Wall decay of chlorine in water distribution system

Chlorination is a widely practised method of disinfection in India. The chlorine consumption in a distribution pipe is due to bulk and wall reactions. The chlorine decay due to wall reactions is particularly significant in old distribution systems with unlined metallic pipes. The estimation of the wall reaction parameter is difficult and is a product of calibration. The approaches adopted for the estimation are pipe column studies and calibration of the chlorine transport model for the water distribution system model. In the present study both these approaches are carried out for a real-life water distribution system. The relative decay of chlorine in cast iron and PVC pipes of different diameters were determined using laboratory experiments. The pipe column studies showed that the chlorine decay rates were significantly higher in CI pipes than in PVC pipes and vary inversely with initial chlorine concentration. The field data on pressure and chlorine residuals was collected for the hydraulic and water quality calibration of the system. The hydraulic calibration of the system was carried out successfully using network flow and a pressure simulation model. Global and zoned wall reaction parameters were estimated using an inverse model applied onto the distribution system. The estimated zoned parameters are marginally better in simulating the chlorine residuals. The wall reaction parameters are found to be inversely proportional to the pipe diameter. The calibration of distribution system for hydraulic flow conditions and water quality is essential for better operation, maintenance and control.