Multi-source drinking water systems and disinfection strategies: evidence from inspections, Madrid

In some drinking water supply systems, the existence of different water sources and disinfection strategies may lead to errors in the identification and control of the residual disinfectant, with a direct impact on water quality and public health surveillance. In the Community of Madrid, the main water operator supplies 95% of the population using chloramination. Chloraminated water requires the determination of combined residual chlorine, whereas water disinfected with sodium hypochlorite requires measuring of free residual chlorine. These disinfectants differ in their properties, analytical methods and interpretation of control parameters. In 2025, 297 sanitary inspection activities were carried out in water supply systems. In 37 of them, alternation between chloraminated public network water and water from self-supplied sources disinfected with sodium hypochlorite was identified, with deficiencies detected in 92% of these cases. Inspections assessed, among other aspects, water traceability, identification of the residual disinfectant, measurement procedures and the actions carried out by operators. The incorrect identification of the residual disinfectant by some operators, affected decision-making, with inspections detecting unnecessary applications of additional disinfection, resulting in an increased risk of ineffective disinfection, formation of by-products, and deterioration of drinking water quality. The presence of different disinfection strategies within the same water supply system may pose public health risks. This experience highlighted that the main risk was not linked to the disinfection techniques themselves, but to deficiencies in operators’ operational knowledge and decision-making, demonstrating that adequate training and a proper understanding of disinfectants are essential to ensure drinking water quality and protect public health.

Key messages

• Errors in residual disinfectant identification may lead to additional disinfection, increasing the risk of ineffective treatment, disinfection by-products and deterioration of drinking water quality.

• Sanitary inspections are an effective tool to detect operational failures in disinfectant control and ensure drinking water quality, particularly in drinking water systems with multiple water sources.