The effects ofdigestate on the growth rates of Euglena gracilis, Chlorella vulgaris, and Dunaliella tertiolecta were investigated to select suitable microalgae for culturing with digestate from methane fermentation. Microalgae were cultured in an aqueous solution containing digestate at concentrations of 5%, 10%, 13%, 20%, 40%, 50%, and 100%, and Cramer-Myers (CM) solution as a control, at photosynthetic photon flux densities (PPFDs) of 75-150 micromol m(-2) s(-1) with continuous illumination at 30 degrees C. The number of cells was monitored daily, and specific growth rates (mu) were calculated as cellular multiplication rates. The maximum mu values of these species were greater in appropriate concentrations of digestate than in CM medium. The maximum mu values were 0.047 h(-1) in 10% digestate for E. gracilis, 0.065 h(-1) in 20% digestate for C. vulgaris, and 0.052 h(-1) in 50% digestate for D. tertiolecta at a PPFD of 150 micromol m(-2) s(-1). The mu of D. tertiolecta were 2.5 and 1.1 times higher than those of E. gracilis and C. vulgaris, respectively, in 50% digestate. These results demonstrated that these species could be cultured at high growth rates with diluted methane fermentation sludge and that, among these species, Dunaliella sp. was suitable for culturing at higher concentration of digestate under relatively low-level light conditions.

In order to design a culture system for microalgal biomass production with a low cost and convenient cell collection, growth performance of mixtures of microalgal cells, including Euglena gracilis, Chlorella vulgaris, and Dunaliella tertiolecta cultured in a volume of 1 L were investigated at a PPFD of 300 µmol m -2 s -1 at the surface of the solution with continuous illumination at 30 °C. Each culture container contained diluted digestate at concentrations of 5, 10, 15, 20, and 50 %. Sample cells for counting cell number were collected daily. Pseudospecific growth rates (µ s ) of each species at each depth were calculated as cellular multiplication rates using number of cells per time. The average µ s of each species was highest in 5 % digestate. The average µ s of all three microalgal species (0.035 h -1 ) was observed in all layers in 5 % digestate solution. The µ s of each species was highest in 5 % digestate (0.048 h -1 , 0.041 h -1 , and 0.022 h -1 , respectively for C. vulgaris, E. gracilis, and D. tertiolecta). In conclusion, E. gracilis, C. vulgaris, and D. tertiolecta showed the highest specific growth rate in 5 % digestate.

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