1.0 INTRODUCTION Jar testing is a method of simulating a full-scale water treatment process, providing system operators a reasonable idea of the way a treatment chemical will behave and operate with a particular type of raw water. Because it mimics full-scale operation, system operators can use jar testing to help determine which treatment chemical will work best with their system’s raw water using the optimal chemical dosages. In many plants, changing water characteristics require the operator to adjust coagulant dosages at intervals to achieve optimal coagulation. Different dosages of coagulants are tested using a jar test, which mimics the conditions found in the treatment plant. Jar testing entails adjusting the amount of treatment chemicals and the sequence in which they are added to samples of raw water held in jars or beakers. The sample is then stirred so that the formation, development, and settlement of floc can be watched just as it would be in the full-scale treatment plant. (Floc forms when treatment chemicals react with material in the raw water and clump together.) The operator then performs a series of tests to compare the effects of different amounts of flocculation agents at different pH values to determine the right size floc for a particular plant. Benefits of jar testing are:a. Better finished water quality b. Longer filter runs c. Overall prolonged filter life d. Lower chemical costs
2.0 OBJECTIVE To determine the optimal coagulant dose which will produce the highest removal of a given water turbidity.
3.0 LEARNING OUTCOME 1. To identify the most common coagulant used in the coagulant process. 2. To determine the most effective and optimum dosage of coagulant for a particular mixing intensity and duration.
3. To understand the complex interrelationships that exists between the chemicals and the constituents of the water being treated, as well as other factors such as pH, temperature, the intensity and duration of mixing. 4.0 THEORY Raw water and wastewater are normally turbid containing solid particles of varying sizes. Particles with sizes greater than 50 µm settle fairly rapidly. The settling velocities of colloidal particles of sizes less than 50 µm are very slow. Thus, these particles are encouraged to collide leading to coalescence of particle to form flocs particles, which are bigger and heavier. These particles will have higher settling velocities and easily settle out. Colloidal particles do not agglomerate by itself due to the presence of repulsive surface forces. A process is needed to suppress these forces so as to allow flocs formation. This process is called coagulation process. It is actually the addiction of chemical coagulant to the raw water or wastewater. Coagulant that normally used are salts of aluminium namely aluminium sulphate and ferric salts namely ferrous sulphate and ferric chloride. The next process that follows the coagulation process is flocculation. It is the process that promotes particles collision due to gentle agitation resulting in agglomeration of smaller non-settleable particles into flocs (bigger particles) which settles easily to produce clarified water. Addition of coagulant aid such as synthetic polymer will accelerate settling.
Majority of ions in surface water consist of negatively charged particle/colloids which are stable in nature (stable = existing in ionized form). They repel other colloidal particles before they collide with one another. The colloids are continually involved in Brownian movement.
5.0 EQUIPMENT AND MATERIALS 1.
Jar test apparatus with six rotating paddles blade
2.
Six (6) beakers
3.
pH meter
4.
Turbidity meter
5.
Pipette
REAGENT Aluminum sulfate (alum) with a known concentration or anionic/cationic coagulant such as ferrous sulfate and ferric chloride
6.0 PROCEDURE 1.
The wastewater sample is prepared.
2.
The temperature, pH and turbidity of water sample is measured.
3.
The prepared wastewater is placed from (1) into six (6) different beakers (Plexiglas graduated beakers) with one litre each.
4.
1 -5 ml of coagulant (alum/ferrous sulfate) is added by using a measuring pipette into beaker 1,2,3,4 and 5 while in beaker 6, no alum was added as it acts as a control sample.
5.
The pH and turbidity of each beaker is measured by using pH and turbidity meter
6.
Each beaker is started stirring rapidly (60 to 80 rpm) for 3 minutes
7.
Then the speed is reduced (30 rpm) for about 20 minutes
8.
The flocculation process is observed and the flock formation is recorded in final 10 minutes by referring to the chart of particle sizes provided in lab sheet. Note: Record the qualitative characteristics of flocks as bad, moderate, good and very good. Cloudy samples indicate bad coagulation while good coagulation refers to rapid flock formation resulting in clear water formation on the upper portion of the beaker.
9.
After the stirring period is over, the stirrer is stopped and the flocks are allowed to settle for about 5 minutes.
10. 500 mL of settled water is separated out into another beaker 11. The temperature, pH and turbidity of the clarified water is determined.
7.0 RESULT AND DATA ANALYSIS Results JAR TEST 1 (Set the coagulant dose) Initial turbidity: 47 NTU Initial pH: 6.8 pH adjustment (base): pH adjustment (acid): Coagulant concentration: 3% JAR No
1
2
3
4
5
6
4.36
4.30
4.23
4.22
4.19
6.29
Coagulant dose (mg/L)
1
2
3
4
5
Control
Agitate (minute)
23
23
23
23
23
23
Fast (rpm)
80
80
80
80
80
80
Slow (rpm)
30
30
30
30
30
30
Settling depth (mm)
6
5
4
6
5
None
Turbidity (NTU)
1
3
8
3
2
45
Fine (Size C)
Fine (Size C)
Very fine (Size B)
Very fine (Size B)
Very fine (Size B)
None
pH
Floc formation (final 10 minutes)*
* Floc formation can be recorded by referring to the measurement scale as provided in lab sheet.
