The Jar Test



The jar test is used to determine dosage requirements for chemicals added to remove small particulates from water or wastewater. Raw drinking water comes from groundwater well supplies or surface water sources such as lakes, rivers, or reservoirs. While groundwater tends to be quite clear, surface water often has a lot of suspended particles that make it appear turbid. Particulates are also often responsible for some of the color, taste, and odor problems associated with raw water supplies. Industrial wastewater may be treated for particulate removal to prepare it for reuse or to recover valuable material from the captured particles.

Turbidity, in the context of water and wastewater treatment, refers to the light-scattering properties of a water sample. It is measured in a turbidimeter and reported in turbidity units (TU). There are a variety of methods for measuring turbidity, but the instrument most commonly used is the nephelometer. A water sample is poured into a clear glass cuvette and inserted into a darkened sample holder. A light beam is passed through the cuvette, and a photocell oriented at a 90o angle to the beam path is used to detect any scattered light. The more particles present, the more light will be scattered and detected. Measurements are reported as Nephelometric Turbidity Units (NTU).

Many of the particles in surface water supplies are colloidal in nature. Colloids are particles that are so small in diameter and mass, that they remain in the water column no matter how much settling time is allowed. Clay and microorganisms typically behave as colloids. Clay colloids tend to have a net negative surface charge, which causes them to mutually repel each other and prevents any contact or agglomeration.



Turbidity Removal

A major goal of water treatment is turbidity removal. The jar test is a simulation of the treatment processes that have been developed to accomplish turbidity removal. A group of chemicals has been identified that can serve as coagulants; their addition brings about the clumping together of colloidal material. The mechanisms by which this is accomplished are quite complex and have been the subject of intense study. Which of four major mechanisms of coagulation predominates will depend on the coagulant used, the dosage, and the raw water characteristics including turbidity, alkalinity, and pH. By charge compression or adsorption, the coagulant may act to reduce the net negative charge on each particle, allowing closer proximity of particles and ultimate contact. In other cases, the coagulant acts by forming a heavy, sticky precipitate that enmeshes the colloids and pulls them down as it settles to the bottom. Alternately, the coagulant may act as a chemical bridge, binding to several colloids at the same time.

There are two key processes that must occur for efficient turbidity removal with coagulants. First, there must be rapid and complete mixing of the colloids and the chemical coagulant. This is typically done in a “flash” or rapid mix chamber for a detention time of about 30-60 seconds. The second important step is the flocculation process, in which clumped particles are mixed slowly to allow larger and larger aggregates (floc) to form.                                                                                         
Mixing speed is maintained at a level sufficient to keep the particles suspended without shearing them. After about 30 minutes of flocculation, the water passes into a long sedimentation tank. During the two to four hours it takes the water to cross this tank with very slow velocity, the floc particles settle by gravity to form a thick sludge blanket at the bottom of the basin.

Alum, ferrous sulfate, and ferric chloride are three common coagulants. Because their mechanism of action is complex, “more” is not always “better”. Sometimes better turbidity removal will be achieved with a low dose than with a higher one. The best dose will also be a function of pH. The optimum pH for alum coagulation is usually between 5.5 and 6.5. There is no way to “calculate” the best dose. It must be determined by trial and error; hence, the jar test. The reaction chemistry varies according to the pH and alkalinity of the test sample. Alum coagulation proceeds according to the following equation if there is enough alkalinity in the water to react with the amount of alum dosed:

Al2(SO4)3 x 14H2O + 6HCO3 2Al(OH)3(s) + 6CO2 + 14H2O + 3SO42-


If there is insufficient alkalinity, the reaction will proceed according to the equation:

Al2(SO4)3 x 14H2O 2Al(OH)3 + 3H2SO4 + 8H2O


An alkalinity test is usually performed before initiating a jar test to determine whether alkalinity supplements might be required.




Turbidity Testing

The jar test is performed on a “gang stirrer”. Six axial mixing blades are operated by a single motor, so mixing conditions can be replicated in six one liter samples simultaneously. Each beaker is filled with a one liter sample of the turbid water. Alkalinity and pH adjustments are made, if required, and a range of coagulant doses is tested. At time zero, the coagulant doses are quickly added, and samples are mixed at about 100 rpm for one minute. Then, the mixing speed is reduced to only 30 rpm, and flocculation is allowed to proceed for about 30 minutes. Finally, all mixing is discontinued, and the floc is allowed to settle for 30-60 minutes. A plot of turbidity readings versus dosage reveals the optimum dose.

