Coagulation and Flocculation: Color Removal
Submitted to: Dr. Hashsham
Research Complex Engineering
Department of Civil and Environmental Engineering
Michigan State University
East Lansing, MI 48824
Authors
Dipa Dey
Amanda Herzog
Vidya Srinivasan
ENE 806
March 14, 2007
i
Acknowledgement
First of all, we would like to thank Professor Syed A. Hashsham who
instructed this course and gave us useful advices for our project. Professor
Hashsham has given us plenty of guidance, which was greatly helpful for the
successful completion of this study. Finally we are thankful to Joseph
Nguyen for his help, valuable information and advices during this study.
May 2, 2007
Dipa Dey
Amanda Herzog
Vidya Srinivasan
ii
CONTENTS
Caption
Page No.
Title page
i
Acknowledgement
ii
Contents
iii
List of Figures and Tables
iv
Abstract
1
Introduction
2
Objective
4
Material and methods
4
Results and discussion
5
Conclusions
10
References
11
Appendix -1
12
Appendix-2
14
Experimental setup
14
Appendix-3
15
Appendix-4
17
Coagulation and Flocculation: Color Removal: Lab Protocol
17
Procedure to determine Alkalinity
19
iii
LIST OF FIGURES AND TABLES
Caption
Page No.
Figure 1: Concentration vs Absorbance for Humic Acid (254 nm wavelength)
5
Figure 2: Effect of Alum Dose on Supernatant Turbidity, pH and Remaining Humic
Acid
6
Figure3: Effect of Initial pH on Humic Acid and Turbidity Removal
7
Figure 4: Effect of Initial pH on Humic acid and Turbidity Removal (Spiked Sample)
8
Figure5: Effect of Initial pH on Humic acid and turbidity Removal (Non-Spiked
Sample)
9
Figure A: Removal of TOC by alum coagulation
12
Figure B: Alum Coagulation diagram and its relation to Zeta Potential
12
Figure C: Color removal domains on Alum coagulation
13
Conventional Jar Test Setup
14
Formation of Flocs
14
Settled Particles
14
Table 1: Trial Run
15
Table2: Run1
15
Table 3: Run 3
16
Table 4: Run 3 and Run 4
16
Table 5: Different Jars with Different Alum Dosages (Spiked)
17
Table 6: Different Jars with Different Alum Dosages (Non Spiked)
17
iv
ABSTRACT
Water is expected to be free from pathogenic organisms, undesirable taste and odor. In addition
it should be clear and free from minerals before supplied as a drinking water. Jar test
experiments on coagulation and flocculation were conducted to remove color from water caused
by humic substances. These tests help to obtain an optimum alum dosage required for color
removal. Water sample were collected from Red Cedar River and spiked with humic acid to a
concentration of 35mg/L. Alum varying from 10-220 mg/l was used as a coagulating agent and
pH ranged from 4-9.5 was studied to obtain the optimum conditions in terms of removal of
humic acid, turbidity and settling of flocs. Results were obtained after doing four runs and
compared to similar published studies. An approximate 70% removal of color was achieved
with an alum dosage of 140 mg/L at pH 6. When reducing cost was considered the optimal
conditions are an alum dosage of 100 mg/L at pH 6.5 which results with a 60% removal.
1
INTRODUCTION
An ideal waterworks management ensures that the water supplied for public distribution is free
from pathogenic organisms, undesirable taste and odor, clear, palatable, of reasonable
temperature, neither corrosive nor scale forming and free from minerals which could produce
undesirable physiological effects. Potable water does not mean pure water but water that is free
from harmful impurities, harmful salts and harmful bacteria. In fact water may contain certain
salts and minerals in limited amounts as they impart good taste and assist in digestion. Therefore,
the water should be tested before it is distributed. The aim of laboratory test is to ensure that
potable water conforming to standards is supplied to consumers. In the present study,
Coagulation and Flocculation experiments or Jar Tests were conducted to remove color from
water due to humic substances using alum as the coagulating agent. These tests help in
determining the optimum alum dosage and pH required for color removal. Chemicals used for
coagulation in water treatment should be cost-effective for coagulating impurities and to help
remove any traces of toxic or other undesirable residues in water.
The jar test is a common laboratory procedure used to determine the optimum operating
conditions for water or wastewater treatment. This method allows adjustments in pH, variations
in coagulant or polymer dose, alternating mixing speeds, or testing of different coagulant or
polymer types, on a small scale in order to predict the functioning of a large scale treatment
operation. A jar test simulates the coagulation and flocculation processes that encourage the
removal of suspended colloids and organic matter which can lead to turbidity, odor and taste
problems. Testing is done for the reasons such as, correct chemical dosage and pH adjustments,
sludge characteristics, turbidity removal, color removal, THM (trihalomethanes) removal, and
control chemical costs.