Data analysis a. Compare the level of turbidity in each sample. Turbidity is a measure of the degree to which the water loses its transparency due to the presence of suspended particulates. The more total suspended solids in the water, the murkier it seems and the higher the turbidity. JAR No pH Turbidity (NTU)
1
2
3
4
5
6
4.36
4.30
4.23
4.22
4.19
6.29
1
3
8
3
2
45
From the results as above, it can be seen that sample 6 which acts as a control has the highest turbidity of 45 NTU whereas for the other 5 samples which was added with coagulant of different dosage have varies turbidity. Sample 3 has the highest among the 5 samples while sample 2 and sample 4 have the same turbidity. The lowest turbidity goes to sample 1 which only result as 1 NTU. The turbidity of the samples are affected by the flocs content in the sample. However, the results might not be as accurate as the wastewater used was taken from the FKAAS lake which its contamination may be affected due to rain just shortly before it was obtained. Surface water plant is susceptible to sudden changes in water quality.
b. With the aid of a graph, show the relationship between pH and turbidity with respect to coagulant dosage.
Graph of Turbidity vs pH 9 8
Turbidity (NTU)
7 6 5 4 3 2 1 0 4.15
4.2
4.25
4.3
4.35
4.4
pH
Graph of pH vs Coagulant Dosage 4.38 4.36 4.34 4.32
pH
4.3 4.28 4.26 4.24 4.22 4.2 4.18 0
1
2
3
4
5
6
5
6
Coagulant Dosage (mg/L)
Turbidity (NTU)
Graph of Turbidity vs Coagulant Dosage 9 8 7 6 5 4 3 2 1 0 0
1
2
3
4
Coagulant Dosage (mg/L)
c. From the graph, get the optimum value for pH and coagulant dose of the coagulant process. From the graph in question 2, the optimum value for pH and coagulant dose are 4.36 and 1mg/L respectively. However, the results may not be accepted as the graph supposed to be U-shape rather than bell shape. Hence it is shown that the results obtained is not accurate. This is due to error such as human and random error. d. Explain the implications of using different dosage of aluminium sulphate in the treatment process. Finely dispersed suspended and colloidal particles producing turbidity and color of the water cannot be removed sufficiently by the ordinary sedimentation process. Adding a coagulant with mixing and stirring the water causes the formation of settleable particles. These flocs are large enough to settle rapidly under the influence of gravity, and may be removed from suspension by filtration. Alum was found to be effective coagulant in reducing solids, organics and nutrients in the dairy industry effluent to reuse it in irrigation. In terms of turbidity the process of increase in turbidity removal can be observed by increment of coagulant concentration, although the removal efficiency is almost steady due to alum dosage increment in high concentration of coagulant.
8.0 DISCUSSION 1. By using aluminium sulphate, the mechanism is : Al3+
+
3H2O
Al(OH)3
+
3H
Describe the mechanism of reaction if the aluminium sulphate is replaced by ferum chloride (FeCl3). The properties of iron with respect to forming large complexes, dose, and pH curves are similar to those of alum. In the presence of alkalinity is: FeCl3 + 3HCO3 + 3H2O Fe(OH)3.3H2O(s) + 3CO2 + 3ClFor the mechanism without alkalinity the reaction will be presence as follow: FeCl3 + 6H2O Fe(OH)3.3H2O(s) + 3HCI
2. How the coagulant works? When coagulant is added into the water, it helps to promote coagulation. The coagulant encourages colloidal material in the water to join together into small aggregates or “flocs” (microfloc). Coagulant neutralizes the electrical charges of particles in the water which causes the particles to clump together. Properly formed floc will settle out of water quickly in the sedimentation basin, removing the majority of the water's turbidity. Hence impurities can be easily removed.
3. Name 3 types of acid and base which are suitable for pH neutralization. a) Acid: sulfuric acid, ferum chloride, hydrochloric acid and tartaric acid. b) Base: sodium hydroxide, magnesium oxide, hydrogen chloride and potassium hydroxide.
4. What are the benefits of using coagulant aids? i.
Improve coagulation;
ii.
Build a stronger, more settleable floc;
iii.
Overcome slow floc formation in cold water;
iv.
Reduce the amount of coagulant required;
v.
Reduce the amount of sludge produced.
vi.
Add toughness to the flocs so that they will not break up during the mixing and settling processes.
5. In what way the dosage of aluminium sulphate in the treatment process can be reduced? By using coagulant aids such as lime and bentonite.
6. Instead of Al2(SO4)3, name another three coagulants that can be best used as coagulant aid. i.
Bentonite (clay)
ii.
Calcium carbonate (CaCO3)
iii.
Sodium silicate (Na2SiO3)
9.0 RECOMMENDATION To obtain more accurate and precise results, make sure wastewater source obtained were not taken right after weather changes such as rain because surface water plants are susceptible to sudden changes in water quality thus affect the results. Make sure gloves are worn to avoid fingerprints on turbidimeter sample cells which will affect turbidity readings. Avoid air bubble in pipette when measuring alum dose. Use distilled water as control before testing turbidity for every samples.
10.0
CONCLUSION In conclusion, the objective to determine the optimal coagulant dose which
will produce the highest removal of a given water turbidity of the experiment is achieved. The optimum value of pH and coagulant dose are 4.6 and 1mg/L respectively. Theoretically, adding coagulant such as alum does help to reduce the turbidity of the wastewater until a certain point. However, the results obtained in our experiment did not show a positive results due to few avoidable errors. For instance, weather factor as the rain affected the contamination of the wastewater hence the results were affected as well and not as desired. This is due to surface water plants tend to treat water with a high turbidity which is susceptible to sudden changes in water quality. In a nut shell, jar testing is a pilot-scale test of the treatment chemicals used in a particular water plant. It simulates the coagulation/flocculation process in a water treatment plant and helps operators determine if they are using the right amount of treatment chemicals, and, thus, improves the plant’s performance.
11.0
REFERENCES Principle of Environment Engineering and Science, McGraw Hill 2004, Mackenzie L.Davis and Susan J.Masten Prentice Hall: Introduction to Environmental Engineering: Fourth edition: McGraw Hill International Editions
12.0
APPENDIX
