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8/12/2019 Disinfection Notes
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Disinfection
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Disinfection
Definition:the selective destruction of disease causing organisms
History:1881: Koch demonstrated that chlorine could kill bacteriain lab
1905: London England first chlorination of a public watersupply following typhoid fever outbreak
1912: Large scale chlorination facilities installed atNiagara Falls, N.Y.
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Disinfection
ChlorineHistorically, dominant methods of disinfection
Chloramines Strictly a secondary disinfectant for drinking water
OzoneUsed widely in France, GermanySome popularity in USA, Canada
Chlorine DioxideUsed to some extent in Europe, rare in USA and Canada
Ultraviolet Light (UV)Has become very popular in last 15 years
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Disinfection
Disinfection Terminology(in drinking water treatment)
Primary Disinfection Disinfectant applied in a water treatment plant to control
microorganismsmakes the water safe to drink
Secondary Disinfection Disinfectant applied to water leaving the treatment plant to
protect against intrusion in the distribution system,
suppress biofilm on pipes
Inactivation Rendering a pathogen harmless (not necessarily killing it)
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DisinfectionAn ideal substance does not exist, but some that are usedinclude:
Conventional:
Chlorine
Chloramines
Chlorine Dioxide Ozone UV
Alternatives: Bromine (swimming pools) Mixed oxidants (MIOX) Iodine (small applications)
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Disinfection
Microorganism CharacteristicsPathogens may be divided into four common groups:
1. bacterial spores2. protozoan spores
resistance to disinfection is f(cell wall properties)3. viruses4. vegetative bacteria
easy to kill
respiration takes place at cell surface
Decreasingorder
accordingto chemicaldisinfectionresistance(not UV!)
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Disinfection
Chemical DisinfectionDisinfection with Chlorine (Chlorination):
Disinfectant capabilities depend on its chemical form inwater
f (temperature, pH, organic content of water)
Gaseous chlorine (Cl2) when added to water rapidlyhydrolyzes to hypochlorous acid (HOCl)
-22 ClHHOClOHCl +++
+
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Disinfection
Reaction proceeds essentially to completion @ pH> 4 Hypochlorous acid subjected to additional reactions:disinfection, reaction with organics, dissociation tohypochlorite ions (OCl-), etc.
Between pH 6 and 9, HOCl decreases, OCl-increases Dissociation of HOCl also temperature dependant
Hypochlorousacid
Hypochloriteion
+
+ HOClHOCl-
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20oC
0oC
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Disinfection
Chemical Disinfection
HOCl much more effective than OCl-at killing
microorganisms Maintain pH at 6 to 7 for optimum disinfection with Cl2 Chlorine usually added as:
Chlorine gas Sodium hypochlorite (NaOCl) (liquid)
more expensive better safety poorer stability (loss of approximately 1% per month) used in smaller plants 5% to 15% available Cl2
ChlorineFree][OCl[HOCl] - =+
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Disinfection
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Disinfection
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Disinfection
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DisinfectionChloramines
When chlorine (Cl2) is added to water and ammonia(NH4
+) is present, react to form chloramines
Chloramines also referred to as combined chlorine poor disinfectants
HOCl + NH3 H2O + NH2Cl(monochloramine)
HOCl + NH2Cl H2O + NHCl2 (dichloramine)
HOCl + NHCl2 H2O + NCl3(trichloramine ornitrogen trichloride)
chloramines
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DisinfectionChloramine formation
Species is a f(ammonia, pH, temperature)
pH 4.5-8.5 monochloramines + dichloramines
pH > 8.5 monochloramine alone pH < 4.5 trichloramine
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DisinfectionNote
Free chlorine = residual chlorine existing in water asHOCl and OCl-
Combined chlorine = residual existing in combination withammonia. i.e.chloramines)
Total chlorine = free + combined chlorine Chlorine demand = the difference between amount
added to a water and the amount remaining after aperiod of time
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DisinfectionBreakpoint Chlorination
In many natural waters, a graph of total residual chlorinevs. applied chlorine looks like this:
This is referred to as the chlorine breakpoint curve
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22Chlorine Residual Curve for Breakpoint Chlorination
Rxns with easily oxidizable substances
A B
Formation of Chloramines
C
D
E
Destruction of Chloramines
Formation of Free residual
reakpoint
FreeResidual
Combined Residual
1
1
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DisinfectionBreakpoint Chlorination
4 phenomena occurring as you increase Cl2dose:
1. A-B Chlorine reacts with immediately with oxidizable substances
such as Fe2+(ferrous ion), H2S (hydrogen sulfide), nitrite,organic compounds
as a result, chlorine is converted to chloride which has nodisinfecting power
there is little or no measured residual total chlorine despitethe applied chlorine dose.
