Review about bioproduction of Volatile Fatty Acids from wastes and wastewaters: Influence of operating conditions and organic composition of the substrate

Volatile fatty acids (VFAs) are a group of carboxylic acids considered as building block chemicals. Nowadays, commercial production of VFAs is performed using fossil fuel sources. As an alternative, acidogenic fermentation of wastes by mixed microbial cultures (MMC) is starting to be considered as a potential bioproduction process that would replace conventional production processes and contribute to the circular economy. Nevertheless, more research is needed to control the VFA production yields and to precisely drive the fermentation process to the production of a certain VFA or a mixture of VFAs, either by modifying the operational parameters or by appropriately tunning the substrate composition. Following this gap, this review starts screening the metabolic routes that yield VFAs by anaerobic fermentation. Subsequently, the effect of different operational parameters on VFA production yield and VFA composition distribution is extensively discussed depending on the organic composition of the waste in terms of proteins, carbohydrates and lipids. To the best of our knowledge, previous review articles analyzed the impact of these parameters for different types of wastes, but without specifically considering their organic composition in terms of proteins, carbohydrates and lipids. Afterwards, energy-based metabolic models are presented as the one of the best modelling approaches to predict VFA composition. Then, polyhydroxyalkanoates (PHAs) production by MMC is described since it is one of the most promising applications of waste derived VFAs. Finally, we highlight the research gaps that should be further investigated to develop a large scale VFA bioprocess based on MMC platform from waste streams.