5.2 settling

Settling has been defined as a unit operation in which solids are drawn toward a source of attraction. The particular type of settling that will be discussed in this section is gravitational settling. It should be noted that settling is different from sedimentation, although some authors consider settling the same as sedimentation.

Strictly speaking, sedimentation refers to the condition whereby the solids are already at the bottom and in the process of sedimenting. Settling is not yet sedi-menting, but the particles are falling down the water column in response to gravity. Of course, as soon as the solids reach the bottom, they begin sedimenting. In the physical treatment of water and wastewater, settling is normally carried out in settling or sedimentation basins. We will use these two terms interchangeably.

Generally, two types of sedimentation basins are used: rectangular and circular. Rectangular settling basins or clarifiers, as they are also called, are basins that are rectangular in plans and cross sections. In plan, the length may vary from two to four times the width. The length may also vary from ten to 20 times the depth. The depth of the basin may vary from 2 to 6 m. The influent is introduced at one end and allowed to flow through the length of the clarifier toward the other end. The solids that settle at the bottom are continuously scraped by a sludge scraper and

Effluent Launder
FIGURE 5.4 Portion of a primary circular clarifier at the Back River Sewage Treatment Plant, Baltimore City, MD.
FIGURE 5.5 Elevation section of a circular radial clarifier. (Courtesy of Walker Process.)

removed. The clarified effluent flows out of the unit through a suitably designed effluent weir and launder.

Circular settling basins are circular in plan. Unlike the rectangular basin, circular basins are easily upset by wind cross currents. Because of its rectangular shape, more energy is required to cause circulation in a rectangular basin; in contrast, the contents of the circular basin is conducive to circular streamlining. This condition may cause short circuiting of the flow. For this reason, circular basins are typically designed for diameters not to exceed 30 m in diameter.

Figure 5.4 shows a portion of a circular primary sedimentation basin used at the Back River Sewage Treatment Plant in Baltimore City, MD. In this type of clarifier, the raw sewage is introduced at the center of the tank and the solids settled as the wastewater flows from the center to the rim of the clarifier. The schematic elevational section in Figure 5.5 would represent the elevational section of this clarifier at the

FIGURE 5.6 Elevation sections of a circular clarifier (a) and a rectangular clarifier (b). (Courtesy of Envirex, Inc.)

Back River treatment plant. As shown, the influent is introduced at the bottom of the tank. It then rises through the center riser pipe into the influent well. From the center influent well, the flow spreads out radially toward the rim of the clarifier. The clarified liquid is then collected into an effluent launder after passing through the effluent weir. The settled wastewater is then discharged as the effluent from the tank.

As the flow spreads out into the rim, the solids are deposited or settled along the way. At the bottom is shown a squeegee mounted on a collector arm. This arm is slowly rotated by a motor as indicated by the label "Drive." As the arm rotates, the squeegee collects the deposited solids or sludge into a central sump in the tank. This sludge is then bled off by a sludge draw-off mechanism.

Figure 5.6a shows a different mode of settling solids in a circular clarifier. The influent is introduced at the periphery of the tank. As indicated by the arrows, the flow drops down to the bottom, then swings toward the center of the tank, and back into the periphery, again, into the effluent launder. The solids are deposited at the bottom, where a squeegee collects them into a sump for sludge draw-off.

Figure 5.6b is an elevational section of a rectangular clarifier. In plan, this clarifier will be seen as rectangular. As shown, the influent is introduced at the left-hand side of the tank and flows toward the right. At strategic points, effluent trough (or launders) are installed that collect the settled water. On the way, the solids are then deposited at the bottom. A sludge scraper is shown at the bottom. This scraper moves the deposited sludge toward the front end sump for sludge withdrawal. Also,

FIGURE 5.7 Removal at the settling zone (a); inboard weir design at outlet zone (b).

notice the baffles installed beneath each of the launders. These baffles would guide the flow upward, simulating a realistic upward overflow direction.

Generally, four functional zones are in a settling basin: the inlet zone, the settling zone, the sludge zone, and the outlet zone. The inlet zone provides a transition aimed at properly introducing the inflow into the tank. For the rectangular basin, the transition spreads the inflow uniformly across the influent vertical cross section. For one design of a circular clarifier, a baffle at the tank center turns the inflow radially toward the rim of the clarifier. On another design, the inlet zone exists at the periphery of the tank.

The settling zone is where the suspended solids load of the inflow is removed to be deposited into the sludge zone below. The outlet zone is where the effluent takes off into an effluent weir overflowing as a clarified liquid. Figure 5.7a and 5.7b shows the schematic of a settling zone and the schematic of an effluent weir, respectively. This effluent weir is constructed inboard. Inboard weirs are constructed when the natural side lengths or rim lengths of the basin are not enough to satisfy the weir-length requirements.

