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.
To read more about the coagulation and flocculation process, you should refer to Lesson 8.
To determine the optimum concentration of coagulant to be added to the source water.
- 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
- 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.
- 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 required, mg/L
Stock solution concentration, mg/L
1 mL added to 1 L sample equals
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:
- 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.
- Measure out the proper quantity of distilled water in the volumetric flask.
- Add the chemical to the distilled water.
- 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.
- Collect a two gallon sample of the water to be tested. This should be the raw water.
- Measure 1,000 mL of raw water and place in a beaker. Repeat for the remaining beakers.
- Place beakers in the stirring machine.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
You should print out two copies of the Data Sheet to fill in during the lab. The first will be used to determine the optimum coagulant concentration. The second may be needed to determine lime or coagulant aid concentration. You will use this data to write your lab report.
Your lab report should include all of the data from your data sheet, organized in an understandable manner. In addition, you should answer the following questions:
- Which chemical dosage(s) achieved optimal coagulation in your jar test? Did this dosage also achieve the lowest turbidity?
- Did the optimal chemical dosage used in your jar test achieve the same turbidity found in the treatment plant at the top of the filters?
- How much turbidity is being removed by the filters at the treatment plant? (This can be discovered by subtracting the filter effluent turbidity from the top of filter turbidity.) Does this suggest that sedimentation time is adequate or inadequate?
- Is the effluent turbidity at the treatment plant within the recommended range?
- How does the type of water source influence your plant's turbidity and its coagulation/flocculation methods?