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PRIMARY TREATMENT
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The primary treatment of many waters, especially those coming from surface sources, is the removal of suspended matter. This removal is usually accomplished in two steps due to the wide range of sizes of matter causing the turbidity of the water.
Screens
Larger solids are usually removed by a variety of screen designs. Screens are used in connection with withdraw intakes from rivers or lakes. The water velocity through the screen should not exceed 2 feet per second and ideally be between 1 and 1.5 feet per second. The area of the openings inn a screen should be 180 to 210 percent of the area of the pipe or channel protected by the screen.
Screens should be made of suitable materials according to expected working conditions and should be located at easily accessible places to facilitate cleaning. Also, provision should be made for disposal of the screenings.
Screens may be designed for coarse or fine matter.
One type of screen is a rotating screen with and endless belt made of fine wire mesh mounted on a chain. When the screen raises above the water level, the debris collected on it is discharged into a trough, usually with the help of a water jet. These kinds of screens are usually located in the inflow channel to the pumps, where they are accessible for inspection and maintenance.
Intakes
While removing water form the source, it is desirable to have a point of inflow well below the water surface but far enough from the bottom to avoid the pickup of suspended matter.
Several levels of intake should be provided to permit water withdrawing from the most suitable quality of water no matter how high or low the level may be. There must be a separate coarse screen for each inflow port.
Other factors affecting the placement of intakes include:
Foundation conditions;
Avoidance of channels used for navigation;
Danger of ice formation and thrust;
Possible formation of sand bars or mud banks; and
Existence of currents that may undermine structure or deposit silt against or on it.
Small intakes in lakes or rivers having relatively little change in elevation throughout the year may be quite simple. Large intakes are usually costly and complex. Using cofferdams, the lake or river bottom must be excavated to provide a firm foundation for the heavy structure. A tunnel or a pipeline sunk in the lake or river bottom may carry the water to the shore.
Clarification by Settling
After water has passed though one or more screens, the next step in preparing for the final stags of treatment is sedimentation, usually with the aid of coagulants. Sedimentation, with or without chemicals is rarely a complete treatment process with exceptions as where clear upland waters are retained in large reservoirs for long periods of time.
Sedimentation and coagulation are important in that these processes prepare waters for filtration and proper preparation of the water is necessary to ensure efficient and trouble-free performance of filters. The aim in both design and operation of a treatment plant should be to combine the several treatment processes into an effective and economical whole.
Plain Sedimentation
Impoundment of surface water in a storage reservoir for a considerable time results in clarification which is sometimes sufficient treatment. More often, however, such storage is a preliminary step to further treatment. Clarification of the water by storage results not only from sedimentation, but also from sunlight and aeration. Plain sedimentation usually has little effect in removing the very small particles suspended in the water, but the larger and heavier particles do settle. The extent of settlement depends on the particles size and the velocity of flow of the water.
Where the water is drawn form a highly turbid source, pretreatment basins may be utilized to remove much of the turbidity and provide water that is relatively uniform in quality.
Sedimentation by Coagulation
In order to remove the very fine particles of suspended matter in the water, it is common practice to add a chemical, called a coagulant, to the water. This coagulant forms as a flow, attracting finely divided particles and the colloidal mater in the water into groups or aggregates that are more easily removed by sedimentation.
The ability of suspended matter in water to settle depends largely on the size and specific gravity of the particles, but it is also influenced by the temperature of the water. The colder the water, the more viscous it is, and the grater the friction that must be overcome by the particle in settling.
Design of Sedimentation Tanks
Although overflow rates, or surface settling rates, may be important in calculating the size of a sedimentation basin. If the allowable overflow rate is 1,000 gallons per day per square foot of tank surface, a flow of 1 mega gallon per day would require a tank having a surface area of 1,000 square feet.
In water treatment, however, the tank capacity is not usually determined by the overflow rate or by the surface loading per unit of surface area, except when high-rate settling basins are used. Detention periods for water treatment basins are usually between 3 and 4 hours and the depths are 3 to 12 feet or more in larger plants.
Cross sectional areas are such as to provide a horizontal velocity of flow ranging from 0.5 to 3 feet per minute and normally about 1 foot per minute.
