When insufficient chlorine is used for disinfection, the total chlorine residual may be made up of combined chlorine residual, causing taste and odor problems. What causes this problem?

• A. Addition of chlorine gas
• B. Increase in pH
• C. Deconstruction of chlorine compounds
• D. Formation of chlorite

Answer: (C)

When the available free chlorine is too low, the chlorine that is present is quickly consumed by reactions with nitrogen-containing compounds, especially ammonia and organic nitrogen. This converts free chlorine into chloramines, which are a form of combined chlorine. These combined chlorine species remain in the water as the residual and are associated with noticeable taste and odor problems, while free chlorine levels stay low and disinfection effectiveness suffers. In this scenario the chlorine compounds are effectively transformed into chloramines rather than staying as free chlorine, which explains why the total residual is dominated by combined chlorine. The other options don’t explain this phenomenon: adding chlorine would increase free chlorine and help, higher pH shifts the chlorine speciation but doesn’t cause the combined residual to dominate, chlorite formation is related to chlorine dioxide processes, and the transformation into chloramines is the mechanism at play here.