Transcript Monitoring and Assessing Water Quality

Jar Testing
Coagulation Dosage
Water Treatment Plants
WQT 134
Aquatic Chemistry II
Determining ALUM Coagulation Rates
http://www.cee.vt.edu/ewr/environmental/teach/wtprimer/jartest/jartest.html
http://www.waterspecialists.biz/html/jar_test.html
Week 8 Objectives
Reading assignment:
Tech Brief: Jar Testing
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Understand how to test pH, turbidity, and color on raw
water sample
Understand Jar Testing/Coagulation chemistry
Understand the role of pH, alkalinity, turbidity, temperature
on coagulation and flocculation application
Key Words
• Coagulation: adding and rapid mixing of chemicals to
remove particles from water. (flash mixing)
• Flocculation: adding and slow mixing of chemicals and
particles to create flocs that settle out of water.
• Turbidity: suspended, dissolved, and colloidal particles in
pretreated water that need to be removed to optimize
treatment efficiency.
Jar Testing Steps
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Fill the Phipps and Bird jar testing apparatus containers with
1000 ml of sample water.
Label each Beaker #1 add 1 liter of water =control, Beaker #2
add 2 ml of alum=2 mg/L, Beaker #3 add 5 ml of alum=5 mg/L,
Beaker #4 add 10 ml of alum=10 mg/L, Beaker #5 add 15 ml of
alum=15 mg/L, Beaker #6 add 20 ml of alum=20 mg/L
Add the coagulant to each container and stir at approximately
100 rpm for 1 minute (record condition of flocs during rapid
mix coagulation).
Reduce the stirring speed to 25 to 35 rpm and continue mixing
for 15 to 20 minutes(record condition of flocs every 5 minutes
on data sheet below).
Determine which coagulant dosage has the best flocculation
time and the most floc settled out.
Test the turbidity of the water in each beaker using a
turbidimeter (record value on data sheet).
Jar Testing Experimental Design
#1
#2
#3
#4
#5
#6
control 2 mg/L 5mg/L 10 mg/L 15 mg/L 20 mg/L
Add 1 liter of water Label each
Beaker #1 =control
Beaker #2 add 2 ml of alum=2 mg/L
Beaker #3 add 5 ml of alum=5 mg/L
Beaker #4 add 10 ml of alum=10 mg/L
Beaker #5 add 15 ml of alum=15 mg/L
Beaker #6 add 20 ml of alum=20 mg/L
Rapid mix 100 rpm for 1 min
Record floc conditions
Slow mix 25 rpm for 15 min
Record floc conditions every 5 min
Let settle for 10 min
Determine optimal coagulant dosage.
Record Turbidity on optimal coagulant dose
Jar Testing Results
• A hazy sample indicates poor coagulation.
• Properly coagulated water contains floc particles
that are well-formed and dense, with the liquid
between the particles clear.
In determining the proper
dosage of alum, the most useful
test is the _______ test:
a. marble
b. jar
c. carbonate
d. pH
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Which of the following is the
main purpose of the
coagulation/flocculation
process?
a. to remove turbidity
b. to soften the water
c. to add oxygen
d. to disinfect.
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The most important raw water
constituent for a surface water
plant is:
a. temperature
b. hardness
c. turbidity
d. pH
a.
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A laboratory procedure for
evaluating coagulation,
flocculation, and
sedimentation is called what?
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25% 25% 25% 25%
Temperature profile
Turbidity removal efficiency
Treatability study
Jar testing
Te
1.
2.
3.
4.
Coagulation and
Flocculation at Water
Treatment Plants
“Ironically, it is easier to clean up dirty water
than to make clean water cleaner. The reason
is because particles must collide before they
can stick together to make larger flocs. More
particles means more collisions.”
Turbidity
• Turbidity – particles (sand, silt, clay, bacteria,
viruses) in the initial source water that need to be
removed to improve treatment.
1. Suspended Solids
2. Colloidal Solids (~0.1 to 1 mm)
3. Dissolved Solids (<0.02 mm)
7 ntu
Raw water
375 ntu 0.02 ntu
backwash Treated
2
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5 ntu
1 ntu
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The turbidity of a water
treatment plant effluent cannot
be above?
Coagulation
• Coagulants tend to be positively charged.
•Due to their positive charge, they are attracted to
the negative particles in the water
•The combination of positive and negative charge
results in a neutral , or lack, of charge
•Van der Waal's forces refer to the tendency of
particles in nature to attract each other weakly if
they have no charge.
Water Treatment Coagulants
Particles in water are negative; coagulants
usually positively charged.
1. Alum- aluminum sulfate
2. Ferric chloride or ferrous sulfate
3. Polymers
What determines the optimum
and most cost-effective amount
of a coagulant to use?:
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1. Beyond that dose, it takes a
very large increase in the
amount of chemical to
produce a small increase in
turbidity removal
2. Below that dose the
coagulant results in poor
settling
3. The treatment plant budget
4. Divide the number of
gallons of water in the
coagulation tank by the
nephelometric turbidity unit
reading to determine the
dosage in mg/L.
