable commercial enzymes for degrading duckweed biomass to fermentable sugars; thirdly, optimisation of pretreatments and enzymatic saccharification; finally, fermentation and optimisation of the ethanol yield. Pond-grown L. minor contained 51.2 % carbohydrate (w/w dry matter) of which 77 % (including glucose, galactose and xylose) is fermentable. A series of enzymatic hydrolyses was used to evaluate the commercial enzymes and optimise conditions for their use in the saccharification of duckweed biomass. Celluclast 1.5L (CE) and Novozyme 188 (BG) were identified as suitable for hydrolysing duckweed cell walls (prepared as alcohol insoluble residues). The additional use of thermophysical pretreatment (steam explosion) results in a dramatic decrease in the amount of enzyme required for quantitative saccharification. A more advanced commercial cellulase cocktail (Cellic® CTec 2; CTec 2) is likely to further reduce the enzyme cost. Methods for the simultaneous saccharification, using CTec 2 and BG, and fermentation of steam exploded duckweed were developed. These resulted in an 80 % ethanol yield at a diluted substrate concentration (1 % w/v). However the ethanol yield decreased dramatically at higher substrate concentrations (to 18 % at 20 % w/v substrate concentration, which is a highly viscous suspension). Further studies involved the development of approaches to address this: (i) increasing the yeast titre in the inoculum or (ii) growing the inoculum on steam-exploded duckweed. These approaches facilitated an ethanol yield of up to 70 % (w/w) at a substrate concentration of 20 % (w/v). Maximising the final ethanol yield is of great importance in reducing the costs of production. The optimized ethanol production process indicates the technical potential for industrial ethanol production from duckweed. Operating costs have also been estimated and are discussed in relation to the potential exploitation of protein as a co-product