CEE 371 Fall 2009

Homework #5   Disinfection

1.      Briefly describe the main factors that affect the effectiveness of disinfection using free chlorine for inactivating pathogens.

2.      Consider the effect of pH on the distribution of the free chlorine species:

a)      Construct a figure showing the percent distribution of HOCl and OCl-1 as a function of pH (3 to 11) using an equilibrium acidity constant (Ka) of 3.16x10-8.

b)      Calculate directly (do not read from the figure) the % HOCl existing at pHs of 5.5, 6.5, 7.5, and 8.5.

c)      For a constant dosage of chlorine, is it better to disinfect at pH below 7.5 or at pH above 7.5?  why?

3.      The hydraulics of a disinfection contact chamber were characterized at design flow using a tracer study based on a step or continuous feed of fluoride at 2.0 mg/L.  Initially, there was no fluoride in the water.  Results of measurements of fluoride concentration in the reactor effluent are shown in the table below.

 Time (min) Effluent Fluoride Conc (mg/L) 0 0 5 0 10 0 15 0.05 20 0.10 25 0.45 30 0.72 35 0.95 40 1.20 50 1.46 60 1.65 70 1.88 80 1.92 90 2.0 100 2.0

a)      Plot the effluent fluoride concentration data and determine the t10 time in minutes.

b)      The water utility would like to exceed the Giardia removal required by US EPA and obtain 1 log inactivation by disinfection.  Determine the disinfectant residual needed if free chlorine disinfection is used and compare to that needed if chloramine disinfection is used.  The disinfection contact chamber characterized in part a) will be used for conditions of: water temperature of 5oC, pH of 7.0.

c)      The tracer data in the table above show that the mean hydraulic detention time (tR) of the contact chamber is 40.3 minutes.  Calculate the ratio of t10 to tR.  How much smaller in volume could the contact chamber be if it exhibited ideal plug flow behavior?

Assigned: 26 Oct 09

Due: 2 Nov 09