Volatile Fatty Acids from Household Food Waste: Production and Kinetics

Household food waste (HFW), which is rich in organic matter, is a good candidate for producing added-value bio-based chemicals, such as volatile fatty acids (VFAs), by acidogenic fermentation processes. However, the lack of design tools, such as appropriate kinetic models, hinders the implementation of this technology because the results of these processes are affected by operational factors. In this work, VFA production by the acidogenic fermentation of HFW under uncontrolled pH levels (4–5) was studied at thermophilic (55 °C) and mesophilic (35 °C) temperature conditions. Batch reactors were used to digest HFW, and VFA production and the individual acid distributions were measured at different fermentation times from 0 to 624 h. The results showed higher individual and total VFA production at 35 °C and 120 h of fermentation time as a consequence of the competition between the VFA production and decomposition reactions. Acetic and valeric acids were VFAs mainly produced as a result of a high content of proteins in the initial substrate, and a small amount of propionic and butyric acids were present. A simplified kinetic model was successfully developed to represent the complex process of VFA formation from the acidogenic fermentation of HFW. A simple mechanism for the production–decomposition of VFAs, corresponding to a zero-order reaction for the first 48 h and a single consecutive reaction from that time on, was proposed. For both mesophilic and thermophilic conditions, the suggested kinetic model was able to predict the individual and total concentrations of VFAs along the fermentation time.