Combined chlorine forms when chlorine mixes with ammonia in water. It's a weaker disinfectant than free chlorine and can cause unpleasant taste and odor in tap water.

Combined chlorine is created when chlorine reacts with ammonia naturally present in water or added during treatment. While less effective than free chlorine, it provides longer-lasting disinfection in distribution systems. Water utilities measure combined chlorine using colorimetric methods. Sources of ammonia can include agricultural runoff or organic matter decomposition. Though it causes fewer disinfection byproducts, combined chlorine's weaker disinfecting power and potential taste issues make its use a balancing act for water providers.