A local water company is concerned with the water quality within its water distribution network (Fig. 7.18). They want to determine the age and the chlorine concentration of the water as it exits the system at different junctions. The water surface at the reservoir is 70 m.
Fig. 7.18
Chlorine is injected into the system at the source of flow, R-1, at a concentration of 1 mg/L. It has been determined through a series of bottle tests that the average bulk reaction rate of the chlorine in the system (including all pipes and tanks) is approximately –0.5 per day. Pump information and stepwise water demand pattern data are as shown in Tables 7.28 and 7.29. Junction and pipe information are given in Tables 7.30 and 7.31.
Tables 7.28 and 7.29
Tables 7.30 and 7.31
The cylindrical tank has a diameter of 15 m. The base and minimum elevations are 99 m. The maximum elevation is 104 m and the initial elevation is 103.4 m.
1. Perform an age analysis on the system using a duration of 300 h and a time step of 2 h. Fill in the results in Table 7.32, indicating the maximum water age at each junction and tank after the system reaches equilibrium (a pattern of average water age vs. time becomes evident). What point in the system generally has the oldest water? Explain why the water is oldest at this location.
2. Perform a constituent analysis using the same duration and time step as in part 1. Fill in the results in Table 7.32, indicating the minimum chlorine concentration for each junction and tank after the system has reached equilibrium (a pattern of concentration vs. time becomes evident). What point in the system has the lowest chlorine concentration? Explain why the chlorine residual is lowest at this location
Table 7.32
3. From the above table and graphs of demand, age, and concentration versus time generated within WaterGEMS, determine the following correlations: (a) Age and chlorine concentration; (b) Demand and chlorine concentration at a junction; (c) Demand and water age at a junction.
4. Why is it necessary to run the model for such a long time? Do you feel that 300 h is too long or too short a time period for testing the model? Why?