23Chlorine Residual Curve for Breakpoint Chlorination
Rxns with easily oxidizablesu bstances
A B
Formation of Chloramines
C
D
E
Destruction of Chloramines
Formation of Free residual
reakpoint
FreeResidual
Combined Residual
1
1
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Disinfection 4 Steps (Contd):
2. B-C Formation of mono- and dichloramines (assuming ammonia is
present!)
Every molecule of free chlorine you add reacts withammonia to form a molecule of combined chlorine
Chloramine species formed is a f (pH, N:Cl2ratio) > pH 8.5 mono
pH 4.5-8.5 mono and di
Also some formation of chloro-organic compounds (e.g.THMs) (usually < 1% at this point. The reaction with ammoniais much faster, and is preferred.)
23Chlorine Residual Curve for Breakpoint Chlorination
Rxns with easily oxidizablesu bstances
A B
Formation of Chloramines
C
D
E
Destruction of Chloramines
Formation of Free residual
reakpoint
FreeResidual
Combined Residual
1
1
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Disinfection 4 Steps (Contd):
3. C-D Once all the ammonia has been used up in forming
chloramines, additional free chlorine oxidizes and destroysthe chloramines (products = nitrogen, nitrate, chloride, and
other products) reduces the total chlorine residual
Cl2+ chloramines Cl-, N2, others
23Chlorine Residual Curve for Breakpoint Chlorination
Rxns with easily oxidizablesu bstances
A B
Formation of Chloramines
C
D
E
Destruction of Chloramines
Formation of Free residual
reakpoint
FreeResidual
Combined Residual
1
1
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Disinfection 4 Steps (Contd):
4. D-E Once most of the chloramines are oxidized (Point D),
additional chlorine added creates an equal free chlorine
residual free chlorine residual = [HOCl] + [OCl-] D known as breakpoint some chloramines may also be present at low
concentration
The presence of free chlorine provides Effective disinfection Some chloro-organic by-product formation
23Chlorine Residual Curve for Breakpoint Chlorination
Rxns with easily oxidizablesu bstances
A B
Formation of Chloramines
C
D
E
Destruction of Chloramines
Formation of Free residual
reakpoint
FreeResidual
Combined Residual
1
1
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DisinfectionChlorine Dioxide (ClO
2
)
Historically had limited application as a disinfectant inNorth America
Applied for taste and odour (T&O) control (historically)
However, in certain circumstances (especially forGiardiacontrol)excellentchoice as disinfectant Effective in destroying phenols Does not form THMs in significant amounts
Pulp & paper industry: largely replaced chlorine for
bleaching of paper products in North America Less environmental impact than Cl2
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DisinfectionChlorine Dioxide (ClO2) Generation
1 method: mix sodium chlorite and chlorine in controlledproportions to form ClO2
yields 85% - 90% ClO2(typical)
want to minimize unreacted chlorine and chlorite resulting solution concentration of ClO2 500-2,000mg/L slowly add to water to dose at 0.5-1.0 mg/L (typical)
NaCl22ClOCl2NaClO222
++
sodium
chloritechlorine
dioxidesodium
chloride
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DisinfectionAdvantages and Disadvantages
Advantages does not react with ammonia much stronger disinfectant than chlorine no strong
disinfection efficiency dependence on pH
strong chemical oxidant taste and odour control, colour does not form THMs (unless excess chlorine present)
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DisinfectionAdvantages and Disadvantages
Disadvantages must be generated on site (unstable) generation system must be well controlled to minimize
excess free chlorine
produces byproducts chlorite (ClO2-), chlorate (ClO3-)
may cause health problems
U.S. limits: ClO2-- 0.8 mg/L; ClO3-- 1.0 mg/L
Ontario limit: ClO2-
- 0.8 mg/L costs more than Cl2
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DisinfectionOzone
Unstable gas, must be generated onsite, usedimmediately
Strong oxidizing gas (strongest common oxidizing agentin water treatment)
reacts with many organic, inorganic molecules More reactive than chlorine, however ozone does not
leave a residual must add an additional chemical for secondary disinfection
Reactions rapidly inactivate organisms Can use it for simultaneous disinfection and oxidation
(flocculation aid, taste and odour control, colour removal,etc.)
