Download Chlorination - Cornell Engineering

Disinfection
Monroe L. Weber-Shirk
School of Civil and
Environmental Engineering
Overview
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Why does getting this right matter?
Disinfection Options
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Poisson Distribution of Pathogens
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Disinfection-by-products
Tastes and odors
Real pathogens
Getting the right dose
Probability of ingesting k pathogens
Implications for dose dependency
WaterBorne Disease Outbreaks
Chlorine sources
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Metals
Water
Ammonia
Organics
The case for Chlorine
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Gas
Bleach
Onsite production
Chemistry
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Chicks law
CT
Problems with Disinfection
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Disinfection mechanisms
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Chlorine
Ozone
Irradiation with Ultraviolet light
Iodine
Silver
Chlorine
It kills stuff
Residual
Recontamination
Regrowth
Hypochlorinators
Chlorine free

Some European cities
Chlorine saves lives…
 If you accept the “Chlorine eliminated Typhoid
Line”
 Then you will likely recommend chlorination as
the first line of defense in the Global South But in small systems (in the Global South)
 Chlorine dose is generally not controlled based on a
target residual dose
 Surface water may currently be untreated and hence
have high turbidity
 that correlates with high chlorine demand
 that contains pathogens embedded in organic particles
WHO on alternative disinfectants:
Iodine, silver, copper, quaternary ammonium compounds
 none of them are considered suitable for long-term use to
disinfect drinking water
 Iodine is difficult to deliver to water and can cause adverse
health effects
 However, iodine, either dissolved in water or in the form of
an iodinated exchange resin, has been used for short-term
water treatment
 Silver and copper are difficult to deliver to water and are
only bacteriostatic.
 Quaternary ammonium compounds are limited in
availability, costly and not effective against viruses and
parasites.
Silver as a Disinfectant…
 Silver is used as a bacteriostatic agent for point-of-use or
household water treatment by storing water in vessels
composed of silver or passing water through porous or
granular filter media impregnated with silver
 Many microbes including viruses, protozoan cysts,
oocysts, and bacterial spores, are not inactivated at silver
concentrations employed for point-of-use drinking water
treatment
 Bacteria may develop silver resistance
 Therefore, silver is not recommended for routine
disinfection of household water
Chlorine Disinfection Mechanisms*
 Oxidation of membrane-bound enzymes for
transport and oxidative phosphorylation
 Oxidation of cytoplasmic enzymes
 Oxidation of cytoplasmic amino acids to nitrites
and aldehydes
 Oxidation of nucleotide bases
 Chlorine substitution onto amino acids (more likely)
 DNA mutations
 DNA lesions
*It is possible that none of these mechanisms have been documented
Chick’s Law
 The death of microorganisms is first order with respect to
time
 Thus, the remaining number of viable microorganisms, N,
decreases with time, t, according to:
dN
  kN
dt
 where k is an empirical constant descriptive of the
microorganism, pH and disinfectant used.
 Integrating with respect to time, and replacing limits (N =
No at t = 0) yields:
1
N
 kt
pC* 
kt
ln
  kt
N  N0e
ln 10 
N0
EPA Pathogen Inactivation
Requirements
Safe Drinking Water Act
SDWA requires 99.9% inactivation for
Giardia and 99.99% inactivation of viruses
Giardia is more difficult to kill with
chlorine than viruses and thus Giardia
inactivation determines the CT
Concentration x Time
EPA Credits for Giardia
Inactivation
Treatment type
Credit
Conventional Filtration
99.7%
Direct Filtration*
99%
Disinfection
f(time, conc., pH, Temp.)
* No sedimentation tanks
EPA Disinfection CT Credits
To get credit for 99.9% inactivation of Giardia:
Contact time (min)
chlorine
pH 6.5
pH 7.5
(mg/L) 2°C
10°C
2°C 10°C
0.5
300
178
430
254
1
159
94
228
134
Inactivation is a function of _______,
time
____________
concentration
pH
temperature
______,
and ___________.
Where did these numbers (to 3 significant digits) come from?
CT equation for Giardia

