DISINFECTION: DRINKING WATER
AND WASTEWATER
Disinfection: destruction of pathogenic organisms
Sterilization: destruction of all microorganisms
Purpose: disinfect to prevent the transmission of water-borne diseases,
i.e., destroy bacteria, viruses, amebic cysts
Possible Disinfectants
Physical:
1. Heat: boiling water; no important pathogens are heat resistant, costly
2. Ultraviolet light: no residual, thin sheets
3. Filtration: e.g., diatomaceous earth
4. Radiation: γ (gamma), ultrasonic, acoustic
Possible Disinfectants, cont.
Chemical:
5. Chemical agents
a) chlorine
b) bromine
c) iodine
oxidants
d) ozone
e) H2O2
f) phenol
g) alcohol
h) acids & bases, pH >11, pH<3
i) metal ions, silver, copper (algal blooms)
Indicator Organisms
Impractical to analyze for all pathogens, so indicators are a “group” of
organisms, like:
- Fecal coliforms: exposure to fecal mammalian fecal matter
Unfortunately for P & P industry:
effluents often contain naturally occurring bacteria, Klebsiella Pneumonia,
which registers as fecal coliforms.
A more specific indicator, Escherichia Coli (E. coli) is being adapted in
many places (including Michigan). Reliable so far……
Physical Disinfection Mechanisms
1. Damage to cell membrane
2. Alteration of cell membrane permeability
- allows for potential escape of vital nutrients: N, P
3. Alteration of colloidal nature of protoplasm
- e.g., coagulation of cell protein
4. Inhibition of enzyme activity
5. Alteration of organism DNA, RNA
Desirable Properties of Disinfectants
1.
Be effective under conditions normally found in wastewater: pH, temp
2.
Non-toxic to humans at concentrations needed to disinfect
3.
Reasonable cost
4.
Safe to transport and apply
5.
Concentration easily measured (dosage)
6.
Maintain a residual (not shown in Table 12.1, 4th Ed.)
Only one agent meets all of the above: chlorine
(next choice probably ozone (O3)
Some facts
• Chlorine is found in the
Earth itself and, as salt, in
the seas which cover
seven tenths of the
planet.
More facts
• Worldwide, waterborne diseases kill over 25,000
people each day. Drinking water chlorination is one of
the most widely used methods to safeguard and
protect drinking water supplies.
• It’s fundamental to the life of plants and animals.
What’s it for?
• Chlorine is also extensively used in the
production of paper products, dyestuffs,
textiles, petroleum products, medicines,
antiseptics, insecticides, food, solvents,
paints, plastics, many other consumer
products, and disinfectants
Comparison of Ozone and Chlorine
1.
O3 more efficient: faster and smaller dosage
2.
O3 more expensive
3.
O3 has no residual
4.
O3 has no taste or odor problems
Chlorine Addition
•
Highly soluble, ~7,000 mg/L
•
Residuals desired (1 mg/L) not a problem
•
Shipped in large steel cylinders: certain risks associated
Note: use of hypochlorite compounds is an alternative which provides
exactly the same disinfectant at lower risk due to handling,
but at higher cost.
Chlorine Chemistry
+
Cl2 + H 2O ← → HOCl + H + Cl
hydrolysis
(fast and complete at pH >3)
−
HOCl ← → OCl + H
ionization
hypochlorous
acid
+
hypochlorite ion
HOCl + OCl = “free available chlorine”
−
not a disinfectant
We could quit here if the wastestream was pure
water……
However,
nitrogen compounds get in the way by reacting
with chlorine
Factors Influencing Disinfection
Efficacy and Microbial Inactivation
• Microbe type: Resistance to chemical disinfectants:
– Vegetative bacteria: Salmonella, coliforms, etc.
– Enteric viruses: coliphages, HAV, SRSVs, etc.
– Protozoan (oo)cysts, spores, helminth ova, etc.
• Cryptosporidium parvum oocysts
• Giardia lamblia cysts
• Clostridium perfringens spores
• Ascaris lumbricoides ova
• Acid-fast bacteria: Mycobacterium spp.
Least
Most
Distribution of Hypochlorous Acid
and Hypochlorite in Water
HOCl more effective
(100:1)
Chlorine Reactions with Ammonia
HOCl + NH 3 ↔ H 2O + NH 2Cl (monochloramine)
HOCl + NH 2Cl ↔ H 2O + NHCl2
HOCl + NHCl2 ↔ H 2O + NCl3
(dichloramine)
(nitrogen trichloride)
(no germicidal value, leaves as gas)
Distribution = f(pH, temp., time, Cl2:NH3)
Breakpoint Chlorination
Nomenclature
free chlorine (available & residual) = HOCl + OCl(gas)
combined = NH2Cl + NHCl2 + NCl3
Relative effectiveness
HOCl
1
OCl0.01
NH2Cl 0.005
NHCl2 0.04
Contact Time – Chick’s Law (1908)
For a given concentration of disinfectant, the longer the contact time,
the greater the kill.
dN
= −kN t
dt
Where:
dNt/dt = rate of change in the concentration of organisms with time
k = inactivation rate constant, T-1
Nt = number of organisms at time t
t = time
Note: departures from this law are common.
Concentration of Disinfectant
Depending on the type of chemical agent, it has been observed that,
within limits, disinfection effectiveness is related to concentration.
'
k =kC
n
Where:
k = inactivation rate constant
k’ = die-off constant
C = concentration of disinfectant
n = coefficient of dilution
Note:
n = 1 both the concentration and time are equally important
n > 1 concentration is more important than time
n < 1 time is more important than concentration