Recycling bioresources is the only way to sustainably meet a growing world population’s food and energy needs. One of the ways to do so is by using agro-industry wastewater to cultivate microalgae. While the industrial production of microalgae requires large volumes of water, existing agro-industry processes generate large volumes of wastewater with eutrophicating nutrients and organic carbon that must be removed before recycling the water back into the environment. Coupling these two processes can benefit the flourishing microalgal industry, which requires water, and the agro-industry, which could gain extra revenue by converting a waste stream into a bioproduct. Microalgal biomass can be used to produce energy, nutritional biomass, and specialty products. However, there are challenges to establishing stable and circular processes, from microalgae selection and adaptation to pretreating and reclaiming energy from residues. This review discusses the potential of agro-industry residues for microalgal production, with a particular interest in the composition and the use of important primary (raw) and secondary (digestate) effluents generated in large volumes: sugarcane vinasse, palm oil mill effluent, cassava processing waster, abattoir wastewater, dairy processing wastewater, and aquaculture wastewater. It also overviews recent examples of microalgae production in residues and aspects of process integration and possible products, avoiding xenobiotics and heavy metal recycling. As virtually all agro-industries have boilers emitting CO₂ that microalgae can use, and many industries could benefit from anaerobic digestion to reclaim energy from the effluents before microalgal cultivation, the use of gaseous effluents is also discussed in the text.

Lutein, a yellow xanthophyll carotenoid, is increasingly recognized for its nutraceutical benefits, particularly in protecting the retina’s macula from age-related degeneration. Microalgae are a promising source of lutein, which can be a primary product or a coproduct in biorefineries. Certain microalgae exhibit lutein levels (up to 1.7%) surpassing those of common dietary sources like kale, spinach, and egg yolk (approximately 0.7–0.9%). Predominantly associated with photosystem II’s light-harvesting complex, lutein is crucial in photosynthesis and cellular defense. However, being quantitatively minor among cellular constituents, lutein necessitates specialized processing for efficient extraction. Although ubiquitous in microalgae, it is not as easily inducible as β-carotene and astaxanthin in Dunaliella salina and Haematococcus pluvialis, respectively. Currently, microalgal lutein production predominantly occurs at the bench scale, presenting challenges in scaling up. Factors like culture medium significantly influence biomass and lutein yields in industrial production, while downstream processing requires cost-effective, food-grade solvent extraction techniques. This review delves into contemporary methods and innovative progress in microalgal lutein production, emphasizing industrial-scale processes from biomass cultivation to final product formulation. A conceptual industrial process proposed in this review shows that two 10 m³ photobioreactors could produce 108 kg dry mass for Chlorella minutissima, which can be processed into approximately 616 g of lutein extract, or over 6000 capsules of finished nutraceutical daily. Despite lutein production via microalgae being in nascent stages at large scales, existing research provides a solid foundation for well-informed scale-up endeavors.

Microalgae are promising for CO ₂ biofixation and nutrient removal from wastewater, while producing high‐value biomass. This study evaluated the mixotrophic cultivation of Chlorella vulgaris using sugarcane vinasse and simulated gas in sequential photobioreactor (PBR) (three PBRs were connected in series, allowing the gas from one reactor to feed into the next). Biomass productivity reached 118.4 mg.L ⁻¹ .d ⁻¹ under 20% vinasse and 2% CO ₂ . Subsequently, the PBRs were coupled in series to increase biomass productivity and enhance CO ₂ absorption by the culture. By coupling the PBRs, there was an increase in biomass concentration (1430, 1810, and 2190 mg.L ⁻¹ ), and COD removal was 61.42%, with maximum CO ₂ biofixation reaching 455.21 mg.L ⁻¹ .d ⁻¹ . A germination assay with lentil seeds indicated that C. vulgaris has the potential for application to agricultural soils after simultaneously fixing CO ₂ from flue gases and reducing nutrients from industrial wastewater.