Laboratory 7
THE JAR TEST
Katrina Gibbons
Michelle DiMeglio
November 8, 2001
CE 221 - Professory Kney
Purpose
The purpose of this laboratory was to conduct
the jar test to gain a more complete understanding of the treatment process
for removing suspended solids from water. Performing the jar test
enabled us to approach the problem in a “hands on” manner, rather than
looking only at the theory behind suspended solid removal.
Theory
Turbidity is essentially a measure of the “cloudiness” of the water. The main reason that water is turbid is the presence of colloidal particles, namely those smaller than approximately 10 micrometers in diameter. It is necessary that these particles be removed from the water before it is safe for public use, but this can be difficult to do, because these particles will not settle out by gravitational forces, due to their small size and weight. In order to utilize the advantage of gravity to its fullest extent, the particles must be made larger so that they will settle out, and the only way to do this is to find a way to get them to stick together, or agglomerate.
There are both repulsive and attractive
forces that hinder and promote agglomeration, respectively. These
forces are called Zeta Potential and van der Waal’s forces, where Zeta
Potential is the repulsive force and van der Waal’s forces act to attract
particles to each other. In order for agglomeration to occur, the
van der Waal’s forces must be greater than the Zeta Potential.
Apparatus & Reagents
- Jar Test Apparatus
- six 1500 mL beakers
- pH meter
- pipettes
- conductivity meter
- turbidimeter
Procedure
1. Made up a 10 g/L solution of alum
2. Made up a 0.1 N solution of NaOH (buffer).
3. Filled each of the six 1500 mL beakers with one liter of river water.
4. Measured the temperature, conductivity, and initial pH of the water.
5. Added alum and NaOH solutions, in equal portions, as specified by the instructor.
6. Followed the following mixing protocol:
a. Rapid mix – 1 minute (100 rpm)
b. Slow mix – 15 minutes (20 rpm)
c. Off, settling – 30 minutes
7. Measured the final turbidity. Took a sample from the center of each one liter sample, about 2 inches down, being careful not to disturb the flocs that have settled.
8. Measured the final pH.
Key Equations:
Al2(SO4)3 * 14.3H20 + 6H20
----> 2Al(OH)3(s) + 14.3H20 + 3H2S04
Al2(SO4)3 * 14.3H20 + 6Na(HC03)
----> 2Al(OH)3(s) + 3Na2SO4 + 14.3H20 +6CO2
Al2(SO4)3 * 14.3H20 +6Na(OH)
----> 2Al(OH)3(s) + 3Na2SO4 + 14.3H20
Figure 1. Diagram of the jar test device
Some "action photos" of the class hard at work:
My personal favorite.
Results
The following table displays the data we collected in the laboratory.
Table 1. Data collected from jar test
Beaker Alum (mL)
Buffer (mL) Turbidity (NTU) Initial
pH Final pH Temp.(deg. C)
Conductivity
1
0.00
0.00
1.05
8.06 7.78
20.5
-
2
0.50
0.50
0.91
8.06 7.64
20.5
189
3
1.00
1.00
1.11
8.06 7.52
20.5
196
4
1.50
1.50
1.02
8.06 7.68
20.5
204
5
2.00
2.00
0.67
8.06 7.58
20.5
207
6
2.50
2.50
0.82
8.06 7.80
20.5
213
7
3.00
3.00
0.90
8.06 7.67
19.6
220
8
3.50
3.50
0.65
8.06 7.80
19.6
225
9
4.00
4.00
0.70
8.06 7.70
19.6
232
10
4.50
4.50
0.60
8.06 7.68
19.6
237
11
5.00
5.00
0.40
8.06 7.96
19.6
241
12
5.50
5.50
0.47
8.06 7.78
19.6
245
From our data, it can be seen that although the turbidity fluctuates slightly, there is a general decreasing trend as the amount of alum added to the sample increases. The initial and final pH of each sample decreased, with the initial pH of all samples at 8.06, and the final pH values fluctuating, but all very close in value (the average is 7.72, with a standard deviation of 0.12).
The following graph plots the concentration
of alum against the normalized turbidity. The normalized turbidity
was calculated by dividing the actual turbidity by the initial turbidity.
Figure 2. Graph of alum conc. vs. Normalized
Turbidity
Figure 2 shows that although turbidity
fluctuates, there is an overall downward trend as more alum is added.
