Cascade Control for Post-Chlorine Dosage During Drinking Water Treatment Under Cyber Attacks

In the water treatment process, the free chlorine residual (FCR) of treated water is a critical metric for the post-chlorination process. However, the stability of this process is significantly compromised by potential cyber attacks targeting communication channels, as well as fluctuations in influent water quality and flow rates. The stringent accuracy requirements of post-chlorination control necessitate a control scheme with fast system response and high robustness under sudden cyber attacks and disturbances. In this context, traditional single-loop feedback control schemes can no longer effectively stabilize the FCR of treated water. This study proposes a cascade (master/slave) feedback control scheme to address the issue of post-chlorine dosage control. The master controller is developed based on a second-order sliding mode (SOSM) control, which utilizes the FCR of the clear-water reservoir inlet as its reference. An internal model control (IMC) approach is then adopted in the slave controller, enabling the FCR of the clear-water reservoir inlet to track the reference acquired from the master controller. It is shown in simulation results that, compared with conventional single-loop feedback control approaches, the proposed cascade control strategy exhibits superior robustness against cyber threats, disturbances, and model uncertainties, along with rapid system response. The proposed strategy thus provides a reliable and efficient solution for maintaining water quality under complex and varying operational conditions. Note to Practitioners—Ensuring stable control of FCR in water treatment systems is critical, particularly when facing challenges such as cyber attacks and fluctuating water conditions. Traditional control methods often fail to meet the robustness and accuracy required in these dynamic environments. This study introduces a cascade control framework combining a master controller (SOSM) with a slave controller (IMC). This system is designed to address disturbances and cyber threats while maintaining fast response times and high accuracy. Practitioners managing water treatment facilities can benefit from this approach by integrating it into existing control systems to enhance resilience and maintain water quality. The proposed method adapts to fluctuating operational demands, offering a practical solution to modern water treatment challenges.