CCl  tcontact  0.2828  pH 2.69  CCl0.15  0.933

tcontact  0.2828  pH 2.69  CCl0.85  0.933
pC* 
tcontact  CCl0.85

0.2828  pH 2.69  0.933
Chicks Law!
T -5

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
T -5
T -5
 pC *
 pC *
CCl = Free Cl2 Residual [mg/L]
tcontact = Time required [min]
pH = pH of water
T = Temperature, degrees C
pC* = -[Log(fraction
remaining)]
Note: These equations are NOT dimensionally correct!
Disinfectant Limitations
Disinfection by products
Tastes and Odors
Real pathogens
Getting the right dose
Disinfection Byproducts
Contaminant
MCLG1 MCL
(mg/L)2 (mg/L)2
Potential Health Effects
from Ingestion of Water
Bromate
zero
Increased risk of cancer
0.010
Sources of
Contaminant in
Drinking Water
Byproduct of drinking water
disinfection (plants that use
ozone)
Chlorite
0.8
1.0
Anemia; infants & young
children: nervous system
effects
Byproduct of drinking water
disinfection (plants that use
chlorine dioxide)
Haloacetic acids
(HAA5)
n/a6
0.060
Increased risk of cancer
Byproduct of drinking water
disinfection
Total
Trihalomethanes
(TTHMs)
none7
---------n/a6
0.10
---------0.080
Liver, kidney or central nervous
system problems; increased
risk of cancer
Byproduct of drinking water
disinfection
Disinfectants
MRDLG
Contaminant
1
MRDL1
(mg/L)
Potential Health Effects
from Ingestion of Water
Sources of
Contaminant in
Drinking Water
(mg/L)2
2
Chloramines
(as Cl2)
MRDLG
=41
MRDL
=4.01
Eye/nose irritation; stomach
discomfort, anemia
Water additive used to control
microbes
Chlorine (as
Cl2)
MRDLG
=41
MRDL
=4.01
Eye/nose irritation; stomach
discomfort
Water additive used to control
microbes
Chlorine
dioxide (as
ClO2)
MRDLG
=0.81
MRDL
=0.81
Anemia; infants & young
children: nervous system effects
Water additive used to control
microbes
Maximum Residual Disinfectant Level (MRDL) - The highest level of a disinfectant allowed in drinking water.
There is convincing evidence that addition of a disinfectant is necessary for control of microbial contaminants.
Maximum Residual Disinfectant Level Goal (MRDLG) - The level of a drinking water disinfectant below which
there is no known or expected risk to health. MRDLGs do not reflect the benefits of the use of disinfectants to control
microbial contaminants.
Tastes and Odors: Taste Thresholds
C
O l
Cl
N H
2
N Cl
H
C
ch l 2
lo
de rofo
ch rm
di loro
ch p
h
ca loro eno
rb be l
io on t nze
do et ne
fo rac
rm h
lo
r
O
H
Taste threshold (mg/L)
id
e
 Complaints of the chlorine taste should not be discounted
 Chlorine taste may prevent some consumers from using
treated water
 Need to convince consumers that
the chemical taste is healthy
1
0.1
0.01
0.001
0.0001
Chlorine Taste Acceptance
 The introduction of chlorine into a community that
has always drunk water without the addition of
chemicals can be difficult.
 Reducing the amount of chlorine added might
increase the social acceptance of chlorination.
 With less chlorine, there is less breakthrough of
chlorinated compounds into the finished water,
and therefore less of a chemical taste.
 If the water doesn’t taste strongly like chemicals,
more people are likely to subscribe to the use of
chlorine for disinfection.
Inactivation of Shielded Pathogens
Many of the studies measuring inactivation
of pathogens by disinfectants were
conducted using dispersed pathogens
What happens if the pathogens are
embedded in an organic particle?
Faecal contamination potentially contains
pathogens embedded in protective organic
matter.
0.36 mg/L
average
free Cl2 at
pH 6
What do you
conclude?
pC*
Cell Associate virus was inside fetal
rhesus kidney derived cells
4
3.5
3
2.5
2
1.5
1
0.5
0
dispersed
cell associated
model
2nd model
0
50
Time (min)
86 100
CCl  t to get pC* of 4 is (86 min)*(0.36 mg/L)=31 (min mg/L)
Conclusions from Virus in Kidney
Cells
The rate of virus deactivation dropped
significantly when the virus particles were
inside kidney cells
The deactivation of embedded virus
particles can not be described by a single
first order reaction (________________)
Chicks Law is violated
What is controlling the rate of virus
deactivation?
Scales of the Embedded Virus
Location
Dispersed
Inside cell with
1000 nm
disrupted cell
wall
1 mm
Inside intact
cell
Deactivation
rate
Very fast
Slow
Very slow
Virus particles are about 20 nm
HOCl are about 0.2 nm
Mass Transport and Chlorine
Protection
Chlorine must diffuse
through cell contents to
reach virus
Organic material inside
the cell reacts with
chlorine before it gets to
the virus
Scale this up to a Faecal Aggregate
 Turbid water could easily
have organic particles that
are 10 or even 100 mm in
diameter
 The amount of organic
matter in a small particle
and the slow diffusion
would provide long term
protection for embedded
pathogens
10 mm
Getting the Right Dose:
WHO on Chlorination
exposed
 Chlorine compounds usually destroy pathogens after 30
minutes of contact time, and free residual chlorine (0.2–0.5
mg per liter of treated water) can be maintained in the
water supply to provide ongoing disinfection.
 Several chlorine compounds, such as sodium hypochlorite
and calcium hypochlorite, can be used domestically, but
the active chlorine concentrations of such sources can be
different and this should be taken into account when
calculating the amount of chlorine to add to the water.
 The amount of chlorine that will be needed to kill the
pathogens will be affected by the quality of the untreated
water and by the strength of the chlorine compound used.
 If the water is excessively turbid, it should be filtered or
allowed to settle before chlorinating it
(___________________________)
Remove particles first!
Pathogen Poisson Process
Probability
Suppose we have an average pathogen
concentration of C in our drinking water
Suppose we drink volume V
What is the probability that we will ingest k
pathogens?
Suppose V=1L, C=2/L, what is probability
of ingesting exactly 2 pathogens?
k
2
CV   CV

