1. Pathogens decrease exponentially when exposed to a disinfectant. The
decrease is described by the Chick’s law:
ln Nt/N0 = -kt
where: N0 = initial number of pathogens ( cells/L)
Nt = number of pathogens after time t (cells/L)
t = duration of disinfection
k = empirical constant (t-1)
The following data were obtained in a disinfection experiment dealing with
inactivation of poliovirus (Floyd et al., 1978. Environ. Sci. Technol. 12: 1031-1035):
Time (sec) N/N0
a. plot –ln(N/N0) versus time
b. calculate the k value
c. calculate the time required for a 1/5000 reduction of poliovirus
a. plotting –ln (N/N0) versus gives a straight line with a slope k
b. calculate k using excel
c. -ln(N/N0) - ln (1/5000)
t = ------------ = ------------
2. As regards the effect of pH on disinfection efficiency, what is the difference
between chlorine and chlorine dioxide?
Chlorine is more effective at low pH
Chlorine dioxide is more effective at higher pH
Why is chloramination recommended for water distribution systems?
Lower THM and haloacetic acids
Do not react with EPS
4. Why does chloramination promote the growth of nitrifying bacteria?
Because the nitrite produced by AOB exert a chloramines demand
What are some advantages and disadvantages of disinfection with ozone?
Advantages: More powerful than chlorine; does not react with ammonia; action not controlled by
Disadvantages: More expensive than chlorine; does not leave any residual
6. What is the ozonation by-product of most concern at the present time?
bromate (BrO3 )
7. Ultraviolet radiation
Give the categories of UV lamps used in disinfection studies
Continuous-wave emission lamps: they include low-pressure (10 to 10 torr) mercury lamps
with a peak emission at 253.7 nm, and medium-pressure (10 to 10 torr) mercury lamps which
emit light at 185nm to more 300nm.
Pulsed emission lamps produce pulsed light (30 pulses/sec) over a wide spectrum
(polychromatic) ranging from 185 to more than 800 nm.
8.What are the main factors affecting the efficiency of UV disinfection ?
Color, suspended solids, photoreactivation
9.Define photodynamic inactivation
Photodynamic inactivation (photochemical disinfection) consists of the use of visible light or sunlight as the
energy source, O2, and a sensitizer dye such as methylene blue, rose bengal, or eosin.
10. Summarize the toxicological problems associated with disinfection (covers chlorine,
chlorine dioxide, ozone, UV)
1) BOD of a wastewater sample:
5 ml of wastewater is added to 300-ml BOD bottle. Add dilution water to the bottle
until obtaining a volume of 300 ml. The initial DO concentration was 7.8 mg/L and
the final DO concentration was 4.3 mg/L after 5-day incubation. (5 pts)
a. calculate the BOD5 of the sample
b. calculate the ultimate BOD ( assuming a k value of 0.1)
a. BOD5 = ---------------- = 210 mg/L
b. Ultimate BOD = ----------- --- = ------------------- = 310 mg/L
1- 10-kt 1 – 10-0.1 x 5.0
2) What are some problems of the BOD test? (3 pts)
Does not take into account algae growth and release of O2
Nitrifiers exert an O2 demand
Overlooks recalcitrant compounds
3. Give the different steps involved in wastewater treatment: (2 pts)
Preliminary treatment, primary treatment, Secondary (or biological) treatment,
4. Explain the difference between carbonaceous BOD and autotrophic BOD (3 pts)
Carbonaceous BOD: Exerted by heterotrophic microorganisms
Autotrophic BOD: Exerted by autotrophic microorganisms such as nitrifiers
5. Give 2 examples of unit operations and 2 examples of unit processes (2 pts)
Unit operations: Filtration, sedimentation
Unit processes: disinfection, adsorption