High-efficiency biofuel production by mixing seawater and domestic sewage to culture freshwater microalgae

Abstract

Although microalgae biofuel has many advantages, its production is faced with difficulties regarding algal cell harvesting and low lipid content. In this experiment, mixtures of seawater and domestic sewage were prepared for the cultivation of freshwater microalgae. The results showed that the proportion of seawater had an important effect on the precipitation and biofuel production of the microalgae. The increasing proportion of seawater in the mixed wastewater improved the sedimentation performance and lipid content of microalgae, but the growth of microalgae was greatly inhibited under 80% seawater stress. Therefore, the highest algal lipid productivity of 10.74 mg L-1 d-1 was achieved in the wastewater containing 60% seawater, which was 2.15-fold higher than that of the control. Furthermore, the fatty acid saturation and the C16-C18 fatty acid proportion of the microalgae cultured in wastewater containing 60% seawater were relatively ideal, which was beneficial for improving the quality of biodiesel. Therefore, wastewater containing 60% seawater was the most favourable for the cultivation of biofuel-producing microalgae. The mechanism by which seawater improved the precipitation performance of freshwater microalgae was also investigated. The addition of seawater to the sewage increased the cell diameter of the microalgae, reduced the zeta potential of microalgal colloids, and promoted the synthesis of extracellular polymeric substances by the microalgal cells. Thus, the precipitation performance of the microalgae cells was effectively improved. The pigment contents and antioxidant enzyme activities of the microalgae were also determined, and it was found that the glutathione peroxidase activity of microalgae increased under certain seawater stress, indicating that glutathione peroxidase played an indispensable role in the removal of reactive oxygen species produced by seawater stress.