High-rate algal ponds in wastewater treatment: a critical look at recent developments

Conventional wastewater treatment systems carry a significant environmental footprint, underscoring the urgent need for more sustainable alternatives. Microalgae-based wastewater treatment systems represent a promising and eco-friendly alternative, by enabling simultaneous wastewater treatment and biomass production. Various system configurations including waste stabilization ponds, photobioreactors, sequential batch reactors, biofilm reactors, column bubble systems, hybrid systems, and high-rate algal ponds, leverage photosynthesis and microalgae–bacteria symbiosis to effectively remove nutrients and organic matter. Photobioreactors provide enhanced control of environmental conditions and optimize biomass production, while sequential batch and biofilm reactors prioritize biomass growth. Column bubble systems utilize granular biomass for efficient treatment and high-rate algal ponds rely on the symbiosis of algae and bacteria to improve treatment efficiency. Raceway pond design is customized to meet specific operational requirements, with nutrient loading and microalgae species selections playing crucial role in determining biomass yield and nutrient uptake. High-rate algal ponds (HRAPs) are engineered systems to optimize and intensify the algal–bacterial symbiotic processes, providing a high-efficiency framework for nutrient removal and biomass production. Environmental conditions such as temperature, light intensity, and pH affect the growth and dominance of different microalgae species in open pond systems. This review critically synthesizes recent findings to identify operational gaps and facilitate scale-up and implementation of nature based solutions in current practices. It focuses on raceway pond systems, examining key parameters governing algal–bacteria symbiosis, biomass production, nutrient removal, and harvesting efficiency. Finally, it provides a comparative assessment with alternative microalgae cultivation technologies in terms of performance and sustainability.