There are several important points about the optimum dose. First, it may change from day to day. If there are high raw water turbidity fluctuations, a jar test will be required with each major change. Further, the “optimum” dose does not always refer to the dose that achieves maximum turbidity removal. If a 10 mg/L increment in dosage produces only a slight improvement in turbidity removal, the increased chemical costs may not warrant the higher dose. Therefore, the optimum dose is more practically thought of as the one that achieves the best turbidity removal “for the money”.

The jar test may be used to test variables other than dosage. For example, all beakers might be tested with 30 mg/L alum, but the initial pH in each beaker might be varied over a range of 5.5 to 7. The effectiveness of coagulant “aids” (polymers which sometimes enhance the agglomeration of colloids) could also be evaluated using a jar test.



Jar Testing Procedures

Deciding on Coagulant Chemistries

  1. Identify which coagulant chemistries you plan to evaluate. Generally speaking, you will evaluate the chemistry that is currently being used if it is an existing application. Keep in mind, however, alternative chemistries may offer advantages such as lower dosage costs, reduced sludge volumes, or improved performance.

  2. Develop a table format which shows the chemistry you are evaluating, cost per pound, best dosage, and the last column should show costs such as cost per unit of water treated or annual cost.

  3. Plan to evaluate the chemistry currently being used, particularly if it is already effective in an existing application.
    Hint: Inorganics (iron, aluminum, calcium, or magnesium salts) are effective at inorganic or metals removal, oil and grease removal, and precipitation. Organic polymers can show specific effectiveness in numerous areas including oil emulsion breaking, and will not generate the sludge volumes that inorganic salts do. Oftentimes, a combination of inorganic and organic coagulants is most effective.

  4. Coagulants are virtually always considered positive or cationic charged chemistries. Flocculants, however, can be anionic or nonionic in charge. The effectiveness or in ineffectiveness of an existing application may give you some insight as to which charged flocculant to evaluate first. Guidelines are available based upon application and pH, but there are exceptions to “normal”.

When flocculants are used alone for applications such as sludge dewatering or improved suspended solids settling, the most appropriate charge will vary from application to application. Sometimes it is necessary to use a combination of two flocculants with opposite charges to achieve the most effective results.



Preparing Coagulant and Flocculant Solutions for Jar Testing

For existing applications, determine the current feedrate of the coagulant and flocculant, if available. Additionally, if the coagulant is being diluted to a certain percent solution, it would be beneficial to know this information as well. Generally speaking, coagulant chemistries are fed neat and are not made down into solutions.

Determine the flocculant solution being prepared. Emulsion polymers require strong sheer force mixing and some age time to be inverted to full activity. Again, determine what the current make-down solution is for the existing application and dosage rate. Base the dosage of the flocculant aid on product dosage, not the make-down solution dosage. Make-down solution dosages for flocculants are normally 0.1 – 1% solutions.



A. Coagulant Solutions

Prepare your coagulant solution based on the information generated in item A and the size of the beakers you plan to use in your jar tests. For example, if your coagulant dosage is within the range of 5 to 100 ppm, you may want to make a 1% solution of the coagulant to add to your 1,000 ml beakers. This solution strength would result in you adding approximately 0.5
mls – 10 mls of your 1% solution to 1000 mls sample water.

However, if your coagulant dosage is several hundred parts per million or greater, it would be better to make a 5 of 10% solution to be added to your 1,000 ml beakers.



Making a 1% Solution of a Coagulant With a Specific Gravity of 1.2

  1. Determine the amount of solution to be made (200 mls).

  2. Amount of coagulant product needed = mls of 1% solution to be made, multiplied by 1%, and divided by specific gravity of coagulant =200 × .01 ÷ 1.2 = 1.67 mls. (2 grams)

  3. Place amount of coagulant needed into approximately 198 mls (198 grams) of distilled water, for a 1% solution.
    Note: If 10% solution is made, 16.7 mls (20 grams) of coagulant would be placed into 180 mls (180 grams) of distilled water.

  4. The solution can be mixed by placing it in a clean sample bottle, securing a lid, and shaking it vigorously for 30 seconds to 1 minute.




