Coagulation is the process by which particles become destabilized and begin to clump together.
Coagulation is an essential component in water treatment operations. Evaluation and optimization of the coagulation/rapid mixing step of the water treatment process includes a variety of aspects. Optimal coagulant dosages are critical to proper floc formation and filter performance. Maintaining the proper control of these chemicals can mean the difference between an optimized surface plant and a poorly run surface plant. Inadequate mixing of chemicals or their addition at inappropriate points in the treatment plant can also limit performance.
Effect on Turbidity
Coagulation by itself does not reduce turbidity. In fact, turbidity may increase during the coagulation process due to additional insoluble compounds that are generated by chemical addition. The processes of flocculation, sedimentation, and filtration should be used with coagulation to reduce suspended solids and turbidity.
Coagulants and Polymers
The coagulation process includes using primary coagulants and may include the addition of coagulant and/or filter aids. The difference between these two categories is as follows:
Typical coagulants and aids are discussed in further detail below:
Chemicals commonly used for primary coagulants include aluminum or iron salts and organic polymers. The most common aluminum salt used for coagulation is aluminum sulfate, or alum.
Alum may react in different ways to achieve coagulation. When used at relatively low doses (<5 mg/L), charge neutralization (destabilization) is believed to be the primary mechanism involved.
At higher dosages, the primary coagulation mechanism tends to be entrapment. In this case, aluminum hydroxide (Al(OH)2) precipitates forming a “sweepfloc” that tends to capture suspended solids as it settles out of suspension. The pH of the water plays an important role when alum is used for coagulation because the solubility of the aluminum species in water is pH dependent. If the pH of the water is between 4 and 5, alum is generally present in the form of positive ions (i.e., Al(OH)2+, Al8(OH)4+, and Al3+). However, optimum coagulation occurs when negatively charged forms of alum predominate, which occurs when the pH is between 6 and 8.
When alum is used and charge neutralization is the primary coagulation mechanism, effective flash mixing is critical to the success of the process. When the primary mechanism is entrapment, effective flash mixing is less critical than flocculation.
Ferric chloride (FeCl3) is the most common iron salt used to achieve coagulation. Its reactions in the coagulation process are similar to those of alum, but its relative solubility and pH range differ significantly from those of alum.
Both alum and ferric chloride can be used to generate inorganic polymeric coagulants. These coagulants are typically generated by partially neutralizing concentrated solutions of alum or ferric chloride with a base such as sodium hydroxide prior to their use in the coagulation process. The resulting inorganic polymers may have some advantages over alum or ferric chloride for turbidity removal in cold waters or in low-alkalinity waters.
Organic polymers tend to be large molecules composed of chains of smaller “monomer” groups. Because of their large size and charge characteristics, polymers can promote destabilization through bridging, charge neutralization, or both. Polymers are often used in conjunction with other coagulants such as alum or ferric chloride to optimize solids removal.
Cost may be a consideration when selecting chemicals. The system should perform an economic analysis when comparing chemicals and not just compare unit cost. For instance, a polymer may cost more per unit than alum, but less polymer may be needed than alum. Therefore, the total cost for polymer may not be much different than the total cost for alum. The following issues may be evaluated as options to consider for treatment process enhancement.
An evaluation of the chemicals used in the treatment process can identify the appropriateness of the coagulation chemicals being used. A thorough understanding of coagulation chemistry is important, and changes to coagulation chemicals should not be made without careful consideration. The following items should be considered when evaluating chemicals and coagulation:
The table below provides some guidelines for selecting the proper chemical based on some raw water characteristics.
Chemical Selection Guidelines Based on Raw Water Characteristics
|Raw Water Parameter||Chemical Consideration|
Alkalinity is a measure of the ability to neutralize acid. Alkalinity levels are typically expressed as calcium carbonate (CaCO3) in mg/L.
|Alkalinity influences how chemicals react with raw water. Too little alkalinity will result in poor floc formation, so the system may want to consider adding a supplemental source of alkalinity (such as lime, soda ash, or caustic soda). Beware that these supplemental sources of alkalinity may raise the pH of the water, and further pH adjustment may be needed to obtain proper floc formation. Systems should discuss this issue with a technical assistance provider or a chemical supplier. One rule of thumb is that alum consumes half as much alkalinity as ferric chloride.|
|Alkalinity < 50 mg/L||This concentration of alkalinity is considered low, and acidic metallic salts, such as ferric chloride or alum, may not provide proper floc formation. Systems may want to consider a high basicity polymer, such as polyaluminum hydroxychloride (PAC1), or an alum/polymer blend.|
|Increase in total organic carbon||More coagulant is typically needed. Remember, organics influence the formation of disinfection byproducts and systems will need to comply with the Stage 1 Disinfection Byproduct Rule. A good resource is the EPA guidance manual Enhanced Coagulation and Enhanced Precipitative Softening Guidance Manual (May 1999).|
|pH between 5.5 and 7.5||Optimum pH range for alum.|
|pH between 5.0 and 8.5||Optimum pH range for ferric salts.|
|pH > 8.5||Ferric salts might work or other high acidic coagulants.|
|Temperature < 5°C||Alum and ferric salts may not provide proper floc formation. May want to consider using PAC1 or non-sulphated polyhydroxy aluminum chloride.|
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.
