Anaerobic digestion (AD) is widely applied for the treatment of organic waste, yet dewatering of the AD digestate produces a liquid effluent rich in ammonium and organic pollutants that can pose environmental risks if discharged untreated or add extra load to wastewater treatment systems. In addition, upgrading biogas produced through AD generates a CO2 stream, which requires appropriate handling to minimize emissions. This study explores the use of microalgae as a sustainable strategy to address both challenges by converting nutrient-rich effluents into valuable biomass while simultaneously enabling CO2 biofixation.  

The project began with a screening phase in which native Nordic microalgal strains, Chlorella vulgaris, Coelastrella sp., Chlorococcum sp., Scotiellopsis reticulata, and Desmodesmus sp.,  were assessed for their growth performance and nutrient removal efficiency in AD effluents from chicken manure, pulp and paper sludge, and food waste. This stage identified strains capable of efficiently assimilating volatile fatty acids and ammonium nitrogen. Based on these results, Coelastrella sp., Chlorella vulgaris, and Chlorococcum sp. were selected for further evaluation of total ammonia nitrogen tolerance in AD effluent of municipal sewage sludge under different dilution conditions. Subsequent experiments investigated the effects of light availability and CO2 sparging on the growth, nitrogen removal and biomass composition of Chlorella vulgaris and Chlorococcum sp. While CO2 supplementation enhanced biomass accumulation and fatty acid production, higher nitrogen removal was achieved under conditions without CO2 sparging. The final stage, cultivations was scaled up  in 4.5 L reactors to assess Chlorococcum sp. performance under different CO2 concentration (0.04, 3, 6, and 9%) and bioreactor configurations. Aeration with 6% CO2 yielded the highest biomass production and was selected for the comparison of the effect of reactor design (bubble column bioreactors,  airlift bioreactors, and bubble column bioreactors containing carriers). Bubble column bioreactors showed superior nutrient removal efficiency, whereas airlift bioreactors supported faster cell growth.  

Overall, this thesis demonstrates that native Nordic microalgae offer strong potential for the treatment of AD effluents while simultaneously utilizing CO2  through biofixation. The findings provide a foundation for integrating microalgal cultivation into waste management systems, mitigating environmental impact while generating valuable biomass with potential for applications such as biofertilizers, biochar, biodiesel and bioplastics production.