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COAGULATION
The use of coagulant is necessary with waters having turbidities greater than 30 to 50 mg/l, but in practice coagulation is used in all water-treatment plants that employ rapid sand filtration. However, coagulation and settling do not provide a complete treatment; filtration is necessary in addition.
Coagulation is accomplished by mixing a suitable chemical or chemicals with the water. Depending on the characteristics of the water and sometimes the chemicals, there may be required a rapid and violent mixing followed by a longer and gentler mixing period, or the rapid mix may not be needed.
The term mixing basin is applied to the tank in which the water and chemicals are mixed together. The term settling basin, or sedimentation basin, is used to designate the tank in which settling takes place.
There are three methods of indicating the amount of chemicals applied to water or sewage. These are grains per gallon (gpg), pound per million gallon (lb per million gal), and milligrams per liter (mg/l). Since there are 7000 grains in a pond, 1 gpg equals 1 lb in 7000 gal, or 1,000,000/7000 = 142.8 lb per million gal. The unit milligrams per liter is now in place of parts per million, though the two are the same for all practical purposes in water treatment. Since 1 gal of water weighs 8.34 lb, 1 gpg is equal to 142.8/8.34 = 17.1 mg/l. Also, 1 mg/l is the same as 8.34 lb per million gal.
The cost of chemical for coagulation is considerable. Also, if the water is not properly clarified by the process of coagulation and sedimentation, the filters require more frequent washing and the used of grater volumes of wash water.
Finally, some waters do not yield readily to coagulation; instead there is formed a fine, feathery floc which is fragile and settles slowly. For these reasons, care is needed in the design of the mixing and settling basins; the water must be properly conditioned for the chemical reaction; and the chemical must be fed accurately.
Chemicals Used for Coagulation
The chemical most generally used in coagulation is aluminum sulfate, Al2 (SO4)3 . 18 H2O, which is also called filter alum or simply alum. Iron salts more or less frequently used include ferrous sulfate, Fe SO4 . 7 H2O; ferric choride, Fe Cl3; ferric sulfate, Fe2 (SO4 )3 , and chlorinated copperas, Fe2 (SO4 )3 . Fe Cl3 . Lime in the form of chalcium hydroxide , Ca (OH)2, is necessary in conjunction with ferrous sulfate and sometimes is used alone; it also may be needed in waters deficient in alkalinity to react with aluminum sulfate.
These chemicals are sued most effectively and economically when the acidity or alkalinity of the water is held within relatively narrow limits. In order to obtain and maintain the desired reaction, it may be necessary to use also acids or alkalis such as sodium carblonate, Na2 CO3, which is generally called soda ash; lime, CaO; or sulfuric acid, H2SO4.
Use of Aluminum Sulfate
Aluminum Sulfate is obtainable either as liquid alum or in the form of lumps, which must be put into a solution prior to being fed into the water. Owing to the considerable water content of liquid alum, long-distance hauling is uneconomical, and it is available only within a relatively short radius of manufacturing plants. Its use, however, simplifies feeding problems and reduces labor required to prepare the solutions for the feeders.
When aluminum sulfate is added to water that contains calcium bicarbonate, the following reaction takes place:
Al2 (SO4)3 + 3 Ca (HCO3)2 > 3 CaSO4 + 2 Al(OH)3 + 6 CO2
When the water contains sodium carbonate, the reaction is:
Al2 (SO4)3 + 3 Na2 CO3 + 3 H2O > 3 Na2SO4 + 2 Al(OH)3 + 3 CO2
The aluminum hydroxide, which is insoluble and colloidal forms the floc. The resulting floc will indicate whether the correct amount of coagulant is being used. Large, feathery flakes generally indicate that the amount of coagulant was too great. Normally, floc particles the size of a pin head are desirable.
Theoretical Requirements
The amount of aluminum sulfate required for coagulation depends on the turbidity and color of the water that is treated. Temperatures below about 35 F or above 70 F may increase the coagulant demand slightly, because of a slower reaction or from reabsorption of the floc.
Experimental work in the laboratory will give an indication of the approximate dosage that is required, but actual results in plant operation may show that some adjustments in this quantity are necessary. For relatively clear water, a minimum dose of 0.3 gpg may be used, whereas highly turbid water may require as much as 5 gpg. The average dosage is about 1 gpg.
