COAGULATING CHEMICALS

The type of chemicals that should be used for coagulating raw water can be determined by using the results from jar tests, plant tests, or by using the data shown in table 9-3(A). Theoretically table 9-3(A) is correct; however, these values can be misleading when applied to some types of raw water. The chemical content of water may have a considerable influence on the optimum pH range for the various coagulants. For example, coagulation with ferrous sulfate is usually best accomplished at relatively high pH values in the alkaline zone. With soil, colored waters, ferric coagulant may sometimes be used with considerable success at pH values of 4.0 or less. Because of this wide variation in the optimum pH range of coagulant (caused by individual characteristics of the raw water), the coagulant dosage and the optimum zone for floc formation should be determined by jar tests, rather than just relying on table 9-3(A).

Table 9-3(A).-Optimum pH Ranges for Common Coagulant

JAR TEST

The jar test is the most common method of determining proper coagulant dosages. When there is a question as to which chemical should be used as a coagulant, it is often necessary to run more than one series of jar tests. Different coagulant chemicals and pH ranges should be used to determine which one produces the most satisfactory results at the lowest cost. The step-by-step procedures for ajar test are as follows:

1. Prepare a standard solution of each coagulant selected for trial by adding 10 grams of coagulant to 1 liter of distilled water.

2. Correct the pH of a sample of raw water to within the optimum range for the coagulant being tested (only if the pH is to be adjusted to the same extent in actual plant operation). Divide the raw water into six 1 liter samples,

^{3 .} Add 0.5 ml of standard coagulant solution to one sample of raw water, 1.0 ml to the second sample, 2.0 ml to the third sample, 3.0 ml to the fourth sample, 4.0 ml to the fifth sample, and 5.0 ml to the sixth sample. The result is a dosage of 5, 10, 20, 30, 40, and 50 mg/1, respectively.

4. Agitate samples in the jar test apparatus at a velocity about equal to the treatment equipment you are using and for the same length of time as the treatment equipment mixing time.

5. Keep the samples at the same temperature as water passing through your treatment equipment.

6. After stirring, let the samples settle for 30 minutes.

7. Siphon off a sample of the supematant and determine the turbidity by using a turbidimeter.

8. The smallest amount of coagulant that produces the lowest turbidity represents the optimum dosage. Multiply the coagulant dosage in mg/1 ( step 5 above) by 8.33 to get the correct chemical feed in pounds per million gallons.

9. Repeat the steps for each chemical used until satisfactory results are obtained.

As to acceptability, the taste and odor of water must be considered from the user's point of view. Tastes and odors of water are most commonly caused by algae, decomposing organic material, dissolved gases, or industrial wastes. Potability is not fleeted by the presence of odors and tastes. On the other hand, palatability is frequently affected, particularly when a substance such as bone or fish oil is present. Tastes and odors that make water unpalatable must be removed. Use of free available chlorine, aeration, and activated carbon can do much to prevent or remove unacceptable tastes and odors from treated water.

The use of free available chlorine is advantageous because most odors and tastes are removed and rigorous disinfection is assured.

Activated carbon is the most widely used single process for taste and odor removal. Aeration and copper sulfate treatment are also used. All three methods are described below. The method used depends upon the substance or substances to be removed and the equipment available. l Activated carbon is an excellent absorbing agent to use in ridding water of unpleasant tastes and odors. It is also an effective agent for removal of organic color. It is insoluble and tends to float unless all particles are wetted thoroughly by being made into a slurry before being added to the water. When continuous flow equipment is being used, the activated carbon should be added to the limestone feeder and added to the water with the limestone slurry. When the batch type of equipment is being used, the activated carbon can be added along with other chemicals in the coagulation tank. Being insoluble, activated carbon does not affect the pH value or chemical characteristics of water. One ounce of activated carbon per 1,000 gallons of water is usually adequate. However, dosages up to 1 pound per 1,000 gallons can be used, depending upon the kind and degree of impurities present. Use of activated carbon in much higher dosages for removal of chemical agents is discussed later in this chapter.

NOTE: Treatment with activated carbon should always be made ahead of, or part of, the coagulation process, so the activated carbon and the various impurities absorbed by it are removed. l Aeration treatment consists of adding

oxygen by exposing the water to air. The process has a two-fold action. Volatile odorproducing materials are released to the atmosphere, and the action of the air upon readily oxidizable materials causes a precipitation of insoluble oxides and hydroxides. Removal of hydrogen sulfide is an example of the formal action, while removal of iron is an example of the latter action. The aeration of water to rid it of the taste and odor of decomposing vegetable matter generally involves both actions. l Copper sulfate controls tastes and odors caused by small living organisms. This treatment is most frequently used in lakes and reservoirs. The copper sulfate is applied by towing a porous sack containing copper sulfate crystals behind a boat or by spraying a copper sulfate solution over the surface of the water. The amount of copper sulfate you should use depends on the type and concentration of organisms present. Dosages must be controlled because amounts greater than 2.0 parts per million (ppm) kill fish in the water. The amount necessary to remove microorganisms has no detrimental effect on human beings. Copper sulfate treatment is rarely used in field water supply for several reasons.

1. The advantage to be derived from treating an entire lake or reservoir frequently does not warrant the expense of the treatment when the length of time the water source is to be used is taken into consideration.

2. The amount of copper sulfate used entails considerations of wildlife, medical effects, and total water chemistry which are beyond the water supply technician's area of operation.

3. Superchlorination and dechlorination with activated carbon are effective for short periods although they are expensive for extended operations,

Temperature must be considered in the treatment of water. Lowering the temperature of water suppresses odors and tastes and, therefore, increases its palatability. In the summer, the temperature of deep lakes and reservoirs decreases sharply from top to bottom. By shifting the depth of intake, it maybe possible to draw relatively cool water even during hot weather. Water should be drawn from the lower depths when possible. Cool water is more viscous than warm water and thus is more difficult to filter. Cool water is more difficult to coagulate and chlorinate effectively than warm water because of slower reactions. Water treatment rates should be reduced when water temperatures are less than 45F.

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