Chlorination & measurement
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Transcript of Chlorination & measurement
CHLORINE MEASUREMENT
Praful Hanmante
Most commonly used for
disinfection of water.
CHLORINE AS DISINFECT
Cl2 when applied to water forms hypochlorous acid and hydrochloric acid
Cl2 + H2O ↔ HOCl + H+ +Cl - [pH dependent reaction]
Ca(OCl)2 + 2H2O ↔ 2HOCl + Ca(OH)2
NaOCl + H2O ↔ HOCl+NaOH
HOCl = Hypochlorous Acid
Ca(OCl)2 = Calcium hypochlorite
Ca(OH)2 = Calcium Hydroxide
NaOCl = Sodium hypochlorite
HOCl ↔ H+ + OCl - [pH dependent reaction]
Killing effiency of HOCl is 40 -80 times larger than OCl - .
Lower pH favors HOCl.
HOCl = Hypochlorous acid
OCl - = Hypochloride ion
Free chlorine = HOCl + OCl -
FREE CHLORINE
HOCl + NH3 ↔ NH2Cl (monochloramine) + H2O
HOCl + NH2Cl ↔ NHCl2 (dichloramine) + H2O
HOCl + NHCl2 ↔ NCl3 (trichloramine) + H2O
Combined chlorine=monochloramine+dichloramine+
trichloramine
Total chlorine = combined chlorine + free chlorine
COMBINED CHLORINE
BREAKPOINT
MEASUREMENT TECHNIQUES
DPD Colorimetric Method
Amperometric Method
Add buf fer and indicator solution to a known volume of water.
Af ter mixing, measure wavelength of l ight transmitted through sample to determine colour change and thus, amount of free residual chlorine in the sample.
AdvantagesPrice.High pH is not a l imiting factor.EPA approved DPD measurement.
DisadvantagesUses reagents. .High maintenance.No pH or temperature output.
COLORIMETRIC METHOD
COLORIMETRIC METHOD
Free Residual Chlorine— (DPD), The reaction takes place at a
buffered pH of 6.3 to 6.6.
Total Residual Chlorine— By adding potassium iodide to the
sample, chloramines in the sample oxidize iodide to iodine,
which then oxidizes the DPD indicator to the magenta color at
a buffered pH of 5.1.
A sensor consisting of a membrane (which allows HOCL to migrate through it), two dissimilar metal electrodes, and an electrolyte are submersed into the water sample.
A free residual chlorine reading is derived based on mV changes experienced by the sensor.
Advantages
Ease of use
No reagents or buffers added
Disadvantages
pH dependence
AMPEROMETRIC METHOD
How does the chlorine sensor work
Current is proportional to diffusion rate, which is proportional to concentration
cathode
membrane
anode
(Ag/AgCl)
electrolyte
(KCl)
electrons
HOCL
HOCL + H+ + 2e- Cl- + H2O
2Ag + 2Cl - AgCl + 2e-
Amperometric sensor measures
only HOCL
PH DEPEDENACE
FREE CHLORINE AND THE PH PROBLEM
Three ways to solve the problem...
Restrict applications to only those having constant pH
Use acid to adjust the pH of the sample
Measure pH continuously and perform automatic correction
350
400
450
500
550
600
650
7.0 7.5 8.0 8.5 9.0 9.5
sen
sit
ivit
y, n
A/p
pm
pH at 25 C
15
The reagent solution...Add an acid to the sample to lower
pH to about 6.
Acid converts OCl to HOCl, which the sensor measures.
Disadvantages
– requires sample conditioning system
– requires reagents acid
reagent
bottle
reagent
pump
sensor
HOCl + OCl HOCl
INTERNAL PH CORRECTION
The Rosemount Analytical approachUse continuous pH correction
Good between pH 6.0 and 9.5.
– Below pH 6.0, don’t need correction.
– Above pH 9.5, usually there is too little HOCl available to measure.
If pH varies less than 0.2 peak-to-peak, don’t need continuous pH correction.
If pH varies more than 0.2 peak-to-peak, use continuous pH correction.
free chlorine
sensorpH
sensor
Free chlorine sensor - 499ACL-01
Easy to replace membrane - no tools required
Range: 0 - 20 ppm
Linear range: 0 - 6 ppm
Linearity: 2% (typical). Following calibration in the linear range, the measured concentration of any point in the linear range is within about ±0.1 ppm of the expected value.