2  2

P 
e
P 
e  0.27
k
k!
k
2!
CV=15
Probability of ingesting
exactly k pathogens
0.12
1
0.1
0.8
0.08
0.6
0.06
0.4
0.04
0.2
0.02
0
Cumulative probability
Probability of k Pathogens
45% chance of not
getting sick!
0
0
10
20
30
Number of pathogens
 What is probability of k < (dose)?
 Let dose = 15
 Find cumulative probability for k=14
Find probability that k>0
0!
0
e
 CV
e
 CV
Pk 0  1  e  CV
0.4
1
0.3
0.8
0.6
0.2
0.4
0.1
0.2
0
0
0
2
Cumulative probability
Pk 0
e  CV
k!
CV 


CV=1
Probability of ingesting
exactly k pathogens
Pk
CV 


k
4
Number of pathogens
For CV = 1, Pk>0 = 0.63
For CV = 0.001, Pk>0 = 0.001
(converge for small CV)
Effect of Pathogen Dose
For CV = 0.001, Pk>0 = 0.001
 What happens if the pathogen dose is 10 rather
than 1?
 Let’s assume that the concentration of this new
pathogen is 10 times as great (CV=0.01)
 What is the probability that you ingest 10 or
more? For CV = 0.01, Pk≥10 = 3x10-27
 Pathogens with an infectious dose of 1 are
potentially quite harmful even at very low
concentrations!
 Pathogens with an infectious dose>1 are not
dangerous at low concentration!
Waterborne Disease Outbreaks in the
US (1985)
 G. lamblia was the most frequently identified pathogen for
the seventh consecutive year, causing three (20%) of 15
waterborne outbreaks.
 In each of the outbreaks, as in well-characterized
waterborne outbreaks of giardiasis in the past, water
chlorination had been maintained at adequate levels to
make outbreaks of bacterial diseases unlikely, but the lack
of an intact filtering system capable of filtering Giardia
cysts, distribution system problems, and mechanical
deficiencies allowed drinking water to become a vehicle of
giardiasis.
Waterborne Disease Outbreaks
(1993)
 The majority of outbreaks (68%) during 1991-1992 were
classified as AGI of unknown etiology
 Water sampling showed the presence of coliforms and/or
deficiencies in chlorination for 91% of these outbreaks
 24 outbreaks (71%) were associated with contaminated
untreated or inadequately treated groundwater
 Two outbreaks were associated with treatment deficiencies
in water systems using UV light for disinfection
 Three protozoal outbreaks during 1991-1992 occurred in
systems that were equipped with chlorine disinfection and
met EPA coliform standards but were not equipped with
filtration
Waterborne Disease Outbreaks
(1993)
 Four of the six surface water systems associated with
WBDOs were equipped with filtration.
 In three of these outbreaks, raw water quality had deteriorated
because of sewage effluents that were not appropriately
diluted as a result of low stream flows during dry weather.
 During the outbreaks associated with these systems, filtration
deficiencies were noted, with elevated turbidity in finished
water.
 Decreased filtration efficiency combined with
deterioration in raw water quality also contributed to the
WBDO in Milwaukee (1993).
It appears that chlorination was unable to provide an
effective barrier when filtration failed
A fatal waterborne disease
epidemic in Walkerton, Ontario
 An estimated 2,300 people became seriously ill and seven died from
exposure to microbially contaminated drinking water in the town of
Walkerton, Ontario, Canada in May 2000
