How does organic matter in water influence chlorine demand?

• A. It reacts with chlorine to form less effective disinfectants
• B. It increases chlorine residual automatically
• C. It has no effect on chlorine demand
• D. It reduces the need for chlorine

Answer: (A)

Organic matter in water consumes chlorine when disinfection is attempted. The chlorine reacts with these organic compounds, forming chlorinated byproducts and other oxidized products. This use of chlorine increases the chlorine demand, meaning more chlorine must be added to achieve and maintain the desired free chlorine residual. In other words, the presence of organics lowers the amount of active disinfectant available and can lead to byproduct formation, so the idea that organics react with chlorine to form less effective disinfectants best captures why chlorine demand goes up. The other choices aren’t accurate: organics don’t automatically raise the residual, they don’t have no effect, and they don’t reduce the need for chlorine.