Once the chlorine demand has been satisfied and maximum levels of combined residual chlorine have been attained, any further additions of chlorine will increase the amount of free residual chlorine proportionally. The point at which this happens is called breakpoint chlorination. For quality control, process control, and compliance with regulations, testing of residual chlorine is essential. There are several manufacturers and many sources of commercial test kits based on the DPD test method. There are also other methods to measure chlorine in treated water, but these methods are not considered to be as good as the DPD method.
Chlorine reacts with organic nitrogenous matter in the water.
The term chlorine demand indicates the amount of chlorine needed to react with the amount of organic matter in the water. Free chlorine or residual chlorine is needed to ensure the destruction of bacteria in the water. Chlorine is not as effective for viruses and ineffective for protozoa cysts. The water pH, temperature, contact time, and the amount of organic matter determine the effectiveness of the chlorine treatment. Odor Several methods are used to remove organic matter that often cause color, taste, and odor.