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.
Feed systems are another important aspect of the coagulation step in typical treatment processes. The figures below show examples of chemical feed systems.
Feed systems need to deliver coagulants into the treatment system at rates necessary for optimal performance. The following aspects of feed systems should be evaluated:
Satisfactory Dispersal/Application Points
Coagulation and mixing also depends on satisfactory dispersal of coagulation chemicals and appropriate application points. Coagulants should be well-dispersed so that optimal coagulation may occur. Enough feed points should be used so chemicals are able to mix completely. The system should evaluate the following items:
Mixing distributes the coagulant chemicals throughout the water stream. When alum or ferric chloride is used to achieve destabilization through charge neutralization, it is extremely important that the coagulant chemical be distributed quickly and efficiently because the intermediate products of the coagulant reaction are the destabilizing agents. These intermediate species are short-lived and they must contact the solids particles in the water if destabilization is to be achieved. When other mechanisms are predominant in the coagulation process, or when organic polymers are being used as the coagulant chemical, immediate distribution of the coagulant chemical is not as critical and less-intense mixing may be acceptable, or even desirable. In some cases, excessive mixing may serve to break up coagulant molecules or floc particles, thereby reducing the effectiveness of subsequent solids removal processes.
The time needed to achieve efficient coagulation varies depending on the coagulation mechanism involved. When the mechanism is charge neutralization, the detention time needed may be one second or less. When the mechanism is sweep floc or entrapment, longer detention times on the order of 1 to 30 seconds may be appropriate.
In general, the lower the coagulant dosage, the faster the mixing should occur because chemical reactions happen very quickly at low dosages. Rapid mixing disperses a coagulant through the raw water faster than the reaction takes place. When alum or ferric chloride are used in lower dosages (for charge destabilization; not sweep floc development), it is important to ensure that they mix very quickly with the raw water to be effective. Engineers have developed methods of determining appropriate mixing rates, called “mixing intensity values” or “velocity gradient” abbreviated as the letter “G.” This value is used to size various mixing mechanisms such as static mixers, impellers, and blades and depends upon the type of mechanism used.
Conversion Factors and Equations for Determining Coagulant Dose
|ac = acre||ha = hectare||mi = mile|
|cfs = cubic feet per second||hr = hour||min = minute|
|cm = centimeter||in = inches||mL = milliliter|
|d = diameter||in3 = cubic inches||ppm = parts per million (mg/L)|
|ft = feet||kg = kilogram||r = inner radius|
|ft3 = cubic feet||L = liter||sec = second|
|gal = gallons||lbs = pounds||Sp. Gr. = specific gravity|
|gpd = gallons per day||mg = milligrams||sq ft = square feet|
|gpm = gallons per minute||MG = million gallons||sq in = square inches|
|gpg = grains per gallon||MGD = million gallons per day||sq m = square meters|
|g = grams||m3 = cubic meters||yd = yard|
1 sq ft = 144 sq in or 144 sq in/sq ft
1 ac = 43,560 sq ft or 43,560 sq ft/ac
1 gal = 8.34 lbs or 8.34 lbs/gal
1 ft3 = 62.4 lbs or 62.4 lbs/ft3
1 grain/gal = 17.1 mg/L or 17.1 mg/L/gpg
1 mg = 64.7 grains or 64.7 grains/mg
1 MGD = 694 gpm or 694 gpm/MGD
1 MGD = 1.55 cfs or 1.55 cfs/MGD
1 ft = 12 in or 12 in/ft
1 yd = 3 ft or 3 ft/yd
1 mi = 5,280 ft or 5,280 ft/mi
1 min = 60 sec or 60 sec/min
1 hr = 60 min or 60 min/hr
1 day = 24 hr or 24 hr/day
1 million = 1,000,000 = 1 x 106
1 ft3 = 7.48 gal or 7.48 gal/ft3
1 liter = 1,000 mL or 1,000 mL/L
1 gal = 3.785 L or 3.785 L/gal
1 gal = 231 in3 or 231 in3/gal
1 g = 1,000 mg or 1,000 mg/g
1 kg = 1,000 g or 1,000 g/kg
1 lb = 454 g or 454 g/lb
1 kg = 2.2 lbs or 2.2 lbs/kg
|Conversion Factors (Metric System)|
1 ha = 2.47 ac or 2.47 ac/ha
1 ha = 10,000 sq m or 1,000 sq m/ha
1 m = 100 cm or 100 cm/m
1 m = 3.28 ft or 3.28 ft/m
1 liter = 1 kg or 1 kg/L
1 MGD = 3,785 m3 or 3,785 m3/MGD
