Demonstration and validation of high-rate algal ponds system for sewage treatment and resource recovery potential

High-Rate Algal Ponds (HRAPs) are increasingly recognized as a promising nature-based solution for advancing the circular economy and sustainable wastewater management. In this study, a demonstration-scale HRAP system was designed, implemented, and monitored in India, characterized by a sub-tropical climatic region, to evaluate its performance in sewage treatment, resource recovery, and climate co-benefits. The system, with a treatment capacity of 50 m3 per day (equivalent to 500 population units), comprised two parallel raceway ponds (each measuring 333.5 m2) operated continuously over a two-year period (2023–2024) to assess long-term operational stability and efficiency. The pilot study demonstrated high treatment efficacy, achieving average removal efficiencies of 87.6% for BOD, 88.1% for COD, 70.1% for total nitrogen, and 71.4% for total phosphorus. Concurrently, biomass production was measured from the system, with an average concentration of 455 mg L−1. The harvested microalgal biomass was further evaluated for its potential as a biofertilizer through agronomic trials on selected vegetables, such as tomato. Results indicated enhanced plant growth across all treatments, with 75% nutrient substitution improving plant height and leaf number, and 50% substitution yielding the highest dry biomass. A carbon footprint assessment was conducted for three construction configurations, reinforced cement concrete, brick, and earthen, incorporating operational energy consumption, embodied emissions, algal CO2 fixation, and avoided fertilizer production. All configurations exhibited net-negative carbon balances, with the RCC-based system demonstrating the shortest carbon payback period of 0.5 years. Overall, the findings demonstrate that HRAP systems present a sustainable, low-energy, and climate-positive approach to decentralized sewage treatment, effectively integrating pollutant removal, biomass valorization, and carbon mitigation. The study underscores the potential of HRAP technology to uphold circular economy principles, particularly in small towns and rural regions within sub-tropical climatic regions.