B. Flocculant Solutions

Make the appropriate percent solution for the flocculant to be tested and the feed equipment to be used. If you cannot determine this information, a good starting point is a 0.1% solution. The calculations for determining the amount of emulsion flocculant product to use are the same as for coagulant; however, the mixing procedure requires much more agitation and mixing.



Making an Emulsion Flocculant Solution

  1. Determine the amount of solution to be made (100 mls).
  2. Amount of flocculant product needed = number of mls of solution to be made, multiplied by the percent solution to be made, divided by the specific gravity of the flocculant =  100 × 0.001 ÷ 1.05 = approximately 0.1 mls (0.1 grams) of flocculant product.
  3. Amount of water for solution = 100 grams – 0.1 gram
    flocculant product = 99.9 grams (99.9 mls) of water.
  4. Place 99.9 mls of water into a plastic 200 ml mix cup.
  5. Using a Braun mixer (or similar), begin mixing the distilled water in the mix cup. A stir rate of 800-1000 rpm is desired.
  6. Shake the emulsion sample for 5-10 seconds until the product is uniform.
  7. Immediately add 0.1 mls of flocculant product into the vortex area of the water as it is mixing.
  8. Mix for a total of approximately 1 minute while turning the mixer section from one side to the other side of the mix cup for approximately 20 second intervals.
  9. Discontinue mixing. Allow emulsion flocculant mixture to drain from the Braun mixer into the cup and clean the mixer with a dry paper towel.
  10. Ideally, turbulent mixing should continue for 30 minutes for most efficient unwinding of the polymer. At least 30 minutes should be provided with occasional mixing.
    Note: If a mixer is not available, invert the emulsion polymer as follows. (This method will yield less efficient inversion and unwinding of the polymer).
    1. Use a container with a lid. Measure the water out and add it to the container.
    2. Rapidly shoot the emulsion into the water using a syringe. With this method, target a 0.1% solution.
    3. Immediately cap the container and vigorously shake for 1 minute.
    4. Provide additional occasional moderate shaking for 30 minutes.

The resulting solution should be white, translucent to opaque, and homogenous.



Making a Flocculant Solution With Dry Polymers

  1. Determine the amount of solution to be made (100 mls).

  2. Generally, a 0.1% solution strength is a good amount, so weigh out 0.1 grams and add to 100 mls. (If you don’t have access to a metric balance, you can come close by using the brass dippers in the test kits as holding 0.1 grams of flocculant. A 0.1 gram plastic dipper will provide about 0.06 grams of dry flocculant. This will vary somewhat by the flocculant being used).

  3. Slowly add the flocculant particles into the vortex of water, mixing at 800-1000 rpm, then offset and continue turbulent mixing for 30 minutes. Alternatively, add slowly to a container, cap it, and vigorously shake for 1 minute, then provide additional moderate shaking for 30 minutes.
    The resulting solution should be clear and homogenous. If fisheyes or globs are present, continue additional mixing.




Using existing plant operating data, determine the range of coagulant dosage and flocculant dosage you would like to evaluate in your jar test. In the first iterations of jar tests, you will probably be changing the coagulant dosages while maintaining a constant flocculant dosage. The proper coagulant chemistry and dosage is what you want to determine first. Sometimes, you may not even apply flocculant until you have correctly identified the most effective coagulant chemistry.

Example: Plant is currently feeding a coagulant at 60 ppm and flocculant at 1 ppm. You have a 4-beaker gang stirrer and plan to dose the coagulants in the jars at 20 ppm increments, starting with 20 ppm. You will dose all the jars with 1 ppm of flocculant. Calculate the amount of 1% solution of coagulant added to 1,000 mls of water to achieve a dosage of 20 ppm.

Determine what 1 ml of a 1% solution into 1,000 mls sample is equal to in mg/L or ppm. 1 ml of a 1% solution = 0.01 grams.

Divide by 1,000 mls and 1 ml in 1000 mls = 0.00001 g/L.

Multiply by 1,000,000 to get mg/L which is 10 mg/L (ppm) or a 1% solution = 10,000 ppm.

Dilute a 1 ml 1% solution with 1,000 mls of water to be tested. 10,000 ÷ 1,000 = 10 ppm.

When using a 1% solution of coagulant and 1,000 ml jars, 1 ml of solution added = 10 ppm. For a 500 ml sample, 1 ml of a 1% solution = 20 ppm.

Therefore, for jar 1, you will add 2 mls, add 4 mls to jar 2, 6 mls to jar 3, and 8 mls to jar 4 to have respectively 20, 40, 60, and 80 ppm of coagulant.

