Photosynthetic microorganisms for nutrient removal and biomass valorization in a circular economy context

Abstract

The growing global population has intensified wastewater generation, leading to increased nutrient discharges into water bodies, including nitrogen (N) and phosphorus (P). These excess nutrients cause eutrophication and threaten aquatic ecosystems. Conventional wastewater treatment processes are energy-intensive and prioritise nutrient removal over resource recovery, operating within a linear economy concept. This dissertation addresses the need for a paradigm shift by integrating a circular economy approach into nutrient removal from cultivation medium through the application of photosynthetic microorganisms, i.e. microalgae and cyanobacteria, for simultaneous nutrient removal followed by biomass valorisation for bioproducts.

The research investigated nutrient removal from phosphate-limited recirculating aquaculture system (RAS) water, efficient biomass-harvesting strategies, integration of biorefinery, and the development of novel biocomposite materials, following a circular economy approach. Experiments were conducted using the microalgal species Tetraselmis suecica, Nannochloropsis oculata, Phaeodactylum tricornutum, Pavlova gyrans and mixed-species consortia.

This first research work demonstrated that T. suecica removed nitrate from phosphatelimited RAS water despite high N:P molar ratios. Further, the harvested biomass was analysed for fatty acid profiles. The second research work utilised a floating photosynthetic biofilm (FPB), which enabled simplified biomass harvesting, with the recovered biomass subsequently processed following a biorefinery approach for lipids and agricultural biostimulant. In the third research work, biodegradable biocomposite films were prepared using cellulose, extracted from N. oculata, combined with spirulina protein in a polyvinyl alcohol matrix.

Collectively, these studies demonstrate the feasibility of a two-pronged approach in which microalgae and cyanobacteria serve as both nutrient-removal agents and feedstocks for value-added bioproducts, including pigments, fatty acids, biostimulants, and biopolymers. The presented work demonstrates the integration of the circular economy principles, showcases nutrient removal and biomass valorisation, and establishes a foundation for future scale-up studies.