Which is NOT a common method
for determining optimum
coagulant effectiveness?:
1. Jar test
2. Zeta potential detector
3. Streaming current
detector
4. Colorimetric method
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Water Treatment Coagulant Alum
Alum- (aluminum sulfate)- particles suspended in
natural, untreated water normally carry a negative electrical
charge. These particles are attracted to the positive charges
created by aluminum hydroxides. Dosage is generally around
25 mg/L.
1. Trivalent Al+3 charge attracts neg – particles
2. Forms flocs of aluminum hydroxide (AlOH3).
3. Impacted by mixing, alkalinity, turbidity
and temp.
4. Ideal pH range 5.8-8.5
Alum CHEMISTRY
Alum- (aluminum sulfate)- made by dissolving
aluminum hydroxide (bauxite or clay) in sulfuric acid
2Al(OH)3 + 3H2SO4 + 10H2O → Al2(SO4)3·16H2O
When ALUM is dissolved in alkaline water, it
undergoes hydrolysis (reacts with water) to produce
a high surface area gelatinous precipitate of
aluminum hydroxide, Al(OH)3 (gibbsite)
(Al(OH)3 sticks the negatives.
When ALUM is reacted with water it hydrolyzes to
form aluminum hydroxide and dilute sulfuric
acid (lowers pH).
-----Need alkalinity adjustment
Alum CHEMISTRY
Alum- (aluminum sulfate)Al2(SO4)3·14H2O
2Al+3 +3SO4-2 +14H2O
2Al+3 + negatively charged colloids
neutral surface charge
WHY IS ALKALINITY SO IMPORTANT??
2Al+3 + 6 HCO3-
2(Al(OH3)(S) + 6CO2
No bicarbonate (low alkalinity, low pH sulfuric acid!):
Al2(SO4)3·14H2O
Optimum pH: 5.5 to 6.5
Operating pH: 5 to8
2(Al(OH3) (S) +3H2SO4-2 +14H2O
When alum is added to water, a
floc is formed from the
combination of alum and
a. alkalinity
b. acid
c. chlorine
d. lime
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The precipitate formed by
coagulation with alum is
aluminum ________.
Bicarbonate
Carbonate
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Sulfate
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Adding Alum to water will cause
the pH of the water to increase.
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2. False
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Alum comes in dry grade as a
minimum of 17.5% pure product,
in liquid form it is 49% pure or
8.23% by weight Al2O3?
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2. False
Overcoming problems of coldwater floc can be corrected by
operating the process at the best
pH for that water temperature,
increasing the coagulant dosage,
1. Adding weighting
or:
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2. Performing the jar test
3. Increasing the number
and strength of floc
particles
4. Increasing the
detention time for floc
formation
Which of the following
conditions most affect
coagulation performance?
a. velocity, chlorine dosage,
detention time, and air
temperature
b. velocity, water temperature,
detention time and
coagulant dosage
c. water temperature, detention
time, air temperature, and
chlorine dosage
d. detention time, velocity, air
temperature, and chlorine
dosage
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With the coming of winter, the
water temperature drops. A
likely operational problem at a
filtration plant with coagulation
100%
is:
1. Floc carryover from the
sedimentation system
2. High chlorine residual
3. High alkalinity
4. Odor
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Water Treatment Coagulant Aids
Activated silica (sodium silicate)- helps improve
coagulation, decreases volume of coagulant necessary.
Typically is sodium silicate.
1. secondary coagulant
2. reduces primary coagulants needed
3. Sodium silicate are alkaline
4. widens pH range for coagulation
5. used at 7-11% of alum
6. Heavier denser floc that settles faster
7. Can be formed on site
8. Corrosion inhibitor (forms a surface coating)
The three most commonly used
coagulants in water treatment
are:
67%
1. Aluminum hydroxide,
lime and sodium
hydroxide
2. Aluminum sulfate,
ferric chloride, and
ferrous sulfate
3. Lime, sodium
hydroxide, and chlorine
4. Soda, lime and chlorine
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Which of the following would
most likely improve the
coagulation/flocculation
process?
a. increase in raw water
hardness
b. decrease in water
temperature
c. increase in water
temperature
d. decrease in raw water
alkalinity
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Water Treatment Coagulant Aids
Polyelectrolytes- are water-soluble organic polymers
that are used as both primary coagulants and
coagulant aids. Act as "bridges" between the alread
formed particles :
• Anionic—ionize in solution to form negative sites
along the polymer molecule.
• Cationic—ionize to form positive sites.
• Non-ionic—very slight ionization.
• effectiveness: particles type, turbidity present, and
the turbulence (mixing) available during coagulation
Which one of the following
chemicals would be most
suitable as a filter aid?
a. alum
b. soda ash
c. sodium hydroxide
d. anionic polymer
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Water Treatment Coagulant/pH
Alkalinity- Alkalinity is a measure of the buffering
capacity of water. These buffering materials are
primarily the bases bicarbonate (HCO3-), and
carbonate (CO32-), and occasionally hydroxide (OH), borates, silicates, phosphates, ammonium,
sulfides, and organic ligands.
Chemicals applied to raise alkalinity
• Lime—CaOH2 accompanies alum or iron salt
• Sodium bicarbonate- NaHCO3- raise alkalinity
• Soda Ash—Na2CO3 -raise alkalinity
• Caustic Soda—NaOH -raise alkalinity