Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser Perspective on anaerobic digestion for biomethanation in cold environments Subhabrata Dev a,b,c , Shouvik Saha a , Mayur B. Kurade a , El-Sayed Salama a,d , Marwa M. El-Dalatony a , Geon-Soo Ha a , Soon Woong Chang b , Byong-Hun Jeon a, ⁎ a Department of Earth Resources and Environmental Engineering, Hanyang University, Seoul 04763, South Korea b Department of Environmental Engineering, Kyonggi University, Suwon 16627, South Korea c Mineral Industry Research Laboratory, Institute of Northern Engineering, University of Alaska Fairbanks, Fairbanks, AK 99775, USA d Department of Occupational and Environmental Health, School of Public Health, Lanzhou University, Lanzhou 730000, Gansu, China ARTICLE INFO Keywords: Anaerobic digestion Biomethane Psychrophilic anaerobes Cold-adapted proteins Cold-adapted lipid Membrane fluidity ABSTRACT The anaerobic digestion (AD) has become an important part of the wastewater treatment plants that regulates the sustainable management of organic wastes with simultaneous production of bioenergy. AD at low tem- peratures using psychrophilic anaerobes with optimum growth temperatures < 20 °C has gained significant attention for improvement of biogas productivity in cold regions. The present review discusses the detailed characteristics of psychrophilic anaerobes, and how the properties of those particular psychrophiles can be utilized towards the cost-effective production of methane at cold environment. The different challenges for AD at low temperature have been described thoroughly. The various strategies such as (a) adaptation of microbial community, (b) optimization of operational parameters, (c) utilization of specialized biodigester design, and (d) modification of downstream process to improve the AD and biomethane production in cold environments have also been summarized. The present review proposes the future technological developments which should be aimed at effective performance of anaerobic digesters to improve biomethanation in cold regions. 1. Introduction Globalization has increased the demand of renewable energy to compensate for depletion of fossil fuels and to mitigate environmental concerns. Anaerobic digestion (AD) is a sustainable process for gen- eration of bioenergy and management of organic waste worldwide, due to its low energy demand [1–3]. Several governmental and non-gov- ernmental organizations have established large numbers of digesters, many of which are either non-functional or have low methane yields due to operational challenges and poor management practices [4]. Temperature is one of the performance bottlenecks for AD and biogas yields, as it influences the initial hydrolysis rate of organic substrates in digesters [1,5,6]. AD is operated at various temperatures depending on climate to reduce external energy demands, including thermophilic (55–60 °C), mesophilic (35–37 °C) and psychrophilic (< 20 °C) [7]. Mesophilic temperature is the most widely used temperature for AD, as it is feasible without external energy inputs for approximately 40% of the world's population living in tropical regions [8]. The enhanced rate of biochemical reactions, higher microbial growth rates (Fig. 1) and increased hydrogen transfer among microbial species increases bio- methanation even at low hydraulic retention times (HRT) [6]. Biogas production in thermophilic conditions has been observed to be 144% and 41% higher than that reported under psychrophilic and mesophilic conditions, respectively [9]. The cold environment (below 5 °C) is present in the significant portion (85%) of the biosphere of Earth such as deep sea, snow, per- mafrost and glacier [10]. Canada, Europe, the northern parts of USA and China, Russia, the southern part of Australia, and New Zealand are temperate regions with moderately variable seasonal temperatures [11]. Therefore, psychrophilic AD operation would be more suitable for these regions than thermophilic or mesophilic AD, if its feasibility were demonstrated. Anaerobic digestion in these regions is a concern due to seasonal variations in wastewater temperature (< 20 °C) [11]. Survival and performance of anaerobic methanogenic microbial communities are also suppressed in cold environments [1]. Only cold-adapted (psychrophilic) microorganisms can perform constantly at temperatures below 20 °C via evolving mechanisms to deal with the accompanying thermodynamic constraints [12]. Cold- adapted microorganisms achieve physiological and ecological success in cold environments based on unique features in their membrane proteins, lipids and genetic responses to thermal shifts. Expression of cold-adapted proteins, lipids and enzymes (e.g., proteases, lipases and https://doi.org/10.1016/j.rser.2018.12.034 Received 22 May 2018; Received in revised form 5 December 2018; Accepted 15 December 2018 ⁎ Corresponding author. E-mail address: bhjeon@hanyang.ac.kr (B.-H. Jeon). Renewable and Sustainable Energy Reviews 103 (2019) 85–95 1364-0321/ © 2018 Elsevier Ltd. All rights reserved. T