Which of the following factors can affect chlorine residual in water systems?

• A. Temperature
• B. Organic matter
• C. pH level
• D. All of the above

Answer: (D)

All of the listed factors—temperature, organic matter, and pH level—significantly influence chlorine residual in water systems.

Temperature affects chlorine's effectiveness and stability. Higher temperatures can increase the rate of chlorine dissipation, thereby reducing its residual levels. Conversely, lower temperatures typically slow down the reaction rates, allowing chlorine to persist longer in the water.

Organic matter also plays a critical role in chlorine residual. When chlorine is introduced to water containing organic materials, it tends to react with these compounds, forming chloramines or other byproducts. This reaction consumes the chlorine, thereby reducing the amount available as a residual.

The pH level of the water is another important factor. Chlorine exists in two forms: free chlorine (hypochlorous acid and hypochlorite ion), and the balance between these forms is affected by pH. At lower pH levels, a larger portion of chlorine is in the form of hypochlorous acid, which is more effective as a disinfectant and helps maintain residual levels. As pH increases, the proportion of hypochlorite ion rises, which is less effective, thus adversely impacting the chlorine residual.

Since all these factors can impact the chlorine residual, the answer that includes them all is indeed