 The Walkerton operators were asked to provide a chlorine residual
(majority to be free chlorine) of 0.5 mg/L after 15 min.
 Evidence at the Inquiry revealed that chlorine dosage practice at Well
#5 was insufficient to achieve a 0.5 mg/L residual even in the absence
of any chlorine demand.
 Although the evidence did not allow for an estimate of the chlorine
demand at the time Well #5 was contaminated, it was reasonable to
assume that the contamination causing this outbreak was accompanied
by a chlorine demand sufficient to consume entirely, or almost entirely,
the low chlorine dose thereby allowing inadequately disinfected water
into the distribution system
Chlorine must have been consumed because there was
an outbreak
Chlorine Sources
 On Site Production (electrolysis)
 Chlorine gas (Cl2)
 Liquid Bleach (NaOCl)
 Calcium hypochlorite (Contains 65%
available chlorine) Ca(OCl)2
Bleach Concentration in terms of
sodium hypochlorite (NaOCL)
Bleach concentration in terms of
Available Chlorine (As Cl2)
Additional Information
(estimated)
Grams per
liter
Density of
the solution
(lb/U.S. gal)
Specific gravity
of the solution
Wt. %
Trade %
Grams per liter
Wt. %
Trade
%
5
5.4
53.9
4.8
5.1
51.4
9.0
1.08
10
11.6
115.8
9.5
11.0
110.4
9.7
1.16
15
18.6
185.7
14.3
17.7
177.0
10.3
1.24
Chlorine
 First large-scale chlorination was in 1908 at the
Boonton Reservoir of the Jersey City Water Works
in the United States
Chlorine
 Widely used in the US
oxidizes organic
matter
 Typical dosage (1-5 mg/L)
 variable, based on the chlorine demand
 goal of 0.2 mg/L residual
 Trihalomethanes (EPA primary standard is 80
mg/L)
 Chlorine concentration is measured as Cl2 even
when in the form of HOCl or OCl-
Chlorine Reactions
Charges
0
+1 -2 +1
-1
Cl2 + H2O  H+ + HOCl + ClHypochlorous acid HOCl  H+ + OCl- Hypochlorite ion
The sum of HOCl and OCl- is called the
____
______ residual
_______
free chlorine
 HOCl is the more
effective disinfectant
 Therefore chlorine
disinfection is more
effective at ________
low
pH
 Dissociation constant is
10-7.5
 HOCl and OCl- are in
equilibrium at pH 7.5
Fraction of Free Chlorine
Chlorine and pH
HOCl  H+ + OCl-
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
HOCl
OCl-
pk
5
6
7
8
pH
9
10
Ammonia Reactions
-3
+1
-3+1 +1
NH3(aq) + HOCl  NH2Cl+ H2O
Combined chlorine
NH2Cl + HOCl  NHCl2+ H2O
 Substitution reactions…
 The combined chlorine maintains its oxidizing
potential
Breakpoint Chlorination
Removal of ammonia by chlorination
-3
+1
0
-1
2NH3(aq) + 3HOCl  N2+ 3Cl- + 3H2O
Oxidizing equivalents of chlorine are
consumed
Does Chlorine Completely oxidize
organic matter? NO!
4HOCl + CH4  CO2 + 2H2O + 4Cl- + 4H+
Oxidation states
Carbon in organic matter (-4)
Carbon in carbon dioxide (+4)
Chlorine in HOCl (+1)
Chloride (-1)
Therefore should take 4 moles of chlorine
(Cl2) per mole of organic carbon
23.6 g chlorine/g organic carbon
Chlorine Demand (mg/L)
Chlorine Demand vs. Total Organic
Carbon
2.5
2
1.5
1
y = 0.51x - 0.16
R2 = 0.93
0.5
0
0
1
2
3
4
Total Organic Carbon (mg/L)
5
0.5 mg chlorine
mg carbon
Reaction with organic compounds
with unsaturated linkages
C
H
C
H
+ HOCl
→
Cl
C
H
OH
C
H
Chlorine doesn’t oxidize the organic carbon
Chlorine maintains its oxidation number
0.5 mg Cl2 mole Cl2 mole Cl 1 12 g C
mole Cl 1