1 m3 = 1,000 L or 1,000 L/m3
1 gal = 3.785 L or 3.785 L/gal
1 gm = 1,000 mg or 1,000 mg/gm
1 kg = 1,000 gm or 1,000 gm/kg
|1. Flow, gpm|
|2. Flow, MGD|
II. Chemical Feeds:
A. Dry Chemicals (Weight-based)
1. Feed Rate, lb/day 2. Dosage, ppm
B. Liquid Chemicals (Volume-based)
1. Feed Rate, gal/day 2. Dosage, ppm
C. Liquid Chemicals (Liquid Weight-based)
1. Feed Rate, lb/day 2. Dosage, ppm
D. Liquid Chemicals (Dry Weight-based)
1. Feed Rate, dry lb/day 2. Dosage, ppm
III. Chemical Doses:
A. Calibration of a Dry Chemical Feeder:
Chemical Feed Rate, lb/day
B. Calibration of a Solution Chemical Feeder:
1. Chemical Feed, lbs/day 2. Chemical Feed, gpm 3. Chemical Solution, lbs/gal 4. Feed Pump, gpd
C. Chemical Feeder Setting:
1. Chemical Feed, lbs/day = (Flow, MGD)(Dose, mg/L)(8.34 lbs/gal) 2. Chemical Feeder Setting, mL/min 3. Chemical Feeder Setting, gal/day 4. Chemical Feeder Setting, %
IV. Coagulation and Flocculation:
1. Polymer, lbs 2. Dose, mg/L 3. Polymer, % 4. Liquid Polymer, gal
Sample Calculations for Determining Flows and Chemical Doses
The following examples demonstrate how the previously presented equations can be used if a system is conducting jar tests or modifying chemical feed practices to improve filter effluent turbidity. Systems may find these examples useful for calculating flow values or determining chemical feed settings.
Example 1: Flow Conversion
To convert a flow from gpm to MGD:
Scenario: If a system's flow is 900 gpm and the flow needs to be converted to MGD, the following equation can be used:
Flow, MGD = 1.3 MGD
Example 2: Chemical Doses
To calculate the liquid alum chemical feeder setting in milliliters per minute:
Scenario: The optimum liquid alum dose based on the jar tests at a particular plant is 12 mg/L. The system wants to determine the setting on the liquid alum chemical feeder in milliliters per minute when the plant flow is 5.3 MGD. The liquid alum delivered to the plant contains 439.8 milligrams of alum per milliliter of liquid solution.
Chemical Feeder Setting, mL/min = 380 mL/min
Example 3: Chemical Dose
To calculate the liquid alum chemical feeder setting in gallons per day:
Scenario: The optimum liquid alum dose based on the jar tests at a particular plant is 12 mg/L. The system wants to determine the setting on the liquid alum chemical feeder in gallons per day when the flow is 5.3 MGD. The liquid alum delivered to the plant contains 4.42 pounds of alum per gallon of liquid solution.
Chemical Feeder Setting, gpd = 120 gpd
Example 4: Chemical Dose
To calculate the polymer fed by the chemical feed pump in pounds of polymer per day:
Scenario: A system wants to determine the chemical feed in pounds of polymer per day from a chemical feed pump. The polymer solution contains 18,000 mg polymer per liter. Assume the specific gravity of the polymer solution is 1.0. During a test run, the chemical feed pump delivered 700 mL of polymer solution during 7 minutes.
Polymer Feed, lbs/day = 5.7 lbs polymer/day
Example 5: Chemical Dose
To calculate the flow delivered by the pump in gallons per minute and gallons per day:
Scenario: A small chemical feed pump lowered the chemical solution in a 4-foot diameter tank 1 foot and 3 inches during a 6-hour period.
Example 6: Chemical Dose
To determine the settings in percent stroke on a chemical feed pump (the chemical could be chlorine, polymer, potassium permanganate or any other chemical solution fed by a pump) for various doses of a chemical in milligrams per liter:
Scenario: The raw water flow rate to which the chemicals are delivered is 315 gpm. The solution strength of the chemical being pumped is 3.8 percent. Assume the specific gravity of the chemical solution is 1.0. The chemical feed pump has a maximum capacity of 97 gallons per day at a setting of 100 percent capacity.
Table 1: Setting for Chemical Feed Pump
Pump Flow, gpm = 315 gpm
Solution Strength, % = 3.8%
Chemical Dose, mg/L
Chemical Feed, lbs/day
Feed Pump, gpd
Pump Setting, % stroke
0.5 1.9 5.9 6.0 1.0 3.8 11.8 12.2 1.5 5.7 17.8 18.4 2.0 7.6 23.7 24.4 2.5 9.5 29.7 30.6 3.0 11.4 35.6 36.7 3.5 13.2 41.2 42.5 4.0 15.1 47.2 48.7 4.5 17.0 53.1 54.7 5.0 18.9 59.1 60.9 5.5 20.8 65.0 67.0 6.0 22.7 70.9 73.1 6.5 24.6 76.9 79.3 7.0 26.5 82.8 85.4 7.5 28.4 88.7 91.4
Figure 1: Chemical Feed Pump Settings for Various Chemical Doses from Table 1 Above