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DisinfectionChemistry of Ozone
Air or oxygen flows between 2 electrodes and electricalspark produces ozone (O3)
1 to 3.5% by weight if ambient air is used
2 to 8% by weight if oxygen is used
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Disinfection
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Disinfection
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Disinfection
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Disinfection
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Disinfection
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Disinfection Ozone is capable of reacting by 2 mechanisms:
Direct reaction of ozone molecule (O3) (pH 6-8) As hydroxyl free radical (OH)
at pH 9+, ozone added to water rapidly decomposes to formthe OH radical
OH much more powerful than ozone itself, but scavengedby carbonate and bicarbonate ions & natural organic matter
Half life of O3approximately minutes 1 hour Half life of (OH) approximately microseconds
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DisinfectionMethod of Application
Apply ozone gas to water in ozone contactor Use porous diffusers to make small bubbles (maximize
contact)
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40
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DisinfectionMethod of Application
Old Rule of thumb application: Try to maintain 0.4 mg/L O3(C) contact for 4 minutes (T) CT = 1.6 mg/L.min
We now dose ozone according to CT requirements(discussed later)
Must destroy any unused offgas since its toxic
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Disinfection Advantages (contd)
Organic by-products produced by ozone are easilybiologically degradable
incorporate biological filtration following ozonation to allow
mineralization of assimilable organic carbon (AOC)
instead of allowing this material to be discharged intodistribution system (= regrowth)
O3+ biological filtration becoming common in Quebec excellent combination of disinfection, organics destruction
Ozone known to inactivate Cryptosporidiumcysts UV is usually more cost-effective for this
GAC
to CO2and H2O
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DisinfectionAdvantages and Disadvantages
Disadvantages most complex disinfection technology cannot be employed as secondary disinfectant due to very
short half-life (no residual)
cannot be purchased in bulk and stored until use generation equipment is capital cost expensive secondary disinfectant should be withheld until after
biological filtration to allow AOC to be converted to CO2and water
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Disinfection
Disinfection (Chlorination) By-Products (DBPs)
Chlorine + organic matter DBPs
DBPs are mostly organochlorine (organohalide)compounds
Organic precursors e.g.humic or fulvic acids from soil, decaying vegetation,
algae
Trihalomethanes (THMs) chloroform, bromodichloromethane,dibromochloromethane, bromoform
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Disinfection
Haloacetic Acids (HAAs) Five common HAAs:
Monochloroacetic Acid Dichloroacetic Acid Trichloroacetic Acid
Monobromoacetic Acid Dibromoacetic Acid
THMs and HAAs some used to be suspectedcarcinogens. Current evidence suggests not, but
regulations remain.
Reducing THMs/HAAs likely lowers other DBPs that aresuspected carcinogens
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Disinfection
DBPs regulated in Ontario:
THMs: 100 g/L (soon to be 80 g/L?)
Bromate (BrO3-): 10 g/L
Chlorite (ClO2-): 1.0 mg/L
HAAs: none, but soon to be 60 g/L?
United States:
THMs, bromate, HAAs, chlorite, chlorate
World Health Organization (WHO): Also includes cyanogen halides, aldehydes, several others
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Disinfection
Formation of DBPs is function of: Type of disinfectant
Cl2= THMs, HAAs, other organochlorine compounds O3= bromate, AOC ClO2= chlorite, chlorate
NH2Cl = no THMs, HAAs, so satisfies regulations recent research shows formation of nitrosamines,
nitromethanes, which may be more toxic than THMs/HAAs
Precursor type and concentration
humic acid fraction, fulvic acid, algae, etc. Disinfectant dose pH Contact time
Temperature
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Disinfection
Control of DBPs (Trihalomethanes) Chlorine reacts with natural organic matter (NOM) to form
THMs (NOM measured as total organic carbon TOC)
Alternatives to reduce formation of THMs:
Improve removal of precursors prior to chlorination (orchange location of chlorine addition)
Use alternative disinfectant
Remove DBPs after formation Only if no other alternativeavailable.
