3. Use the information in problem 1 and theexperimental data from a tracer study shown in the table below toanswer the following questions.
t (min) | C (mg/L) | t (min) | C (mg/L) | t (min) | C (mg/L) |
0 | 0 | 30 | 0.174 | 85 | 3.245 |
5 | 0 | 35 | 0.305 | 105 | 4.085 |
10 | 0 | 45 | 0.654 | 125 | 4.570 |
15 | 0 | 55 | 1.219 | 145 | 4.823 |
20 | 0.028 | 65 | 1.916 | 165 | 4.968 |
25 | 0.073 | 75 | 2.629 | 185 | 5.064 |
a).  (2 pts) To determinethe required free chlorine for disinfection of the filtered water,a tracer (fluoride ion, dosage = 6.0 mg/L) study (step-dose method)was used to determine t10. What is thet10?
b). (15 pts) During the year the watertemperature mostly ranges from 5 to 20oC. Assume the pHvalue of water is relatively stable at about 8.0 at 5oCand about 7.5 at 20oC during the disinfection process.What is the maximum difference (mg/L) of required free chlorine dueto the range of water temperature?
c).  (9 pts) The minimumrequired chlorine residual in the plant effluent is 0.2 mg/L. Doesthe minimum free chlorine calculated in part b satisfy therequirement? Assume the distribution pipeline is an ideal PFR andfree chlorine decays following a first-order reaction with areaction rate constant k (0.12 hr-1 at5oC and 0.15 hr-1 at 20oC) and theaverage velocity of treated water along the pipe is about 3 ft/s.If a chlorine residual above 0.6 mg/L is too objectionable to beaccepted, what will be the minimum pipe distance from the plant tothe nearest consumers required to address the odor/taste issue incold weather? What will be the maximum service distance to ensureminimum chlorine residual 0.1 mg/L in destination in warmweather?
d).       (6 pts) The plant(based on the above conditions) will provide service to someconsumers only 2 mile (of pipeline) away. The treatment plant willbuild an additional tank at the end of the pipeline to temporallystore treated water between the consumers and the plant. Assume thetank is an ideal CMR and the decay rate is 0.12 hr-1based on a first-order reaction (independent from the temperature)and the pipeline remains as 2 mile. What should be the minimumhydraulic residence time of this tank?
