CEE 462
Disinfection
November 23, 2021
Today’s class
• Field trip tomorrow!
• Visualize the topic
• Review Tempe’s water treatment plants
• Principles and design standards
• Disinfection
• Design exercise: Disinfection
• Grand challenges: Cyber
• Extra: Note change in dates for HW9 and HW10
Visualize the topic
Principles and design
standards
Topics for today
• Types of pathogens
• Types of disinfectants
• Log removal
• Concentration and time, i.e., CT
• Disinfectant demand
• Disinfection practice
• Regulatory context
• Disinfection design
By the end of this lecture, you should be able to
1.
2.
3.
4.
Describe the term disinfection
List the pathogens of concern for drinking water treatment
List the common disinfectants used for water treatment
Specify whether the disinfectants can be used for primary
disinfection, secondary disinfection, or other
5. Describe an unintended consequences of disinfection
Raw water
to Water
Distribution System
Raw water quality
Surface water vs. groundwater
River vs. lake/reservoir
• Alkalinity
• Arsenic
• Calcium & magnesium
• Chloride
• Conductivity, Total dissolved solids
(TDS)
• Hydrogen sulfide
• Iron & manganese
• pH
• Nitrate
• Sodium
• Total hardness
• Total organic carbon
• Turbidity
• Pathogens!!!
Disinfection
Disinfection is the inactivation of
pathogens
• Primary disinfection: inactivation of
pathogens in water
• Secondary disinfection: prevention of
pathogen regrowth by residual
disinfectants
Disinfection is not sterilization
Waterborne outbreak: Norwalk virus, Oak Creek
Canyon, 1989
Waterborne outbreak: Cryptospridium,
Milwaukee, 1993
Largest reported
waterborne disease
outbreak in the US
Waterborne outbreak: Cryptospridium,
Milwaukee, 1993
Waterborne outbreak: E. coli, Walkerton, 2000
The Walkerton Inquiry identified that outbreaks in
drinking water usually is the results of the failure
of multiple elements, or barriers, in the water
treatment system.
Waterborne outbreak: E. coli, Walkerton, 2000
Types of pathogens
• Bacteria (enteric)
• Viruses (enteric)
• Protozoa cysts (enteric)
• Also non-enteric bacteria, amoeba, algal toxins…
Types of disinfectants
Rates of disinfection
Disinfection is a complex set of reactions dependent on:
• Chemistry of the disinfectant
• Biological characteristics of the pathogens
• Interaction between disinfectant and pathogen
• Influence of the medium, i.e., water (temperature, pH,
electrolytes, interfering substances)
Log removal
Initial number = 1,000,000 bacteria
· 90% (1 Log) = 100,000 remain
· 99% (2 Log) = 10,000 remain
· 99.9% (3 Log) = 1,000 remain
· 99.99% (4 Log) = 100 remain
· 99.999% (5 Log) = 10 remain
Log removal (LR) and % removal
LR = log(Craw/Ctreated)
% removal = 100 – (100/10LR)
C,raw
1.00E+06
C,treated 1.00E+05
LR
1
% removal
90
Disinfection chemistry: Chick’s Watson equation
𝑑𝑁
−
= 𝑘 ′ 𝐶𝑁𝑡
𝑑𝑡
𝑁𝑡 = 𝑁0
C= concentration of disinfectant
k' = coefficient of specific lethality (disinfection rate
constant), L/mg-min
No, Nt, concentrations of pathogens at time 0, t
t = time, min
′ 𝐶𝑡
−𝑘
𝑒
𝑁𝑡
′ 𝐶𝑡
−𝑘
=𝑒
𝑁0
Rates of disinfection
If you have less disinfectant, you need
more contact time for the same % of
inactivation.
