Question

3. Use the information in problem 1 and the experimental data from a tracer study shown in the table below to answer 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 determine the 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 the t₁₀?

b). (15 pts) During the year the water temperature mostly ranges from 5 to 20^oC. Assume the pH value of water is relatively stable at about 8.0 at 5^oC and about 7.5 at 20^oC during the disinfection process. What is the maximum difference (mg/L) of required free chlorine due to the range of water temperature?

c).   (9 pts) The minimum required chlorine residual in the plant effluent is 0.2 mg/L. Does the minimum free chlorine calculated in part b satisfy the requirement? Assume the distribution pipeline is an ideal PFR and free chlorine decays following a first-order reaction with a reaction rate constant k (0.12 hr^-1 at 5^oC and 0.15 hr^-1 at 20^oC) and the average 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 be accepted, what will be the minimum pipe distance from the plant to the nearest consumers required to address the odor/taste issue in cold weather? What will be the maximum service distance to ensure minimum chlorine residual 0.1 mg/L in destination in warm weather?

d).        (6 pts) The plant (based on the above conditions) will provide service to some consumers only 2 mile (of pipeline) away. The treatment plant will build an additional tank at the end of the pipeline to temporally store treated water between the consumers and the plant. Assume the tank is an ideal CMR and the decay rate is 0.12 hr^-1 based on a first-order reaction (independent from the temperature) and the pipeline remains as 2 mile. What should be the minimum hydraulic residence time of this tank?

Answer 1

For water purification different chamicals are used.