Successful practices for valorization of microalgal biomass for inclusion in circular economy

Global wastewater generation is enormous and continuously increasing, becoming a huge challenge to humanity. The use of wastewater for cultivation of microalgae has emerged as a potential cost-effective strategy for large-scale production of microalgae biomass. This strategy also offers an effective approach for removing nutrients from wastewater, making its treatment sustainable, eco-friendly and energy efficient. Among the different types of wastewater, wastewater from anaerobic digestion plants deserves attention, since this process generates a large amount of residue called digestate, reaching up to 90 % of the amount of substrate. Using the anaerobic digestate to cultivate algae is a way to reduce its harmful impact on the environment, improve the economic profitability of anaerobic digestion facilities and convert digestate from a waste to a beneficial by-product. By selecting and reviewing 129 articles published in Scopus between 2010 and 2024, the present review investigates and evaluates the possibilities of increasing the biomass value of microalgae cultivated in wastewater as a growth medium and highlights the importance of pre-selection of appropriate (non-toxic) wastewater to obtain usable “safe” microalgal biomass that can be further utilized to produce high added value products. The results indicated that the biomass of microalgae grown in non-toxic wastewater, including digestate, can have a number of economically and environmentally sustainable applications, the most important of which are its use as a feed, feed ingredient, ingredient of fish feed, as well as for the production of health-related compounds (proteins containing essential amino acids, polyunsaturated fatty acids, carotenoids, phycobiliproteins, polysaccharides, etc.), biofertilizers and biostimulants for agriculture. Cultivation of microalgae for the simultaneous treatment of non-toxic wastewater and obtaining biomass and high-value products can be an effective and prospective approach for the valorization of microalgae biomass within the modern concept of а circular bioeconomy.