13 Disinfection F11

7/27/2019 13 Disinfection F11

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13-1 Disinfection_F11

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DISINFECTION (3rd DC 240; 4th DC 294)

Definition

Disinfection - Killing or inactivating pathogens- To destroy organisms that cause diseases in humans and domestic animals.

Pathogens: Disease-producing microorganisms

Sterilization: destruction of all living organisms- Drinking water need not be sterile

Purpose of Disinfection

- To protect public health by preventing the spread of water born diseases bydisinfecting drinking and wastewater to an acceptable level.

Three categories of human enteric pathogens:1) Bacteria2) Viruses3) Protozoa (amebic cysts)

- Purposeful disinfection must be capable of destroying all three.

Diseases and Pathogens (Examples)

Water Born Disease Pathogens Note

Typhoid Salmonella typhi BacteriaBacillary dysentery Shigella dysenteriae BacteriaCholera Vibrio cholera BacteriaGiardiasis Giardia lambia ProtozoaCryptosporidiosis Cryptosporidium parvumis ProtozoaAmoebiasis Naglaria ProtozoaAdenovirus infection Adenovirus VirusGastroenteritis Astrovirus VirusGastroenteritis Parvovirus Virus

Removal of Pathogenic Microorganisms by Water Treatment Processes

- Removal of pathogenic microorganisms and other microorganisms in conventional water treatment processes include:

a. Coagulation/flocculationb. Sedimentationc. Filtrationd. Inactivation by chemicals, (e.g., lime, soda ash)e. Natural die-away of organisms in an unfavorable environment


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Disinfection by chemical disinfectants

Considerations as a water disinfectant

1) Effectiveness

- ability to kill the types and numbers of organisms within reasonable (practicable):a) contact time

b) temperature (over an expected range)c) water quality variations

2) Availability

- constant and readily available supply of disinfectant at reasonable cost and in form conveniently, safely and accurately applied.

a) reasonable cost ($)

b) readily, safely availablec) constant supplyd) convenient form

3) Toxicity

- cannot render the water toxic or objectionable (aesthetically or otherwise) for human consumption

a) non-toxic to humans and domestic animalsb) palatable, non-objectionable

4) Handling

a) convenient, safe, and easy to store, transport, and handle.b) conveniently, safely, easy, and accurately apply

5) Persistency (maintaining killing power)

a) persist in residual concentrations as safeguard against recontamination

6) Assay technique (for process control)

a) practical, easy, reproducible

b) reasonable costc) rapid and accurate


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Disinfectants

1) Chlorine, Cl2C hypochlorous acid, HOClC hypochlorite, OCl-

2) ChloramineC monochloramine, NH2ClC dichloramine, NHCl2

3) Chlorine dioxide, ClO24) Ozone, O35) Others

C Bromine (Br), Iodine (I2), Iodide (I-)

Chemistry of Chlorination

Chlorinea. Chlorine gas is soluble in water

solubility = 7160 mg/L at 20C, 1 atm

b. Chlorine reacts with:1) Water2) Ammonia and3) Other compounds:

a) Organic nitrogen

b) Organic compounds (phenol, humic substances)c) Nitritesd) Iron and manganesee) Hydrogen sulfide and cyanide.

Basic reactions with water

Formation of Hypochlorous acid, HOCl

- Chlorine gas hydrolizes in water rapidly, almost completely, to form hypochlorous acid:- Hypochlorous acid is a weak acid and dissociate poorly at pH< 6.

Cl2(g) + H2O HOCl + H+ + Cl- (1)

{HOCl} {H+} {Cl-}Keq = ------------------------ = 4 x 10

-14 (2){Cl2} {H2O}

{ } = [ ]{ } = activity[ ] = M = activity coefficient


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{HOCl} 4 x 10-14---------- = --------------{Cl2} {H

+} {Cl}

{HOCl}/{Cl2} increases, as {H+} decreases

Note: pH = - log {H+}

Formation of Hypochlorite ion, OCl

- Hypochlorous acid ionizes to (dissociates into) hypochlorite ion and hydrogen ions inthe reversible reaction:

HOCl H+ + OCl (3)

{H+} {OCl}

Ka = ----------------- = 2.7 x 10-8 = 107.5 (4){HOCl}

{OCl} 2.7 x 108---------- = -----------------{HOCl} {H+}

{OCl } / {HOCl} decreases, as {H+} increases,

- HOCl is approximately 80100 times as effective as OCl.- Chlorine exists predominantly as HOCl at pH 4 - 6.

