INTRODUCTION

Chlorination of water supplies and wastewater treatment plant effluents is mainly done for the destruction of disease-producing microorganisms. Chlorine has many other uses in the wastewater treatment plant operation including odour control and fly and ponding control (in trickling filters). By reacting with ammonia, iron, manganese, sulphide and some organic substances, chlorine can provide additional wastewater treatment. Unfortunately, if the use of chlorine is not controlled carefully it might cause problems. When used as a control on the disease causing bacteria, the idea is to disinfect and not sterilize the effluent. Disinfection is the process of killing disease-producing bacteria. Process of killing all living organisms is called as sterilization. If an attempt were made to sterilize the effluent, the biological life in the receiving waters would likely be destroyed. The best process for monitoring the disinfection process and preventing harm to aquatic life by over-chlorination is to maintain a specific chlorine residual level in the effluent.It will also increase the quality of water by reacting with ammonia, iron, manganese, sulphide and some organic substances. The residual chlorine is sustained in water to promote the primary purpose of chlorination. This method of determination depends upon the oxidising power of free and combined chlorine residuals. At pH 8 or less, chlorine will release free iodine from potassium iodide solution. The released iodine is titrated against standard sodium thiosulphate solution using starch indicator.

2KI + Cl₂ → 2KCL+I₂

I₂ + Starch → Blue Colour

I₂ + 2Na₂S₂O₃ → Na₂S₄O₆ + 2NaCl + 2Na

Relevant Indian Standard for Break point Chlorination Test :

1. IS 3025 (Part 28)-1986: Method of Sampling and Test (Physical and Chemical) for Water and Wastewater : Chlorine Demand, First Revision.