Semi-quantitative microbiological risk assessment for urban washing and cleaning activities

Abstract Increasing water scarcity and urban growth have intensified the need for sustainable water reuse strategies, especially in urban environments. This study assesses the feasibility and health and safety of using reclaimed water in municipal cleaning operations—namely street and gutter, vehicle, and container washing—by the public utility AGERE in Braga, Portugal. A semi-quantitative microbiological risk assessment was applied to evaluate potential health risks for five exposure groups: hygiene and cleaning workers, urban solid waste workers, urban collection and environmental management department workers, operational centre workers, and pedestrians. Three water quality scenarios were modelled, based on potential Escherichia coli concentrations representing varying treatment levels. The methodology combines hazard identification, vulnerability scoring for each receptor group, and damage estimation based on failure probability and severity. Risk levels are derived from the interaction of these factors, enabling the identification of high-risk activities and groups. The highest risk was observed for vehicle washing workers under the secondary + disinfection scenario (hazard 7), with a value of 5.86, classified as acceptable but close to the critical threshold. In contrast, the lowest risk was 2.98 (negligible), associated with advanced treatment (hazard 3) applied to urban solid waste workers. Lower hazard levels, associated with advanced treatment, led to negligible risk for all receptors. The results highlight the importance of tailoring water treatment quality to specific reuse applications and reinforce the need for safety protocols, hygiene measures, and operational training. This study provides empirical support for the safe implementation of non-potable water reuse in urban cleaning, offering guidance for regulatory development and practical planning in municipal water management.