cee 371 print version water and wastewater systems · 2011-06-17 · removal log removal n, if n0 =...

CEE 371 Lecture #13 11/21/2009

Lecture #13 Dave Reckhow 1

CEE 371Water and Wastewater

Updated: 21 November 2009

Print version

Water and Wastewater Systems

Lecture #13

David Reckhow CEE 371 L#13 1

Drinking Water Treatment: DisinfectionReading: Chapter 7, pp.242-249

Dr. John Snow


CEE 371 Lecture #13 11/21/2009


Cholera in London & Dr. John SnowDuring an outbreak of cholera in London in 1854, John Snow plotted on a map the location of all theJohn Snow plotted on a map the location of all the cases he learned of. Water in that part of London was pumped from wells located in the various neighborhoods. Snow's map revealed a close association between the density of cholera cases and a single well located on Broad Street.

Removing the pump handle of the Broad Street well put an end to the epidemic. This despite the f t th t th i f ti t th t h l


fact that the infectious agent that causes cholera was not clearly recognized until 1905.

John Snow's map showing cholera deaths in London in 1854 (courtesy of The Geographical Journal). The Broad Street well is marked with an X (within the red circle).


CEE 371 Lecture #13 11/21/2009


The “Index Case”The woman living at 40 Broad Street (thought to be Sarah Lewis wife of(thought to be Sarah Lewis, wife of police constable Thomas Lewis) lost both her five-month old child and husband to cholera.In the four to five day interval between her child's onset of diarrhea on August 28-29, 1854 and subsequent death on September 2, 1854, Mrs. Lewis had soaked the diarrhea-soiled diapers in


soaked the diarrhea soiled diapers in pails of water. Thereafter she emptied the pails in the cesspool opening in front of her house.

Economic pressures & engineeringWhy was the cesspool at 40 Broad Street not maintained?

Such neglect was increasingly common in London, due in part to economic circumstances. At the time of the Broad Street pump outbreak, London had about two hundred thousand cesspools. For many years, the contents of the cesspools were sold as agricultural manure to be used as fertilizer in the many farms that surrounded London. The money earned from manure sales would then be used to maintain the cesspools. Yet during the nineteenth century as London's population grew ever more rapidly, farms were forced to move further from the central city. Transportation costs increased, adding to the expense of acquiring cesspool-based manure. Starting in 1847, another change took place that undercut the sale of cesspool manure. Solidified bird droppings (or guano) were brought in as fertilizer from South America at a price far below cesspool manure.

With no economic incentive to sell their feces, poor l ld t h t i t th t t


people would empty human wastes into the streets, or directly into the London waterways.

Most lacked public health understanding of how disease was spread, as did many medical and health officials of the times. In the absence of manure sales, cesspools became expensive to clean. As a result, they were poorly maintained and infrequently emptied. Over time this neglect lead to cracks and crevices, which offered opportunities for the spread of enteric pathogens. Such spread of Vibrio choleraemay have occurred at 40 Broad Street.

CEE 371 Lecture #13 11/21/2009


Disinfection

1-2 punch of filtration1-2 punch of filtration & chlorination

Greenberg, 1980, Water Chlorination, Env. Impact & Health Eff., Vol 3, pg.3, Ann Arbor Sci.

David Reckhow CEE 371 L#20 7Melosi, 2000, The Sanitary City, John Hopkins Press

US Death Rates for Typhoid Fever

Disinfection of PWSOne of the greatest achievements in public g phealth during the 20th century

US Centers for Disease Control (CDC)One of the greatest engineering feats of the 20th century


National Academy of Engineering

CEE 371 Lecture #13 11/21/2009


DisinfectionKill or inactivate pathogens

Bacteria, viruses protozoaMethods

Heat: boil waterExpose to UV lightAdd Chemical Oxidants

Small scale, for emergencies

Slowly becoming more common


Chlorine (Cl2, HOCl or OCl-)Chloramines (NH2Cl or NHCl2)Ozone (O3)Chlorine Dioxide (ClO2)

