Coagulation

 

 

Introduction

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:

  1. Primary coagulants: Primary coagulants are used to cause particles to become destabilized and begin to clump together. Examples of primary coagulants are metallic salts, such as aluminum sulfate (referred to as alum), ferric sulfate, and ferric chloride. Cationic polymers may also be used as primary coagulants.

 

  1. Coagulant Aids and Enhanced Coagulants: Coagulant aids and enhanced coagulants add density to slow-settling floc and help maintain floc formation. Organic polymers, such as polyaluminum hydroxychloride (PACl), are typically used to enhance coagulation in combination with a primary coagulant. The advantage of these organic polymers is that they have a high positive charge and are much more effective at small dosages. Even though they may be more expensive, a smaller amount may be needed, thereby saving money. Organic polymers also typically produce less sludge.

 

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.

 

 

Chemicals

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:

  1. What is the protocol for low-turbidity water? The primary coagulant should never be shut off, regardless of raw water turbidity.

 

  1. Are chemicals being dosed properly with regard to pH, alkalinity, and turbidity? Is dose selection based on frequent jar testing or other testing methods such as streaming current monitoring, zeta potential, or pilot filters? Relying exclusively on past practice may not be enough. The system may want to consider doing a jar test while the plant is running well to see how floc in the jar should look.

 

  1. Do standard operating procedures (SOPs) exist for coagulation controls? Systems should develop SOPs and establish a testing method that is suited to the plant and personnel. SOPs should be based on the consensus of all operators to ensure shared knowledge and experience. Also, all processes should be documented as they are performed so they may be reproduced in the future.

 

  1. Are the correct chemicals being used? Is the best coagulant being used for the situation?
    Changing coagulant chemicals or adding coagulant aids may improve the settleability of the flocculated water and in turn optimize performance. Coagulants may also be changed seasonally. The system should be carefully evaluated before full-scale plant changes of chemicals are made. If the system does change chemicals and needs an immediate response, the operator may need to purge the chemical feed line, particularly if the chemicals are far (several hundred feet or more) from the point of application.

 

  1. Does the pH need to be increased through supplemental alkalinity? Adding a supplemental source of alkalinity, such as lime or soda ash, may be necessary for proper floc formation. However, adding lime (or other alkali supplements) and iron- or aluminum-based coagulants at the same point can degrade turbidity removal performance. The coagulant works on the high pH lime, the same as it does with naturally occurring turbidity or alkalinity. Therefore, the addition of lime typically creates the demand for more ferric- or alum based coagulant and the operator will probably add more coagulant in response to this demand. More coagulant can cause the pH to decrease, and more lime is typically added to compensate. Although finished water quality may be adequate when the raw water is stable, the plant pays a high cost in chemicals and sludge removal. This particular procedure is not foolproof and may not be effective at all when raw water characteristics change rapidly. One solution to this issue is to shift the feed line locations. Moving the coagulant line as far downstream as practicable from the lime addition point may allow the turbidity from the lime to fully dissolve. Placing the lime line well downstream of the coagulant addition point may allow for the coagulation of DBP precursors at a lower, more efficient pH before the lime addition elevates pH. Note that this mode of operation will not work for lime softening plants.

 

  1. Do operators have the ability to respond to varying water quality conditions by adjusting coagulation controls? Systems should provide operators with learning opportunities so that they are able to react to unusual situations quickly and appropriately. Heavy rains or lake turnover may happen rarely, but noting indicators of these events will help with planning. For example, a sudden drop in pH may occur prior to the first heavy rain reaching the intake. Systems should use this as a trigger to change the coagulant dosage.

 

  1. Are chemicals used before manufacturer recommended expiration or use-by dates? Does the chemical supplier operate an ISO 9000 production facility and provide quality certification? Chemical purity is important in all treatment systems.

 

  1. Are chemicals being added in the correct order? The order of chemical addition is very important, because certain chemicals interfere with others. Jar tests should be used to develop optimal sequences. The system may also want to consider changing the location of chemical feed points. For instance, some utilities have found that optimum water quality was achieved when a coagulant was fed in raw water and a polymer was fed prior to filtration.

 

  1. Is the chemical feed system operating properly? Operators should consider checking the accuracy of chemical feed systems at least once daily or once per shift. The system may want to install calibration columns on chemical feed lines to verify proper dosage or provide some other form of calibration. Systems should not set the chemical feed pumps to operate at maximum stroke and feed rates, which can damage the pumps.

