Jar Test Procedure
Introduction
Coagulation/flocculation is the process of binding small particles in the water
together into larger, heavier clumps which settle out relatively quickly. The larger
particles are known as floc. Properly formed floc will settle out of water quickly in
the sedimentation basin, removing the majority of the water's turbidity.
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. The first step of the jar test involves adding coagulant to the
source water and mixing the water rapidly (as it would be mixed in the flash mix
chamber) to completely dissolve the coagulant in the water. Then the water is
mixed more slowly for a longer time period, mimicking the flocculation basin
conditions and allowing the forming floc particles to cluster together. Finally, the
mixer is stopped and the floc is allowed to settle out, as it would in the
sedimentation basin.
The type of source water will have a large impact on how often jar tests are
performed. Plants which treat groundwater may have very little turbidity to
remove are unlikely to be affected by weather-related changes in water
conditions. As a result, groundwater plants may perform jar tests seldom, if at all,
although they can have problems with removing the more difficult small suspended
particles typically found in groundwater. Surface water plants, in contrast, tend to
treat water with a high turbidity which is susceptible to sudden changes in water
quality. Operators at these plants will perform jar tests frequently, especially after
rains, to adjust the coagulant dosage and deal with the changing source water
turbidity.
Purpose
To determine the optimum concentration of coagulant to be added to the source
water.
Materials
Volumetric flask (1,000 mL)
Analytical balance
Coagulants and coagulant aids
Magnetic stirrer (optional)
A stirring machine with six paddles capable of variable speeds from 0
to 100 revolutions per minute (RPM)
Beakers (1,000 mL)
Pipets (10 mL)
Watch or clock
Turbidometer and sample tubes
Stirring Machine
Procedure
1. Decide on six dosages of the chemical(s).
You should use the chemicals in use at the treatment plant you
visit. These chemicals may include coagulants, coagulant aids,
and lime.
The dosages should be in a series with the lowest dosage being
lower than the dosage currently used in the plant and the
highest dosage being higher than the dosage currently used in
the plant. Insert the six dosages into your data sheet.
If pre-lime has to be fed, it is usually best to hold the amount
of lime constant and vary the coagulant dosage.
2. Prepare a stock solution of the chemical(s).
It is not necessary to know the purity (strength) of the
chemicals you use since the strength will be the same for plant
operation. All results of the jar tests are in parts per million or
milligrams per liter. (1 ppm = 1 mg/L).
You will need to prepare a stock solution for each type of
chemical used. The strength of the stock solution will depend
on the chemical dosages which you decided to use in step 1.
The table below shows what strength stock solution you should
prepare in each circumstance.
Approximate dosage Stock solution 1 mL added to 1 L
required, mg/L concentration, mg/L sample equals
1-10 mg/L 1,000 mg/L 1 mg/L
10-50 mg/L 10,000 mg/L 10 mg/L
50-500 mg/L 100,000 mg/L 100 mg/L
For example, if all of your dosages are between 1 and 10 mg/L, then
you should prepare a stock solution with a concentration of 1,000
mg/L. This means that you could prepare the stock solution by
dissolving 1,000 mg of the chemical in 1 L of distilled
water. However, this would produce a much larger quantity of stock
solution than you need and would waste chemicals. You will probably
choose instead to dissolve 250 mg of the chemical in 250 mL of
distilled water.
Once you decide on the strength and volume of stock solution to
prepare, the procedure is as follows:
1. Weigh out the proper quantity of the chemical using the
analytical balance.
Put an empty weigh boat on the balance and tare it.
Then add the chemical slowly to the weigh boat until the
desired weight has been achieved. It is much easier to
add chemical to the weigh boat than to remove it, add
the chemical very slowly and carefully.
2. Measure out the proper quantity of distilled water in the
volumetric flask.
3. Add the chemical to the distilled water.
4. Mix well.
If lime is used, it is best to use a magnetic stirrer since
lime is not completely soluble in water. In other cases,
magnetic stirrers can still be useful.
3. Collect a two gallon sample of the water to be tested. This should be
the raw water.
4. Measure 1,000 mL of raw water and place in a beaker. Repeat for the
remaining beakers.
5. Place beakers in the stirring machine.
6. With a measuring pipet, add the correct dosage of lime and then of
coagulant solution to each beaker as rapidly as possible.
The third column of the table in step 2 shows the amount of
stock solution to add to your beaker. Two examples have been
explained below.
If you have prepared a 1,000 mg/L stock solution, then 1 mL
of the stock solution added to your 1,000 mL beaker will result
in a concentration of 1 mg/L. So, if you wanted to have a
chemical concentration in your beaker of 4mg/L, you would
add 4 mL of stock solution.
If you prepared a 100,000 mg/L stock solution and wanted to
achieve a chemical dosage of 150 mg/L, then you would need
to add 1.5 mL of stock solution to your beaker.
7. With the stirring paddles lowered into the beakers, start the stirring
machine and operate it for one minute at a speed of 80 RPM. While
the stirrer operates, record the appearance of the water in each beaker.
Note the presence or absence of floc, the cloudy or clear appearance
of water, and the color of the water and floc.
The stirring equipment should be operated as closely as
possible to the conditions in the flash mix and/or flocculation
facilities of the plant. Mixing speed and time may vary at your
plant from the times and speeds listed in this and the following
step. Record any alterations on your data sheet.
8. Reduce the stirring speed to 20 RPM and continue stirring for 30
minutes. Record a description of the floc in each beaker 5, 10, 15, 20,
25, and 30 minutes after addition of the chemicals.
9. Stop the stirring apparatus and allow the samples in the beakers to
settle for 30 minutes. Record a description of the floc in each beaker
after 15 minutes of settling and again after 30 minutes of settling.
10. Determine which coagulant dosage has the best flocculation time and
the most floc settled out. This is the optimal coagulant dosage.
A hazy sample indicates poor coagulation. Properly coagulated
water contains floc particles that are well-formed and dense,
with the liquid between the particles clear.
11. Test the turbidity of the water in each beaker using a turbidometer.
Pipet water out of the top of the first beaker and place it in a
sample tube, making sure that no air bubbles are present in the
sample. (Air bubbles will rinse while turbidity will sink.)
Carefully wipe the outside of the sample tube clean. Place the
sample tube in a calibrated turbidometer and read the turbidity.
Repeat for the water from the other beakers.
The least turbid sample should correspond to the optimal
coagulant dosage chosen in step 10.
12. If lime or a coagulant aid is fed at your plant in addition to the
primary coagulant, you should repeat the jar test to determine the
optimum dosage of lime or coagulant aid. Use the concentration of
coagulant chosen in steps 10 and 11 and alter the dosage of lime or
coagulant aid.
13. Using the procedure outlined in step 11, measure the turbidity of
water at three locations in the treatment plant - influent, top of filter,
and filter effluent.