The detention period of a tank is the time that would be required for the flow of water to fill it in the absence of outflow. Thus, if normal flow is 2 million gallons per day and the tank capacity is 0.5 million gallons, the detention period will be:
2,000,000 / 500,000 = 4 hours
Usually the detention period is selected on the basis of the characteristics of the water, and the required capacity of the tank is then computed.
With the exception of very small plants, two or more tanks are desirable. While flow variations in water treatment are not normally very great, flexibility is desired and multiple basins permit maintenance, repairs, and cleaning without interrupting operations output.
Tank Size
Usually the overflow rate and a depth are assumed, and the required product of the width and the length are calculated. Standard sizes of removal of solids equipment will govern the tank width to a minor extent. For instance, to treat 2, 250,000 gpd with a 3-hour detention time and an assumed depth of 12 feet, the required surface area will be 3,125 feet and the width and length may be 40 feet and 78 feet respectively. This surface is better provided by two tanks each 20 by 78 feet.
The diameter of a circular tank also depends on the overflow rate, the required volume, and the depth. As for the rectangular tanks, equipment is made in certain standard sizes. Generally the tank bottom is cone-shaped, with a 45o gradient. With these conditions, the volume may be found by the following formula:
V = d2 (0.011d + 0.785 h)
Where V = volume of circular tank, in cubic feet;
d = diameter of the tank, in feet; and
h = vertical depth at wall, or sidewater depth, in feet.
A value of h is assumed and the equation can be solved for d or V if the orther is known or assumed.
Rectangular Tank Example:
A rectangular settling tank without mechanical equipment is to treat 400,00 gpd of raw water. The sedimentation period is to be 4 hours, the velocity of flow 3 inches per minute, and the depth of the water and sediment 14 feet. If an allowance of 4 feet for sediment is made, what should be the length of the basin and the width of the same?
The required length:
240 minutes detention time * 0.25 feet per minute velocity of flow = 60 feet
Volume of water to be treated in each period of 4 hours is:
400,000 * (4/24) = 66,700 gallons or 8,920 cubic feet.
Since the length of the tank has been calculated as 60 feet the cross sectional area must be:
8,920 /60 = 148.7 square feet.
The depth of water when there is 4 feet of sediment is 10 feet and the required width of the tank is therefore:
148.7 / 10 = 14.9 feet
A Circular Tank Example:
A circular settling tank with standard mechanical sludge-removal equipment is to handle 750,000 gpd of coagulated water. If the sedimentation period is to be 4 hours and the depth of the tank is to be 10 feet, what should be the diameter of the tank?
Volume of water to be handled during 4 hours:
750,000 * 4/24 = 125,000 gallons or 16,710 cubic feet.
If 10 feet depth is wanted and a diameter of 30 feet is assumed for the first trial, the volume found is:
V = d2 ( 0.011 d + 0.785 h) = 302 (0.011 *30 + 0.785*10) = 7,270 cubic feet.
Second trial is necessary to get closer to the required volume, 45 feet diameter is used this time:
V = 452 (0.011*45 + 0.785*10) = 16,900 cubic feet
Construction Features of Sedimentation Tanks
Inlets and Outlets
The design of a tank inlet should be such as to facilitate an even and uniform flow to the outlet. At the inlet of a rectangular tank the usual practice is to provide a baffle wall which distributes the inflow across the entire width of the tank.
Extensive experimental work indicates that there is little benefit from baffles place at intervals in a rectangular tank, since eddies and current are often created.
In a circular tank, the feed is often through the center column and thence outward through ports in the column. The water then flows radially to a peripheral weir which does not require a baffle.
Sludge Removal
The advantage on frequent removal of the sludge is avoidance of settle solids decomposition and possible taste and odor production. A disagreeable and costly task of cleaning a tanks containing several feet of deposited solids is avoided by tanks designed with mechanical devices for continuous removal of the sludge.
Tank Level Controls
To reduce the flow through the sedimentation tank correspondingly, a tank-level controller should be provided. This controller must reduce or increase the inflow to the tank in the same rate as the outflow is reduced or increased to avoid the creation of eddies or currents.
The volume rate of water through a sedimentation tank should be reasonably uniform. Also, the tanks may discharge almost directly to the filters, which should be operated at a fairly uniform rate.
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