Integrated Microalgae Processing with The Water-Energy-Food-Climate Nexus Toward a Circular Bioeconomy

Abstract

This thesis provides original empirical findings that halotolerant microalgae, particularly Chlorella vulgaris, and freshwater species Scenedesmus T. obliquus CCAP 276/3A, can easily serve as twin-role agents for cleaning wastewater and crop biofertilisation, with measurable impacts in the water, energy, food, and climate sectors. Ionic media (Na⁺ and Cl⁻ at 1–2 g/L NaCl concentration) increased nitrate and phosphate reduction by 90% or more, and chloride uptake was 57.3%, demonstrating biodesalination potential. Comparison assays revealed Scenedesmus T. obliquus CCAP 276/3A removed 96.2% nitrate and 93.6% phosphate at low ionic strength, and Porphyridium purpureum absorbed 67.2% chloride at full-marine ionic strengths. In plant growth experiments, Chlorella vulgaris CCAP 211/21A increased tomato height by 33.7% and radish root extension by 41.3% relative to chemical fertilisers. Nutrient- rich post-growth biomass demonstrated biofertilisation potential in 13 plant species, with a potential effect on root biomass and shoot extension, at germination. Modelling suggests that there is potential for as much as 758,000 tonnes of CO₂ to be sequestered, 192 GWh/year bioenergy generation, and 75% reduction in synthetic fertilisers if algae systems were scaled within a Saudi Arabian context. Methodologically, the study employed a photobioreactor in controlled pilot-to- laboratory environments. Salinity gradients and medium formulations were used in simulating freshwater, brackish, and saline media. Mechanically processed biomass was employed in testing the growth of vegetables and tubers, e.g., lettuce, tomato, zucchini, and radish, in a triplicate factorial format in greenhouse environments. Statistical tools, including ANOVA and t-tests, confirmed that algae-based fertilisers performed on par with or better than synthetic treatments. Laboratory and literature data were synthesised into a Saudi Arabian context using a material flow analysis and scenario forecasting model that incorporated reuse efficiency, CO₂ offsets, biomass yield, Circularity Index, and nutrient recovery rates. The model used national wastewater infrastructure datasets from 13 regions in Saudi Arabia, enabling region- specific projections of impact and scalability. Report on (a) influence of salinity on bioremediation potential of algae, (b) on bioremediation and biodesalination potential, (c) effectiveness of algae biomass as biofertiliser and biostimulant with multiple plant species, and (d) effectiveness of using the strategy in a Saudi Arabian context for integrating within a Water-Energy-Food- Climate (WEFC) nexus perspective. This thesis effectively integrates empirical data, systems modelling, and regional infrastructure analysis to put forward scientifically credible, policy-ready algae-based solutions. It outlines a seven-phase deployment schedule (2025–2060) in harmony with Saudi Vision 2030 and Net-Zero 2060 with targets of an 80% national rate of reuse, 830,000 tonnes CO₂ offset each year, and 150,000 green algae jobs. A novel RCI was established and applied to gauge closed- loop performance, rising from 0.24 to 0.75 by 2040 in the best-case scenario. Riyadh, Makkah, and the Eastern Region were identified as Tier 1 deployment zones due to high volumes of effluent but low reuse efficiency. Integration of the microalgae process has the potential to be used with wastewater treatment and energy infrastructure. The study reframes algae not as ancillary organisms but as central agents of sustainability, resilience, and economic diversification. These findings constitute a significant contribution to the emerging field of circular bioeconomy engineering in arid regions.