In order to obtain a complete reaction with aluminum sulfate, the water must contain from 6 to 10 mg/l of alkalinity of each grain per gallon of coagulant. Theoretically 1 gpg of aluminum sulfate reduces de alkalinity of the water by 7.7 mg/l, but the reduction varies with different waters. In addition, there should be an excess of alkalinity ranging from 10 to 50 mg/l. If this reserve alkalinity is not present, the reaction may be incomplete and some of the aluminum sulfate may pass through the filter in solution as residual alum.
Best results are usually obtained with aluminum sulfate when the pH value is between 6.5 and 8.5 but coagulation will also take place when the pH value is 5.5. If the water to be treated with aluminum sulfate is deficient in alkalinity, the necessary adjustment can be made by adding lime or soda ash. Theoretically, 1 gpg of aluminum sulfate combines with 8.3 mg/l of 90 percent soda ash or with 5.96 mg/l of 95 percent hydrated lime. Thus, either 0.5 grain of soda ash or 0.35 grain of lime is required for each grain of aluminum sulfate used for coagulation. Also, 1 mg/l of soda ash is practically equivalent to 1 mg/l of alkalinity.
When the water is colored and aluminum sulfate is use as the coagulant, it may be necessary to produce first a color floc, which is a compound formed by the aluminum of the coagulant and the organic material causing color.
With most colored waters, this floc is formed best when the pH value is considerably below 6.5. Therefore, it is sometimes advantageous to lower the pH value of the water by adding sulfuric acid before the aluminum sulfate is applied.
If soda ash or lime is also to be added in order to give the water the necessary alkalinity to permit complete coagulation of the aluminum sulfate or to eliminate corrosiveness, the color floc should be removed by filtration before the acidity is neutralized.
Use of Ferrous Sulfate
Ferrous sulfate, which is also called copperas (not to be confused with copper sulfate), is used extensively as a coagulant with lime in waters that are not colored. This substance is generally cheaper than aluminum sulfate.
In the case of waters that are highly colored, however, the addition of ferrous sulfate and lime will not give satisfactory results. Because of the high alkalinity that would be produced by this treatment, compounds that tend to set the color would be formed before a suitable floc for removing the color could be produced. In general, ferrous sulfate is most valuable in large treatment plants, where constant laboratory supervision is possible. Ferrous sulfate and lime may be applied either dry or in solution in the same manner as aluminum sulfate.
The lime may be introduced either before or after the copperas. Where the ferrous sulfate is added first to water containing calcium bicarbonate, the immediate reaction is that indicated below:
Fe SO4 + Ca (HCO3)2 > Fe(HCO3)2 + Ca SO4
Then, when the lime is added, the reaction that takes place is the following:
Fe (HCO3)2 + 2 Ca (OH)2 > Fe (OH)2 + 2 CaCO3 + 2 H2O
If the lime is added first, the reaction that occurs when the ferrous sulfate is applied is indicated below:
Fe SO4 + Ca (OH)2 > Fe(OH)2 + Ca SO4
In either case, ferrous hydroxide is formed. This hydroxide is oxidized by the dissolved oxygen in the water, as shown below:
4 Fe (OH)2 + 2H2O + O2 > 4 Fe (OH)3
The ferric oxide forms the floc.
In general, the dosages of ferrous sulfate will be about the same as, or slightly higher than, the amount of aluminum sulfate that would be required. For each grain of ferrous sulfate per gallon of water, it is necessary to have about 0.5 mg/l of dissolved oxygen and at least 0.4 gpg of lime. The required amount of lime increases with the turbidity and for very turbid water the amounts of lime and ferrous sulfate may be equal.
Use of Chlorinated Copperas
When chlorine is added to a solution of ferrous sulfate, the two react chemically to form ferric sulfate and ferric chloride. The reaction may be as follows:
6 Fe SO4 + 3 Cl2 > 2 Fe2 (SO4)3 + 2 Fe Cl3
Theoretically, 1 lb of chlorine is required to react with 8 lb of ferrous sulfate, but it has been found in actual practice that 1 lb of chlorine will react with about 7.3 lb of ferrous sulfate. The combination of ferric sulfate and ferric chloride is known as chlorinated copperas. Each of these ferric compounds is a good coagulant, and their combination often is quite effective.
Chlorinated copperas is a valuable coagulant for removing color, especially where the water has a low pH value. First, chlorinated copperas is applied in sufficient quantities to give a good flow. When the mixing is bout half completed, and alkaline coagulant, which may be aluminum sulfate and lime or chlorinated copperas and lime, is added.
