Two-Stage Anaerobic Digestion for Green Energy Production: A Review

Anaerobic digestion (AD) is a biotechnological process in which the microorganisms degrade complex organic matter to simpler components under anaerobic conditions to produce biogas and fertilizer. This process has many environmental benefits, such as green energy production, organic waste treatment, environmental protection, and greenhouse gas emissions reduction. It has long been known that the two main species (acidogenic bacteria and methanogenic archaea) in the community of microorganisms in AD differ in many aspects, and the optimal conditions for their growth and development are different. Therefore, if AD is performed in a single bioreactor (single-phase process), the optimal conditions are selected taking into account the slow-growing methanogens at the expense of fast-growing acidogens, affecting the efficiency of the whole process. This has led to the development of two-stage AD (TSAD) in recent years, where the processes are divided into a cascade of two separate bioreactors (BRs). It is known that such division of the processes into two consecutive BRs leads to significantly higher energy yields for the two-phase system (H2 + CH4) compared to the traditional single-stage CH4 production process. This review presents the state of the art, advantages and disadvantages, and some perspectives (based on more than 210 references from 2002 to 2024 and our own studies), including all aspects of TSAD—different parameters’ influences, types of bioreactors, microbiology, mathematical modeling, automatic control, and energetical considerations on TSAD processes.