Promoting sustainable domestic wastewater management through Nature-based Solutions in a water-scarce Greek Island

Water scarcity and the increasing demand for sustainable wastewater management have intensified interest in decentralized treatment systems that enable safe water reuse, energy recovery, and environmental protection. In Mediterranean and semi-arid regions, reclaimed wastewater is increasingly used for agricultural irrigation, raising concerns related to treatment robustness under variable climatic conditions, the fate of conventional and emerging contaminants, and potential impacts on soil health, crop productivity, microbial communities, and human health. These challenges are addressed in the present study by evaluating a full-scale integration of anaerobic systems and nature-based solutions to promote water reuse for agriculture within a circular water management framework in Lesvos Island, Greece.

The methodology combined long-term process monitoring, advanced chemical analysis, ecotoxicological risk assessment, monitoring antibiotic-resistant bacteria/genes and disinfection and controlled agronomic experiments. Domestic wastewater was treated for over 1000 days using an upflow anaerobic sludge blanket (UASB) reactor operated under ambient conditions, followed by a two-stage vertical subsurface flow constructed wetland designed to enhance solids removal, organic matter degradation, and nitrification. The quality of the reclaimed effluent was assessed for conventional pollutants and a broad spectrum of contaminants of emerging concern (CECs). Subsequently, reclaimed water was applied in real-scale and pilot irrigation trials, where soils, crops, and associated microbial communities were systematically monitored using physicochemical analyses, high-throughput DNA sequencing, and crop growth assessments. Human health risks were evaluated through exposure-based risk characterization using measured concentrations in reclaimed water and agricultural matrices.

The integrated system demonstrated high operational robustness despite pronounced seasonal fluctuations in temperature and hydraulic loading. The UASB reactor achieved substantial removal of suspended solids and COD while producing biogas, with methane yields strongly influenced by temperature. The constructed wetlands provided effective polishing, resulting in overall removals exceeding 90% for organic matter and solids and near-complete ammonium oxidation, producing effluents compliant with EU Class A water reuse standards. Nutrients were partially retained, supporting the fertigation needs. Chemical screening revealed that most CECs were significantly reduced during treatment, although some persistent compounds remained detectable at low concentrations. Nature-based treatment achieved higher ARB removal than conventional systems, while ARGs persisted despite UV and chlorination. Irrigation with reclaimed water enhanced crop biomass and soil moisture without compromising soil physicochemical properties. Microbial analyses showed moderate but structured shifts in bacterial and fungal communities, indicating functional adaptation rather than ecological disruption. Human health risk assessment indicated negligible risk under current reuse practices.

Overall, this investigation demonstrates that the integration of anaerobic treatment with constructed wetlands provides a reliable, energy-positive solution for decentralized wastewater treatment and agricultural reuse. The findings confirm that reclaimed water can be safely reused with minimal environmental and health risks when supported by appropriate treatment and monitoring. This work supports the implementation of circular water reuse strategies and provides a scientifically robust basis for scaling up nature-based solutions in water-stressed regions.

Acknowledgement 

This work was supported by CARDIMED project (https://www.cardimed-project.eu/), which has received funding from the European Union’s Horizon Programme under Grant Agreement ID: 101112731