Acidogenic Fermentation Towards Valorisation of Organic Waste Streams into Volatile Fatty Acids

Anaerobic digestion (AD) has been applied over the years for the treatment of a wide variety of organic streams. The advantages of this process over other biological treatments relate mainly to the low cost of bioreactors operation, when compared to aerobic processes, since AD-based treatments offer a cost-effective solution for the treatment of high-strength organic wastes, through economic return of bioenergy (methane) and by-products generation. AD is a sequential biochemical process in which the complex organic components present in the carbon waste, such as polysaccharides, proteins, and lipids, are hydrolysed, broken down, and fermented into intermediate products that are subsequently reduced into methane and carbon dioxide. These stages are commonly referred to as hydrolysis, acidogenesis and methanogenesis. In conventional AD applications, these transformations take place together in a single reactor system, where a delicate balance between the several groups of microorganisms is kept, although they differ widely in terms of physiology, nutritional needs, environmental sensitivity, and growth and substrate uptake kinetics.1 Problems in the stability and control of conventional AD systems have led to the technological development of new solutions, such as phase separation, firstly proposed by Poland and Gosh.2 Two-phase anaerobic digestion implies a system configuration with separate reactors for acidogenesis and methanogenesis connected in series, thus allowing for the optimisation of each process separately. The first phase (acidogenic fermentation) leads to the production of intermediate products mostly volatile fatty acids (VFA), whereas the second (methanogenic phase) results in the conversion of these intermediates into stable end products, mainly methane and carbon dioxide. Although both anaerobic processes with and without phase separation are well developed, there is little information on adequate design and operation of acidogenic reactors.3 Therefore microbiological and kinetic aspects on the operation of acidogenic reactors with Acidogenic Fermentation Towards Valorisation of Organic Waste Streams into Volatile Fatty Acids