What indicates that chlorine demand is satisfied?

When chlorine is added to water for disinfection purposes, it reacts with various materials present, including organic compounds, inorganic substances, and microorganisms. The point at which chlorine demand is satisfied signifies that there are no longer reactants in the water consuming chlorine. This is indicated by the halt of the reaction of chlorine with these materials.

Once chlorine demand is met, any additional chlorine added to the water will not be consumed by these compounds and will remain in the water, which means its concentration can start to rise. This is fundamental for maintaining residual chlorine levels in the distribution system, ensuring ongoing disinfection.

The other options provide different scenarios which do not specifically identify the satisfaction of chlorine demand. For example, all bacteria being killed does not necessarily indicate that chlorine demand is met, as some microorganisms may still react with chlorine. Similarly, water clarity is influenced by various factors unrelated to chlorine demand satisfaction, and a rising concentration alone does not confirm that all chlorine reactions have ceased.