Microalgal solutions in Nordic conditions : industries transition toward resource recovery?

Abstract: Microalgal solutions can through photosynthesis recover greenhouse gas (CO2) and nutrients from industrial waste, reducing climate footprint and eutrophication. An added value to the process is algal biomass containing lipids, proteins, and carbohydrates with commercial potential for biofuel, feed, and fertilizer. Microalgal cultivation in Nordic conditions is challenged by strong seasonality in light and temperature that can compromise biomass stability. To make microalgal cultivation sustainable and competitive with conventional feedstock, large-scale outdoor cultivation using waste streams is necessary but limits control over seasonal fluctuations in environmental conditions. In this thesis, I used a polyculture approach in outdoor large-scale cultivations with industrial waste resources, to study biomass production and quality in an annual, seasonal, and diurnal perspective. Research focused on the biomass potential for nutrient recovery and carbon capture from industries, year around stability and quality. Production was tested in the South Baltic Region using a brackish water polyculture grown for five years in a green wall panel (GWP) fed with cement industry flue gas (CO2 source). In a second setup, a freshwater polyculture was cultivated seasonally in raceway ponds (RWP), with an additional waste resource from landfill leachate water (nitrogen source).  Stable biomass performance and CO2 recovery up to 10 g m-2 d-1 was achieved for five years over seasons in the GWP with high protein in autumn and winter, whereas lipids remained stable throughout the annual cycle. Laboratory experiments confirmed naturally occurring diurnal shifts in temperature as superior lipid boosters compared to conventional nitrogen limitation. Stability of overall performance could be explained by flue gas recirculation mode, lack of contamination and polyculture complementarity of the two green algal strains that dominated throughout the five years. The use of multiple waste streams in the RWP added complexity to the cultivation as leachate water composition varied, resulting in a diverse green algal polyculture. Seasonality in nitrogen recovery rate was explained by total nitrogen and light. Results indicate stability of biomass and resource recovery in Nordic conditions using local polycultures in large-scale outdoor cultivation and periods of lower biomass production can be compensated by high quality metabolites such as proteins and lipids.