In water and wastewater treatment operations, the processes of
coagulation and flocculation are employed to separate suspended solids from water. Although
the terms coagulation and flocculation are often used interchangeably, or the single term
"flocculation" is used to describe both; they are, in fact, two distinct processes. Coagulation and
flocculation processes are used to improve removal of solids in both drinking water and
wastewater treatment, through destabilization and aggregation of suspended material. Floc
formation and electric charges are the basis of coagulation. When a coagulant is added to the
water and mixed thoroughly, a thick gelatinous precipitate is formed which is insoluble in water.
2
This precipitate is called as “floc”. As the floc settles down, it coalesces with the colloidal
particles. Also, the floc is positively charged and the colloidal particles are negatively charged.
The flocs thus attract the colloidal particles and hence make them settle down.
Flocculation is the action of coagulant or polymers to form bridges between the flocs and bind
the particles into large agglomerates or clumps. Bridging occurs when segments of the polymer
chain adsorb on different particles and help particles aggregate. An anionic flocculants reacts
with a positively charged suspension, thus adsorbing on the particle and hence causing
destabilization either by bridging or charge neutralization. In this process it is essential that the
flocculating agent be added by slow and gentle mixing to allow for contact between the small
flocs and to agglomerate them into larger particles. The newly formed agglomerated particles
are quite fragile and can be broken apart by shear forces during mixing. Care must also be taken
to avoid overdosing the polymer as doing so causes settling/clarification problems. Anionic
polymers are lighter than water, hence, increasing the dosage will increase the tendency of the
floc to float and not settle.
Once suspended particles are flocculated into larger particles, they can usually be removed from
the liquid by sedimentation, provided that a sufficient density difference exists between the
suspended matter and the liquid. Such particles can also be removed or separated by media
filtration, straining or floatation. When a filtering process is used, the addition of a flocculants
may not be required since the particles formed by the coagulation reaction may be of sufficient
size to allow removal. The flocculation reaction not only increases the size of the floc particles,
but also affects the physical nature of the floc, making these particles less gelatinous and thereby
easier to dewater.
3
OBJECTIVE
The objective of the jar test experiments is to determine the optimal conditions for removal of
color due to humic substances using aluminum sulfate octadecahydrate as the coagulant agent.
MATERIAL AND METHODS
Surface water was obtained from the Red Cedar River, East Lansing, which had a concentration
of humic acid of approximately 25 mg/L and an alkalinity of 219 mg/L as CaCO3. An additional
10 mg/L of humic acid was spiked in the Red Cedar River samples to adjust the humic acid to
approximately 35 mg/L or a 70% absorbance. The coagulant a was prepared as a stock solution
of 10 mg/mL aluminum sulfate octadecahydrate (Al2(SO4)3•18H2O) . The jar test was executed
with the conventional apparatus with twelve 2-liter beakers (Appendix-2). Each run was mixed
for 160 rpm for 1 min, 70 rpm for 15 min, 30 rpm for 35 min and settled for 2 hours. The
absorbance was measured by the Spectronic Genesys 2 spectrophotometer at 254 nm using 1 cm
quartz cells. A standard curve was generated using tap water and a humic acid range of 0-40
mg/L to evaluate the relationship of concentration vs. absorbance (Fig 1.). The turbidity was
measured using the Turbidimeter 2100A.
4
RESULTS AND DISCUSSION
A total of 4 jar test experiments were conducted by varying parameters such as pH and alum
dosage. The main objectives of the experiments were to determine the optimum alum dosage and
pH for efficient color and turbidity removal. Changes in pH, turbidity and color were noted for
each run. To enhance the absorbance to 70% which was determined using the standard curve
(Fig 1), the Red Cedar River sample was spiked with humic acid to a concentration of 35mg/L.
The first run determined the optimum alum dosage required for color removal, by varying the
alum dosage and fixing the initial pH of the sample. Alum dosages varying from 10-220 mg/L
were chosen. The effects of the remaining absorbance and turbidity, including final pH were
compared in figure 2. The percentage of humic acid remaining in the sample decreased with an
increase in alum dosage. The highest alum dosage of 220 mg/L resulted in about 76% removal of
humic acid (Fig 2). These results were compared with results from previous studies
(Appendix-1, Fig A) and were in significant correlation. Turbidity showed a gradual decrease
with increase in alum dosage which for alum dosage of 220 mg/L resulted in approximately 86%
removal.
