Disinfection

1. The following data were obtained in a batch chlorination experiment. Plot the data (as residual chlorine concentration versus applied chlorine dosage)

and determine the "breakpoint" dosage.

                  Dosage (mg/L) : 1.00     2.00     3.00     4.00     5.00     6.00     7.00

                  Residual (mg/L) :     0.80     1.55     1.95     1.25     0.50     0.85     1.95

2. When HOCl (Cl₂) is added to water, it will react with ammonia to form chloramine:

NH₃ + HOCl → NH₂Cl + H₂O

A solution containing 0.5 mg/L of ammonia is dosed with 4.0 mg/L of Cl₂ in a batch contact vessel. If the rate constant for the reaction is k = 5x10⁶ L/mol^-s, estimate the time required to reduce the ammonia concentration by 50%.

3 The relationship between chlorine concentration and contact time required for 99% kill of E.coli is given by

C^0.86 t = 0.24

If a chlorine concentration of 4.0 mg/L is used, what time is required for 99.9% kill? You may assume that disinfection kinetics may be represented by dN/dt = - k' C^0.86 N. If a mixed-flow contactor is used, what residence time would be required for 99.9% kill?

4. A groundwater supply has been designated as vulnerable to fecal contamination, and the state has specified a disinfection level of 99.99% ("4-log") virus inactivation. The peak hourly pumping rate is 2000 gal/min from the well field through a 4200-foot pipe with a 16-in diameter to a reservoir in the town. The water temperature is 10oC. What chlorine residual is required in the water at the outlet of the pipe? From Table 11.10, p.439 in Viessman, et al., the C-t product is 6.0 mg-min/L. Assume that the contact time is equal to the hydraulic residence time in the pipe.

5. When ultraviolet light is used for disinfection, the rate of die-off of bacteria may be expressed as rN = - k' I N, where Iis the light intensity. Unfortunately, intensity decreases exponentially with penetration depth, i.e., I= I0 e?a x. The average intensity over a depth, x, may be expressed as I = (I0/ax) [1 - e?a x]. A disinfection system is designed for a 99.9% kill rate when the water depth is 2 inches. During operation, however, the water depth is 2.5 inches. What is the resulting percent kill? The absorption (extinction) coefficient ("a") has been measured as 0.05 cm^?1.