Long-term performance of low-cost free chlorine sensors to monitor on-site water reuse

Water scarcity increasingly challenges the supply of sufficient quantities of safe water for human consumption. On-site water reuse systems can contribute to mitigating the effects of water scarcity by closing water cycles locally. However, broader adoption of on-site water reuse is constrained by the high cost of water quality monitoring. This work demonstrates the successful design and validation of low-cost chlorine amperometric sensors for long-term monitoring using Prussian blue-coated screen-printed carbon electrodes. The study is relevant for practice, as testing was conducted in a water reuse system treating real greywater and municipal wastewater. The sensors achieved a relatively stable sensitivity of 8.55 ± 1.19 μA/(ppm⋅cm²) for hypochlorous acid after a 3-day stabilization period and maintained a sensitivity of 3.72 μA/(ppm⋅cm²) after 1 month. The sensors exhibited a linear dynamic range from 0.3 to 5 ppm for hypochlorous acid (<i>R</i>² > 0.95). This study highlights the potential of Prussian blue-coated sensors as a practical solution for continuous chlorine monitoring in on-site water reuse applications. The findings encourage further research to improve durability and scalability to optimise the sensors for applications where low-cost sensing solutions are needed without personnel on-site for regular sensor maintenance and calibration.