Determine the amount of 0.1% flocculant solution added to 1,000 ml jars to achieve 1 ppm dosage.

1 ppm = (X mls of 0.1% solution multiplied by 0.001 divided by 1,000 mls of sample) multiplied by 1,000,000.

X = 1 ml of 0.1% flocculant solution, therefore, we are going to add 1 ml of the 0.1% flocculant solution to achieve 1 ppm dosage to all four beakers.




Adding the Coagulants and Flocculants to the Beakers and Evalulating the Effectiveness

  1. After the beakers have been filled to the 1,000 ml mark with the water to be tested, begin stirring the water at 100 rpm or maximum speed on the gang stirrer.

  2. Add the coagulant dosages to the beakers as previously determined; in this example, 20, 40, 60, and 80 ppm dosages.

  3. Allow the coagulant to mix at the rapid speed of 100 rpm for approximately 2 minutes, or try to duplicate the amount of agitation and mix time provided by the existing plant treatment system. The timeframe should be correlated with the retention time associated with the actual injection pint of the coagulant in the current application. Generally, the turbulence and mixing achieved at the injection point in the application is much greater than can be generated in a jar test. Square jars provide more turbulence than round jars.

  4. During this fast mix procedure, observe the jars very closely to determine which dosage yields the first floc or formation of particles. Make note which dosage showed this characteristic.

  5. As the 2-minute rapid mix time comes to an end, observe which dosage yields the largest floc size and clarity of water.

  6. After the 2-minute rapid mix time, reduce the speed to a slower mix of approximately 30-40 rpm. Allow the jars to mix at this lower speed for approximately 3 minutes or the times correlating to the plant system. During this period, continue to evaluate the floc size and clarity of water.

  7. At the end of the slow mix, turn the stirrers off completely and allow the floc to settle to the bottom of the jar, or possibly float, depending on the treatment application. Make note of which dosage yields the most rapid settling or floating rate, largest floc particles, and the clarity of water. After the jars have set for approximately 2 to 5 minutes, you can extract some water from the jars and run a turbidity analysis to determine more accurately which jar yields the best clarity water.

    Note: The above procedure was first run without any flocculant to determine the most cost-effective coagulant dosage. You should run the same procedure again and add the flocculant at the end of the 2-minute rapid mix period, but prior to reducing the speed to 40 rpm. After adding the flocculant at the high rapid mix speed, allow for an additional 30 seconds to 1 minute mixing to ensure that the flocculant is completely dispersed in the jar. Then reduce the speed to 30-40 ppm and continue to evaluate the water quality in the jar using the same procedure stated above.

  8. Evaluating Flocculant Chemistries. After the most effective coagulant has been determined, it is sometimes beneficial to determine which flocculant will yield large, stable floc particles and more rapid settling or flotation. Keep in mind that coagulant is the primary chemistry used for charge neutralization or precipitation and to initiate the formation of floc particles. The flocculant chemistry is used to increase the size of the floc particles produced and improve settleability, flotation, and clarity of the water. Also look for fines that remain. Good flocculation will leave minimal fines.

    1. Determine the most effective range of coagulant chemistry.

    2. Pick a coagulant dosage and vary the flocculant dosage in the four jars. For example, you may decide to try a slightly lower coagulant dosage than what your best jar test had previously shown without flocculant aid and vary the flocculant dosages in increments of 0.1 to 0.5 ppm.

    3. Evaluate the jars based on the parameters previously stated.

    4. Evaluate different flocculants with varying charges and percent actives.

  9. Determination of the Most Cost-effective Program. Using a combination of operating costs generated on your cost comparison dosage sheet and performance characteristics using the jar test evaluation sheet, determine the most effective program for the client. Keep in mind, if you recommend an inorganic to replace an organic program, there may be increased costs associated with additional sludge haul-off or dewatering. Other items, such as the need for pH adjustment, should also be considered.



