Water Quality Modeling of Chilliwack's Water Distribution System in Preparation of the Emergency Chlorination Test

The City's water distribution system supplies a population of 76,000 from seven groundwater wells without using any chemical constituent. On August 27, 2007 an emergency chlorination test was conducted for two weeks by injecting chlorine to the distribution system. The objective of this chlorination test was to determine if enough chlorine residual can be observed in the whole distribution system in the event of a contamination. The first step of this study was to build the water quality model with the kinetic reaction parameters. Based on the model predictions from two demand scenarios (Average Day Demand and Maximum Day Demand), a set of 49 sampling locations was proposed prior to the test to ensure that chlorine is properly propagated in the entire system. The field measurements collected during the chlorination test show that 68 % of the sampling locations received the chlorinated water within 24 hours, 90 % of the sampling locations received the chlorinated water within two days, and 100 % were covered in four days. The model predictions were fairly accurate as the simulation results showed that between 56 % and 75 % of all the junction nodes will receive the chlorinated water within one day, and 95 % of all junction nodes are covered in four days. Several sampling locations situated in the Fairfield Island area with measured water age over 72 hours can still have chlorinated water within 24 hours if three hydrants flowing at 60 L/s are opened for 24 hours. The chlorination system can thus be used as an emergency standby system with the capability to effectively disinfect the entire system in the event of an emergency. This study demonstrated that a complex water quality network model can be used as a practical tool for the planning of sampling design to meet water quality requirements.