Humic Acid Concentration, mg/l
40
30
20
10
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Absorbance
5
Figure 1: Concentration vs Absorbance for Humic Acid (254 nm wavelength)
A substantial decrease in humic acid content was observed beyond an alum dosage of 30 mg/L
(Fig 2). Alum dosages higher than 100 mg/L did not result in a significant decrease in turbidity
or absorbance, which was in accordance with a previous study carried out with different alum
10
1.0
9
0.9
8
0.8
pH
7
0.7
6
0.6
5
0.5
4
0.4
3
% Absorbance
Measured pH and Turbidity (NTU)
dosages and color causing components (Appendix-1, Fig A)
0.3
% Absorbance
2
0.2
Remaining Turbidity, NTU
1
0.1
0
0.0
0
50
100
150
200
Alum Dosage, mg/L
Figure 2: Effect of Alum Dose on Supernatant Turbidity, pH and Remaining Humic Acid
The alkalinity of the river water sample was measured to be 219 mg/L as CaCO3. If the water is
poorly buffered, any addition of alum results in a drop in pH. In our study, addition of 220 mg/l
of alum resulted in a drop of approximately 1 pH unit.
6
10
0.6
0.5
8
Final pH
0.4
6
0.3
% Absorbance
4
0.2
2
% Absorbance
Measured pH or Turbidity (NTU)
Initial pH
0.1
Remaining Turbidity, NTU
0
0.0
4
5
6
7
8
9
10
Initial Adjusted pH
Figure3: Effect of Initial pH on Humic Acid and Turbidity Removal
The second run determined the optimum pH for color removal by varying the initial pH of the
sample and fixing the alum dosage. Minor changes in turbidity and absorbance removal were
observed for alum dosages higher than 140 mg/L, therefore, 140mg/L was selected as the
optimum alum dosage. A gradual decrease in color and turbidity removal was observed between
pH 4 and 6. Beyond a pH of 6, there was minimal change in turbidity and absorbance. A dip in
the final pH curve (between 6 and 6.5) shows that there exists an optimum pH for color removal.
At a pH of 6, the color removal was about 72%. Any increase in pH beyond 6.5, did not result in
significant color removal. From Fig B (Appendix-1), it is evident that our study followed the
trend as depicted in the charge neutralization zone and did not show any significant color
removal in the sweep coagulation zone. Figure C (Appendix-1) shows that about 40% removal
for 100cu was obtained for a pH of around 6.
7
The third and fourth runs were with spiked and non spiked samples respectively. Spiked and non
spiked samples were used to compare the effects of the optimum alum dosage and pH on the
addition of humic acid and the natural occurring humic substances. The main objective of run
four and five was to further optimize the experiment, where the alum dosage is reduced and the
initial pH is adjusted closer to neutrality. This would assist in reducing the cost of alum required
and the time in adjusting the pH to 7 after treatment. The alum dosage was reduced to 100 mg/L
and the initial pH values used ranged from 5 to 7.5 for each jar respectively. Such a set up
allowed us to verify if effective color and turbidity removal can be obtained by using a lower
alum dosage and a pH closer to 7. There was a steep decrease observed in turbidity and color
removal in Fig 4, for pH values ranging from 5 to 6, while the change is much more gradual after
a pH of 6. Therefore pH 6 is the optimum for an alum dosage of 100mg/L.
8
0.50
0.45
7
Final pH
Initial pH
0.40
6
0.35
5
% Absorbance
0.30
4
0.25
3
0.20
Remaining Turbidity, NTU
% Absorbance
Measured pH or Turbidity (NTU)
Alum Dose = 100 mg/L
0.15
2
0.10
1
0.05
0
0.00
5
6
7
8
Initial adjusted pH
Figure 4: Effect of Initial pH on Humic acid and Turbidity Removal
(Spiked Sample)
8
10
0.50
Alum Dose = 100 mg/L
8
0.40
Initial pH
0.35
Final pH
6
0.30
0.25
% Absorbance
4
0.20
% Absorbance
Measured pH or Turbidity (NTU)
0.45
0.15
2
0.10
Remaining Turbidity, NTU
0.05
0
0.00
5
6
7
8
Initial Adjusted pH
Figure5: Effect of Initial pH on Humic acid and turbidity Removal
(Non-Spiked Sample)
The non spiked samples reacted similarly to the spiked samples (Fig 5) resulting in a pH of 5.7 to
be an optimum pH for effective color and turbidity removal. An alum dosage of 100 mg/L at a
pH between 5.5 and 6 is the optimal conditions for effective color and turbidity removal.
