IslamicUniversityofGazaEnvironmentalEngineeringDepartment
Water Treatment WaterTreatment
EENV4331
Lecture7:Disinfection
Dr.Fahid Rabah
1
7.DisinfectioninwaterTreatment
f f f 7.1DefinitionofDisinfection :
Disinfection is the destruction of pathogenic microorganisms. Disinfectionisthedestructionofpathogenicmicroorganisms.
7.2Disinfectionmethods:
A.Chemicaldisinfection:
Chlorination Chlorination
Ozonation
B.Nonchemicaldisinfection:
Heat
UltraVioletradiation(UV)
2
( )
7.DisinfectioninwaterTreatment
7.3DefinitionMicroorganismsofconcerninclude: g
Microorganismsofconcerninclude:
TypeSize,m
Viruses0.01to0.1
Bacteria0.1to5
Cryptosporidiumoocysts 3to5
Giardia cysts6to10
Protozoan10to25
Indicatororganismsareoftenusedtoassessthepresenceorabsenceof
pathogens
CommonindicatororganismsarecoliformsEcoli
3
7.4Disinfectionbychlorination y
7.4.1IntroductiononChlorination:
Chlorineisthemostwidelyuseddisinfectantbecauseitiseffectiveatlow
concentrations,cheapandformsresidualifappliedinsufficientdosage.
The principal chlorine compounds used in water treatment are: Theprincipalchlorinecompounds usedinwatertreatmentare:
*Chlorine(Cl
2
),
*Sodiumhypochlorite(NaOCl),
*C l i h hl it [C (OCl) ] d *Calciumhypochlorite[Ca(OCl)
2
],and
*chlorinedioxide(ClO
2
).
Chlorine(Cl
2
)canbeusedingas orliquid form. (
2
) g q
TheCl
2
gasisliquefiedbyhighpressure(510atm)totheliquidform.
4
7.4Disinfectionbychlorination y
7.4.2ChemistryofChlorineinwater:
Chlorine gas reacts readily with water to form hypochlorous acid and Chlorinegasreactsreadilywithwatertoformhypochlorous acidand
hydrochloricacid:
Cl
2
+H
2
OHOCl +HCl
Th d d h hl id h di i i ld h hl i i Theproducedhypochlorous acid thendissociatestoyieldhypochloriteion:
HOCl H
+
+OCl
TherelativedistributionofHOCl andOCl
isafunctionofpHandtemperature
see(Fig7.1).BothHOCl andOCl
areexcellentdisinfectantsbutHOCl is
moreeffective.
BothHOCl &OCl
reactwithammoniaifexistsinwatertoproducechloramines: p
NH
3
+HOCl NH
2
Cl +H
2
O(monochloramine)
NH
2
Cl+2HOClNHCl
2
+H
2
O(dichloramine)
NHCl
2
+ 3HOCl NCl
3
+ H
2
O (Trichloramine) NHCl
2
+3HOClNCl
3
+H
2
O(Trichloramine)
Note: Chloraminesaregooddisinfectants
BothHOCl &OCl
reactwith reducingcompoundssuchasFe
+2
, Mn
+2
,NO
2
,
and the chlorine will be reduced to the non effective chlorid ion Cl
5
andthechlorinewillbereducedtothenoneffectivechlorid ionCl .
6
RelativeamountofHOCl andOCl
asafunctionofpHat0
o
and20
o
. Figure7.1
BothHOCl &OCl
reactwith reducingnaturalorganicmatersproducing
trihalomethanes (THMs)including:
*Chloroform(CHCl
3
),
*Bromoform (CHBr
3
),
*bromodichloromethane (CHCl
2
Br), b o od c o o e a e (
2
),
*dibromochloromethane (CHClBr
2
).
The THMs are carcinogenic compounds and TheTHMsarecarcinogeniccompoundsand
theirtotalconcentrationindrinkingwatershouldnotbemorethan0.1mg/l.
THMs are one of the disinfection by products DBPs that should be minimized or THMsareoneofthedisinfectionbyproductsDBPs thatshouldbeminimizedor
removedbeforesupplyingthewatertotheconsumers.