mg C
70.9 g Cl2 mole Cl2 mole C 12 mole C
 Disclaimer – There is
no solid connection
between chlorine
demand and turbidity
 But – organic matter is
often associated with
particulate matter
Chlorine Demand (mg/L)
Chlorine Demand vs Turbidity
 If using the standard
dose of 2 mg/L, then
no residual above 15
NTU!
2
1.5
1
Based on 6
watersheds in
western Oregon
0.5
0
0
5
10
Turbidity (NTU)
15
The Case for Chlorine
It kills stuff
Residual
Recontamination
Regrowth
Effect of Chlorination on
Inactivating Selected Bacteria
Bacteria
Cl2 Concentration
(mg/l)
Time
(min)
Ct Factor (mgmin/l)
Reduction(%)
Campylobacter jejuni
0.1
5
0.5
99.99
Blaser et al, 1986
Escherichia coli
0.2
3
5
99.99
Ram and Malley,
1984
Legionella pneumophila
0.25
60-90
18.75
99
Kuchta et al, 1985
Mycobacterium chelonei
0.7
60
42
99.95
Carson et al, 1978
Mycobacterium fortuitum
1.0
30
30
99.4
Pelletier and
DuMoulin, 1987
Mycobacterium
intracellulare
0.15
60
9
70
Pelletier and
DuMoulin, 1987
0.5-1.0
5
3.75
99.6-100
Salmonella typhi
0.5
6
3
99
Shigella dysenteriae
0.05
10
0.5
99.6-100
Staphylococcus aureus
0.8
0.5
0.4
100
Bolton et al, 1988
Vibrio cholerae(smooth
strain)
1.0
<1
<1
100
Rice et al, 1993
Vibrio cholerae (rugose
strain)
2.0
30
60
99.999
Rice et al, 1993
Yersinia enterocolitica
1.0
30
30
92
Paz et al, 1993
Pasteurella tularensis
Reference
Baumann and
Ludwig, 1962
Korol et al, 1995
Baumann and
Ludwig, 1962
Effect of Chlorination on
Inactivating Selected Viruses
Viruses
Cl2
Concentration
(mg/l)
Time
(min)
Ct factor
(mg-min/l)
Reduction
(%)
Reference
Adenovirus
0.2
40-50 sec
0.15
99.8
Clarke et al, 1956
Coxsackie
0.16-0.18
3.8
0.06
99.6
Clarke and
Kabler, 1954
Hepatitis A
0.42
1
0.42
99.99
Grabow et al,
1983
0.5-1.0
30
22.5
--
Keswick et al,
1985
Parvovirus
0.2
3.2
0.64
99
Churn et al, 1984
Poliovirus
0.5-1.0
30
22.5
100
Keswick et al,
1985
Rotavirus
0.5-1.0
30
22.5
100
Keswick et al,
1985
Norwalk
Effect of Chlorination on
Inactivating Selected Protozoa
Cl2
Concentration
(mg/l)
Time
(min)
Ct Factor
(mg-min/l)
Reduction
(%)
Cryptosporidium
parvum
80
90
7200*
90
Korich et al,
1990
Entamoeba
histolytica
1.0
50
50
100
Snow, 1956
--
--
68-389
99.9
AWWA, 1999
0.5-1.0
60
45
99.99
de Jonckheere
and van de
Voorde,
1976
Protozoa
Giardia lamblia
Naegleria fowleri
Reference
The Case for a Residual
 Disinfect any recontamination
 Prevent bacteria growth in the treated water
 Do pathogens grow in water?
 “The real reason for maintaining residuals during
treatment and distribution is to control
microbiological growths when the water is
biologically unstable.”
 Control those non-pathogenic slime-forming
organisms
 “Current practice in North America tries to kill all
microorganisms whenever possible”
Protection Against Recontamination
In order to be effective the following
requirements must be met
The amount of chlorine demand must not
exceed the residual
The pathogens must be dispersed (not
associated with other particles)
This is unlikely if the contamination is faecal
