Chapter 25
Drinking Water Treatment
Charles P Gerba
1. Why is it important to reduce the amount of biodegradable organic matter and
nutrients during water treatment?
Because they can result in growth of bacteria in the distribution system and interfere
with the effectiveness of disinfectants.
2. Describe the major steps in conventional treatment of drinking water.
Disinfection (chlorination), coagulation, flocculation, sedimentation, filtration and
disinfection (chlorination).
3. What group of waterborne pathogens is most effectively removed by filtration?
Why?
Protozoan parasites because of their large size.
4. What methods can be used to assess the growth of bacteria in water?
Assimilable organic carbon, coliform growth response, biodegradable dissolved
organic carbon
5. Which pathogenic microorganisms are the most difficult to remove by
conventional water treatment and why?
Enteric viruses because of their resistance to disinfection and inability to be removed
by filtration. Cryptosporidium and Giardia because of their extreme resistance to
some disinfectants such as chlorine.
6. Why is coliform regrowth in distribution systems a problem?
Interferes with its use as an indicator of fecal contamination.
7. Why does the HPC increase after ozonation of drinking water?
The ozonation breaks down organic matter making it more biodegradable.
8. A water treatment plant is required to remove a 3 log10 of Cryptosporidium
oocysts from the water source. What treatment processes do you recommend?
What if you have to remove 5 log10? See Chapter 26 for Cryptosporidium
resistance to disinfectants.
Conventional drinking water treatment should achieve 3 log removal (see Table
25.4). To obtain another 2 log removal UV light or ozonation would be required in
addition to the conventional treatment.
9. To determine the AOC of drinking water two 40-ml flasks are inoculated with
500 colony forming units (CFU) of Pseudomonas flurorescens strain P-17. After
9 days the concentration in one flask has reached 2.4 X 105 CFU/ml and 1.2 X 105
CFU/ml. Calculate the average AOC in this drinking water sample.
From Example Calculation 25.1
AOC = µg carbon/liter) = (Nmax X 1000))/Y
Nmax = 2.4 X 105 + 1.2 X 105/2 = 1.8 X 105
Y = yield coefficient (CFU/mg carbon) = 4.1 X 106 (Calculation 25.1)
Thus AOC = (1.8 X 105)(1000)/ 4.1 X 106 = 43.9 µg/acetate-carbon equivalents/liter