 

  1. Are chemicals properly mixed, particularly chemicals that are diluted? The system may want to consider an automatic mixer in the chemical tank to provide thorough mixing.

 

 

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

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.

 

 

 

Jar Testing

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.

 

 

Materials

Stirring Machine
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.

 

  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.

 

  1. Measure out the proper quantity of distilled water in the volumetric flask.

 

  1. Add the chemical to the distilled water.

 

  1. 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.

 

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

 

  1. Place beakers in the stirring machine.

 

  1. 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.

 


    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.

 

  1. 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.

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.

 

  1. 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.

 

 

 

Feed Systems

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:

  1. Is redundancy a consideration? Redundancy should be built into the feed systems so that proper feeding of chemicals can be maintained if primary systems fail or malfunction.

 

  1. Do chemical feed pumps have sufficient dosage range? Feed systems should be sized so that chemical dosages can be changed to meet varying conditions.

 

  1. Are chemical feed systems and solution piping checked regularly? Preventive maintenance is critical for avoiding process upsets due to equipment breakdown.
    Coagulant lines should be flushed out frequently to prevent buildup. Where possible, chemical feed lines should be easy to take apart for quick replacement or simpler maintenance.

 

  1. Is a diaphragm pump used? A continuous pump allows coagulants to be added in a way that avoids pulsed flow patterns.

 

  1. Does the plant stock repair parts for all critical equipment? Repair parts with a long leadtime for delivery should be reordered as soon as possible after removal from inventory.

 

 

 

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:

  1. Is dispersion taking place? Coagulation reactions occur rapidly, probably in less than 1 second. When injecting at hydraulic jumps, weirs, or flumes, the coagulant should be distributed uniformly across the width of the flow.

 

  1. Where are coagulants being added? Generally, metal salts should be introduced at the point of maximum energy input. Low-molecular weight cationic polymers can be fed with metal salts at the rapid mix or at second stage mixing following the metal salt. Highmolecular weight nonionic/anionic floc/filter aids should be introduced to the process stream at a point of gentle mixing. Most polymer feed solutions should be provided with a “cure time” or “aged” before use. Use of an inline blender with carrier water aids in further dispersal at application. Most polymers have specific preparation instructions and should not be added directly in the raw, concentrated form in which they are received.

 

  1. Is rapid mixing equipment checked frequently? Systems should check the condition of equipment and ensure that baffling provides for adequate, even flow.

 

 

 

Rapid Mixing

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

Abbreviations
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

 

 

Conversion Factors

AREA:

1 sq ft = 144 sq in or 144 sq in/sq ft

1 ac = 43,560 sq ft or 43,560 sq ft/ac

 

DENSITY:

1 gal = 8.34 lbs or 8.34 lbs/gal

1 ft3 = 62.4 lbs or 62.4 lbs/ft3

 

DOSAGE:

1 grain/gal = 17.1 mg/L or 17.1 mg/L/gpg

1 mg = 64.7 grains or 64.7 grains/mg

 

FlOW:

1 MGD = 694 gpm or 694 gpm/MGD

1 MGD = 1.55 cfs or 1.55 cfs/MGD

 

LENGTH:

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

 

TIME:

1 min = 60 sec or 60 sec/min

1 hr = 60 min or 60 min/hr

1 day = 24 hr or 24 hr/day

 

UNITS:

1 million = 1,000,000 = 1 x 106

 

 

VOLUME:

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

 

WEIGHT:

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)

AREA:

1 ha = 2.47 ac or 2.47 ac/ha

1 ha = 10,000 sq m or 1,000 sq m/ha

 

LENGTH:

1 m = 100 cm or 100 cm/m

1 m = 3.28 ft or 3.28 ft/m

 

DENSITY:

1 liter = 1 kg or 1 kg/L

 

FLOW:

1 MGD = 3,785 m3 or 3,785 m3/MGD

 

VOLUME:

1 m3 = 1,000 L or 1,000 L/m3

1 gal = 3.785 L or 3.785 L/gal

WEIGHT:

1 gm = 1,000 mg or 1,000 mg/gm

1 kg = 1,000 gm or 1,000 gm/kg

 

 

Formulas

I. Flows:

1. Flow, gpm
or
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.

 

 

 

 

 

 

OR

 

 

 

 

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