What Is Contact Time (Ct) in Drinking Water Treatment?

To effectively disinfect drinking water, chlorine must be present in the water at a sufficient concentration for a duration that is long enough to deactivate viruses and bacteria. Contact time (Ct) refers to the amount of time the chlorine has/takes to react with the microorganisms in the water.

What Chlorine Contact Time Is Required for Drinking Water?

The World Health Organisation (WHO) recommends a contact time value of 15 mg.min/l for water with a pH of less than 8.

How Is Chlorine Contact Time Calculated and Controlled?

In theory, the process of finding the optimum Ct is straightforward, by measuring the residual chlorine at the outlet of the tank and the flow rate. However, there are other factors that need to be considered, including the influence of baffles in the tank (which can affect the tank’s efficiency), as well as pH and temperature.

A few months ago, Pi asked customers and contacts if we should develop a device that could calculate Ct in real time and control the chlorine setpoint to ensure a minimum contact time value. Based on the feedback, Pi is proud to introduce CTSense, a stand-alone, or an add-on control to any CRIUS® 4.0 analyser.

How Does CTSense Support Contact Time Compliance?

Simply put, the CTSense takes in (or measures) signals for flow, pH, temperature and chlorine. It allows a factor for the mixing efficiency of the contact tank and uses the formula below to calculate a Ct value.

Not only that, but it can use that value to alter the chlorine residual setpoint to achieve a desired Ct, using a range of PID based feed forward and feedback control algorithms. It is also available with integrated or existing pH and temperature signals.

If you’d like to know more about CTSense, click here for the datasheet. Alternatively, give us a call on +44 (0)1254 422835.