This value, often referred as CT
(i.e., concentration × time), is the
critical design parameter for
disinfection of a given contaminant
CT varies between disinfectant and pathogens
For viruses
Disinfectants summary
Disinfectant
Issue
Combined
Chlorine
Chlorine Dioxide
Ozone
Ultraviolet Light
Bacteria
Excellent
Good
Excellent
Excellent
Good
Viruses
Excellent
Fair
Excellent
Excellent
Fair
Protozoa
Fair to Poor
Poor
Good
Good
Excellent
1o Disinfectant
Frequency
Most Common
Common
Occasional
Common
Emerging Use
Cost Rank
1 = lowest
2
3
4
5
Residual Regs
4 mg/L
4 mg/L
0.8 mg/L
--
--
Byproducts
THMs, HAAs
Traces of THMs,
HAAs
Chlorite
Bromate
--
Typical Dose
1 – 6 mg/L
2 – 6 mg/L
0.2 – 1.5 mg/L
1 – 5 mg/L
20 – 100 mJ/cm2
Source
Gas, liquid or dry
hypochlorite,
delivery or onsite
generation
Same as Cl2,
ammonia
delivered as
liquid or solid
Onsite generation
from chlorine or
chlorite
Onsite generation
using electricity
and air or oxygen
Low or medium
pressure lamps
Effectivene
ss
Free Chlorine
Sensitivity of Organisms
For Log removal = 2
C. parvum = Cryptosporidium parvum
Chlorine chemistry
Chlorine gas reacts with water to form hypochlorous acid and
hydrochloric acid
Cl2+H2O → HOCl + HCl
Hypochlorous acid dissociates to hypochlorite in a pH
dependent fashion
HOCl → H+ + OClBoth HOCl and OCl- are oxidizing agents. HOCl is the
strongest of the two, but both are considered free chlorine.
Free chlorine reacts with inorganic reducing compounds such
as Fe+2, Mn+2, NO-2, and is reduced to the non-oxidizing ClWhen ammonia is present, HOCl and OCl- react with
ammonia to produce chloramines
NH3 + HOCl → NH2Cl + H2O (monochloramine)
NH2Cl + 2HOCl → NHCl2 + H2O (dichloramine)
NHCl2 + 3HOCl → NCl3 + H2O (Trichloramine)
Chlorine chemistry
Chlorine gas reacts with water to form hypochlorous acid and
hydrochloric acid
Cl2+H2O → HOCl + HCl
Hypochlorous acid dissociates to hypochlorite in a pH
dependent fashion
HOCl → H+ + OClBoth HOCl and OCl- are oxidizing agents. HOCl is the
strongest of the two, but both are considered free chlorine.
Free chlorine reacts with inorganic reducing compounds such
as Fe+2, Mn+2, NO-2, and is reduced to the non-oxidizing ClWhen ammonia is present, HOCl and OCl- react with
ammonia to produce chloramines
NH3 + HOCl → NH2Cl + H2O (monochloramine)
NH2Cl + 2HOCl → NHCl2 + H2O (dichloramine)
NHCl2 + 3HOCl → NCl3 + H2O (Trichloramine)
Chlorine chemistry
Breakpoint
Dosage : amount of chlorine added
Demand: amount consumed by oxidation
Residuals: amount remaining after oxidation
Chlorine dosage
Chlorine demand
The residual level before breakpoint are combined residuals
(chloramines)
The residuals after breakpoint are combined + free chlorine
residuals.
Addition of chlorine after breakpoint is in the form of free chlorine
(HOCl and OCl-).
CT values and regulations is based on free chlorine
Chlorine residual
Residuals must be at least 0.20 mg/L at the farthest tap in the
system, and must NOT exceed 4.0 mg/L
Types of disinfectants
Disinfectant
Issue
Combined
Chlorine
Chlorine Dioxide
Ozone
Ultraviolet Light
Bacteria
Excellent
Good
Excellent
Excellent
Good
Viruses
Excellent
Fair
Excellent
Excellent
Fair
Protozoa
Fair to Poor
Poor
Good
Good
Excellent
1o Disinfectant
Frequency
Most Common
Common
Occasional
Common
Emerging Use
Cost Rank
1 = lowest
2
3
4
5
Residual Regs
4 mg/L
4 mg/L
0.8 mg/L
--
--
Byproducts
THMs, HAAs
Traces of THMs,
HAAs
Chlorite
Bromate
--
Typical Dose
1 – 6 mg/L
2 – 6 mg/L
0.2 – 1.5 mg/L
1 – 5 mg/L
20 – 100 mJ/cm2
Source
Gas, liquid or dry
hypochlorite,
delivery or onsite
generation
Same as Cl2,
ammonia
delivered as
liquid or solid
Onsite generation
from chlorine or
chlorite
Onsite generation
using electricity
and air or oxygen
Low or medium
pressure lamps
Effectivene
ss
Free Chlorine
Ozone
Ozone is the strongest oxidant used for disinfection
O3 is generated on-site by passing dry O2 through high voltage electrodes
UV disinfection
UV between 250-270 nm damage DNA.