Formulas and Conversions

Percent Solution Table


%* lb/gal oz/gal
1 0.084 1.3
2 0.170 2.7
3 0.258 4.1
4 0.348 5.6
5 0.440 7.0
6 0.533 8.5
7 0.629 10.1
8 0.726 11.6
9 0.825 13.2
10 0.929 14.9
* Approximate % by weight




Ounces (fluid) x 29.57 = mL
Ounces (dry) x 28.35 = grams
Cubic Ft. x 7.5 = gallons
Gal x 8.34 = lbs
Gal x 3785 = mL
Gal/Hr x 63 mL/min
Grains/Gal x 17.1 = ppm
Grams x 15.43 = grains
MGD x 694 = gpm
10,000 ppm = 1%
Pounds x 453 = grams
ppm x 8.33 lb/MG
Quarts x 946 = mL
Cubic Ft x 62.4 = Pounds
Pounds x 7000 = Grains
Gal x 3.785 = Liters
1 Mile = 5280 ft
2.31 ft. of water = 1 psi
0.433 psi = 1 ft/water


Feed Rate Formulas


In the following calculations:
pi = 3.14, L = Length, W = Width
D = Diameter, H = Height

Description: lb/hr

Area = A(sq ft.):
Rectangle: V= L x W

Circle: V=Description:

Description: gal/hr

Volume = V( cu. ft.):
Rectangular Tanks: V= L x W x H

Circular Tanks: V= Description:

Description: PPM

In Pipes: V= Description:


Description: PPM


GPM = GPM of plant
* 6 min collection for dry feeder

Divide pipe diameters by 12 to convert from inches to feet. Example: 8 inch pipe divided by 12 equals 0.66 feet.




Basic Lab Equipment For Small Water Treatment Plants

Equipment applicable to both surface and ground water plants.




Copy of latest edition of "Standard Methods for the Examination of Water and Waste Water", American Public Health Association, Inc., 1015 Eighteenth St., N.W., Washington, DC 20036.





Colorimeter or Spectrophotometer capable of performing various tests to determine the chemistry of both raw and finished water. Tests could include: Chlorine(free and total), Iron, Manganese(high and low range), Aluminum (if you use Alum), Nitrates (if you are in a farming area), Phosphorous (if you are using a orthophospate for corrosion control or dirty water complaints), and Flouride (SPADNS Method). A good colorimeter or spectrophotometer should be capable of performing all of the tests listed in addition to several other parameters which could be applicable to your plant. Contact your state Department of Health to determine the parameters required at your plant. Hach and LaMotte, as well as other suppliers, offer good quality equipment as well as technical expertise.


As required..... Reagent sets for tests to be performed with colorimeter or spectrophotometer, EPA approved if possible.




Bench type pH Meter, preferable with temperature compensation (Hach, LaMotte, Fisher, Orion and others). The pH Meter should include a stand for the probe and an electric stirrer. Also, a supply of pH 4, pH 7 and pH 10 buffer solutions should be on hand for calibrating the meter.
1......................... Thermometer, -10 to 110 degrees °C.


As required... Alkalinity, Hardness and CO2 tests are usually performed using one of two titration methods (digital titrator and buret). The digital titrator method (Hach) requires a digital titrator and the appropriate reagent set. The buret method may be purchased as a complete outfit, (LaMotte and others), or the components purchased separately. When purchased separately the following items are required: Buret support with large white porcelain base and a buret holder for assembling and steadying automatic burets. Each separate test will require one automatic buret, with stopcock, 50 or 100 mL, complete with rubber bulb and reservoir and the appropriate reagents for each test.


1......................... Balance, general laboratory, triple beam, in grams (Ohaus Dial-O-Gram, Ohaus Cent-O-Gram or others)
1......................... Technical weights, set, metric, class C, 1 gram to 1000 gram, for balance calibration.
1......................... Stopwatch
6......................... Measuring pipets, 10.0 mL capacity, 0.1 mL subdivisions.
6......................... Measuring pipets, 1.0 mL capacity, 0.01 mL subdivisions.
2......................... Pipet fillers.
2......................... Graduated cylinders, 100 mL Capacity, 1.00 mL subdivisions.
1......................... Graduated cylinder, 500 mL Capacity, 5.00 mL subdivisions.
1......................... Graduated cylinder, 1000mL Capacity, 10.00 mL subdivisions.
4......................... Erlenmeyer flasks, 250 ml, wide mouth.
4......................... Casserole, porcelain, 210 mL.
6......................... Glass stirring rods, 3 mm diameter


The following items are required in plants treating surface water, or in plants using ground water which has been determined to be under the influence of surface water. These items are in addition to the equipment already listed.


1......................... Turbidimeter (Hach, Tumer, LaMotte, HF, etc.)


As required... Primary turbidity standards. Turbidimeters must be standardized using a standard approved by your state Department of Health.


