CEE 371 Water and Wastewater Systems · asdfasdf David Reckhow CEE 371 L#14 40. CEE 371 Lecture #14...

CEE 371 Lecture #14 11/21/2009

Lecture #14 Dave Reckhow 1

CEE 371Water and Wastewater

Updated: 21 November 2009

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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


CEE 371 Lecture #14 11/21/2009


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


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%


CEE 371 Lecture #14 11/21/2009


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


concentration; this is the “C” in “Ct” add to the water

Chlorine Demand IISummation of at least 3 typesyp

Chlorine demand = inorganic demand + fast organic demand + slow organic demand

Inorganic demand is based on known stoichiometry – see next slidestoichiometry see next slideBoth types of organic demand are highly variable

As a first approximate you can use:Fast + slow organic demand = 0.25*TOC

Where all concentrations are in mg/LDavid Reckhow CEE 371 L#14 6

CEE 371 Lecture #14 11/21/2009


Inorganic Chlorine Demand IMajor inorganic ions responsible & reactionsj g p

Manganese: 1M/M

Iron: 0.5M/M

Sulfide: 4M/M when sulfate is product

+−+ ++→++ HClMnOOHMnHOCl 3222

−−++ ++→+ OHClFeFeHOCl 21

2132

21

Ammonia: 1.5 M/M, when nitrogen gas is produced


+−−− ++→+ HClSOHSHOCl 544 24

+−+ +++→+ HOHClNNHHOCl 21

221

21

221

421 2111

Inorganic Chlorine Demand IIMolar Calculation

Where all concentrations are in moles/liter

Mass-based Calculation

][5.1][4][5.0][][ 422 +−++ +++=− NHHSFeMnDemandInorganic

)(67)(88)(630)(31 22 +−++ +++ NHHSFeMnDemandInorganic

Where all concentrations are in mg/L


)(6.7)(8.8)(63.0)(3.1 4+++=− NHHSFeMnDemandInorganic

CEE 371 Lecture #14 11/21/2009


QuestionYou have a raw water with the following gcharacteristics

TOC = 2 mg/L, Fe+2=0.5 mg/L, Mn+2=0.2 mg/L, HS-=0.1 mg/L, NH4

+=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


Cl2 gas: larger installations1 ton cylindersy

With small (150 lb) vertical tanks in background

Requires separate sealed room orsealed room or bldg.


Avon, CO

CEE 371 Lecture #14 11/21/2009


Commercial Chlorinatorfeed


Fig 7-18; pg. 236 in H&H

Large chlorinatorchlorinator

Vacuum created at aspirator injectorinjector


Figure courtesy of Wallace & Tiernan Co.

CEE 371 Lecture #14 11/21/2009


Dose controlFlow pacingp g

Small home system

F ll l


Fig 7-19; pg. 237 in H&H

Full-scale municipal system

Dose controlFeed back system

Adjusts for varying chlorine demand


Fig 7-19; pg. 237 in H&H

CEE 371 Lecture #14 11/21/2009


Hypochlorite Dosingdafd


Chlorine tanksLeft side is currently feedingRight side is on reserve

CEE 371 Lecture #14 11/21/2009


Chlorine feeding systemg y

CEE 371 Lecture #14 11/21/2009


Dose adjustment knob

CEE 371 Lecture #14 11/21/2009


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)


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


CEE 371 Lecture #14 11/21/2009


Chloramines: Breakpoint CurveMCA = monochloramineDCA = dichloramine

The Breakpoint

Combined Free

DCA = dichloramineFRC = free residual chlorineNmg

Clmg−− 26.7

NmgClmg−− 24


MCA DCA FRC

Chlorine Dose

Compare with Fig 7-17 in H&H, pg. 234

Northampton’s Ground Storage4.0 MGTwo Concentric cells

Can be isolated to service one while keeping the other in service


in service

NaOCl added just prior to entry

CEE 371 Lecture #14 11/21/2009


Raw Water

Reservoirs &Reservoirs &Transmission Mains

Clearwell

9 Sept 06

CEE 371 Lecture #14 11/21/2009


Clearwell

9 Sept 06

Clearwell

9 Sept 06

CEE 371 Lecture #14 11/21/2009


Clearwell

9 Sept 06

Clearwell

29 Sept 06

CEE 371 Lecture #14 11/21/2009


ClearwellFrom the plant site

29 Sept 06

ClearwellDropping a panel pp g pinto position

29 Sept 06

CEE 371 Lecture #14 11/21/2009


Clearwell

29 Sept 06

Northampton Ground Storage

Finished water storage at plantFinished water storage at plantKnow as a “Clearwell”

View from Outer ringUnder construction

David Reckhow CEE 371 L#9 3429 Sept 06

CEE 371 Lecture #14 11/21/2009


ClearwellExit portalp

29 Sept 06

ClearwellIn outer ring, looking SW?g, g

29 Sept 06

CEE 371 Lecture #14 11/21/2009


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


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

Outer cell is considered less

gQ = 6.5 MGD max plant flowC = 0.5 mg/LTemp = 0.5oCpH = 7.5

efficient based on length to width ratio

22:1 for outer cell38:1 for inner cell

1.0 log inactivationCt = 79 mg/L - mint10 ≥ 157 min


CEE 371 Lecture #14 11/21/2009


CT for Giardia & Free Chlorine

Portions of H&H Table 7-4 are


Table 7 4 are extracted from this table

Source: EPA, 1999, Guidance Manual for Disinfection Profiling & Benchmarking

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CEE 371 Lecture #14 11/21/2009


Also below

Storage profile

542 ft surface elevation, the washwaterpumps can no longer draw


longer draw suction

Tracer Test ITracer Test Conditions

Depth = 26.5 ftVolume = 2,073,200 galQ = 3000 gpm = 4.32 MGDFluoride concentration

k d 0 034 /

tR = 11.52 hr

Background = 0.034 mg/LTarget = 1.33 mg/L

Temperature = 10.1 CpH = 8.14


CEE 371 Lecture #14 11/21/2009


Outer Cell DataStep DosepDecember 13, 2007

Lag time = 6.6 min=0.00458 days


Testing for Ct criteriaStep dose tracer testp

tR = 11.52 hr = 691.1 mint10 = 318 mint10/tR = 0.46

At 6.5 MGD


min459319.05.6

0732.2=== d

MGDMGtR

min211)46.0(4591010 ==⎟⎟

⎠

⎞⎜⎜⎝

⎛=

tracerRR t

ttt

Greater than 157 min

CEE 371 Lecture #14 11/21/2009


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