Effective drinking water disinfection requires chlorine to remain in the water at an adequate concentration for a sufficient period. This exposure allows the chlorine to deactivate viruses and bacteria. Contact time (Ct) describes the period during which chlorine reacts with microorganisms present in the water.
Image Credit: Shutterstock.com/Alter-ego
What Chlorine Contact Time Is Required for Drinking Water?
For water with a pH below 8, the World Health Organization (WHO) recommends a contact time value of 15 mg·min/L.
How Is Chlorine Contact Time Calculated and Controlled?
In principle, determining the appropriate Ct value is relatively simple. The calculation involves measuring the residual chlorine concentration at the tank outlet together with the water flow rate. However, several additional factors must also be taken into account, including the effect of tank baffles, which can influence the tank’s hydraulic efficiency, as well as variations in pH and temperature.
Several months ago, Pi asked its customers and contacts whether the company should develop an instrument capable of calculating Ct in real time and adjusting the chlorine setpoint to maintain a minimum contact time value. Following this feedback, Pi introduced CTSense as either a standalone unit or an add-on control system for any CRIUS® 4.0 analyzer.
How Does CTSense Support Contact Time Compliance?
CTSense receives or measures signals for flow rate, pH, temperature, and chlorine concentration. The system also allows users to enter a factor representing the mixing efficiency of the contact tank. It then applies the formula below to calculate the Ct value.
Contact Time = Chlorine Residual x Retention Time
(mg x min/l) = (mg/l) x (mins)
In addition, CTSense can use the calculated value to adjust the chlorine residual setpoint and achieve the required Ct. It does this through a selection of PID-based feed-forward and feedback control algorithms. The system can also work with integrated pH and temperature measurements or with signals from existing instruments.
This information has been sourced, reviewed, and adapted from materials provided by Process Instruments (UK).
For more information on this source, please visit Process Instruments (UK).