Pioneering circular pathways: Improved recycling of nutrients in industrial wastewater into valueadded products

Traditional industrial processes are linear, focusing on extraction and generating significant waste. The food industry, a major water consumer, produces large volumes of organic, nutrient-rich wastewater, much of which remains untreated or is sent to treatment facilities. With population growth, climate change, and rising industrial demand placing pressure on water and nutrient supplies, integrating circular economy principles—such as water reuse and nutrient recovery—into food production is essential. Biological water recycling methods using bacteria, algae, fungi, or yeast offer a more sustainable alternative to physical and chemical treatments while also producing valueadded biomass. Of these microbes, fungi are best suited to removing the high organic loads present in food production wastewater, generating biomass rich in lipids, proteins, polysaccharides, and biochemicals. This not only reduces pollution and treatment costs but also provides valuable biobased materials. Additionally, filamentous species can flocculate microalgae, easing settling and dewatering and reducing the reliance on chemical- and energy-intensive methods. Based on four original articles, this doctoral thesis focuses on nutrient removal and valorisation of food production wastewater into value-added biomass. It examines the growth potential of ten fungal species in five nutrient-rich waste streams from the dairy, bakery/confectionery, recirculating aquaculture, and malting industries (Article II). A distinctive aspect of this research is the exploration of novel waste streams, such as recirculating aquaculture sludge and steeping waters from malting, and its focused on filamentous fungal species, which have received little attention in this context. To assess fungal growth potential in submerged cultures, a novel method for measuring oxygen uptake rates (OUR) using a manometric BOD apparatus was developed (Articles II & III). The most suitable wastewater stream for fungal growth was identified, along with the fungal species that exhibited the highest growth potential. Those species with the potential for high-value biomass and by-product production were selected and cultivated in the chosen wastewater. Nutrient removal was measured to assess the system’s potential for wastewater valorisation through fungal growth. Additionally, to evaluate its ability to produce value-added biomass, biomass yields and composition—including elemental content, proteins, amino acids, and fatty acids—were analysed (Articles III & IV). This analysis provided insights into the potential applications of the resulting biomass, including its use in biofuels, animal and human supplements or cosmetics. Additionally, the research explored fungalassisted microalgal harvesting as another application, employing a novel approach that utilises fungal filaments in suspension rather than conventional pre-pelleted fungi or co-cultivation techniques (Articles I & IV).Beyond the laboratory scale, this study explores the broader implications and practical considerations of implementing fungal-based wastewater treatment in food production waste streams. Key aspects such as process monitoring, biomass utilisation pathways, integration into existing treatment infrastructure, and the potential for industrial scale-up are discussed. While the findings are based on controlled lab-scale experiments, they demonstrate the feasibility of this approach. With further optimisation and pilot-scale testing, fungal-based wastewater reuse and valorisation of agro-industrial side streams could play a significant role in advancing sustainable, circular food production practices by reducing environmental impacts while generating valuable bioproducts.