CEE 371 Lecture #14 11/21/2009 Lecture #14 Dave Reckhow 1 CEE 371 Water and Wastewater Updated: 21 November 2009 Print version Water and Wastewater Systems Lecture #14 David Reckhow CEE 371 L#14 1 Drinking Water Treatment : Chlorination Reading : Chapter 7, pp.233-238, 259-262 Forms of Chlorine applied to water Chlorine gas Cl 2 Sodium Hypochlorite liquid (Hypo) NaOCl Calcium Hypochlorite solid Traditional method Becoming more common Ca(OCl) 2 Other forms Organic-N based compounds and resins David Reckhow CEE 371 L#14 2
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CEE 371 Lecture #14 11/21/2009
Lecture #14 Dave Reckhow 1
CEE 371Water and Wastewater
Updated: 21 November 2009
Print version
Water and Wastewater Systems
Lecture #14
David Reckhow CEE 371 L#14 1
Drinking Water Treatment: ChlorinationReading: Chapter 7, pp.233-238, 259-262
Forms of Chlorine applied to waterChlorine gasg
Cl2
Sodium Hypochlorite liquid (Hypo) NaOCl
Calcium Hypochlorite solid
Traditional method
Becoming more common
Ca(OCl)2
Other formsOrganic-N based compounds and resins
David Reckhow CEE 371 L#14 2
CEE 371 Lecture #14 11/21/2009
Lecture #14 Dave Reckhow 2
Chlorine Cont.The hypochlorous acid
HOCl H + OCl+ -↔
on o
f tot
al (α
)
0 4
0.5
0.6
0.7
0.8
0.9
1.0
α0=HOCl/CT α1=OCl-/CT
The hypochlorous acid ionizes to hypochlorite.
Although both hypochlorous acid and hypochlorite are disinfectants, hypochlorous acid is much more powerful
pH4 5 6 7 8 9 10 11
Frac
tio
0.0
0.1
0.2
0.3
0.4
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acid is much more powerful. The equilibrium reaction is:
][]][[101016.3 5.78
HOClOClHxKa
−+−− ===
QuestionAt pH 8.5, the percent of the total free p , pchlorine that is in the most effective form is:A. 0%B. 9%C. 27%D. 50%E. 73%F. 91%G. 100%
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CEE 371 Lecture #14 11/21/2009
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Chlorine reacts quickly with substances in water so
Chlorine demand Iq y
that the effective residual is always less than the doseChlorine residual = chlorine dose – chlorine demand
The effective concentration; this is This is what you
Chlorine demand is usually measured for a particular water and it may depend on the contact time and dose
It may be estimated from known water quality
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concentration; this is the “C” in “Ct” add to the water
+=0.3 mg/LWhat is the chlorine demand?A 0 3 mg/LA. 0.3 mg/LB. 0.9 mg/LC. 3.7 mg/LD. 4.2 mg/LE. 5.5 mg/L
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Cl2 gas: larger installations1 ton cylindersy
With small (150 lb) vertical tanks in background
Requires separate sealed room orsealed room or bldg.
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Avon, CO
CEE 371 Lecture #14 11/21/2009
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Commercial Chlorinatorfeed
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Fig 7-18; pg. 236 in H&H
Large chlorinatorchlorinator
Vacuum created at aspirator injectorinjector
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Figure courtesy of Wallace & Tiernan Co.
CEE 371 Lecture #14 11/21/2009
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Dose controlFlow pacingp g
Small home system
F ll l
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Fig 7-19; pg. 237 in H&H
Full-scale municipal system
Dose controlFeed back system
Adjusts for varying chlorine demand
David Reckhow CEE 371 L#14 14
Fig 7-19; pg. 237 in H&H
CEE 371 Lecture #14 11/21/2009
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Hypochlorite Dosingdafd
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Chlorine tanksLeft side is currently feedingRight side is on reserve
CEE 371 Lecture #14 11/21/2009
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Chlorine feeding systemg y
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Dose adjustment knob
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Chloramines I Inorganic chloramines are formed by the reaction of free chlorine with ammonia The reaction is stepwise giving monochloraminewith ammonia. The reaction is stepwise, giving monochloramine (equation 1) followed by dichloramine (equation 2). The dichloramine is quite unstable, forming nitrogen gas (equation 3) and some nitrate. This decomposition is responsible for the classic breakpoint chlorination phenomenon.
NH3 + HOCl --------> NH2Cl + H2O (1)
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3 2 2 ( )
NH2Cl + HOCl --------> NHCl2 + H2O (2)
NH2Cl + NHCl2 --------> 3H+ + 3Cl- + N2 (3)
Chloramines IIAdvantagesg
Less reactive so that it persists longer in distribution systemsMay be better at penetrating biofilms on pipe wallsForms smaller amounts of disinfection byproducts (DBPs)
DisadvantagesWeak disinfectant; appropriate as a secondary disinfectant onlyMay lead to tastes & odorsMay stimulate nitrification
Can be isolated to service one while keeping the other in service
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in service
NaOCl added just prior to entry
CEE 371 Lecture #14 11/21/2009
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Raw Water
Reservoirs &Reservoirs &Transmission Mains
Clearwell
9 Sept 06
CEE 371 Lecture #14 11/21/2009
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Clearwell
9 Sept 06
Clearwell
9 Sept 06
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Clearwell
9 Sept 06
Clearwell
29 Sept 06
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ClearwellFrom the plant site
29 Sept 06
ClearwellDropping a panel pp g pinto position
29 Sept 06
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Clearwell
29 Sept 06
Northampton Ground Storage
Finished water storage at plantFinished water storage at plantKnow as a “Clearwell”
View from Outer ringUnder construction
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ClearwellExit portalp
29 Sept 06
ClearwellIn outer ring, looking SW?g, g
29 Sept 06
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Clearwell or Ground StorageMulti-purposep p
Chlorine contact tank for achieving “Ct”Giardia controls
3 log Giardia is more restrictive than 4 log virus when using chlorine2.5 log credit given for Giardia (clarification + filtration), leaving 0.5 log for CtNorthampton has decided to see 1.0 log for CtNorthampton has decided to see 1.0 log for Ct
Buffering system flowsFire FlowBackwash Storage
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Evaluation of Clearwell (cont.)Ct requirements Design conditions
Design is for maintenance of Ct even when one of the two concentric tanks is taken out of service“t” is normally evaluated for peak hourly flow