Chapter 26
Disinfection
Charles P Gerba
1. Of the non-spore-forming bacteria, which microbial group is the most
resistant to thermal inactivation in water?
Mycobacterium and Mycobacterium avium (see Table 26.2).
2. What is thermal reduction time? D value?
This is also known as Thermal Death Time (TDT) and is the time necessary to kill
a know number of organisms at a specific temperature. The D value or decimal
reduction time is the time required to destroy 90% of the organisms. The value is
numerically equal to the number of minutes required for the survivor, as a
function of the time curve, to transverse one log (see Figure 26.1).
3. Why are all microorganisms not inactivated according to first-order kinetics?
This may be due to a number of possible factors, including genetic differences in
resistance to various environment stresses (e.g., differences in cell membrane
making them more resistant to disinfectants), adsorption of the microbes
particulates, and clumps of microbes.
4. How long would you have to maintain a residual of 1.0 mg/L of free chlorine
to obtain a Ct of 15? A Ct of 0.1?
15 minutes, 0.1 minute.
5. Why is chlorine more effective against microorganisms at pH 5.0 than at pH
9.0?
Because of the greater formation of hypochlorous acid at pH 5.0.
6. Which chlorine compound is most effective against biofilms? Why?
Chloramines, because they can penetrate the biofilm.
7. What factors interfere with chlorine disinfection? Ultraviolet light?
Particulate matter (turbidity, clays, organic detritus), soluble organic matter,
organic and inorganic nitrogenous compounds.
8. What is the main site of UV light inactivation in microorganisms? What group
of microorganisms are the most resistant to UV light? Why?
The nucleic acid. Viruses. The small size of nucleic acid and some of them such
as the adenoviruses can use the host repair enzymes to repair the UV light damage
to the nucleic acid.
9. At what pH is iodine most effective against protozoan parasites? Why?
At low pH iodine is more effective against some protozoan cysts because they are
more sensitive to I2 than HOCl.
10. What is photoreactivation? Are all microorganisms capable of
photoreactivation? Why?
This is when the UV-light-damaged bacteria repair the nucleic acid by the
activation of photoreactivating enzyme, which binds and then splits the thymine
dimmers. No. Not all bacteria have the enzyme needed to repair the damaged
nucleic acid.
11. What are two sources of ionizing radiation? How does ionizing radiation kill
microorganisms?
Radioactive materials (cesium-127 and cobalt-60) and high-energy electron
beams. They inactivate microorganisms either directly or indirectly by production
of free radicals.
12. Why does suspended matter interfere with the disinfection of
microorganisms?
It interferes with the disinfectant reaching the microorganism. This may happen
by the suspended matter reacting with the disinfectant, protection of the
microorganism when adsorbed to the suspended matter, or the suspending matter
may coat the organism (the microorganism is inside of a clump of suspended
matter).