Molasses wastewater treatment and lipid production at low temperature conditions by a microalgal mutant Scenedesmus sp Z-4

Background: 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 degrees 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. Results: 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 degrees C). The results showed that algal growth was inhibited at 4, 10, and 15 degrees C compared to that at the optimal temperature of 25 degrees C. However, decreased temperature had no significant effect on the total cellular lipid content of algae. Importantly, lipid productivity at 10 degrees C was compromised by more net energy output relevant to biodiesel production, which demonstrated that the low temperature of 10 degrees 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 degrees 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. Conclusions: 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.