Primary purpose for drinking water treatment

By far the most common

Application PointsPrimary Disinfection

removal or inactivation of pathogens by “treatment technique” or TT approach

CT conceptdone in the treatment plant, sometimes as a first stepcan be: free chlorine, ozone, chlorine dioxide or UV light

Secondary DisinfectionAdded as the last step just prior to entry into distribution system


intended to maintain a residual of disinfectant throughout the distribution system

Minimize growth on pipe walls, some protection against re-contamination, or maybe just a “sentinel”

usually free or combined chlorine, sometimes chlorine dioxide

CEE 371 Lecture #13 11/21/2009


PrePre--DisinfectantDisinfectant

Point of Addition of DisinfectantsDAF

ee s ecta ts ecta t

Intermediate DisinfectantIntermediate Disinfectant

Coagulant

Settling

Direct Filtration

Rapid Mix Flocculation


Intermediate DisinfectantIntermediate Disinfectant

Dist.Sys.

Clearwell

PostPost--disinfectantdisinfectantFiltrationFiltration

GACAds.

Chick’s Law

In the early 1900's Dr Harriet Chick postulated that theIn the early 1900's Dr. Harriet Chick postulated that the death of the microorganisms was a first order process. So, for a given disinfectant and concentration:

kNdtdN

−=


This can be separated and integrated (with N = No at t = 0) to yield:

kteNN −= 0 ktNN

−=⎟⎟⎠

⎞⎜⎜⎝

⎛

0

lnor:

CEE 371 Lecture #13 11/21/2009


Chick-Watson LawThe fraction inactivated is a function of the specific lethality (λ) of the disinfectant-organism couple and the disinfectant concentration (C )

Many studies have found that n is in the range of 0.8 to 1.2 for most microorganisms. In engineering practice, it is usually assumed that n is unity, thus the equation becomes:

tCNN nλ−=⎟⎟

⎠

⎞⎜⎜⎝

⎛

0

lnnCk λ= so

y y q


{ } λxCt x

3.2log =

CtNN λ−=⎟⎟

⎠

⎞⎜⎜⎝

⎛

0

ln Ck λ=and

Chick-Watson IIUse of Ct values for various “log removals” is general practice

Here is how Ct corresponds to specific lethality of Chick’s Law (for n=1)

% Removal

Log Removal

N, if N0 = 10,000/L

Ct

90 1 1000 2.3/λ

Model is not always accurate, but it is usually a good first approximation


99 2 100 4.6/λ99.9 3 10 6.9/λ

99.99 4 1 9.2/λ

CEE 371 Lecture #13 11/21/2009


Specific Lethality (λ) at 20oCGeneral hierarchy

Disinfectants: O3>ClO2>HOCl>OCl->NHCl2>NH2ClOrganisms: bacteria>viruses>protozoa

Disinfectant E. coli Poliovirus I Entamoeba histolytica Cysts

O3 2300 920 3.1

Units:L/mg-min


HOCl 120 4.6 0.23 ClO2 16 2.4 OCl- 5.0 0.44 NHCl2 0.84 0.00092 NH2Cl 0.12 0.014

Some may change with pH, dose; all are affected by temperature

Chick-Watson Law: HOCl & Giardia1 0Direct

N/N

0

0 4

0.6

0.8

1.0

1 mg/L HOCl2 mg/L HOCl4 mg/L HOCl1 log removal2 log removal

Specific Lethality = 0.23plot

David Reckhow CEE 371 L#13 16Time (min)

0 5 10 15 20 25 300.0

0.2

0.4

1 log

2 logs

CEE 371 Lecture #13 11/21/2009


Chick-Watson Law: HOCl & GiardiaLog

10g

plotN

/N0 0.1

1

10

1 mg/L HOCl2 mg/L HOCl4 mg/L HOCl1 log removal2 log removal3 log removal

Specific Lethality = 0.23

1 log


0 5 10 15 20 25 30

0.001

0.01 2 logs

3 logs

Ct values for Giardia lamblia cystsH&H, Table 7-4, pg.245


CEE 371 Lecture #13 11/21/2009


Ct values for VirusesFor Viruses at various temperaturesp

pH 6-9 H&H Table 7-5, pg 245


How and where to determine “t”For all but ideal PFRs, ,there exists a range of residence times

