7/27/2019 W13-CIVE375-Lab4

1/7

Department of Civil and Environmental Engineering

Winter 2013 | CIVE 375: Water Quality Engineering

Lab 4: Coagulation, Flocculation and Sedimentation

Introduction

Chemical coagulation and flocculation are important processes, used prior to sedimentation, to increase

solids removal within a drinking water treatment plant.

Much of the suspended matter in water is colloidal in nature. Colloidally suspended particles cannot be

removed by gravity sedimentation alone as they are negatively charged and the repulsion of particles from

one another keeps them in solution. The most common method of removing them is the combined

treatments of coagulation and flocculation followed by sedimentation.

A chemical coagulant is added to a solution to suppress the charge on the particles and allow flocculation

to occur. Flocculation is the process of colloidal particles colliding and forming larger particles which are

susceptible to gravitational forces and can thus be settled out.

The laboratory test commonly used for determining the approximate dosage of a particular chemical that

is required for treatment is called the "jar test". The water to be tested is placed in jars or beakers. Various

amounts of chemicals are put into the jars, and the ingredients are stirred to cause the formation of the

floc. The floc is then allowed to settle in each jar, and the jar in which the least amount of chemical

produced a good settleable floc is chosen as the one with the most economical dosage. Normally this

dosage would then be used in the column test. However, due to time limitations, a coagulant dosage for

the column test has already been prescribed in this lab.

A settling column analysis is commonly performed to evaluate the % removal vs. depth of settling tank as

this relationship is non-linear and thus difficult to describe mathematically.

Procedure

This laboratory will be completed as a lab section group effort.

The suspension to be worked with is a prepared suspension of clay. The coagulant is a ferric sulphate

solution at a concentration of 2 mg/mL.

7/27/2019 W13-CIVE375-Lab4

2/7

W13-CIVE375 Lab 4 2/7

A. Jar Test

1. Select one person to be responsible for timing the steps of the jar test procedure.2. Measure 1 L of sample in a graduated cylinder and transfer to the jars under the stirrers.3. Number the jars in sequence and record the experimental dosage to be used in each. The trial

dosages should be 0 (control), 1, 2, 4, 6 and 10 mL of coagulant (at concentration 2 mg/mL).

4. Start the stirrer at 100 rpm immediately before adding the coagulant.5. Add the coagulant to each jar simultaneously (use 5 pipettes and 5 people).6. Continue rapid mix for one minute.7. Reduce the speed of the stirrer to 30 rpm and stir at that rate for 15 minutes. Watch the jars

carefully and record the time at which the first floc appears in each jar (with respect to the end of

rapid mix). Floc will appear as discretely visible particles, but may be very small.

8. At the end of the slow-stirring perioda. stop the stirrer

b. record the floc index (see table in observations)c. allow the floc to settle for 20 minutes

Be careful not to disturb the jars during the settling period

9. After the settling period, test the supernatant of all the samples for pH (pH meter) and turbidity.10.Record all data on the sheet provided.

B. Settling Column

A settling column (0.265 m inside diameter 2.54 m depth) has been prepared with a suspension of clay.

1. Take 50 mL samples (using a graduated cylinder) from all ports2. Immediately filter the samples using the vacuum apparatus and prepared filters (take care to use

the correctly labeled filter).

3.

Rinse out the graduated cylinder.4. Stop air mixing and repeat steps 1 to 3 at time intervals of 5, 10, 20, 30, 45 and 60 minutes.

These will be assigned to individual groups by the technician.

5. Place the used filter discs in the appropriate evaporating dish and place in the 105C oven.Weighing will be performed by the technician.

Be very careful not to mix up the filter discs.

Safety Procedures

1. Solutions should be handled with normal care to avoid prolonged exposure to skin and eyes andto avoid ingestion. Wash and dry hands before leaving laboratory.2. Keep hands and clothing clear of rotating spindles on the jar-testing apparatus.3. Avoid contact with internal surfaces of 105C drying oven. If burned, flush area with cold water

until burning sensation subsides.

7/27/2019 W13-CIVE375-Lab4

3/7

W13-CIVE375 Lab 4 3/7

Assignment

1. What causes particles to remain in a colloidal suspension? How does a coagulant aid in the settling ofsuch a suspension?

2. Explain clearly why the settling velocity of floc is non-linear in nature. That is the floc settlingvelocity changes in time.

3. Create a plot of coagulant added versus turbidity. What is the optimal dose of coagulant (in mg/L)?Give clear reasons why you chose this dose.

4. Prepare a solids percent removal contour plot for the column test (see Figure 1). Plot each measured%removal value on the depth-time grid (depth of port below free surface versus time of sample).

