Valorization of wastewater for the production of bioproducts through the use of microalgal and cyanobacterial systems

Global population growth increases the demand for fresh water, leading to a significant increase in wastewater production, mainly from industrial processes. Traditional wastewater treatment systems are often expensive and not always effective in removing contaminants. Microalgae represent a sustainable solution for wastewater treatment due to their mixotrophic growth capacity: they use light as an energy source and both atmospheric CO2 and organic carbon as carbon sources. This characteristic makes microalgae promising phycodepurative agents, especially for nitrogen- and phosphorus-rich wastewater. This biomass not only reduces the carbon footprint of wastewater treatment, but also represents a valuable resource for the production of high-value compounds such as proteins, natural pigments (e.g. phycocyanin and astaxanthin) and starch. The integration of wastewater treatment with photosynthetic biomass production positions microalgae as an ideal and versatile system within the circular bioeconomy. In this study, we conducted a comparative study of four Chlorella strains, evaluating the optimal light conditions and photoperiods for growth, growth performance under autotrophic and mixotrophic conditions, and the primary metabolic pools produced (e.g. carbohydrates, proteins, pigments). Of the strains analysed, Chlorella vulgaris was the most efficient in terms of growth (productivity = 0.3 g/L d-1) and production of high-value biomass (starch = 4.61 μg/mg FW). The ability of Chlorella zofingiensis and Arthrospira platensis to grow and purify dairy waste was evaluated and their production of high-value compounds was assessed. Furthermore, a carbon balance study was carried out to analyse the effectiveness of the system. The results suggested that the cyanobacterium A. platensis has the best purification capacity with 83.6% of total organic carbon (TOC) removed from dairy waste, all of which was converted to biomass (1.79 g/L). Finally, we analysed the role of light stress, associated with the growth of C. zofingiensis on dairy waste, in promoting the production of the metabolite astaxanthin.