Plug Flow

Non-ideal Flow


Completely Mixed Flow

⎟⎠⎞

⎜⎝⎛ −=

−Rt

teCC 10

Rtt

eCC−

= 0

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛−⎟⎟

⎠

⎞⎜⎜⎝

⎛+

=⎟⎟⎠

⎞⎜⎜⎝

⎛+− k

tt

R

Rekt

CC1

0 11

1

⎟⎟⎠

⎞⎜⎜⎝

⎛+−

=k

tt

ReCC1

0

Step

Puls

e When k=0

CEE 371 Lecture #13 11/21/2009


t10 conceptUS EPA regulatory approachg y pp

Use the t10 value90% of water has a residence time greater than t10

10% of water has a residence time less than t10

A “conservative” or safe approachProtection of public health

Value ranges from:Value ranges from:100% of tR for PRF10.5% of tR for CSTR (-ln(0.9))In between for all “real” reactors


Determining t10Conduct tracer studyy

Add a conservative substance to tank inlet at a particular time

Fluoride is good; doesn’t change, just moves with the water, non toxicCan be either a pulse (slug), step-up, or step-down

Monitor concentration of conservative substance in tank outlet

Data AnalysisPrepare graph of concentration vs timeIdentify when concentration reaches 10% of “breakthrough” valueDavid Reckhow CEE 371 L#13 22

CEE 371 Lecture #13 11/21/2009


Case Study I: Amherst Ozone Contactor

Four chambersUnder/over baffled

Fluoride Tracer testStep feed @ t=0

2.4 mg/LAdd d i lAdded to inletMeasure F- at outlet vs time


Amherst O3 Contactor II

Fluoride tracerAtkins WTP, Ozone Contactor

C/C

0

0.6

0.8

1.0

1.2Fluoride tracer study

Q=1000 gpmV=22,980 galC0=2.4 mg/L


0 20 40 60 800.0

0.2

0.4Fluoride DataIdeal PFR Ideal CSTR

⎟⎠⎞

⎜⎝⎛ −=

−Rt

teCC 10 Data from :Teefy, 1996

[AWWARF Report]

CEE 371 Lecture #13 11/21/2009


Amherst O3 Contactor III0 5

Atkins WTP, Ozone Contactor

C/C

0

0.3

0.4

0.5

Fluoride DataIdeal PFR Ideal CSTR

Calculation of t1014 min or 65% of tR


0 5 10 15 20 25 300.0

0.1

0.2 Ideal CSTR

14 min 23 min2.5 min

C/C0=10%

Data from :Teefy, 1996 [AWWARF Report]

Ct values for VirusesFor Viruses at various temperaturesp

pH 6-9 H&H Table 7-5, pg 245


CEE 371 Lecture #13 11/21/2009


Amherst O3 Contactor IVUse of t10 for disinfection compliance10 p

Conventional treatment requires 2 log virus inactivation by disinfectionFor ozone 0.9 mg/L – min is worst case (0.5oC, in H&H table 7-5)With a if t = 14 min then we need to have 0 065With a if t10 14 min, then we need to have 0.065 mg/L ozone residual at outlet of tank

David ReckhowCEE 371 L#13

27

( )L

mgt

CtC L

mgrequired 065.0

min14min9.0

10min ===

Treatment vs SourcesSurface water Groundwater

Major water quality concerns

PathogensTurbidityColor & TOCTaste & odor

Major water quality concerns

Fe/MnHardnessArsenic, perchlorateVOCs & pesticides

Typical treatment“conventional” coagulation-filtrationSome use advanced treatment

Typical treatmentDisinfection onlySofteningAerationPressure filtration


CEE 371 Lecture #13 11/21/2009


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cee 371 print version water and wastewater systems · 2011-06-17 · removal log removal n, if n0 =...

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