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- Use of the microalgae-bacteria Consortium in photobioreactors for the treatment of wastewater from the paper pulp industryPublication . Sátiro, Josivaldo Rodrigues; Albuquerque, António João Carvalho de; Simões, Rogério Manuel dos SantosThis thesis investigated the application of microalgae–bacteria consortia in photosynthetic and granular systems for the treatment of pulp and paper industry wastewater, characterized by low biodegradability and reduced nitrogen content. The study evaluated how inoculum concentration affects start-up, hydraulic retention time (HRT), and initial reactor sizing. Optimal microalgae-to-bacteria ratios (1:1, 1:5, 3:1) were identified: PBR5 (1:5) promoted higher COD and nitrogen removal, PBR6 (3:1) enhanced phosphorus removal, and PBR3 (1:1) improved biomass formation. Excessive microalgae hindered granule formation, while an optimized HRT of 16 h with 8 h cycles was determined. In photobioreactors, higher bacterial proportions accelerated organic matter degradation (up to 85% COD removal) and promoted efficient flocculation (>90%), whereas higher microalgae ratios improved phosphorus removal (up to 86%) and lipid accumulation in the biomass (22%). The symbiotic interaction also enabled nitrogen removal above 85% via nitrification and assimilation. A hybrid system combining photobioreactors and constructed wetlands treated raw industrial effluent, achieving 89% COD, 69% total nitrogen, 59% total phosphorus, and 81% phenolic compound removal. The system maintained high operational stability (600 mgVSS/L biomass) and excellent settleability (89.7%), demonstrating the potential of nature-based solutions and integration into circular economy strategies. Aerobic granular sludge systems with microalgae–bacteria consortia (AB-AGS) were evaluated in sequential reactors, showing rapid granulation (>82% of particles >1.0 mm in 15 days), high compactness, and structural stability, with phosphorus removal above 92% and significant organic matter and nitrogen elimination. Operational comparisons highlighted differences in settleability: one system reduced the sludge volume index (SVI) from 31.84 to 4.59 mL/g, while another maintained 27.42 mL/g, indicating slower but functional compaction. The latter also exhibited higher lipid accumulation (23.3 ± 1.8%), more than double that observed in reactors inoculated solely with bacteria (9.3 ± 1.7%), demonstrating the potential of AB-AGS as an integrated biorefinery for biofuel precursors without chemical additives. Overall, the results confirm that integrating microalgae–bacteria consortia into photosynthetic, hybrid, and granular systems is an effective and sustainable approach for treating complex industrial wastewater. These systems enable high pollutant removal, generate valuable biomass, and reduce reliance on energy-intensive processes. Although conducted at laboratory scale, the findings provide a solid foundation for future pilot-scale validation and potential industrial implementation.