Chloramines- Chloramine can be formed by first adding a small quantity of ammonia to the water, then adding chlorine.

1. Reaction with ammonia- Ammonium ion exists in equilibrium with ammonia and hydrogen ion

NH4+ NH3 + H

+

2. Chlorine reacts with ammonia in water to form chloramines

NH3 + HOCl NH2Cl + H2O (3)monochloramine

NH2Cl + HOCl NHCl2 + H2O (4)dichloramine

NHCl2 + HOCl NCl3 + H2O (5)trichloramine(nitrogen trichloride)


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3. Chlorine Residuals and Chlorine Demand

a. Free available chlorine residual (FAR): HOCl, OCl-

- Residual chlorine existing in water as hypochlorous acid or hypochlorite ion.- Hypochlorous acid is primary disinfectant, hypochlorite ion is somewhat less effective. Thus the power of free chlorine residual

decreases with increasing pH.

b. Combined available chlorine residual (CAR): NH2Cl, NHCl2, NCl3, organic N-Cl

- Residual existing in chemical combination with ammonia (chloramine) or organic nitrogen compounds.- Effectiveness of CAR is significantly less than that of FAR.

c. Total Available Chlorine Residual (TAR): TAR = FAR + CAR

d. Chlorine demand- is the amount of chlorine that reacts with inorganics (Fe2+, Mn2+, NO2

-, and NO3-) and

organic impurities.

Chlorine Demand = (Chlorine added to water) - (TAR)

- This amount must be satisfied before free available chlorine is formed.- Difference between the amount of Cl2 added to a water and the quantity of free and combined available chlorine remaining at the

end of a specified contact period.

Example(Sato): Chlorine added to water (chlorine dosage) is 7 mg/L, free chlorineresidual is 0.1 mg/L, monochloramine is 0.3 mg/L, and dichloramine is 0.2 mg/L.

1) What is total available chlorine residual (TAR)?2) What is the chlorine demand?

Solution

Total Available Chlorine Residual (TAR) = FAR + CAR = 0.1 mg/L + (0.3 + 0.2) mg/L= 0.6 mg/L

Chlorine Demand = (Chlorine added to water) - (TAR)

= 7 mg/L 0.6 mg/L = 6.4 mg/L


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Removal of microbes (or spores) in a treatment plant

a) Log removal (LR) in a plant is given by

Influent concentrationsLR = log (----------------------------------)

Effluent concentrations

b) Percent (%) removal in a treatment plant is

100% removal = 100 - ----------

10LR

Example 4-29 (4th DC 297)

A city measured the concentration of aerobic spores in its raw and finished water asan indicator of plant performance. Spores are often plentiful in water supplies and areconservative indicators of how well a plant is able to remove Cryptosporidium. The citydata are as follows:

(Solution)

1) Calculate weekly average:

Day of Week Raw Finished

Sunday 200,000 16

Monday 145,000 4

Tuesday 170,000 2Wednesday 150,000 8

Thursday 170,000 10Friday 180,000 2

Saturday 180,000 3

(Spores/L)

Day of Week Raw Finished

Sunday 200,000 16Monday 145,000 4

Tuesday 170,000 2

Wednesday 150,000 8

Thursday 170,000 10

Friday 180,000 2

Saturday 180,000 3

AVERAGE 170,714 6.43

(Spores/L)


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170,714Log Removal (LR) = log (--------------- ) = 4.42

6.43100

% Removal = 100 - --------- = 99.996104.42

Example(Sato): What is the equivalent percent reduction for a 3 log reduction of E. coli?

(Solution)100

% removal = 100 - --------10LR

100% removal = 100 - -------- = 99.90 %

103

Example(Sato): What is the log reduction of E. colithat is equivalent to 99.90% reduction?

(Solution)100

% removal = 100 - --------- = 99.90 %10LR

Solve for LR, LR = 3

There is a 3 log inactivation of E. coli.


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Disinfection Kinetics (4th DC 295)

- Under ideal conditions, when an exposed microorganism contains a single site vulnerable to a single unit of disinfectant, the rate ofdie-off follows Chick's law.

