Comparative evaluation of full-scale treatment wetland, high-rate algal pond, and activated sludge system for municipal wastewater treatment

Nature-based solutions (NBS) provide sustainable alternatives to conventional activated sludge (CAS) systems for municipal wastewater treatment, yet comprehensive full-scale comparisons remain limited. This study evaluates the simultaneous long-term performance of three full-scale treatment systems operating in parallel and receiving the same influent: CAS system, a treatment wetland (TW), and a high-rate algal pond (HRAP). Performance was monitored over two growing periods, focusing on the removal of organic matter, suspended solids, and nutrients. Additionally, HRAP system was tested with varying hydraulic retention times (HRT) and biomass recycling strategies. Results indicate that while CAS system and TW provided consistent organic matter removal (97% and 89% BOD 5 removal, respectively), HRAP efficiency was limited by insufficient separation of algal biomass from the effluent. HRAP achieved significantly higher ammonium‑nitrogen removal (86 ± 24%) than CAS and TW. Both HRAP and TW significantly reduced total phosphorus (53 ± 29% and 56 ± 26%, respectively). Operating the HRAP under prolonged retention times (35 days) and with biomass recycling demonstrated stable performance. Removal of suspended solids and BOD 5 was significantly improved, reaching effluent concentrations of 39 ± 30 mg/L and 18 ± 9 mg/L, respectively; however, nutrient removal remained unchanged. The study confirms that while CAS is compact and efficient, HRAP and TW offer viable, low-energy alternatives with added benefits of resource recovery and lower environmental footprints. These findings support the integration of algal and wetland technologies into decentralised treatment schemes, contributing to circular economy goals in the water sector.