9
CONCLUSIONS
This experiment yielded the following conclusions:
•
At alum dosage of 140 mg/L the color is removed by 70%; concentrations higher than
140mg/L had little significance in color removal after this point.
•
A maximum of 70% of color removal was achieved at an initial pH of 6. Approximately
40% in color reductions occurred below or above pH 6.
•
When cost is considered the optimal conditions for color removal is an alum dosage of
100 mg/L at pH 6.
10
REFERENCES
1. American Water Works Association. Water Quality and Treatment: A Handbook of
Community Water Supplies, 4th Ed. 1990. McGraw-Hill
2. Syed Hashsham. Coagulation and Flocculation: Color Removal, Lab Report. 1993.
3. http://www.waterspecialists.biz/html/about_coagulation___flocculati.html
4. http://www.ctre.iastate.edu/educweb/ce523/Notes/CoagFloc.doc
11
APPENDIX-1
Figure A: Removal of TOC by alum coagulation
12
Figure B: Alum Coagulation diagram and its relation to Zeta Potential
Figure C: Color removal domains on Alum coagulation
13
APPENDIX-2
Experimental setup
Conventional Jar Test Setup
Formation of Flocs
Settled Particles
14
APPENDIX-3
Table 1: Trial Run
Trial Run
pH
Turbidity (NTU)
Temperature (o C)
Alkalinity
Color
Initial fixed pH
After Settling
Spiked Sample (+ alum dosage)
10
50
100
Raw Water Raw Water Spiked w/ Humic Acid
8.13
8.08
5.1
4.1
219
0.49
N/A
233
0.627
7.5
pH
8.19
7.88
7.56
Temperature
22.3
22.3
22.3
Turbidity Absorbance
5.4
0.661
6.7
0.526
2.1
0.303
Table2: Run1
Run 1
Raw Water
pH
Turbidity
Color
Alkalinity
Temp
8.06
3.1
0.473
17.8
23.3
After Setteling the samples
Alum Doses
Turbidity pH
0
4.5
10
7
20
9.4
40
7.5
60
5.1
80
3.1
100
1.6
120
1.4
140
0.9
160
1.1
180
0.6
200
0.9
220
0.6
Absorbance
8.05
7.98
7.89
7.766
7.368
7.278
7.146
7.083
7.06
6.996
6.833
6.83
6.72
0.752
0.726
0.762
0.704
0.513
0.373
0.28
0.252
0.202
0.202
0.194
0.178
0.174
Background=0.063
Alkalinity(ml)
22.6
16.2
17.8
29.1
24.9
19.5
14.5
10
9.1
8.2
7.8
8.3
7.5
15
Table 3: Run 3
Run 2
Alum Dosage: 140 mg/L
Initial pH
Initial pH Final pH Turbidity Absorbance
4
4
3.8
2.6
0.568
4.5
4.5
4.07
2
0.52
5
5
4.46
2.4
0.42
5.5
5.5
4.59
3.4
0.331
6
6
5.1
1.2
0.159
6.5
6.5
6.2
0.8
0.165
7
7
6.76
1
0.208
7.5
7.5
6.88
0.8
0.216
8
8
7
0.8
0.225
8.5
8.5
7.07
1
0.245
9
9
7.22
0.9
0.255
9.5
9.5
7.61
0.9
0.285
Table 4: Run 3 and Run 4
Run 3 and 4
alum dosage = 100 mg/L
WITHSPIKED
Initial adjusted pH Turbidity
5
7
5.5
4.5
6
1
6.5
1.6
7
2.6
7.5
2.5
WITHOUTSPIKED
Initial adjusted pH Turbidity
5
4.8
5.5
8.5
6
1.2
6.5
1.5
7
1.2
7.5
1.2
pH
4.58
4.86
5.9
6.43
6.85
7.08
Absorbance
0.357
0.205
0.172
0.24
0.289
0.291
pH
4.45
4.66
5.76
6.25
6.48
6.95
Absorbance
0.446
0.331
0.15
0.168
0.181
0.207
16
APPENDIX-4
Coagulation and Flocculation: Color Removal: Lab Protocol
Standard Curve:
1. Using the spectrophotometer find the absorbance of tap water (to calculate background)
2. Spike 1L of tap water with 1 mg of Humic acid (salt) and then use the spectrophotometer
to find the absorbance.
