Early warning of NPS using intra-urban wastewater-based epidemiology

Introduction

New psychoactive substances (NPS) evolve rapidly and challenge conventional surveillance systems. Wastewater-based epidemiology (WBE) enables the detection of emerging drug use patterns, but treatment plant-level approaches face limitations for NPS due to low concentrations and degradation

Objectives

To design an intra-urban early warning system based on WBE to enhance early detection of NPS and support timely public health decision-making

Methods

We developed a model integrating sociodemographic data, drug use indicators, and the sewer network of Barcelona using geographic information systems. The network was represented as a graph, and an optimization algorithm was applied to identify sampling sites that maximize detection capacity while balancing population coverage, proximity to “hotspots,” and minimization of interference (e.g., hospitals). Two strategies were evaluated: population coverage (4 sites) and hotspot-focused surveillance (5 sites). Coverage and implementation costs were estimated

Results

Optimal sampling locations were identified in districts with a higher likelihood of NPS use. The population-based strategy achieved coverage of up to 14.5% of the target population, while the hotspot-focused strategy covered up to 24.7% of identified hotspots. Estimated annual costs ranged from €127,000 to €154,000, with sample collection accounting for the largest share

Conclusions

This study presents an innovative, data-driven and scalable approach for early detection of NPS at the urban level. Combining intra-urban WBE with optimization algorithms enables non-intrusive, neighborhood level surveillance that is fully anonymized and independent of healthcare access or testing behaviours. This approach generates timely, objective, and actionable intelligence, strengthening early warning systems and improving preparedness for emerging public health threats

Key messages

• Intra-urban WBE with optimisation enables earlier, more precise detection of NPS, strengthening urban early warning systems and supporting timely public health action.

• A scalable, anonymised and data-driven approach improves detection of emerging drug use beyond treatment plants, enhancing preparedness and targeted responses.