Effects of temperature and pretreatment conditions on mixed-acid fermentation of water hyacinths using a mixed culture of thermophilic microorganisms

The MixAlco™ process biologically converts biomass to carboxylate salts that may be chemically converted to a wide variety of chemicals and fuels. This study investigated the use of water hyacinths as a feedstock, comparing digestibility after each of four different pretreatments at two fermentation temperatures (40 and 55 °C). Water hyacinths were treated with excess lime (0.3 g Ca(OH)2/g dry biomass). Short-term treatment occurred for 1 and 2 h at 100 °C. Long-term treatment occurred for 4 and 6 weeks at 50 °C. Treated water hyacinths were fermented with marine microorganisms for 28 days and acid concentration (g/L), conversion (g volatile solids (VS) digested/g VS fed), and selectivity (g acid/g VS digested) were measured. All pretreatments out performed fresh feedstock fermentations. The 40 °C fermentations exhibited greater acid yields and selectivity than the 55 °C. The 1-h hot-lime pretreatment exhibited the best overall outcomes at approximately 250%, 200%, and 125% increases relative to the fresh water hyacinths in total acid, conversion, and selectivity, respectively. The results show that with a gentle 1-h hot-lime pretreatment, water hyacinths can be fermented to produce liquid fuels, thus creating an economic value to water hyacinths that are cleared from choked waterways.