3. Continue spiking tap water for concentration 5, 10, 15, etc (in increments of 5) until the
absorbance reaches a reading of approximately 95%.
4. Minus the background (step 1); plot Humic acid concentration (mg/L) vs Absorbance
Run 1: (spiked)
1. Measure the initial pH, temperature, turbidity and alkalinity (refer to the procedure at the
end of the handout) of the river water sample (Red Cedar)
2. Spike river water sample with Humic acid to an absorbance to 70%; Note: concentration
of Humic acid to be added can be determined from the standard curve plot.
3. Measure pH and adjust to 7.5
4. Make up Alum Dosages with distilled water:
Table 5: Different Jars with Different Alum Dosages (Spiked)
Error! Not a valid link.
5. Add the alum dosage to 1L of river water for each jar.
6. Mix at 100+ RPM for 1 min
7. Mix at 70 RPM for 15 min
8. Mix at 30 RPM for 35min
9. Let settle for 30 min.
10. After settling measure supernatant pH, turbidity, and absorbance (degree of color
removal).
11. Plot the effect of alum dose on supernatant turbidity, pH, and remaining humic acid.
Run 2: (spiked)
1. From run 1 chose an appropriate alum dosage.
2. Adjust pH from 4-9.5 (increments of 0.5) which is the initial adjusted pH
3. Add the chosen alum dosage
17
4. Mix at 100+ RPM 1 min
5. Mix at 70 RPM for 15 min
6. Mix at 30 RPM for 35 min
7. Let settle for 30 min
8. After settling measure supernatant pH, turbidity and absorbance (degree of color
removal).
9. Plot the effect of initial pH on Humic acid and turbidity removal
Run 3: (not spiked)
1. Measure the initial pH, temperature, and turbidity of the river water sample (Red Cedar)
2. Adjust pH to 7.5
3. Make up Alum Dosages with distilled water:
Table 6: Different Jars with Different Alum Dosages (Non Spiked)
Error! Not a valid link.
4. Add the alum dosage to 1L of river water for each jar.
5. Mix at 100+ RPM for 1 min
6. Mix at 70 RPM for 15 min
7. Mix at 30 RPM for 35min
8. Let settle for 30 min.
9. After settling measure supernatant pH, turbidity, and absorbance.
10. Plot the effect of alum dose on supernatant (of the river water sample only) turbidity, pH,
and remaining humic acid.
11. Compare plot from run 1 with run 3
Run 4: (not spiked)
1. Measure the initial pH, temperature, and turbidity of the river water sample (Red Cedar)
2. Use the appropriate alum dosage used in run 2.
3. Adjust pH from 4-8 (increments of 0.5) which is the initial adjusted pH
4. Add the chosen alum dosage
5. Mix at 100+ RPM 1 min
6. Mix at 70 RPM for 15 min
7. Mix at 30 RPM for 35 min
18
8. Let settle for 30 min to 1 hour
9. After settling measure supernatant pH, turbidity and absorbance (degree of color
removal).
10. Plot the effect of initial pH on Humic acid and turbidity removal
11. Compare run 2 with run 4.
Procedure to determine Alkalinity
1. Use pH 7.0 buffer and adjust the pH meter to 7.0. Use pH 4.0 buffer and adjust the pH meter
to 4.0.
2. Titrate 100 mL of sample with 0.02 N H2SO4 to obtain pH 4.5 by vigorously stirring
towards the end of the titration step. This is done in order to break the surface and to obtain
rapid equilibrium between CO2 in solution and CO2 in the atmosphere.
3. Total Alkalinity is measured as CaCO3 in mg/L= (ml of titrant) X 10
To prepare 0.02N H2SO4, Use the following formula:
Nf*Vf=Ni*Vi
Where:
Nf: Final normality desired
Vf: Final total volume
Ni: Initial normality
Vi: Initial volume
Note:
The addition of coagulants to waste water causes chemical reactions where repulsive forces
surrounding the colloids are neutralized, thereby resulting in floc formation. These particles can
be removed by filtration or sedimentation. Soon after settling, the appearance of each jar is
observed. If no jar appears clear, increase alum dosage in the next set of experiments. Deficient
dosage causes the jar to look cloudy with insignificant floc formation/settling. An excess amount
of alum results in dense floc formation and poor settling. The jar with the optimum alum dosage
19
will have settled floc at the bottom and a relatively clear supernatant. The clearest sample is
determined by measuring it’s turbidity. If turbidity values are not satisfactory, the experiment
needs to be repeated with different alum dosages.
20