Psychrophilic anaerobic digestion: A critical evaluation of microorganisms and enzymes to drive the process

Anaerobic digestion (AD) technology is limited by low-temperature conditions preventing the adoption of simple, unheated digesters during cold periods. Decrease in temperature has a detrimental effect on various AD operating parameters and imposes a strong negative impact on microbial growth and enzymatic activity, particularly among mesophiles and thermophiles. Psychrophiles, however, have evolved a range of genotypic and phenotypic adaptive features, which have enabled them to overcome barriers associated with cold environments. Consequently, much attention has focused on psychrophilic microorganisms and their metabolites such as enzymes, as biological augmentation tools for psychrophilic anaerobic digestion (PAD) due to their important economic and environmental benefits. However, such bioaugmentation capabilities, inherent in psychrophiles, are not fully exploited as only a few studies have been targeted towards their impact on PAD. Therefore, this review provides a holistic synopsis of the current state of knowledge on biological strategies to enhance AD performance, which may serve as a stepping-stone for future PAD-related research. The AD process under psychrophilic conditions and a global perspective of PAD is discussed. In addition, various adaptation strategies employed by psychrophiles to survive cold stress are identified. Finally, biological approaches including bioaugmentation as well as enzymatic biomethanation employed to address the problem of PAD are highlighted. Special focus is given to potential strategies that can be employed to improve the applicability of biological additives at low temperatures.