Cultivation of Microalgae Nannochloropsis for Bioremediation of Lactose-Enriched Dairy Waste Streams and Co-Production of High-Value Biomass

The EU is one of the world's largest milk producers (ca. 150 million metric tons). Dairy processing generates significant amounts of nutrient-rich wastewater and by-products, which pose significant environmental problems due to high organic matter contents (e.g., lactose, proteins and fats), BOD level and COD level. Microalgae biotechnology is an attractive solution for dairy waste treatment due to its potential to achieve simultaneous bioremediation and co-generation of useful biomass and high-value compounds in a circular economy model. The main objective of this thesis was to develop a sustainable and cost-effective microalgae-based strategy for lactose enriched dairy waste stream (specifically whey powder solution) using Nannochloropsis, an autotrophic lipid-rich microalgal genus with a typically high omega-3 polyunsaturated fatty acids (ω-3 PUFAs) content in the form of eicosapentaenoic acid (EPA or C20:5). The application of Nannochloropsis can alleviate dairy systems reliance on a linear “collect-treat-discharge” practice of handling waste and instead promote a more sustainable, cost effective and circular practice whereby valuable ‘waste’ resources are continuously recovered and reused. The study presents an integrated approach which addresses fundamental research gaps in Nannochloropsis cultivation on dairy waste streams (such as the mechanism of lactose assimilation) and transfer these insights into the development of an effective pre-treatment and growth strategy to attain optimal bioremediation. The specific objectives of the study were: (1) to evaluate the mechanism of lactose assimilation in Nannochloropsis under different trophic modes and their intrinsic capacity for metabolising dairy waste, (2) to assess the effect of different waste pretreatment regimes (e.g. salinity intervention and sterilisation techniques) on physicochemical and biological characteristics of the waste and their ability to support microalgal growth, (3) to understand the critical role that phycospheric bacteria and their interaction with microalgal cells play in driving growth and bioremediation performance, (4) to assess the effect of cultivation on waste on the proximate and lipid composition of resulting microalgal biomass to determine end-user applications, and (5) using critical insights from Objective 1-4, to develop an innovative multiple-stage growth strategy which harnesses the power of probiotic bacteria in the wastewater and symbiotic bacteria in microalgal co-culture in order to maximise Nannochloropsis growth, bioremediation efficiency, and lipid productivity on the waste. Overall, this thesis demonstrated the potential of Nannochloropsis-based strategies for the effective bioremediation of lactose-rich dairy waste streams and co-generation of valuable products for circular bioeconomy development, such as ω-3 PUFAs, and β-galactosidase enzymes. Future studies can combine the two-step strategies developed in the thesis with other nutrient-feeding (e.g. batch, fed-batch, and continuous) and biological strategies (e.g. adaptive laboratory evolution) to further optimise growth and bioremediation performance. Lab-scale discoveries made in the study should also be substantiated at pilot scale and supported with predictive process modelling and robust trials using diverse waste streams generated throughout the dairy processing chain (e.g. CIP cleaning) in order to validate performance consistency and commercial scalability.