Chlorination Breakpoint in Water Treatment

Chlorination is a common disinfection method used in water treatment to kill harmful microorganisms like bacteria and viruses. When chlorine is added to water, it reacts with various substances present, including organic matter, ammonia, and other inorganic compounds. These reactions occur in stages.

Initially, the added chlorine reacts with reducing agents and oxidizes inorganic substances. As more chlorine is added, it reacts with ammonia to form chloramines (combined available chlorine). Chloramines also have disinfectant properties but are less potent than free chlorine.

Adding even more chlorine leads to the oxidation and destruction of these chloramines and other organic compounds. As the chlorine dose increases, the amount of chlorine residual initially increases, then decreases (as chloramines are destroyed), and finally starts increasing again when all ammonia and oxidizable compounds are consumed.

This point, where the demand for chlorine by impurities is met and further addition of chlorine results in the presence of free available chlorine (like hypochlorous acid, \( \text{HOCl} \), and hypochlorite ion, \( \text{OCl}^- \)), is known as the break-point.

Understanding Break-Point Chlorination

Break-point chlorination is the process of adding chlorine to water up to the point where the chlorine demand is satisfied, and any additional chlorine remains as free residual chlorine. This free residual chlorine is a strong disinfectant and provides a residual effect in the distribution system, protecting water quality.

The question asks for the dose beyond which any further addition of chlorine appears as free residual chlorine. This specifically describes the definition of reaching the break-point dose.

Other Chlorination Terms

• Pre-chlorination: This involves adding chlorine to raw water before other treatment processes like sedimentation and filtration. Its purposes include disinfection, oxidation of organic matter, removal of algae, and improving coagulation. It is done early in the treatment process, not necessarily up to the break-point.
• Double chlorination: This simply means adding chlorine at two different points in the treatment process. The doses might be pre-chlorination and post-chlorination, or similar combinations. It doesn't define a specific dose level like the break-point.
• Super chlorination: This involves adding a very high dose of chlorine, significantly beyond the break-point dose, to ensure complete disinfection and oxidation of all impurities. This is often followed by dechlorination to reduce the residual chlorine level before the water enters the distribution system.

Based on the definition provided in the question, the dose of chlorination beyond which any further addition appears as free residual chlorine is known as Break-point chlorination.