The pathogens must be susceptible to chlorine
Growth of Bacteria in Water
Distribution Systems
 Consumption of dissolved oxygen
 Increased heterotrophic plate counts or coliform
counts
 This does not imply a health risk
 Decreased hydraulic capacity of the pipes
 Formation of taste/odor compounds
 Geosmin, mercaptans, amines, tryptophans, sulfates
 Increased rates of pipe corrosion
I don’t have any evidence that this biological
growth has a significant public health impact
Pathogen Growth in Distribution
Systems (CDC)
 Biofilms are coatings of organic and inorganic
materials in pipes that can harbor, protect, and
allow the proliferation of several bacterial
pathogens, including Legionella and
Mycobacterium avium complex (MAC).
 Mycobacterium avium complex, MAC, is an
opportunistic bacterial pathogen, is resistant to
water disinfection (much more so than Giardia
cysts), and grows in pipe biofilms
I need more data here! Is this a real public health threat?
WHO on Regrowth
 There is no evidence to implicate the occurrence of most
microorganisms from biofilms (excepting, for example,
Legionella, which can colonize water systems in buildings)
with adverse health effects in the general population
through drinking water, with the possible exception of
severely immunocompromised people
 Water temperatures and nutrient concentrations are not
generally elevated enough within the distribution system to
support the growth of E. coli (or enteric pathogenic
bacteria) in biofilms.
 Thus, the presence of E. coli should be considered as
evidence of recent faecal contamination.
http://www.who.int/water_sanitation_health/dwq/en/gdwq3_4.pdf
WHO on Regrowth (2)
 Viruses and the resting stages of parasites (cysts,
oocysts, ova) are unable to multiply in water.
 Relatively high amounts of biodegradable organic
carbon, together with warm temperatures and low
residual concentrations of chlorine, can permit
growth of Legionella, V. cholerae, Naegleria
fowleri, Acanthamoeba and nuisance organisms in
some surface waters and during water distribution
http://www.who.int/water_sanitation_health/dwq/en/gdwq3_7.pdf
Where is the original research for these conclusions?
Life without Chlorine
Amsterdam stopped chlorinating in 1983
Recommendations for Chlorine as
Sole Treatment
 Low turbidity (<30 NTU) and low chlorine
demand for effective use; pre-treat turbid water
(WHO)
 Pre-treat means using a particle removal technology
first
 Low turbidity probably means good quality
groundwater (from a spring or from a well)
 Surface water would generally not meet the NTU
requirement
 Chlorine requires process control (feedback based
on residual chlorine concentration)
Hypochlorinator
Float
1.0 m
1.5” PVC
overflow tube
Transparent
flexible tube
1.78 m
(0.5”)
1.05 m
PVC needle
valve
0.5” PVC tube
Water in the distribution tank
Example Conservation of Mass
Changing Volume
Float
¶
r V ×n
ˆ dA = r dV
ò
ò
¶t cv
cs
Vor Aor
Vor Aor
¶
=dV
ò
¶t cv
Ares dh
dV


