Decentralized wastewater treatment and water renewal via natural systems: Performance evaluation from household to school-based scale

The transition toward energy-neutral and resource-efficient wastewater management is a central pillar of the 2024 EU Urban Wastewater Treatment Directive. This study evaluates the multi-year performance of the Natural Treatment System for Wastewater (NTSW): a decentralized technology utilizing a multi-stage purification process with compost, denitrification, phosphorus-elimination, and active carbon beds. Three full-scale installations (one household and two school-based systems) were evaluated, encompassing 2.5 years of continuous water quality monitoring and a 4-year assessment of compost accumulation to validate the scaling of the technology from pilot to real-world conditions. The results demonstrate that NTSW systems predominantly produce high-quality reclaimed water (BOD 5 < 3 mg/L, NH 4 -N < 1 . 5 mg/L) that not uncommonly aligns with the stringent criteria for surface waters intended for human consumption after further adequate treatment. This high purity enabled water reuse for toilet flushing, landscape irrigation, and groundwater recharge. Crucially, the technology exhibited exceptional energy efficiency, with specific consumption ranging from 0.05 to 0.17 kWh/m 3 (0.6–3.4 kWh/PE/y)–values significantly lower than conventional activated sludge plants. Furthermore, the integrated passive composting process achieved significant nutrient recovery, generating 150–1500 kg of stabilized dry matter annually while supporting local biodiversity through integrated open-water reservoir. By bridging the gap between passive nature-based solutions and high-performance water renewal, the NTSW technology offers a resilient, low-carbon alternative for decentralized wastewater management, directly supporting circular economy goals and climate change adaptation especially in water-scarce regions.