What must disinfection residual NOT fall below for more than 4 hours according to regulations?

In the context of water disinfection, maintaining a minimum disinfection residual level is critical for ensuring that the water remains safe for consumption. Regulations specify that, for more than four hours, disinfection residual must not fall below 0.2 mg/L. This value is based on guidelines aimed at preventing pathogen regrowth and ensuring effective disinfection in the water distribution system.

Having a disinfection residual higher than this threshold ensures that sufficient disinfectant remains to neutralize any potential pathogens that may enter the water supply. It acts as a safeguard against microbial contamination that could pose health risks to consumers. Falling below this level for an extended time could indicate a compromised disinfection process and increase the risk of waterborne diseases. Therefore, the correct answer reflects the critical regulatory requirement designed to protect public health.