1........................ Six position stirring apparatus for "Jar Tests" with variable speed control and light base (Phipps & Bird).



Griffin beakers, 1000 mL (Phipps & Bird) or
Square 2000 mL laboratory jars (B-Ker2, Phipps & Bird)




Jar Testing For Potassium Permanganate Demand

Stock Solutions

(Strong Stock Solution)
5 grams potassium permanganate dissolved in 500 mL distilled water.

(Test Stock Solution)
1 mL strong stock solution thoroughly mixed in 100 mL distilled water.

Each 10 mL of the test stock solution added to a 1000 mL sample equals 1 ppm.

If you have a six position stirrer:

Using a graduated cylinder, measure 1000 ml . of the sample to be tested into each of the six beakers. Dose each beaker to simulate plant practices in pre-treatment, pH adjustment, coagulant, etc. Do not add carbon or chlorine. Using a graduated pipet, dose each beaker with the test stock solution in the following manner.


1 1.0 0.10 no pink
2 1.5 0.15 no pink
3 2.0 0.20 no pink
4 2.5 0.25 no pink
5 3.0 0.30 pink
6 3.5 0.35 pink


Stir the beakers to simulate the turbulence where the KMnO4 is to be added and observe the color change.

As the iron and manganese begin to oxidize, the sample will turn varying shades of brown, indicating the presence of oxidized iron and or manganese. Samples which retain a brown or yellow color indicate that the oxidation process is incomplete and will require a higher dosage of KMnO4. The end point has been reached when a pink color is observed and remains for at least 10 minutes. In the preceding table a pink color first developed in beaker #5 which had been dosed with 3 mL/0.3 ppm. If the first jar test does not produce the correct color change, continue with increased dosages.

When applying potassium permanganate to raw water, care must be taken not to bring pink water to the filter unless you have "green sand". Also, permanganate generally reacts more quickly at pH levels above 7.0.

A quick way to check the success of a KMnO4 application is by adding 5 mL of the test stock solution to 1000 mL finished water. If the sample turns brown there is iron or manganese remaining in the finished water. If the sample remains pink, oxidation is complete.

With proper application, potassium permanganate is an extremely useful chemical treatment. As well as being a strong oxidizer for iron and manganese, KMnO4 used as a disinfectant in pre-treatment could help control the formation of trihalomethanes by allowing chlorine to be added later in the treatment process or after filtration. Its usefulness also extends to algae control as well as many taste odor problems.



Jar Testing For Direct Filtration

The potential economy both in capital outlay and operating costs makes direct filtration an attractive treatment process. Although the decreased costs associated with reduced chemical consumption and reduced sludge load have been emphasized less than the initial capital economy in plant construction, they represent on-going savings that continue for the life of the plant. Pilot plant investigations are required in order to establish the design criteria for a direct filtration plant. However, pilot plant investigations should not be undertaken unless the raw water can be treated by direct filtration.

The procedure is the following:

Step 1--

Determine the raw water turbidity and record.

Step 2--

Filter the raw water through Whatnan #40 filter paper and record.

Step 3--

Fill the four jars with raw water to the 2-liter mark and decide on dosages.

Step 4--

Measure out the coagulant and polymer doses.

Step 5--

With the Phipps & Bird Stirrer at maximum speed, pour in the coagulant and polymer (test to determine sequence); stir at maximum speed for 30 to 40 seconds.

Step 6--

With stirring continuing at about 50 rpm take a 200ml sample.

Step 7--

Filter through Whatman #40 filter paper (discard paper).

Step 8--

After 3 or 4 minutes of the stirring at 50 rpm take another 200 ml sample.

Step 9--

Filter this later sample through Whatman #40 filter paper (discard paper).

Step 10--

Read and record turbidities of all samples.

Step 11--

Plot data on arithmetic scale paper.




Jar Test Procedure


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 determine the optimum concentration of coagulant to be added to the source water.






Stirring Machine




  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.

  1. 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

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.
  1. Collect a two gallon sample of the water to be tested. This should be the raw water.

  2. Measure 1,000 mL of raw water and place in a beaker. Repeat for the remaining beakers.

  3. Place beakers in the stirring machine.

  4. 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.

  1. 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.

    Description: Stirring.


  1. 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.

  2. 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.


Description: Settling.


  1. 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.

  1. 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.

  1. 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.

  2. 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.