Chicks law (Chick, 1908)

- the number of organisms destroyed in a unit time is proportional to the number oforganisms remaining (First-order reaction).

=

N

tN

dN

dtkN

where N = the number of organismsk = rate constant, time-1 (1st order rate constant)

Factors affecting the kill

1) Form of chlorine2) pH3) Concentration4) Contact time5) Type of organism6) Temperature

- Chlorine concentration and contact time relationship can be expressed by:

Cn tp = constant

whereC = concentration of chlorine, mg/L

n = constant for a given system (experimentally derived), dependent on type ofdisinfectant species being used, related to its power to kill.

tp = time required for given percent kill, min.

constant = constant for a given system (experimentally derived), which is specific for typeof microorganisms being killed.For example, Polio melitus would have a much higher constant value than E. coli.


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Water Disinfection (Chlorine Disinfection)(3

rdDC 243; 4

thDC 296)

- Microorganisms kill by disinfectants appears to follow the CT concept.

The CT concept

- The product of disinfectant concentration (C) and contact time (T) yields a constant.

- widely used in the Surface Water Treatment Rule (SWTR) as a criteria for cyst and virusdisinfection.

Empirical expression:

CT = 0.9847 C0.1758 pH 2.7519 temp -0.1467 (114) (3rd DC 243; 4th DC 296)

where

C = disinfectant concentration (mg/L)T = contact time between the microorganism and the disinfectant (min)pH = - log {H+}temp = temperature, C

- The relationship in Eqn 114 means that the combination of concentration and time (CT) required to produce a 3-log reduction inGiardia cysts by free chlorine can be estimated if the free chlorine concentration, pH, and water temperature are known.

Table 3-20 (3rd DC 244); Table 4-20 (4th DC 298)

Table 3-20 (3rd DC 244) or Table 4-20 (4th DC 244) presents the required CTvalues (in mg/L min) for inactivation of Giardiacysts by free chlorine at 10C.

- If the free chlorine concentration, pH, and water temperature are known, the combination of concentration and time (CT) required toproduce a 3-log reduction in Giardiacysts by free chlorine can be estimated.

EPA used empirical data and a safety factor to develop the table. Therefore, thenumbers in the table do not exactly match Eq. 114.

Generally,

for a conventional coagulation plant, 0.5-log inactivation of Giardiais required for an untreated surface water, 3-log inactivation is required


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Example : Determine the re

Given:Water treatment for thAt Peak hourly flow ratContact time = 50 min,------------------------------

Approach

C Use the CT (concentratioC

Apply Surface Water Tre

C For disinfection of Ga) 3 log inactiva

b) 2.5 log inactivatio

C The chlorine dosage

Steps

1) Determine log inactivation

3 - 2.5 = 0.5 log inactiv2) Determine contact time

At a peak hourly flow rcontact time T = 50 mi

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uired residual chlorine concentration?

disinfection of Giardia, Q = 5.0 MGD,

pH = 7, temperature = 10C.--------------------------

time) concept:

tment Rule (SWTR):

ardia,ion required (plant efficiency).n is allowed for treatment prior to disinfecti

must not exceed 1.8 mg/L to minimize TH

equired by disinfection

ation

te = 5 MGD


n.

formation.


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3) Determine CT from Table

Noting that the chlorine dosa

CT = 20 (mg/L)(min) at pH =

4) Determine chlorine dosageC = CT / T = 20 (mg/L)

The residual chlorine concent

Breakpoint chlorination

Definition:- The application of chlorine t

- Chlorination of a water to the extent that

Breakpoint chlorination is req- to obtain a free chlorine res

supply)

Breakpoint chlorination Curve

12

20 (4th DC 298)

e must not exceed 1.8 mg/L to minimize T

7

(min) / (50 min) = 0.40 mg/L

ration needed is 0.40 mg/L.

water to the point where free residual chlo

all the ammonia is converted to N2 (or a higher oxidation state).

iredidual for better disinfection (if ammonia is p

(3rd DC247; 4th DC 302)

- When chlorine is added to water containing rammonia, you yield a curve similar to Figure4-47 (4th DC 302), or Fig. 11.16 : VH, p. 400)

-


M formation,

rine is available.

esent in a water

ducing agents and1-13 (3rd DC, p.247), Fig.

