Contact tanks are commonly used to disinfect drinking water prior to distribution. These tanks are usually open chambers split by a series of baffles. Sub-dividing the chambers helps to control the flow of water through the tanks and improves the chlorine disinfection process. To be effective, chlorine disinfection requires a minimum residence time for water to remain in the tanks. In addition to maximizing the residence time provided by a tank, care must also be taken to eliminate "dead-spots" where water can remain for many days.
CFD analysis provides a highly effective and cost efficient method for testing design options, and CFD is fast replacing older methods which rely on simplified descriptions of the flow inside the tanks.
Complex inlet configurations and unusual tank designs contribute to the need for the fully three- dimensional simulation of flow within the tanks.
CFD can be used to simulate the injection of a chlorine dose (or dye injection) and can be used to compute Residence Time Distribution (RTD) curves,; also known as “breakthrough” curves. The RTD curves can then be used to assess the main residence time parameters (e.g., the T10/T ratio) for the dose and the predicted performance of the contact tank can be established and alternative designs investigated.