Draw contours of equal percentage removal. It is recommended to draw the contours by hand. Your

graph should contain at least five contour lines. Clearly explain any discrepancies in your graph. If

you found the contours difficult to plot, you may remove certain suspect data points, but be sure to

explain how and why you did this.

5. Determine the overall solids removal in the settling column at a depth of 1.25 m and time of 25 minusing the %removal contour plot. A horizontal depth-line is drawn at the required depthD. A

vertical time-line is drawn at the required time. %removal values are defined along the time-line

from a depth of 0 toD. The defined values are intersection points with contours or interpolated end-

points. The overall solids %removalR is defined as

2 3 11 1 2 2...

2 2 2

i i ir d rrd r d

RD D D

rr

80%

72%

64%

58%

51%

80%

60%

48%

42%

38%

76%

72%

70%

66%

61%

38%

28%

23%

25%

Figure 1: Example of percent removal contours. Each circle represents a measured %removal ofsuspended solids at a specific port and time. Note that the measured values shown are ideal. Actual

measured values will exhibit more variability and require judgement when contouring the trends of%removal.

7/27/2019 W13-CIVE375-Lab4

4/7

W13-CIVE375 Lab 4 4/7

where di are the depths or distances between %removal values ri. Using the column test results

presented in Figure 2, the overall solids removal at depth = 1.50 m and time = 20 min is given by

0.38 100 0.27 70 0.35 60 5 0.5 50 4361.6%

1.5 2 1.5 2 1.5 2 1.

70

5

0

2

6 0R

Also note that iD d .

6. Explain how the overall solids removal calculation is used to design a sedimentation reactor (tank).

Submission Format:

Include a title page with lab number, student names, group number, lab section and date. Answer the assignment questions using numbered sections that correspond to the question numbers. Write technically (clear, concise, and complete) using full sentences and good quality tables and

figures as required.

Include an appendix containing your original lab data sheet and legible example hand calculations. Report sources outside of materials provided by the teaching team.

0.0

0.5

1.0

1.5

2.0

2.5

0 10 20 30 40 50 60

depth(m

)

t (min)

30% 40% 50% 60% 70%

r1 = 100%

r2 = 70%

r3 = 60%

r4 = 50%

r5 = 43%

d1

d2

d3

d4

D

Figure 2: Example of determining overall solids removal at a specific time and depth in a column test

using the %removal contours. A horizontal depth-line is drawn at the required depthD. A vertical time-line is drawn at the required time. The rvalues are %removal values defined along the time-line from

depth of 0 toD. The defined values are intersection points with contours or interpolated end-points. The

dvalues are the depths or distances between selected rvalues.

7/27/2019 W13-CIVE375-Lab4

5/7

W13-CIVE375 Lab 4 5/7

Observations

Include this observation sheet in the appendix of your report.

A. Jar Test

Jar # Dose

(mL)

Coagulant

(mg/L)

pH Time to

First Floc

Floc Index

(0-12)

Turbidity

1 0

2 1

3 2

4 4

5 6

6 10

Floc Index 0 2 4 6 8 10 12

Description no floc smoky pin point fair good excellent very heavy

B. Settling Column

Please record your data on the Class Summary Sheet on lab computer.

Port # 1 2 3 4 5

Depth from free surface (m) 0.15 0.65 1.15 1.65 2.15

Time

(min)

Port Sample

Volume (mL)

Filter Weight

(g)

Dry Weight

(g)

Suspended

Solids (mg/L)

0 1

2

3

4

5

7/27/2019 W13-CIVE375-Lab4

6/7

W13-CIVE375 Lab 4 6/7

Time

(min)

Port Sample

Volume (mL)

Filter Weight

(g)

Dry Weight

(g)

Suspended

Solids (mg/L)

5 1

2

3

4

5

Time

(min)

Port Sample

Volume (mL)

Filter Weight

(g)

Dry Weight

(g)

Suspended

Solids (mg/L)

10 1

2

3

4

5

Time

(min)

Port Sample

Volume (mL)

Filter Weight

(g)

Dry Weight

(g)

Suspended

Solids (mg/L)

20 1

2

3

4

5

7/27/2019 W13-CIVE375-Lab4

7/7

W13-CIVE375 Lab 4 7/7

Time

(min)

Port Sample

Volume (mL)

Filter Weight

(g)

Dry Weight

(g)

Suspended

Solids (mg/L)

30 1

2

3

4

5

Time

(min)

Port Sample

Volume (mL)

Filter Weight

(g)

Dry Weight

(g)

Suspended

Solids (mg/L)

45 1

2

3

4

5

Time

(min)

Port Sample

Volume (mL)

Filter Weight

(g)

Dry Weight

(g)

Suspended

Solids (mg/L)

60 1

2

3

4

5