Disinfection Notes

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.


Rxns with easily oxidizablesu bstances

A B


C

D

E



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.)



A B


C

D

E



reakpoint

FreeResidual

Combined Residual

1

1


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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



A B


C

D

E



reakpoint

FreeResidual

Combined Residual

1

1


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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



A B


C

D

E



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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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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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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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

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Disinfection Notes