.


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Zone I, A-B:a. Chlorine reacts first with reducing agents present and develops no measurable

residual.b. Reducing agents (those common to water and wastewater include nitrites, ferrous

ions, hydrogen sulfide, cyanide, manganese ions)

c. The chlorine dosage at B is the amount required to meet the demand exerted by thereducing agents.

Zone II, B-C:a. The addition of chlorine in excess of that required up to point C results in theformation of chloramines.

b. Chloramines established show an available chlorine residual and are effective asdisinfectants.

c. When all the ammonia has been reacted with, a free available chlorine residualbegins to develop (point C on the curve).

Zone III, C-D:a. As the free available chlorine residual increases, the previously produced

chloramines are oxidized.b. This results in the creation of oxidized nitrogen compounds (e.g., nitrous oxide,

nitrogen, nitrogen trichloride), which in turn reduce the chlorine residual.

c. At D (Break Point)- Once most of the chloramines are oxidized, additional chlorine applied to the water

creates an equal residual (as indicated by the rising curve at point D)

Zone IV, D-E:a. Beyond the breakpoint, all added residual is free available chlorine.- some resistant chloramine can still be present beyond D, but their relative importance

is small.


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Chemical Reactions of Chlorine in Natural Waters

=================================================================Rxn No. Reaction-------------------------------------------------------------------------------------------------------------------Zone I, A - B:

2Fe(HCO3)2 + Ca(HCO3-)2 + Cl2 2Fe(OH)3 + CaCl2 + 6CO2

MnSO4 + 4NaOH + Cl2 MnO2 + 2Na2SO4 + 2NaCl + 2H2O

H2S + Cl2 S + 2H+ + 2Cl -

Zone II, B - C:

NH3 + HOCl NH2Cl + H2O monochloramine

Zone III, C - D:

NH2Cl + HOCl NHCl2 + H2O dichloramine

NH2Cl + NHCl2 N2(g) + 3H+ + 3Cl - N removal

NHCl2 + 2HOCl + H2O NO3- + 5H + + 3Cl - nitrate formation

Zone IV, D - E:

Cl 2(g) + H2O HOCl + H+ + Cl - hypochlorous acid

HOCl + H2O OCl- + H3O

+ hypochlorite

NHCl2 + HOCl NCl3 + H2O N-trichloride

NH2Cl + NH2Cl NH3 + NHCl2 ammonia formation-----------------------------------------------------------

Reference:Saunier, B.M. and Selleck, R.E. Kinetics of Breakpoint Chlorination in Continuous flow systems. Paper presented at 96th Annual

Conference AWWA, New Orleans, LO (June, 1976).


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Chloraminated Potable Water Storage Tanks(Opflow, p.14-16, December 2008)

Problem: Ammonia-Oxidizing Bacteria (AOB)

- Trouble arises when AOB converts free ammonia to nitrite and then to nitrate.

- When chloraminated water reaches 15C (59F), AOB start growing rapidly.

- AOB consume nearly all of the free ammonia in the tank, causing an acceleration ofchloramines into chlorine and more free ammonia (by auto-decomposition).

NH3 + 2 O2 NO3- + H+ + H2O

NH3 + HOCl NH2Cl + H2O

- As the AOB continue to consume free ammonia, most of the residual chloramine inthe tank can be destroyed in just a few days.

Solution:

1) Fast response- Interrupting the rapid formation of nitrite and corresponding chloramine loss as soon aspossible

- Twice-weekly water testing when water temperature reach 15C or higher (on totalchlorine, free chlorine, ammonia, nitrite, nitrate, pH, DO).

2) Boost the chlorine dosage

- e.g., to boost 1 Mgal of water by 0.5 mg/L of chlorine requires 3.3 gal of 12.5 % sodium hypochlorite

3) Adequate mixing throughout the tank


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Trihalomethanes (THMs)- Disinfection Byproducts (DBP)

Common THMs:CHCl3 chloroformCHBr3 bromoform

CHCl2Br bromodichloromethaneCHClBr2 dibromochlormethane

Formation and Control of Trihalomethanes

Formation- THMs are formed by the reaction between chlorine and dissolved organics (humic

substances) during the chlorination of surface waters.

