A treated wastewater is to be disinfected using aqueous chlorine. The following data were obtained from laboratory tests conducted on the wastewater. The temperature was 12 oC, the pH was 7.4, and the initial fecal coliform concentration was 5 x 105 organisms per 100 mL of wastewater.
Residual fecal coliform count (number/100 mL) at the following contact times
Chlorine dose (mg/L) 10 min. 15 min 30 min 60 min
1 16,000 5,400 1,100 270
2 4,050 1,600 190 48
4 560 210 34 11
6 112 59 16 7
8 45 29 10 3
10 29 16 6 1
(a) Using appropriate plots of the data, determine values of n and K for a desired effluent concentration of 200 fecal coliforms/100 mL. Show all plots used to generate n and K.
(b) If the state of Iowa mandates a contact time of 30 minutes at T = 12 oC, calculate the chlorine requirements in lb/d for a flow of 10 MGD using the equation you developed for part (a).
(c) Estimate the contact time to achieve an effluent concentration of 200 fecal coliforms/100 mL using a Cl2 dose of 3 mg/L if the wastewater temperature is 28 oC. Assume the activation energy (Ea) at pH = 7.4 is 6,500 cal/mole. If necessary, use R = 1.99 cal/mole-K. Does your answer make sense? Why or why not?
(d) Assuming Chick's law describes disinfection kinetics, determine the rate coefficient (k) for a Cl2 dose of 4 mg/L. Does Chick's law describe these data well? If not, what would you suggest to better describe the kinetics of disinfection at a given Cl2 dose?