Enhancing Food Waste Hydrolysis and the Production Rate of Volatile Fatty Acids by Prefermentation and Hydrothermal Pretreatments

The effects of prefermentation pretreatment (PF) alone and prefermentation combined with hydrothermal (HT) pretreatments (PF-HT) on the production of volatile fatty acids (VFAs) by the anaerobic fermentation of food waste (FW) were investigated. The results showed that PF could enhance the dissolution and hydrolysis of organic materials. The greater levels of soluble organic materials (soluble sugar, 10.8 g/L; and soluble protein, 1.61 g/L) were achieved after the PF-HT pretreatment. In the subsequent anaerobic fermentation tests, for the PF and PF-HT groups, the production rates of VFAs increased 1.3 times and 2.0 times, respectively, as compared to the control over the first 5 days. The maximum concentrations of VFAs for the PF and PF-HT groups on the 15th day of fermentation were 32.9 and 35.5 g COD (chemical oxygen demand)/L, respectively, with corresponding increases in the yields of VFAs of 12% and 21% over the control group. It is suggested that the high content of lipid and cellulose in the FW prevented any further improvement in the yields of VFAs. Butyric (Bu) and acetic acids (Ac) were the prevalent VFAs, regardless of the pretreatment used. However, the metabolic pathways for the VFAs production were different for the two pretreatments: with PF pretreatment, the FW was converted into VFAs via lactic acid, but with PF-HT pretreatment, it was transformed directly into VFAs. Economic analysis indicates anaerobic fermentation with appropriate PF and HT pretreatments is an earning-effective approach for FW to bioenergy such as VFAs.