- Chlorine reacts with humic substances commonly found in raw surface waters to forms trihalomethanes (principally,chloroform and bromodichloromethane).

Precursor + HOCl THMstime*

* reaction time (detention time)

PrecursorsDissolved organics; e.g., humic acids, fulvic acids

- Decaying vegetation produces the humic substances, i.e., humic and fulvic acids, referred to as precursors.

Water Quality Standard (MCL)

Total trihalomethanes (THMs), 0.1 mg/L (interim) (Source: Safe Drinking Water Act of 1996)

2. Control of THM Formation (Remedial Actions)- Alternatives for reducing the production of THMs

1) Change the point of chlorine application

- Moving the point of chlorine application to later stages in water treatment (theeasiest method for reducing THM formation)

- Not before coagulation process- Breakpoint chlorination prior to chemical coagulation is no longer common practice (bacause of trihalomethane formation.)

2) Improve the removal of precursors prior to chlorination, by aeration, adsorption usingactivated carbon.

- If necessary, activated carbon can be applied during early treatment stages to adsorb humic substances, in addition, improvedcoagulation can be implemented to enhance the removal of organic substances.

3) Use an alternative disinfectant, e.g., UV, O3


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Wastewater Disinfection(C

Plot of ( )t to

Nvs. 1+0.23 C t

N

where No = number of coliforN t = number of coliforC t = total (amperometrt = residence time, min

( )( )

-6log10 - log1 6 0

log100 - log1 2 0slope

= =

Therefore,

( )

3

log 3 log(1 0.23 )

(1 0.23 )

tt

o

tt

o

NC t

N

NC t

N

= +

= +

- The reductions of coliform o

a chlorinated primary treatedbe expressed by the relations

N

NC tt

ot= +

( . )1 0 23

3

whereNo = number of coliform orN t = number of coliform orC t = total (amperometric) c

t = residence time, min

18

lorine Disinfection)

organisms at time t = oorganisms at time t

ic) chlorine residual at time t, mg/L

3

=

ganisms in

effluent canhip:

anisms at time t = oanisms at time thlorine residual at time t, mg/L



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Example: What is the required total chlorine residual (mg/L)?

Given:No = 10,000 org/100 mLN t = 200 org/100 mL

t = 15 min

(Solution)

N

NC tt

ot= +

( . )1 0 23

3

solve for Ct

C

N

N

t

mg Lt

t

o=

=

=

1 3 1 3

1

0 23

200

10 0001

0 23 15

0 78

/ /

.

,

. ( )

. /

Pocatello WWTP Chlorine contact basin---------------------------------------------------------------------------Chlorine dose 8 mg/LChlorine demand 5 mg/LAt the end of the chlorine contact basin 1 mg/LDischarge to the Portneuf River

after dechlorination using SO2


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Calculating Chlorine Dosage

1. The chlorine dosage at a water treatment plant is 2.0 mg/L. The flow rate at the plant is700,000 gpd. How many pounds per day of chlorine are required to disinfect the plantswater supply?

Conversion formula is: (mg/L)(MGD)(8.34) = lb/d

(2.0 mg/L)(0.7 MGD)(8.34) = 11.68 lb/d

Thus, 11.68 lb/d of chlorine required.

2. Disinfection at a treatment plant requires 280 lb/d of chlorine. If calcium hypochlorite(65 percent available chlorine) is used, how many pounds per day of calcium hypochloritewill be required?

(0.65)(X lb/d) = 280 lb/d

280 lb/dX lb/d = ------------- = 430.77 lb/d

0.65

Thus, 430.77 lb/d of calcium hypochlorite required.

3. A water supply requires 30 lb/d of chlorine for disinfection. If sodium hypochlorite with10 percent available chlorine is used, how many pounds per day of sodium hypochlorite

are required?

(0.1)(X lb/d) = 30 lb/d

30 lb/dX lb/d = ------------- = 300 lb/d

0.1

Thus, 300 lb/d of sodium hypochlorite is required.


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Dechlorination

a. Dechlorination is required to detoxify a discharge after chlorination.b. Usually accomplished by adding sulfur dioxide, SO2 in aqueous solution

- added at the discharge end of the chlorination chamber.- the oxidation-reduction reaction between the monochloramine residual and sulfur dioxide is very rapid.