- can use activatedcarbon
- aeration in reservoirsfor volatile THMs
(dubious health benefit)
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Disinfection
Improve Removal of Precursors Prior toChlorination
Optimize chemical coagulation possible to reduce THM Formation Potential (THMFP) by
20-70% (typically 30-50%)
Examples: optimize alum dose for TOC removal, not turbidity (may
require increased dose) = enhanced coagulation
Maybe change to optimum coagulant for TOC removale.g.alum ferric chloride
add powdered activated carbon (PAC) to adsorbprecursors (remove PAC in subsequent coag/flocc/filtration)
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Disinfection
Examples (contd): change location of Cl2addition until after sedimentation
and filtration
if possible (e.g.zebra mussel control) But MUST ensure sufficient CT
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Disinfection
Use Alternative Disinfectant to Reduce DBPs Must identify advantages/disadvantages for alternative
disinfectants
Factors to consider:
Potential to form different DBPs (e.g. ClO2-, ClO3-) Disinfection effectiveness Cost Operational issues (more complexity, etc.)
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Disinfection
Alternatives to chlorine include: Chlorine dioxide (forms chlorite, chlorate) Ozone (may form bromate, AOC) Chloramines (only for secondary disinfection)
UV (doesnt form THMs/HAAs)
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Disinfection
Drinking Water Disinfection Rules
General Concepts:
Disinfection requirements are based on sufficientlyinactivating some target pathogen
target should be most resistant pathogen likely to be inwater
Targets Cryptosporidium parvumoocysts Giardia lambliacysts viruses (typically hepatitis A and rotavirus)
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Cryptosporidium Giardia
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Disinfection
General Concepts (contd):
Monitoring for such target pathogens is impractical (tooexpensive, time-consuming)
Instead, monitor disinfectant concentration and contact
time to ensure pathogen control based on inactivationkinetics
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Disinfection
General Concepts (contd):
Its impossible to inactivate all organisms
Chick-Watson Inactivation Kinetics:
N = number of surviving organisms C = disinfectant concentrations T = time
k = inactivation kinetic constant
kCTN
oN=
log
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Disinfection
Lets talk in terms of log inactivation 1-log means 10% are surviving (90% inactivation)
2-log means 1% are surviving (99% inactivation) 3-log means 0.1% are surviving (99.9% inactivation) etc.
Therefore Log inactivation = -kCT Log inactivation directly proportional to CT
kCTNo
N=log
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Disinfection
The value of k is a function of the organism, so use thek for the target organisms
Regulations specify the CT that is required for a minimumlog inactivation needed, based on surveys of ambientconcentrations of pathogens
kCTNo
N=log
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DisinfectionGeneral Concepts (contd):
Common Requirements: 2-log Cryptosporidiumreduction 3-log Giardiareduction 4-log virus reductionNote: reduction = removal + inactivation, where removal is
physical removal via sedimentation/filtration, andinactivation is disinfection
*Read the Ontario Procedure for Disinfection
Tables of CT needed to achieve the above underdifferent conditions (pH, temperature) are given.Engineers design for adequate CT (i.e.dose and contacttime)
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Disinfection
Disinfection must therefore supply:
0.5-log inactivation of Giardia(= 3.0-2.5 log)
2.0-log inactivation of viruses (= 4.0-2.0 log)
Impractical and costly for water treatment plants tomonitor for Giardia and viruses Use tables of CT values corresponding to inactivation
See example following next slide.
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CT values (mg/Lmin) for 90% (1 log) Inactivation ofGiardia
Water TemppH
Free Cl2
0.5 C 5 C 10 C 15 C6789
4970
101146
355072
104
26375478
19283659
Chloramines 1300 730 620 500
Chlorine Dioxide 21 8.4 7.4 6.3Ozone 0.97 0.63 0.48 0.32
6-9
6-9
6-9
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Example:
0.5 mg/L free chlorine
10o
C, pH 7.5
How much contact time is needed to meet Ontariodisinfection requirements in a conventional plant(i.e. coag/flocc/sed/filtration) treating surface water?