Use of Ferric Sulfate
Prepare in free-flowing granular forms, ferric sulfate is applied with relative ease, and it produces a good floc over a rather wide range of pH values. It has a particular advantage where manganese is present in sufficient amounts to require removal. When ferric sulfate is used with lime, the following reaction occurs:
Fe (SO4)3 + 3Ca (OH)2 > 3 CaSO4 + 2 Fe (OH)3
The ferric hydroxide forms the desired floc, just as with all other iron compounds.
Chemical Aids to Coagulation
Lime
The principal use of lime in coagulation is to provide artificial alkalinity in waters that are treated with aluminum sulfate or ferrous sulfate.
Lime may be used as a coagulant without additional chemicals for waters having a high content of magnesium compounds. A large proportion of the magnesium is thus changed to a bulky flocculent mass. However, when waters containing magnesium are coagulated with lime, they have a high alkalinity.
Lime may be purchased in the form of either quicklime, CaO, or hydrated lime, Ca(OH)2. Quicklime contains from 75 to 99 percent of pure calcium oxide, but it must be slaked with water before it is used. Hydrated lime, on the other hand, is slaked lime that is ready for use, and it contains between 80 and 99 percent of calcium hydroxide.
Quicklime is cheaper than hydrated lime, and therefore it is used almost entirely in large plants. However, hydrated lime is usually preferred for small plants, because the saving in the cost of chemical by using quicklime is not enough to compensate for the costs of the labor and equipment that are required for slaking the quicklime.
The effective strength of the quicklime, or the percentage of pure CaO that it contains, must be known in order to computer the required dosage. The computations may be based on the fact that 75.7 lb of pure CaO is equivalent to 100 lb of pure Ca (OH)2. Also, 1 gpg of pure Ca (OH)2 will combine with 10.16 mg/l of free carbon dioxide, CO2. These proportions are based on the ratio of the molecular weights of the chemicals and may be computed in the following manner:
The molecular weight of CaO is 40.08 + 16.00 = 56.08, and the molecular weight of Ca (OH)2 is 40.08 + 32 + 2.02 = 74.10. Therefore, 74.10 lb of Ca (OH)2 is equivalent to 56.08 lb of CaO, or the number of pounds of pure CaO that would be equivalent to 100 lb of pure Ca (OH)2 is (56.08/74.10 ) x 100 = 75.7. The molecular weight of CO2 is 12.01 + 32 = 44.01, and that of Ca (OH)2 has just been determined, is 74.10. Therefore, 1 part of Ca (OH)2 will combine with 44.01 /74.10 = 0.594 part of CO2. Since 17.1 mg/l is equivalent to 1gpg, then 1 gpg of hydrated lime will combine with 17.1 x 0.594 = 10.16 mg/l of CO2.
Soda Ash
Soda ash which is essentially Sodium Carbonate, Na2 CO3, is used to provide artificial alkalinity in water that is to be treated with aluminum sulfate, and it also reduces permanent hardness. It is very soluble in and easily applied to water. The commercial product should contain 98n percent of sodium carbonate. If an excess of soda ash is used, the excess will combine with carbon dioxide to reduce the corrosiveness of the water.
Therefore, exact dosages are not essential for good treatment. For this reason soda ash is particularly useful in small plants that lack precise control facilities, even though its cost is considerably greater than that of lime. It is useful to know that 1 gpg of soda ash will combine with 7.2 mg/l of carbon dioxide.
Other Chemical Used in Coagulation
Sulfuric Acid, H2 SO4, is used infrequently to acidify waters prior to treatment. Sodium Hydroxide, NaOH, which is also known as caustic soda, is an effective alkali. Also, when used to produce alkalinity, it does not increase the hardness of the water.
Sodium silicates, consisting of sodium oxide, Na2O, and silica, SiO2, in various proportions, are use in conjunction with aluminum sulfate to produce large floc particles. In the so-called high-rate upward-flow clarifiers, such large particles are expecially desirable after being compacted and agglomerated by slow agitation.
Determining Coagulant Dosages
Jar tests for aluminum sulfate
After a treatment plant has been in operation of some time, the records of chemical dosages employed for various turbidities and conditions of the water form an excellent guide to the required doses of coagulant.
Records of chemical dosages should, therefore, be made daily and should be plotted on charts in order to facilitate their use. For new plants or for unusual conditions in old plants the required dosages may be determined approximately by theoretical computations as indicated in the preceding explanations. However, the computed dosages should be checked by means of laboratory tests.
The hydrogen ion determination and other relatively simple tests will usually provide sufficient information for the control of coagulants in the operation of the plant. The laboratory test commonly used for determining the approximate dosage of a particular chemical required in a plant is called the jar test. It may be used for any of the various coagulants mentioned, but it is here described only for aluminum sulfate, which is the most common coagulant.