AnotherdangerousDBPs isthehalogenatedaceticacidsHAAs asitmaycause
cancer.
THMs andHAAscanbeminimizedbyremovingtheorganicmatterbefore
disinfection.THMs andHAAs canberemovedfromwaterbyGAC.
7
7.4Disinfectionbychlorination y
7.4.3Breakpointchlorination:
Asillustrated intheprevioussection,chlorinereactswiththesubstancesexisting
inwater.Figure7.2 showsthestagesofthesereactions.
OnFig7.2,Thechlorinedosageispresentedon thexaxisandtheresidual O g , e c o e dosage s p ese ed o e a s a d e es dua
chlorineispresentedontheyaxis.
Whenchlorineisaddeditreactsfirstwiththereducingcompoundssuchas
Fe
+2
Mn
+2
NO
2
and the chlorine will be reduced to the none effective Fe ,Mn ,NO ,andthechlorinewillbereducedtothenoneeffective
chlorideionCl
(fromzerotopointAonthefigure).
WhenaddingmorechlorineitwillreactwithNH
3
toformchloraminesasshown
in the chlorine chemistry ( from point A to B) inthechlorinechemistry(frompointAtoB).
Whenaddingmorechlorinesomechloraminesareoxidizedtonitrogengasand
thechlorineisreducedtothenoneeffectiveCl
ion.(frompointBtoC).
d dd f hl ll d d f l bl hl ( ) Continuedadditionofchlorinewillproducedfreeavailable chlorine (atpointC).
pointCiscalledthebreakpoint.
8
7.4Disinfectionbychlorination y
9
Breakpointchlorination Figure8.2
7.4Disinfectionbychlorination y
Thechlorineaddediscalledthedosage.
The amount used to oxidize the materials existing in water is called the demand Theamountusedtooxidizethematerialsexistinginwateriscalledthedemand.
TheFreeresidual =dosage demand
TheresidualbetweenpointsA toCiscalledcombinedresidual becausethe
hl i i i h f f hl i d hl i chlorineisintheformofchloraminesandchloroorganics.
FrompointCandupafreeresidualchlorinestarttoappear.
inwaterinadditiontothecombinedresidual.
ThefreeChlorineresidualiscomposedofunreactedformsof
chlorineHOCl andOCl
.
Thetotalresidualafterthebreakpoint=free +combined. p
Sincethefreeresidualismuchmoreeffectiveindisinfection,alltheregulations
requireafreeresidualofatleast0.20mg/l atthefarthesttapinthesystem.The
residual chlorine in the produced water is typically 2 5 mg/l just at the outlet residualchlorineintheproducedwateristypically2 5mg/ljustattheoutlet
ofthetreatment.
sincefreeresidual appearsonlyafterthebreakpoint,soweneedtodecidethe
breakpoint dosage Thus the required dosage = breakpoint dosage + free residual
10
breakpointdosage.Thustherequireddosage=breakpointdosage+freeresidual
Example 7.1:
Referring to the Figure, if a dosage of 1.8 mg/L is applied, determine:
(1 ) Theamount freechlorineresidual (2 )Theamount of combinedresidual (1. ) The amount free chlorine residual (2. )The amount of combined residual,
(3. )The amount of total residual, (4.) What is the chlorine demand?
Destruction of
Chloraminesand Presenceof
0.5
chlorine residual
by reducing
compounds
Chloramines and
chloro-organic
compounds
Presence of
chloro-organic
compounds not
destroyed
0.4
g
/
L
)
Formation of chloro-organic
0.3
s
i
d
u
a
l
(
m
g
compounds and chloramines
free residual
0.2
h
l
o
r
i
n
e
r
e
s
Breakpoint
Combinedresidual
0.1
0
C
Combined residual
0 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0.6 0.4 0.2
Chlorine added (mg/L)
From the figure:
1. The of free chlorine residual at a dosage of 1.80 mg/L=0.40 mg/L.
2. The combined residual at the breakpoint =0.21mg/L.
3. The Total residual =0.40+0.21mg/L =0.61 mg/L. g g
4. Chlorine demand = 1.40 mg/L
7.4Disinfectionbychlorination y
7.4.4CTconcept:
h hl ff d d h l ThechlorinationefficiencyisdeterminedusingtheCTvalue.
