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As the world's population grows, the demand for fresh water, food, fuel, energy and modern technology increases tremendously. Not only does this require enormous amounts of resources but it also increases the amount of waste and especially wastewater. These wastewaters contain, based on their origin, not only compounds that can cause environmental problems and health issues but also a huge amount of unused resources, e.g. nitrogen and phosphorus. Traditional methods for wastewater treatment and nutrient recovery are often inefficient or expensive. Microalgae are part of promising new technologies that can help to clean water in a sustainable way while also recovering nutrients.

In my thesis work, I was investigating this opportunity even further, dividing the challenge into two different subprojects, i.e. the removal of heavy metals from aqueous solutions and the improvement of microalgal biomass for the production of biopolymers.

In the first project, consisting of Papers 1 & 2, we investigated Nordic microalgae regarding their ability to remove cadmium (Cd2+), copper (Cu2+) and lead (Pb2+) from aqueous solutions. Furthermore, several microalgae were also immobilized on a waste-based polymer, synthesized from castor oil and sulfur, to improve the removal capacity even further. For a full characterizaton of the removal process, the corresponding kinetics and isotherm models were calculated. While several strains showed really good removal properties, one of the most common strains, Chlorella vulgaris (13-1), performed excellently. Both when free and after immobilization, this strain was not only able to tolerate high concentrations of heavy metals, it also removed up to 98% of the heavy metals.

In the second project, Nordic microalgae were first exposed to different carbon sources, which are commonly found in waste streams. These experiments, designed as a proof of concept, showed that those alternative carbon sources can be utilized under mixotrophic conditions. Afterwards, the tested microalgae were grown in real waste streams from the pulp and paper industry and the municipality. Again, Chlorella vulgaris (13-1) performed excellently while showing an increased carbohydrate fraction in its biomass. These carbohydrates were extracted, analyzed, and fed to extremophile bacteria producing polyhydroxybutyrate (PHB) from microalgal sugars.

Overall, this thesis work shows the potential of microalgae to treat wastewater streams of industrial and municipal origin. They can be used not only to remove pollutants but also as a raw material for the production of bioplastics. The research perfomed in this thesis project can support the development of new innovative, biobased technologies for the treatment of waste streams and the transition from fossile-based to biodegradable polymers in a sustainable manner.