Investigating the interaction of pretreatment and saccharification processes for bioethanol production from olive mill solid waste

Statistical Design of Experiments was used to investigate the interaction between the pretreatment and saccharification processes for ethanol production from olive mill solid waste (OMSW). Sixty-four experimental runs were performed to test for four pretreatment factors: microwave (MW) temperature and duration, organic acid (OA) type and concentration, and two enzymatic factors: enzyme load and Tween 80 addition. Response variables were: loss of biomass and sugars during pretreatment, cellulose saccharification efficiency, formation of fermentation inhibitors, yeast growth, and ethanol production. Saccharification efficiencies were 17 %–94 %, and ethanol production was 6–15 g/L. Hydroxymethyl furfural (HMF) and furfural concentrations were less than 1.1 g/L throughout the runs. Within the investigated ranges, microwave temperature, OA type, and enzyme load had the most significant effects on ethanol production, with a strong interaction between OA type and temperature. Enzyme load had no interaction with other factors. Furthermore, although a maximum ethanol yield (13.4 g/L) required 180 °C, 0.6 mol/L maleic acid, and 22 FPU/g treated OMSW, 11.4 g/L ethanol can be achieved with only 0.4 mol/L maleic acid and 16.5 FPU/g enzyme load, lowering costs. A shorter MW treatment (4 min) and no Tween 80 addition can also be used, reducing costs further since these factors had no significant effect on ethanol yields. The key result was that pretreatment and saccharification processes can be optimized separately.