Electrolyte life: about three months
Variopol quick disconnect option makes replacing the sensor easy.
cable
cable grip
machined
plug,
receives
cable grip
O-rings
MNPT
threads,
pipe tape
not shown
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Determination of total chlorine No single sensor can be used to determine all the different forms
of total chlorine.
The only way to measure total chlorine is to convert it into a form the sensor can measure.
Requires a sample conditioning system. Most manufacturers use an off-line system.
acetic acid buffer
with potassium iodide
total chlorine iodine
iodine sensor
to waste
The concentration of iodine produced by the reaction between total chlorine
and potassium iodide is directly proportional to the concentra-tion of total
chlorine in the sample.
Can be used as disinfectant
Chloramines: effective vs. bacteria but NOT viruses .
Chloramines not powerful enough to form THMs.
Last a lot longer in the mains than free chlorine ,
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CHLORAMINES
Activated carbon
C + 2Cl2 + 2H2O → 4HCl + CO2
When dissolved in water, sodium bisulfite (SBS) is formed
from SMBS (Sodium metabisulfite) :
Na2S2O5 + H2O → 2 NaHSO3
2NaHSO3 + 2HOCl → H2SO4 + 2HCl + Na2SO4
DECHLORINATION
22
Dechlorination
Remember: You’re measuring the absence of chlorine, not chlorine.
chlorine
sensor
dechlorination
chemicals
chlorinated water
chlorinated water sample
(for calibrating and checking
sensor response)
dechlorinated
water sample
Installation
Linear velocity in basin is
typically not high enough
for a good measurement.
Basins and tanks:
Rarely is there enough flow
through a tank to provide
adequate stirring.
better
sample point
Best practice is to use an
extracted sample and the
FCL arrangement for flow
control
FLOW
Loss of flow
Air bubbles
Membrane fouling
FACTOR AFFECTING
Maintenance
Current depends on diffusion rate through
the membrane.
chlorine
membrane
cathode
clean
surface
fouled
surface
A dirty or fouled membrane blocks the passage of chlorine. The result is low
readings and sluggish response to concentration changes.
26
Sample flow Diffusion rate (ie sensor current) depends on the concentration of
chlorine at the surface of the membrane.
As the sensor operates, it depletes the chlorine at the membrane surface.
Diffusion from bulk liquid is not adequate to replace loss, so current drops.
cathode
membrane
sample
cathode
membrane
sample
cathode
membrane
sample
NO FLOW
27
Sample flow (cont’d)… Flowing sample keeps the concentration of chlorine at the
surface equal to the bulk concentration.
Sensor current is proportional to the concentration of chlorine in the bulk solution.
cathode
membrane
sample
FLOW
cathode
membrane
sample
28
Sample flow is important
Normally, sensor current is a non-linear function of flow.
There is a minimum flow above which the sensor current is more or
less independent of flow.
Each type of sensor has an optimum flow.
sensor
current
flow
minimum flow rate
29
Automatic cleaning systems
wastewater
aeration basin
oxygen
sensor sensor
sensor cable
spray nozzle
membrane
water or air line
normal operation
spray cleaner activated
recovery period
normal operation
Commonly used with oxygen sensors.
30
3 ELECTRODE
CAMPARISION
33
Drinking water filter plant
T
OZozone
generator
screens
raw water
flocculantozone destruct
T
ozone contact
chamber
settling basin
backwash
pump
to screens
backwash reclaim pond
NaOH
NaOClpHCl
T PC
mixer
contact
tank
T
34
Distribution system
filter plant
NaOCl
Cl
Cl
35
Groundwater source
NaOCl
Cl
Cl
well
36
Desalination/wastewater reclamation plant
NaOCl
media filter
TC
C
C
Cl
RO modules
calcite
filter
dCl
NaHSO3
membrane filter
pH Cl T
reject
*SCS921A
**FCL
*
**PC
37
Waste treatment plant
raw
waste
grit chamber
primary sedimentation
tankscreens
to sludge systemto sludge
system
aeration basin
secondary
sedimentation
tank
dechlorination
tank
blower
NaOCl
activated
sludgeto sludge
system
to
aeration
basin
chlorine contact
chamber
DODO
CldCl
T
Cl typically total chlorine