Calculations
Turbidimeter used in the laboratory
Normalized Turbidity = Actual (measured)
Turbidity / Initial Turbidity
Normalized Turbidity was calculated in
Excel:
Initial Turbidity = 1.84 NTU
Table 2. Calculation of Normalized
Turbidity
Beaker Actual
Turbidity (NTU) Normalized Turb. (NTU)
Alum (mL)
1
1.05
0.571
0.00
2
0.91
0.495
0.50
3
1.11
0.603
1.00
4
1.02
0.554
1.50
5
0.67
0.364
2.00
6
0.82
0.446
2.50
7
0.90
0.489
3.00
8
0.65
0.353
3.50
9
0.70
0.380
4.00
10
0.60
0.326
4.50
11
0.40
0.217
5.00
12
0.47
0.255
5.50
Amount of alum needed on a monthly basis:
5.0 mL alum x 3.7854
L x 6.5 million gal x 30 days
= 3.69 x 10^9 mL/month
1 L water
1 gal
day
month
3.69 x10^9 mL x
1L x
1 gal x 8.34 lb
= 8.13 x 10^6 lb alum/month
month
1000 mL 3.7854 L
gal
Monthly cost of alum treatment:
8.13 x 10^6 lb alum x
$196 x 1 short ton
= $796,740/month
month
short ton 2000 lb
Discussion and Conclusions
Based on the data, we concluded that, although turbidity generally declines as the amount of alum added to the water increases, there is a point where more alum should not be added. This is because alum will make the water more acidic, and it is for this reason that buffer is added at the same time, and in the same amount as the alum. After analyzing the data, we have decided that the optimum dosage of alum for this system is approximately 5.0 mL of alum. We reached this conclusion based on the fact that the turbidity is at a minimum at this point, at 0.40 NTU.
To apply this conclusion to a more large-scale, realistic situation, we consider the fact that the Easton Water Treatment Plant treats about 6.5 million gallons of water per day. Based on this fact, we calculated that approximately 8.13 million pounds of alum would be needed on a monthly basis to treat the water. We also found that the cost of alum is roughly $196 per short ton, so the monthly cost is about $796,740.
Questions Asked
1. In your own words, discuss attractive forces and repulsive forces with respect to van der Waal’s forces and Zeta Potential.
In the discussion of coagulation and flocculation,
there are two main forces, one attractive and one repulsive, that play
a role in whether flocs will form or not. The repulsive force is
an electrostatic force called the Zeta Potential. The Zeta Potential
is a function of the charge per unit area (q), the thickness of the water
surrounding the shear surface through which the charge is effective (d),
and the dielectric constant of the liquid (D). Contrastingly, van
der Waal’s forces are weak attractive forces between the electrons of neighboring
molecules. These two types of forces are related in that the van
der Waal’s forces have an inverse relationship to the Zeta Potential, 1/d6,
to be exact. From this relationship, it can be seen that by decreasing
the thickness d, the Zeta Potential will be decreased, and it will then
be possible for van der Waal’s forces to dominate. When van der Waal’s
forces are the dominating force, flocs will be formed by the particles,
enabling them to settle out.
2. How can the distance of the effective charge of a particle, with a diamter of less than 1 micrometer, be reduced? By distance, I am referring to the distance (d) in the formula for Zeta Potential, given in the lab.
By adding a positive charge to the water
with the coagulant, the effective charge of the particles is upset, and
the polar orientation of the water molecules is changed. The water
molecules are thus pushed away from the particle, and the distance of the
effective charge of the particle is decreased.
3. Why is the jar test performed and why is it conducted so frequently? Why not just test the water once and be done with it?
The jar test is performed to determine
the best conditions for removing turbidity from water. It is conducted
several times, adding different amounts of coagulant each time, to determine
which concentration of the coagulant in the water will give optimal results
in terms of removing turbidity. This optimum coagulant level is different
for different samples of water, depending on characteristics of the water
such as pH and initial turbidity, so, as a body of water changes the jar
test results will change as well.
4. What are the various coagulants used in flocculation and coagulation?
The coagulant used for our jar test was
Alum. Other coagulants that could be used to aid in the coagulation/flocculation
process include Ferric Chloride and Ferrous Sulfate.
5. What do coagulant aids do, and what are a few reasons for their use?
Coagulant aids introduce positively charged
ions to the water, which reduces the repulsive force, or zeta potential,
between the turbid particles. This allows the attractive forces,
or van der Waals forces, to overcome the repulsive force, causing the particles
to move together. The particles form flocs, which can then settle
out of the water.
Coagulants are used to allow tiny colloidal
particles to settle out of water, thus reducing the turbidity of the water.
These tiny particles are too small to settle out on their own, so coagulants
help to reduce the turbidity of water by causing the particles to form
groups and settle out.
6. What will happen if alum is added without a buffer?
If alum is added without a buffer, the
water will become too acidic for public use. This is because the
addition of alum, in the form of Al2(SO4)3 without a buffer forms
sulfuric acid, H2SO4. When a buffer such as sodium bicarbonate or
sodium hydroxide is added, the acid formed is neutralized