C=Freeresidualchlorineconcentrationinthechlorinationtank,mg/L
T=contacttimeinthestoragetank,min.
ThechlorinationefficiencyisdeterminedaccordingtothevalueC*T
SeetheUSEPATablefortherequiredvaluesofCTtoachievecertain
valueofmicroorganismsinactivation. g
forexample,aCTvalueof67(min*mg/L)isneededat20
o
CandapH
of7.5andaresidualchlorineof1mg/Ltoachieve3loginactivationof
Giardia cysts In this case T = (67/1) = 67 min Giardia cysts.InthiscaseT (67/1) 67min.
Thedisinfectionisachievedinsidethechlorinationtank.Toincreasethe
contacttimeor(thedetentiontime)inthetank,bafflewallsareusually
used This will increase the CT value available in the tank and achieve the used.ThiswillincreasetheCTvalueavailableinthetankandachievethe
designCTvalue.
13
illustrationoftheinactivationunits
10
nloginactivation
=N
0
/N
r
OrnLogInactivation=logN
0
LogN
r
Where,
n = number of log inactivation n=numberofloginactivation
N
0
=initialconcentrationofmicroorganisminwater,Cells/100ml(before
treatment)
N
r
=Remainingconcentrationofmicroorganisminwater,Cells/100ml(Aafter
treatment)
Example7.2
TheInitialnumberofGardia cycts inwater is=10
6
Cell/100ml.Calculate
thenloginactivationforthegivenremainingconcentrationateach
inactivationlogs.
A ) N = 10
4
A)N
r
=10
nLogInactivation=logN
0
LogN
r
=log10
6
log10
4
=64=2loginactivation
B)N
r
=2.3X10
3
L I i i l N L N nLog Inactivation=logN
0
LogN
r
=log10
6
log(2.3X10
3
)
= 63.362 = 2.638 log inactivation 6 3.362 2.638loginactivation
7.4Disinfectionbychlorination y
7.4.5Predictingchlorinationefficiency:
N
TheefficiencyofchlorinationcanbepredictedusingChicksWatsonformula:
t C k'
0
t
n
e
N
N
=
/ infectant d of ion concentrat C
constant off die k' =
L mg es
100 / (t) time at organisms of number N
dilution of t coefficien n
/ infectant, d of ion concentrat C
=
=
ml Cell
L mg es
i ti t t t
100 / organisms, of number N
100 / (t), time at organisms of number N
0
t
=
=
ml Cell initial
ml Cell
16
min time, contact t =
7.4Disinfectionbychlorination
7.4.5 Predicting chlorination efficiency: 7.4.5Predictingchlorinationefficiency:
Example7.3:
TheconcentrationofGiardia cystsindrinkingwateris10
4
/100ml.Afreechlorine
Residual of 1 2 mg/L has been created in the chlorination tank The detention time in the Residualof1.2mg/Lhasbeencreatedinthechlorinationtank.Thedetentiontimeinthe
chlorinationtankis57min.WhatisthefinalconcentrationofGiardia cysts?Whatisthe
InactivationpercentandtheLog inactivationinthiscase?.Taken=1.0,andk=0.103.
l C ll N
e
100 / 7 10 10 * 07 1 * 10
10 * 07 . 1 e
N
N
3 4
3 684 . 0 57 * 2 . 1 * 10 . 0
0
t
= = =
N
N N
on inactivati Percent
ml Cells N
t
t
% 9 . 99 999 . 0
10
7 . 10 10
100 / 7 . 10 10 * 07 . 1 * 10
4
4
0
3 4
= ~
=
= =
ml Cell N N N
on inactivati Log on inactivati n
N
100 / 9989 7 10 10
97 . 2 7 . 10 log 10 log log
10
4
4
0
= = =
= =
ml Cell N N
ml Cell N N N
on inactivati n
removed
t removed
100 / 9989
10
1
1 10
10
1
1
100 / 9989 7 . 10 10
97 . 2
4
log
0
0
=
|
.
|
\
|
=
|
.
|
\
|
=
= = =
17
. \ . \
