Fig. 1

General scheme of steps involved in both AD (with biogas as the final product) and AF (with mainly SCFAs as final products). Complex organic matter is degraded by hydrolytic enzymes produced by fermenting and facultative anaerobic bacteria (Hydrolysis). Hydrolysed products are fermented to volatile SCFAs such as acetic, propionic, butyric, valeric and caproic acids as well as alcohols, longer chain fatty acids, lactate, CO₂, and formate (Acidogenesis). The hydrolysing and fermenting bacteria belong to the phyla Firmicutes, Bacteriodetes, Spirochaetes, Actinobacteria, Chloroflexi, and Proteobacteria. Intermediates are further metabolized to methanogenic substrates H₂, formate, and acetate due to the activity of specialized syntrophic consortia as well as acetogens (acetogenesis). Hydrogenotrophic and acetoclastic methanogens complete the process by converting these compounds to methane and CO₂ (methanogenesis) [21]. In order to accumulate SCFAs acetogenesis and methanogenesis must be prevented. SCFAs = short-chain fatty acids