Pilot-scale cultivation of water-net in secondary effluent using an open pond raceway for nutrient removal and bioethanol production.

Although microalgae are seen as a potential alternative source of energy and other materials currently sourced from petroleum, issues still remain with industrialization because of difficulties in developing commercially operational microalgal production systems. We set up a pilot-scale study that involved the simultaneous reuse of wastewater and production of algae under different light and harvesting conditions. We cultivated Hydrodictyon reticulatum (H. reticulatum), a type of water-net algae, using secondary effluent from a wastewater treatment plant in a raceway open pond combined with an underwater light device. Experimental results showed that the underwater light device maintained some level of underwater light throughout the year. Photoinhibition of algal growth only occurred when the sunlight exceeded 1270 μmol m s. During the non-harvest process, the maximum algal concentration was 1700 mg L at 20 days after the experiment began, the maximum specific growth rate was 0.18 d and the maximum productivity was 21.3 g m d. Conversely, periodic harvesting decreased the concentration of nutrients in the effluent more as the days of cultivation increased, but the productivity of algae also decreased to 11.7 g m d. The maximum yield of bioethanol using three kinds of fermentation strains was 93.5% and, thus, the commercial value of H. reticulatum as a raw material for energy production was excellent.