Microalgae–Microbe Consortia for Agricultural Wastewater Remediation and Biofertilizer Valorization: A Circular Bioeconomy Perspective

Agricultural intensification generates substantial volumes of wastewater enriched with nitrogen, phosphorus, agrochemicals, and suspended solids. Conventional treatments often fail to effectively eliminate these complex pollutants, leading to eutrophication, groundwater contamination, and degradation of soil health. This chapter examines the integrated application of microalgae and plant growth-promoting microorganisms (PGPMs) for the remediation of agricultural wastewater and the production of sustainable biofertilizers, in line with circular bioeconomy principles. Microalgal species, such as Chlorella vulgaris, Scenedesmus obliquus, Chlamydomonas reinhardtii, and Nannochloropsis oculata, have demonstrated excellent potential for recovering nutrients and degrading various organic and inorganic pollutants in wastewater. When co-cultured with PGPMs, including nitrogen-fixing bacteria, phosphate-solubilizing fungi, and IAA-producing endophytes, these consortia enhance pollutant removal, biofilm stability, and biomass productivity. The resulting biomass is rich in macronutrients, phytohormones, and bioactive compounds, making it highly suitable as a biofertilizer to improve crop productivity and restore soil fertility. This chapter also discusses the mechanisms of phycoremediation, the performance of algal–microbial systems, and the development of low-cost cultivation strategies, such as bamboo-based biofilm reactors and photo-sequencing batch systems, which are especially suited to the Indian rural context. Further discussions include harvesting technologies, biomass processing, and strain optimization using omics and synthetic biology tools. This chapter also assesses the techno-economic viability, environmental impact, and regulatory frameworks necessary for practical implementation. Field trials of Indian cropping systems provide strong evidence for the feasibility of this approach among small and marginal farmers. In conclusion, the microalgae–PGPM-based strategy offers a regenerative and climate-resilient approach to nutrient recovery and wastewater valorization, thereby promoting sustainable agriculture in the Global South.