For free chlorine residual

SO 2(g) + H2O HSO3- + H+

HOCl + HSO3- Cl- + SO4

2- + 2H +------------------------------------------------------------------------

SO2 + HOCl + H2O Cl- + SO4

2- + 3H+

For monochloramine

SO2(g + H2O HSO3- + H +

NH2Cl + HSO3- + H2O Cl

- + SO42- + NH4

+ + H +-----------------------------------------------------------------------------------

NH2Cl + SO2(g) + 2H2O NH4+ + SO4

2- + Cl- + 2H+

- Any excess sulfur dioxide applied reduces dissolved oxygen as:

2SO2 + O2 + 2H2O 2SO42- + 4 H+

c. Other dechlorination compounds:C sulfite, sodium metabisulfite


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Ultraviolet Radiation (4th DC 303)

Source: http://www.wedeco.com/us/index.php?id=91294

Source: http://www.wedeco.com/us/index.php?id=91294

Table 4-22 (4th DC 304): Spectral ranges of interest in photochemistry

UVA: causes changes to the skin that lead to tanningUVB: can cause skin burning and is prone to induce skin cancerUVC: is extremely dangerous since it is absorbed by proteins and can lead to cell

mutations or cell death

Range name Wavelength range (nm)

Near infrared 700 - 1,000Visible 400 - 700

UltravioletUVA 315 - 400UVB 280 - 315

UVC 200 - 280

Vacuum ultraviolet (VUV) 100 - 200


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Note: Ozone absorbs UV radiation in the 220-330 nm region. UV light is most strongly absorbed by DNA at 253.7 nm (primarily between 200 nm

and 300 nm). Low-pressure Hg arcs at 253.7 nm, disinfection UV has been found to be very effective for the disinfection of Cryptosporidium,

Giardia, and viruses. U.S. EPA has established UV dose requirements

See Table 4-23 (4th DC 305) shows the U.S. EPA requirements.

UV dose

- The inactivation of microorganisms by UV is directly related to UV dose.

The average UV dose is calculated by:

D = I t

where D = UV doseI = average intensity, mW/cm2t = average exposure time, s

Satos experimental unit


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Survival Fraction (similar to Log removal, LR)

The survival fraction is calculated by:

Survival fraction = log (N / No)

where N = organism concentration after inactivationNo = organism concentration before inactivation

The major factor affecting the performance of UV disinfection systems is influentwater quality:

1) Particles, turbidity, and suspended solids

- can shield pathogens from UV light or scatter UV light to prevent it from reaching the targetmicroorganisms, thus reducing its effectiveness as a disinfectant.

2) Organic and inorganic compounds

Some organic compounds and inorganic compounds (such as iron and permanganate) canreduce UV transmittance by absorbing UV energy, requiring higher levels of UV to achieve thesame dose.

Thus, it is recommended that UV systems be installed downstream of the filters.


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UV application to Combined Sewer Overflow (CSOs)

Source: Muller et al. Water Environment & Technology, p. 36-39 (2011)


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Ozone (Journal AWWA, Vol. 100, No. 11, p.100-111 (2008).

Ozone (O3)

- is a highly reactive (unstable, strong oxidant), odorous, colorless gas.

- must be generated onsite via air or oxygen electrolytic processes.

- is used as a disinfectant to kill bacteria, viruses, and protozoa (including GiardiaandCryptosporidium) .

- is also used to control manganese, iron, and naturally occurring taste-and-ordersubstances.

Ozonation:

does not form THMs (or HAASs, Haloacetic acids)

can form bromate if source waters contain bromide.o Bromate is regulated at 10 ug/L (due to cancer risk)o http://www.epa.gov/enviro/html/icr/gloss_dbp.html

http://water.epa.gov/drink/contaminants/basicinformation/disinfectionbyproducts.cfm

Ozone reacts with natural organics in source waters, converting natural organics to smaller assimilable organic carbon (AOC) molecules

that can be bacterial nutrients.

Thus the systems using ozone often include biologically active filters (BAFs) orother biologically active processes to reduce AOC levels prior to entering thedistribution system in order to prevent biological re-growth in the distribution system.

Example of O3 treatment - Murano Water Treatment Plant, Osaka, Japan

- Chlorine is added after ozonation

13 Disinfection F11

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