Answer: need 3-log Giardiaand 4-log virus reduction.Physical removal gets 2.5/2-log removal credit for Giardiaand viruses, respectively. Need remaining 0.5-log/2-logGiardia/virus inactivation by chlorine.
CT for 0.5-log Giardiainactivation
(0.5 mg/L Cl2, pH 7.5, 10oC)
CT for 2-log virus inactivation
(any Cl2, pH 7.5, 10oC)
22 mgmin/L 3 mgmin/L
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CT for 0.5-log Giardiainactivation
(0.5 mg/L Cl2, pH 7.5, 10oC)
CT for 2-log virus inactivation
(any Cl2, pH 7.5, 10oC)
22 mgmin/L 3 mgmin/L
The Giardia requirement controls (for Cl2, it always does!)
Given: 0.5 mg/L Cl2
Need: CT = 22 mg/L
Therefore contact time = 22/0.5 = 44 minutes
COMPLICATION #1: Chlorine decays. We almost always
use the effluent concentration from the process as C. But, if you know the decay rate, you are allowed to
integrate (see following)
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Disinfection
ChlorineConc.
Time in the tank
Area under the decay curve is CT
Ceffluent
Hydraulicretention time
You get more CT if you consider decayin your CT calculation
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COMPLICATION #2: Not every element of water spends44 minutes in the tank (there is a residence time
distribution).
Solution: use the t10 (time representing the 10thpercentile
fastest water through the process. i.e. 90% of the waterspends more than this time)
t10obtained from tracer tests, computational fluid dynamics,or conservative baffle factors (below).
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Superior baffling (t10maybe 70% hydraulic residence time)
Poor baffling (t10maybe 10% hydraulic residence time)
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So in previous question, if required t = 44 minutes, and
Q = 10 MLD (6.94 m3/min)
Baseline volume required =6.94
44 = 305
3
Assuming baffle factor of 0.7:
Actual tank volume needed = 305 m3 0.7 = 436 m3
COMPLICATION #3: For chlorine (only), CT requirements
are a function of C (see example 11.10).
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Disinfection
Use CT tables to select an appropriate CT value
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Disinfection
assume
2.0
mg/L
initially
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Disinfection
At pH =7, Temperature = 5 oC, assuming C = 2.0 mg/L:
CT for 0.5 log inactivation = 28 mg/Lmin
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Disinfection
Determine t10for the peak hourly flowrate
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Disinfection
90
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Disinfection
Determine t10for the peak hourly flowrate
From Fig. 1.17(b) t10= 90 min @ 3.0 MGD
But we assumed C = 2.0 mg/L, so iterate using C = 0.31 asthe new guess.
mg/L0.31min90
minmg/L28C =
=
f
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Disinfection
assume
0.31
as new
guess
Di i f ti
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Disinfection
t10= 90 min (this doesnt change!)
The new answer (0.25) is close to the previous iteratedanswer (0.31), so we have converged on our solution.
We need 0.25 mg/L of chlorine after 90 minutes to disinfect
the water.
(We then need to figure out what chlorine dose will provide0.25 mg/L after 90 minutes!) How?
mg/L0.25min90
minmg/L23C =
=
Di i f ti
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Disinfection
Groundwater Disinfection for Drinking Water
Primary pathogens of concern are fecal viruses larger pathogens (e.g. Giardiacysts) removed by natural
filtration
In Ontario, there are 2 classes of groundwaters: protected groundwaters groundwaters under the direct influence of surface waters
(GUDI)
GUDI waters are identified using a long list of criteria Proximity to surface sources Evidence of contamination (coliform, turbidity, etc.) Others (see Disinfection Procedure in supplemental notes)
Di i f ti
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Disinfection
GUDI waters are treated as surface waters require 2-log Crypto, 3-log Giardiaand 4-log virus
control
CT concept may be applied to groundwaters for pure groundwaters, must disinfect for 2-log virus
inactivation for GUDI, need same total reductions as for surface
waters (2/3/4-log Crypto/Giardia/viruses)
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End of Disinfection