Hydrogen production by fermentative bacteria and photosynthetic organisms is depressed under psychrophilic conditions. Here we describe single-chamber microbial electrolysis cells (MECs), which were enriched successfully at 4 °C or 9 °C and operated at these temperatures for the production of hydrogen. The rates of hydrogen production varied from 0.23 ± 0.03 to 0.53 ± 0.04 m³ H₂ m⁻³d⁻¹, and the hydrogen yield was in the range 2.66 ± 0.22 − 2.94 ± 0.02 mol H₂ mol⁻¹acetate. The energy efficiency based on electrical input ranged from 154 ± 9 to 200 ± 18% using an applied voltage of 0.6 V or 0.8 V; CH₄ was not detected in any of the reactors. The separate MECs were first operated at 25 °C and methanogenesis was found to occur in them, however no CH₄ was produced if the cells were operated at 4 °C or 9 °C. 16S rRNA gene clone libraries constructed for Archaea proved that the growth of hydrogenotrophic methanogens such as Methanobrevibacter arboriphilus cultured in MECs at 25 °C was effectively inhibited under psychrophilic conditions, and the growth of these organisms was initially suppressed in these MECs maintained at low temperatures. Bacterial 16S rRNA gene clone library analyses showed that temperature significantly influenced the community structure of the anode biofilms in MECs, and Geobacter psychrophilus was found to be the dominant psychrotolerant population in MECs maintained at 4 °C and 9 °C. Our study also demonstrated that frequent inoculation using pre-acclimatized suspensions of microbial fuel cells (MFCs) greatly facilitated the rapid formation of psychrophilic biofilms in MECs at low temperature.

Simultaneous wastewater treatment and lipid production by oleaginous microalgae show great potential to alleviate energy shortage and environmental pollution, because they exhibit tremendous advantages over traditional activated sludge. Currently, most research on wastewater treatment by microalgal are carried out at optimized temperature conditions (25–35 °C), but no information about simultaneous wastewater treatment and lipid production by microalgae at low temperatures has been reported. Microalgal growth and metabolism will be inhibited at low temperature conditions, and satisfactory wastewater treatment performance will be not obtained. Therefore, it is critical to domesticate and screen superior microalgal strains with low temperature adaptability, which is of great importance for wastewater treatment and biodiesel production. In this work, simultaneous wastewater treatment and lipid production were achieved by a microalgal mutant Scenedesmus sp. Z-4 at the low temperature conditions (4, 10, and 15 °C). The results showed that algal growth was inhibited at 4, 10, and 15 °C compared to that at the optimal temperature of 25 °C. However, decreased temperature had no significant effect on the total cellular lipid content of algae. Importantly, lipid productivity at 10 °C was compromised by more net energy output relevant to biodiesel production, which demonstrated that the low temperature of 10 °C was favorable to wastewater treatment and energy recovery by Scenedesmus sp. Z-4. When molasses wastewater with optimal COD concentration of 8000 mg L−1, initial inoculation ratio of 15%, and C/N ratio of 15 was used to cultivate microalgae, the maximum removal rate of COD, TN, and TP at 10 °C reached 87.2, 90.5, and 88.6%, respectively. In addition, lipid content of 28.9% and lipid productivity of 94.4 mg L−1 day−1 were obtained. Scenedesmus sp. Z-4 had good adaptability to low temperature conditions, and showed great potential to realize simultaneous wastewater treatment and lipid production at low temperatures. The proposed approach in the study was simple compared to other wastewater treatment methods, and this potential novel process was still efficient to remove COD, N, and P at low temperatures. Thus, it had a vital significance for the wastewater treatment in low temperature regions.