Microalgae-based technologies, usually configured as high rate algal ponds (HRAP), are efficient, sustainable, and cost-effective alternatives for wastewater treatment due to their high removal efficiencies at low energy demand, ability to recover nutrients and ease of operation. HRAPs and other photobioreactors have been intensively studied in recent years for the treatment of high-strength wastewaters, which are mainly characterised by high and unbalanced (in terms of microalgae requirements) concentrations of organic carbon and nutrients. This review critically evaluated research papers that used microalgae-based systems for the removal of carbon and nitrogen from high-strength wastewaters. These systems can provide removal efficiencies up to 100% for organic matter and ammonium nitrogen. Relatively large area requirements, high evaporative losses, ammonia inhibition, poor light penetration and scattering, carbon dioxide limitation, and unbalanced nutrient ratios rank among the main current limitations of these technologies. Optimisation strategies, including modifications in bioreactor design and operation, can broaden their full-scale application for the treatment of high strength wastewaters.

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