Disinfection of water (potable or recycled) with a chemical disinfectant (chlorine or chloramine) requires time for the chemical to react with and kill the target microbial pathogens. Ideally, the time for the reaction is provided in a purpose-built reactor or contact tank as this is specifically designed for this purpose and provides a controllable process.
The reality however is quite different. Most water supplies rely on disinfection to occur in clear water tanks and reservoirs of various size, with various arrangements of inlet and outlet structures and varying levels of short circuiting. The effective contact time is often less than is assumed.
Many water utilities use a target free or total chlorine residual in their treatment systems to achieve a level of disinfection that they understand to be adequate. This approach to disinfection can be improved in most cases.