Anaerobic Thermophilic Mixed Culture Fermentation Processes

Abstract

Advantageous properties, such as higher hydrogen production, high substrate degradation rate, and efficient heat utilization for the treatment of hot wastewater, favor the thermophilic mixed culture fermentation (MCF) over mesophilic MCF. In this chapter, the typical metabolic reactions in thermophilic MCF are summarized in Sect. 2 according to the multistep process of hydrolysis/acidogenesis, acetogenesis/homoacetogenesis, and methanogenesis. The operational conditions, such as pH, H2 partial pressure, and reactor configuration, may change the microbial community or the metabolic pathway in thermophilic MCF and are reviewed in Sect. 3. Lastly, the metabolites both in the headspace and in liquid solutions are always a mixture in thermophilic MCF, which have to be concentrated and purified before utilization. There are several conventional technologies to separate the metabolites and recover energy, including biogas upgrading, two-stage fermentation, gas stripping, electrodialysis (ED), and microbial fuel cells. These typical technologies, along with other novel technologies, such as production of sole metabolite and medium-chain carboxylic acids production, are reviewed in Sect. 4. The energy cost of thermophilic biogas plants was estimated as just 10% of the energy produced, which implies that the extra energy cost for operating at a thermophilic temperature is marginal. Therefore, the coupling of the process and development of novel technologies are necessary in thermophilic MCF to promote its worldwide application.