The eater that is to be tested is placed in jars or breakers that have a capacity of 500 to 1000 ml. Various amounts of chemicals are put into the jars, and the ingredients are stirred rapidly to cause the formation of the floc. The floc is then allowed to settle in each jar, and the jar in which the least amount of chemical produced a good, settleable floc is chosen as the one with the most economical dosage.
Jar Test for Turbid Alkaline Waters
If the water contains sufficient natural alkalinity to permit the use of aluminum sulfate without the addition of line or soda ash, the jar test may be made in the following manner: Six jars, each containing the same amount of the sample of water, are placed in a stirring machine. A different amount of coagulant is added to each jar, the water is stirred for about 15 min, and then the liquid is allowed to stand so that the floc may form.
The smallest dosage of aluminum sulfate that produces a good floc is the desired dosage for that particular water.
Tests for Turbid Water of Low Alkalinity
When a comparatively large dosage of aluminum sulfate alone fails to produce a good loc in water and its is known that the water is of low alkalinity, the necessary procedure is as follows: A test is first made to determine the alkalinity of the water, and enough soda ash or lime is then added to provide the alkalinity required for a satisfactory reaction with aluminum sulfate.
Equipment for Feeding Chemicals
In order to fed chemicals to the water regularly and accurately, some type of feeding equipment must be used. This is of two general types, dry feed and solution feed. Which will be the preferable for any specific plant depends on a number of factors. First is the character of the chemical; for instance, activated carbon and lime cannot be fed in solution form. The amount to be fed is also important; a dosage of ½ mg/l for 360,000 gpd requires only 1 oz of chemical per hour and, for accuracy, this must be fed as a weak solution by a liquid feeder.
A third factor considers the facilities available at the plant; if the plant is large, personnel will be available to make up solutions of the proper strength and have then available when needed.
Size of the plant may also influence both the type of chemical used and the method of feed. In a large plant, which uses a great deal of coagulant, the chemical should be purchased in its cheapest form and the plant should be equipped to prepare and handle the chemical in that form. On the other hand, if only a small amount of coagulant is used, the chemical will be purchased in the form that can be applied most conveniently and economically.
Another important factor in feeders is accuracy. Equipment should be capable of feeding the chemical within close limits, both for effectiveness of treatment and for economy.
Dry Feeding Equipment
In most dry feeders, the chemical is measured either by volume or by weight. For using volumetric feeders, the chemical must be reasonably uniform in particle size, because material ranging from lumps down to dust cannot be fed accurately by volume. With gravimetric feeders, particle size is of less importance.
Even when dry-feed machines are used, the chemical is dissolved or suspended in water before application.
Solution Feeding
In using solution feeders, solutions or suspensions of definite strength are so prepared that, when specific amount of solution are applied, the quantity of chemical is easily determined. A constant-level tank with an adjustable orifice is used for applying large amounts of solution. A float controls the level on the orifice, permitting accurate control of the dosage. Pumps are also used, a measured amount of solution of known strength being drawn at each stroke of the piston or plunger. For the most part, this type of feeder is applicable only to smaller installations.
Another method of application is to utilize devices in connection with meters measuring the flow of the water to be treated.
These must be of material that is not affected by the relatively strong concentration of chemical that is used. Tanks are generally of concrete to resist corrosion, though metal tanks protected with epoxy are also used. The dissolving boxes should be equipped with pipe manifolds for applying the water to the chemical after it has been weighed and placed in the tanks. Most of the coagulating chemicals, such as aluminum sulfate and ferrous sulfate, are prepared in solutions having a strength of about 5 percent; therefore, the solution tanks should be sized to meet plant operating conditions and the amount of chemicals used daily.
Storage of Chemicals
Provisions for storage and handling of chemicals vary greatly with the size of the plant. A plant treating 1 mgd of water and using 1 gpg of aluminum sulfate as a coagulant will require only 142 lb per day, and a 5-ton truckload will last for 2 months. Storage of 5 to 10 tons in bags may be provided, and handling the chemical will require very little daily labor.
On the other hand, a plant using 3 tons of aluminum sulfate daily must provide storage for 25 or more tons, since the shipment may be in bulk in carload lots or by heavy trucks. In the design of the plant, provision must be made for storage and for easy and economical handling of the chemical to the solution tanks or to the hoppers of the dry-feed machines.
Often, handling from storage bins will be by screw or belt conveyors. Weighing equipment should always be provided, both to check the weight of the chemical as it is delivered and to ensure that proper amounts are used in feeding.
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