Prevents replication of microbes
Intensity: amount of UV delivered to the organism
UV dose = Intensity x residence time
Multiple Barrier Principle
The best way to achieve a healthy public water supply is to put in place multiple barriers that keep
water contaminants from reaching drinking water
1. Source protection keeps the raw water as clean as possible to lower the risk that contaminants will get
through or overwhelm the treatment system.
2. Treatment often uses more than one approach to removing or inactivating contaminants (e.g., filtration
may be followed by chlorination, ozonation, or ultraviolet radiation).
3. Securing the distribution system against the intrusion of contaminants and ensuring an appropriate
free chlorine residual throughout is highly likely to deliver safe water, even when some earlier part of
the system breaks down.
4. Monitoring programs, including equipment fitted with warning or automatic control devices, are critical
in detecting contaminants that exist in concentrations beyond acceptable limits and returning systems
to normal operation.
5. Well-thought-out, thorough, and practiced responses to adverse conditions, including specific
responses for emergencies, are required when other processes fail or there are indicators of
deteriorating water quality.
Report of the Walkerton Commission of Inquiry, part 2, chap. 3
Focus of this course
Multiple Barrier Principle
Disinfection by-products
Disinfection by-products
DBPs are minimized
by using alternative
forms of disinfectants
(ozone, UV), by
removing precursors,
or by removing DBPs
using GAC
Carcinogens
TTHM standard in
drinking water: 80 ppb
Conclusions
• Disinfection is (potentially) the most critical step of drinking
water treatment
• Design consideration based on free chlorine concentrations
(C) and the allowed residence time (T)
• Chlorine is the most widely used disinfectants because it is (i)
cheap, (ii) efficient, and (iii) leaves residuals
• Too much disinfectants, or too much organic matter, leads to
DBPs
Types of disinfectants
Disinfectant
Issue
Combined
Chlorine
Chlorine Dioxide
Ozone
Ultraviolet Light
Bacteria
Excellent
Good
Excellent
Excellent
Good
Viruses
Excellent
Fair
Excellent
Excellent
Fair
Protozoa
Fair to Poor
Poor
Good
Good
Excellent
1o Disinfectant
Frequency
Most Common
Common
Occasional
Common
Emerging Use
Cost Rank
1 = lowest
2
3
4
5
Residual Regs
4 mg/L
4 mg/L
0.8 mg/L
--
--
Byproducts
THMs, HAAs
Traces of THMs,
HAAs
Chlorite
Bromate
--
Typical Dose
1 – 6 mg/L
2 – 6 mg/L
0.2 – 1.5 mg/L
1 – 5 mg/L
20 – 100 mJ/cm2
Source
Gas, liquid or dry
hypochlorite,
delivery or onsite
generation
Same as Cl2,
ammonia
delivered as
liquid or solid
Onsite generation
from chlorine or
chlorite
Onsite generation
using electricity
and air or oxygen
Low or medium
pressure lamps
Effectivene
ss
Free Chlorine
Design exercise
Disinfection design
Giardia: total LR = 3
Viruses: total LR = 4
If Cryptosporidium present, then additional log removal required; 1 to 2.5 log
removal
Disinfection design
If Cryptosporidium present, then additional log removal required; 1 to 2.5 log
removal
Disinfection design, continued
When calculating CT dosage, consider
• T = use the residence time based on peak flow rate
• C = use lowest measured daily residual
• Note: NOM, iron, and other chemicals can exert a disinfectant
chemical demand
• Temperature and pH = use the lowest temperature and
highest pH value
• CT is pathogen dependent = use CT value for the most
resistant pathogen known in your system
Disinfection design exercise 1
A utility detects Cryptosporidium parvum in their
influent water. Based on their treatment train,
they need to provide a 2-log inactivation of C.
parvum through disinfection. The utility uses
chlorine dioxide and has a 60 min residence time
clearwell tank to increase the contact time
between pathogen and disinfectant.
What will be the required chlorine dioxide dosage
in the summer (20oC)?
How is the chlorine dioxide dosage adjusted for
disinfectant demand?
Disinfection design exercise 2
From client: Design flow of 2 m3/s, 15°C water temperature, well-protected
reservoir with no Cryptosporidium detected.
One of your design options is conventional treatment. The pH after filtration is 7.
The filtered water contains NOM that is expected to exert 1 mg/L of chlorine
demand. Determine the chlorine dose given a chlorine contact basin with residence
time of 10 min.
Grand challenges
Environmental
Technological
Social
Climate change
Water reuse
Access to water
Cyber
Water distribution
Equity & justice
Cultural
Organization
Extra