dt
dt
Vor Aor  Qor
dh
Ares  K or Aor
dt
1.0 m
1.5” PVC
overflow tube
Transparent
flexible tube
1.78 m
(0.5”)
1.05 m
h0
PVC needle
valve
0.5” PVC tube
Water in the distribution tank
Orifice in the needle valve
2 gh  0
Integrate to get h as f(t)
Finding the chlorine depth as f(t)
 Ares
K or Aor
h
2g
 Ares
K or Aor
2g

h0
dh

h
t
 dt
0
2  h1/ 2  h01/ 2   t
 1/ 2 tK or Aor 2 g
h   h0 

2 Ares

 1/ 2
Aor
h   h0  tK or
Ares





g 

2 
2
2
h 
h0  tK or
Aor
Ares
g
2
Finding Q as f(t)
Q  K or Aor
2 gh
h 
h0  tK or
Q  K or Aor

2g 

Aor
Ares
g 

2 
h0  tK or
Aor
Ares
g
2
Find Aor as function of initial target flow rate
Aor 
Q0
K or
2 gh0
Set the valve to get desired dose initially
Surprise… Q decreases linearly!
Q  K or Aor
Q
2g 

2 gh0 

K or Q0
K or

2g 

h0  tK or
h0  tK or
g 

2 
Aor
Ares


K or Ares 2 gh0 

Q0
g
Q

Q0
Aor 
Q0
K or

h0 

Ares 2 h0 

tQ0
Q
 1
Q0
2 Ares h0
1
h0




2 gh0
tQ0
Relationship between Q0 and Ares?
Flow at Q0 for 4 days (tdesign) would empty reservoir
Q0tdesign  Ares hres
Q
1 t hres
 1
Q0
2 tdesign h0
Q0
hres

Ares
tdesign
CCl2
CCl2
0

1 t hres
 1 

2 tdesign h0





Hypochorinator Fix
http://web.mit.edu/d-lab/honduras.htm
Conclusions
Reflections
What is one explanation for why Biosand
filters don’t perform as well as expected?
Calicivirus - An Emerging
Contaminant in Water
 Annual estimates among adults in the U.S. reveal
approximately 267 million episodes of diarrhea leading to
612,000 hospitalizations and 3,000 deaths
 In the last 3 decades it has become increasingly clear that
viral agents are responsible for much of this public health
burden
 Human caliciviruses (HuCVs) have been estimated to
cause 95–96% of nonbacterial gastroenteritis outbreaks
(Fankhauser et al., 1998; K.Y. Green et al., 2000).
 These viruses are considered ubiquitous in nature and
stable in the environment, thereby increasing their
propensity to spread and cause disease
Calicivirus
Four genera
Lagovirus
Vesivirus
Norwalk-like viruses (Noroviruses or NLVs)
Sapporo-like viruses (Sapoviruses or SLVs)
Single structural protein that makes up the
viral capsid
27-40 nm in diameter
Cause
disease in
humans
Norovirus: Infectious Dose
 Human volunteer feeding studies have determined
the number of viral particles needed to initiate
Huffman, D. E., K. L. Nelson, et al. (2003).
infection is 10 to 100
 The infectious dose may well be the result of mass
transfer limitations in the gut
 i.e. the virus needs to be transported to the villi in the
gastric mucosa and attach to initiate infection
 Thus it is likely that 1 viral particle is sufficient to
Weber-Shirk
initiate infection
Norovirus: Clinical Illness and
Diagnosis
 The most common symptoms of NLV infections include
mild to moderate diarrhea, abdominal cramps, and nausea
(Adler and Zickl, 1969; Hedberg and Osterholm, 1993).
 Other symptoms may include headache, malaise, chills,
cramping, and abdominal pain. Stools typically do not
contain blood or mucus.
 The onset of illness is generally within 24 to 28 h of
exposure with a relatively short duration of illness (12 to
60 h).
 Adult infection with NLV can be distinguished from
bacterial pathogens such as Salmonella and Shigella due to
its characteristic projectile vomiting (Adler and Zickl,
1969; Caul, 1996)
Norovirus: Inactivation
A 1-log reduction in the RTPCR signal of
Norwalk virus was observed after treatment
with 2 mg/L monochloramine at pH 8 for 3
h
Norwalk virus appears to be fairly resistant
to free chlorine and monochloramine