Sustainable Cultivation of Microalgae using Diluted Anaerobic Digestate as a Nutrient Medium Algae-Based Systems

In this study, the cultivation of two microalgal species using anaerobic digestate as a nutrient medium was investigated. Several pretreatment methods were applied to the anaerobic digestate including hydrogen peroxide treatment, filtration using polyester filter bags, and supernatant extraction; the latter proved to be the simplest and most effective method in reducing the turbidity and COD of the diluted digestate while maintaining sufficient nutrients for the cultivation of microalgae. Initially, the microalga Neochloris oleoabundans was cultivated using diluted anaerobic digestate supernatant on the bench-scale; however, when scaled up to 100 L raceway ponds, the culture was contaminated with other algal species. The microalga Scenedesmus dimorphus was then cultivated using anaerobic digestate supernatant on the bench-scale. It was found that 50-100 mg N/L dilutions were sufficient to maximize the specific growth rates while still producing relatively high biomass concentrations. The microalgae cultivation was scaled up to 100 L raceway ponds using 100 mg N/L dilution at 454 and 317 μmol/m2/s light intensities and 50 mg N/L dilution at 384 and 234 μmol/m2/s light intensities. The maximum biomass concentration attained was 432 mg/L in the 100 mg N/L and 454 μmol/m2/s culture. Furthermore, nitrogen was removed at 65-72% efficiency with complete ammonia removal. Phosphorus removal efficiencies were in the 63-100% range whereas the presence of bacteria in the unsterilized nutrient media resulted in COD removal efficiencies in the 78-82% range. Finally, the effect of mixing on the growth of the microalga S. dimorphus was assessed by cultivating this microalga in a raceway pond at 0.1, 0.2, and 0.3 m/s water surface velocities. It was concluded that increasing the mixing velocity yielded higher biomass concentrations and growth rates. However, by balancing the power required to operate the pond at different velocities with the potential energy yield from biodiesel synthesis, it was concluded that running the pond at 0.1 m/s surface velocity was the only case with a positive net energy yield.