CE 374 Spring 2013 Environmental Lab # 2
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Coagulation and Sedimentation
Remember that the typed report should be no more than two pages written in memo format
1. Learning Objectives
1.1. Define Coagulation and Sedimentation.
1.2. Conduct a Jar Test
1.3. Determine Coagulation dosages required for flocculation.
1.4. Data Analysis
2. Purpose and Scope of Experiment
2.1. The purpose and scope of this lab experiment is to determine the proper coagulation (Alum) dosage and pH required for good coagulation and flocculation of water.
3. Background and Theory (Equations, Figure, Table, Definitions, Terminology)
3.1. Coagulation is one of the processes in water and wastewater treatment. The object of coagulation is to turn the small particles of color, turbidity, and bacteria into larger flocs, either as precipitates or suspended particles. These flocs are then conditioned so that they will be readily removed in subsequent treatment processes. During coagulation a positive ion is added to water to reduce the surface charge to the point where the colloids are not repelled from each other. A coagulant is the chemical that is added to the water to accomplish coagulation.
3.2. Aluminum ((Al3+) is one of the most commonly used coagulants that meet all of the following coagulants’ requirements
3.2.1. Trivalent cation: the most efficient cation to neutralize the negative charge in natural water.
3.2.2. Nontoxic: produce safe water to the environment.
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3.2.3. Insoluble in the neutrual pH: the coagulant that is added must precipitate out of solutions so that high concentrations of the ion are not left in the water. Such precipitation greatly assists the colloid removal process.
3.3. When alum is added to a water containing alkalinity, the following reaction occurs:
For each mole of alum added it uses six moles of alkalinity and produces six moles of carbon dioxide. The above reaction shifts the carbonate equilibrium and decrease the pH. However, as long as sufficient alkalinity is present and CO2(g) is allowed to evolve, the pH is not drastically reduced and is generally not an operational problem. When alkalinity is not sufficient to neutralize the sulfuric acid production, the pH may be greatly reduced:
If the above reaction occurs, line or sodium carbonate may be added to neutralize the acid.
pH and dose are the two additional important factors in coagulation. The optimum dose and pH must be determined from laboratory tests. The optimum pH range for alum is approximately 5.5 to 6.5 with adequate coagulation possible between pH 5 to pH 8 under some conditions. Jar Test is one of the most effective tools available to surface water plant operators. Jar test is intended to simulate conditions in any water treatment plant. It is a good indicator of how well the coagulation flocculation process is working.
4. Methodology (or Procedure)
4.1. Put 1000 mL of Sample into each jar of flocculator.
4.2. Pipette 0 mL of diluted alum solution into Jar # 1. [Note: This is the control sample.]
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4.3. Pipette 10 mL of diluted alum solution into Jar # 2
4.4. Pipette 20 mL of diluted alum solution into Jar # 3
4.5. Pipette 30 mL of diluted alum solution into Jar # 4
4.6. Pipette 40 mL of diluted alum solution into Jar # 5
4.7. Pipette 50 mL of diluted alum solution into Jar # 6
4.8. Turn on the flocculator with 100rpm for 30 seconds
4.9. Back down the flocculator to 30rpm for for 10 minutes
4.10. Back down the flocculator to 20rpm for the other 10 minutes
4.11. Record visual observation
4.12. Measure the pH and turbidity of the sample from each Jar
5. Data Analysis
5.1. Plot the following graphs:
5.1.1. Alum-Dose (mg/L) versus Turbidity (NTU)
5.1.2. pH versus Turbidity (NTU)
6. Apparatus
6.1. Flocculator/Mechanic Mixers with 4 to 6 Jars
6.2. Volumetric pipettes
6.3. Spectrometer
6.4. pH meter
7. Regents
7.1. 10g/L Alum Sulfate Solution
7.2. Sample: Pond Water
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Reference
Clair N. Sawyer, Perry L. McCarty and Gene F. Parkin. Chemistry for Environmental
Engineering and Science. 5th ed. New York: McGraw Hill, 2003.
David, Vernon, et al. Water Chemistry Laboratory Manual. 3rd ed. John Wiley & Sons,
1980.
Andrew D. Eaton, Lenore S. Clesceri and Arnold E. Greenberg. Standard Methods for the
Examination of Water and Wastewater. 19th ed. Washington D.C: American Public
Health Association, 1995.
Davis Cornwell. Introduction to Environmental Engineering. 3rd ed. McGraw Gill, 1998.