We want to end up with a robust polyculture of different algae that grow well and efficiently deplete the nutrients provided in the effluent from the Veas wastewater treatment plant under the daily and seasonally variable conditions that can be expected. Since the cost for harvesting is often a deal breaker at full scale, we also want the polyculture to be easily separated from the water at the end of the cultivation period.
Selecting strains
Of the more than 1700 algae strains in the Norwegian Culture Collection of Algae (NORCCA), we selected 48 strains that were believed to be able to thrive in nutrient-rich wastewater. All of these strains were filamentous since these typically form larger structures that can be easily harvested by, for instance, sieving. We estimated the growth rates of all the strains in both their normal growth medium as well as in sterile Veas effluent using very small growth vessels (0.3 mL) for each strain. After several rounds of testing at different temperatures (8, 12 and 16°C) and light intensities (5, 50 and 200 µmol/m2∙s) we selected four strains from four different genera of green algae (Oedogonium sp., Stigeoclonium sp., Vaucheria sp. and Zygnema sp. – excluding a well-growing but potentially toxin-producing cyanobacteria strain Phormidium sp.) for further testing in polyculture. When grown in polyculture at 16°C, three of the four strains dominated the polyculture after about two weeks of cultivation in sterile Veas effluent, with the cold-loving Vaucheria sp. strain being clearly outnumbered. The polyculture seemed to be growing significantly better than the mono-culture control.
The experimental setup for controlling the light conditions using custom-made lids with 96 LED lights. Photos: Luka Supraha.
Left: Example of a well plate with 96 growth vessels with eight different strains cultivated in triplicates in both Veas effluent and control medium. Right: Cultivation of the polyculture and a monoculture control in 1-L flasks with sterile Veas effluent and pH control. Photos: Luka Supraha.
Under the microscope: star shaped Stigeoclonium and filaments of Oedogonium and Zygnema. Photos: Luka Supraha.