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, polymersco 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
Stirring Machine
Procedure
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.
Appx dosage, mg/L soltn concentration, mg/L 1 mL added to 1 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:
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 chem
