EN-301
Environmental Engineering-I
Disinfection
Dr Atif Mustafa
Professor & Chair
Department of Environmental Engineering
NED University of Engineering & Technology
What is Disinfection?
• Operations aimed at killing/removing
pathogens
• Approximately 90% of bacteria and viruses
are removed by coagulation, settling and
filtration
• Additional disinfection is practiced to follow
standards and provide protection against
regrowth
• Disinfection is not the same as sterilization
• Sterilization implies the destruction of all
living organisms
Properties of Good Disinfectant
1. They must destroy the kinds and numbers of
pathogens that may be introduced into water within a
practicable period of time over an expected range in
water temperature
2. They must meet possible fluctuations in
composition, concentration, and condition of the
water to be treated
3. They must be neither toxic to humans and
domestic animals nor unpalatable or otherwise
objectionable in the concentrations required for
disinfection
Properties of Good Disinfectant
4. Their strength or concentration in the treated
water must be determined easily, quickly, and,
preferably, automatically
5. Their cost must be reasonable
6. They should be safe and easy to store, transport,
handle, and apply
7. They should persist in a sufficient concentration
to provide reasonable residual protection
Disinfectants Used
Chlorine
Ultraviolet (UV) Radiation
Ozone
Chlorine
• Chlorine is the most common disinfecting
chemical used
• The term chlorination is often used
synonymously with disinfection
• Chlorine may be used as
– an element (Cl2), as sodium hypochlorite
(NaOCl),bleach
– as calcium hypochlorite [Ca(OCl)2], or
– as chlorinated lime (CaOCl2)
NaOCl Storage Facility
Chlorine Storage Tanks
Each tank holds ~900 kg of chlorine gas
Ultraviolet (UV) Radiation
• The energy associated with electromagnetic
radiation may conceptually be thought of as
photons
• The energy is related to the wavelength of the
radiation (Einstein, 1905):
E = hc/λ
where E = energy in each photon, J
h = Plank’s constant, 6.6x10-34 J · s
c = speed of light, m/s
λ= wavelength of radiation, m
Ozone
• Ozone is a pungent, unstable gas
• It is a form of oxygen in which three atoms of
oxygen are combined to form the molecule O3
• It is very unstable, therefore it is generated at
the point of use
• Ozone may be generated by photochemical,
electrolytic and radiochemical methods
Ozone
• It is most commonly generated by a discharge
electrode
• Either pure oxygen, purchased as liquid
oxygen (LOX), or the oxygen in the air, is
dissociated by the impact of electrons from the
discharge electrode
Ozone Generator
Most Common Disinfectants
Type of Regulatory Typical Source
Disinfectant Limit on Application
residuals Dose (mg/L)
(mg/L)
Free chlorine 4 1-6 Delivered as
liquid gas in
tank cars, 1
ton and 68 kg
cylinders as
calcium
hypochlorite
powder for
very small
applications
Type of Regulatory Typical Source
Disinfectant Limit on Application
residuals Dose
(mg/L) (mg/L)
Monochloramine 4 2-6 Same source as
for chlorine.
Ammonia is
delivered as aqua
ammonia
solution, liquid
gas in cylinders or
as solid
ammonium
sulphate. Chlorine
and ammonia are
mixed in
treatment process
Type of Regulatory Typical Source
Disinfectant Limit on Application
residuals Dose (mg/L)
(mg/L)
Chlorine 0.8 0.2 – 1.5 Same
dioxide sources as for
chlorine.
Chlorite as
powder or
stabilized
liquid solution
. ClO2 is
manufactured
with an on-
site generator
Type of Regulatory Typical Source
Disinfectant Limit on Application
residuals Dose
(mg/L)
Ultraviolet - 20 – 100 Low-pressure
Light mJ/cm2 high intensity UV
lamps (254 nm)
in contactors
Ozone - 1-5 mg/L Manufactured
on-site using a
corona
discharge in very
dry air or pure
oxygen. Oxygen
is usually
delivered as a
liquid.
Disinfection Kinetics – Chick’s Law
• It is a model that describes disinfection process
• The rate of kill is considered to be a first order
reaction
dN = -kN
dt
Where dN/dt = rate of change in number of
microorganisms with time (mo/volume/time)
k = Chick’s law rate constant (time-1)
N = Concentration of microorganisms (mo/time)
Breakpoint Chlorination
• The term “Breakpoint Chlorination” is
applied to the process whereby enough
chlorine is added to react with all
oxidizable substances such that if
additional chlorine is added it will
remain as free chlorine
Breakpoint Chlorination
Breakpoint Chlorination
• As chlorine is added, readily oxidizable
substances such as Fe2+, Mn2+ and organic
matter react with chorine and reduce most of
it to chloride ion
• The added chlorine then reacts with
ammonia to form choloramines between
points A and B
Breakpoint Chlorination
• Between point B and breakpoint, point C some
chloramines will be converted to nitrogen
trichloride and remaining cholroamines will be
converted to nitrous oxide (N2O) and nitrogen (N2 )
and chlorine will be reduced to chloride ion
• With continued addition of chlorine after
breakpoint, it will increase free available chlorine
(unreacted hypochlorite)
Types of Disinfection
1. Primary Disinfection (Inactivation of
microorganisms in water)
2. Secondary Disinfection (Maintaining disinfectant
residual in the distribution system)
• Disinfection at the start of the Water Treatment
Plant
– Assists in oxidation of inorganics
– Prevents growth of algae
– Prevents large population of bacteria from developing
within the filter media
Chlorine Contact Basin
Plan
Plan
Section
Section
Rectangular contact basin Circular contact basin
UV Disinfection System
Disinfection Byproducts
• Chlorine reacts with natural organic matter (NOM) to
form a number of carcinogenic byproducts
• Examples are trihalomethanes (THMs), haloacetic
acids (HAAs), haloacetonitriles, haloketones,
haloaldehydes, chloropicrin, cyanogen chloride, and
chlorophenols
• THMs and HAAs occur most frequently and
generally represent the highest concentrations of
the organic contaminants
Disinfection Byproducts
• If bromide is present, ozonation will form the
hypobromite ion (OBr ) which, in turn, forms,
hypobromous acid that will react with NOM to
form brominated byproducts.
• There are no known disinfection byproducts that
result from UV radiation
New York City’s Ultra-Violet Drinking Water
Disinfection Plant - the World's Largest