5.2.1 Flow-Through Velocity and Overflow Rate of Settling Basins

Figure 5.7a shows the basic principles ofremovalofsolidsinthesettlingzone. A settling column (to be discussed later) is shownmovingwiththehorizontalflowof the water at velocity vh from the entrance of the settling zone to the exit. As the column moves, visualize the solids inside it as settling; when the column reaches the end of the zone, these solids will have already been deposited at the bottom of the settling column. The behavior of the solids outside the column will be similar to that inside. Thus, a time to in the settling column is the same time to in the settling zone.

A particle possesses both downward terminal velocity vo or vp, and a horizontal velocity vh (also called flow-through velocity). Because of the downward movement, the particles will ultimately be deposited at the bottom sludge zone to form the sludge. For the particle to remain deposited at the sludge zone, vh should be such as not to scour it. For light flocculent suspensions, vh should not be greater than 9.0 m/h; and for heavier, discrete-particle suspensions, it should not be more than 36 m/h. If A is the vertical cross-sectional area, Q the flow, Zo the depth, W the width, L the length, and to the detention time:

The detention time is the average time that particles of water have stayed inside the tank. Detention time is also called retention time. Because this time also corresponds to the time spent in removing the solids, it is also called removal time. For discrete particles, the detention time to normally ranges from 1 to 4 h, while for flocculent suspensions, it normally ranges from 4 to 6 h. Calling V the volume of the tank and L the length, to can be calculated in two ways: to = Z0/vo and to = V/Q = (WZoL)/Q = AsZoIQ. Also, for circular tanks with diameter D, to = V/Q = ( ^ Zo)/Q =

AsZoIQ, also. Therefore,

where As is the surface area of the tank and Q/As is called the overflow rate, qo. According to this equation, for a particle of settling velocity vo to be removed, the overflow rate of the tank qo must be set equal to this velocity.

Note that there is nothing here which says that the "efficiency of removal is independent of depth but depends only on the overflow rate." The statement that efficiency is independent of depth is often quoted in the environmental engineering literature; however, this statement is a fallacy. For example, assume a flow of 8 m /s and assert that the removal efficiency is independent of depth. With this assertion, we can then design a tank to remove the solids in this flow using any depth such as 10-50 meter. Assume the basin is rectangular with a width of 106 m. With this design, the flow-through velocity is

8/(10 )(10 ) = 8.0(10 ) m/s. Of course, this velocity is much greater than the speed of light. The basin would be performing better if a deeper basin had been used. This example shows that the efficiency of removal is definitely not independent of depth. The notion that Equation (5.8) conveys is simply that the overflow velocity qo must be made equal to the settling velocity vo—nothing more. The overflow velocity multiplied by the surface area produces the hydraulic loading rate or overflow rate.

In the outlet zone, weirs are provided for the effluent to take off. Even if vh had been properly chosen but overflow weirs were not properly sized, flows could be turbulent at the weirs; this turbulence can entrain particles causing the design to fail. Overflow weirs should therefore be loaded with the proper amount of overflow (called weir rate). Weir overflow rates normally range from 6-8 m /h per meter of weir length for light flocs to 14 m /h per meter of weir length for heavier discrete-particle suspensions. When weirs constructed along the periphery of the tank are not sufficient to meet the weir loading requirement, inboard weirs may be constructed. One such example was mentioned before and shown in Figure 5.7b. The formula to calculate weir length is as follows:

weir Rate

5.2.2 Discrete Settling

Generally, four types of settling occur: types 1 to 4. Type 1 settling refers to the removal of discrete particles, type 2 settling refers to the removal of flocculent particles, type 3 settling refers to the removal of particles that settle in a contiguous zone, and type 4 settling is a type 3 settling where compression or compaction of the particle mass is occurring at the same time. Type 1 settling is also called discrete settling and is the subject in this section. When particles in suspension are dilute, they tend to act independently; thus, their behaviors are therefore said to be discrete with respect to each other.

As a particle settles in a fluid, its body force fg, the buoyant force fb, and the drag force fd, act on it. Applying Newton's second law in the direction of settling, fg - fb - fd = ma (5.10)

where m is the mass of the particle and a its acceleration. Calling pp the mass density of the particle, pw the mass density of water, Vp the volume of the particle, and g the acceleration due to gravity, fg = ppgVp and fb = pwgVp. The drag stress is directly proportional to the dynamic pressure, pwv2/2, where v is the terminal settling velocity of the particle. Thus, the drag force fd = CDAppwv2/2, where CD is the coefficient of proportionality called drag coefficient, and Ap is the projected area of the particle normal to the direction of motion. Because the particle will ultimately settle at its terminal settling velocity, the acceleration a is equal to zero. Substituting all these into Equation (5.10) and solving for the terminal settling velocity v, produces v = 3 g fc^ (5.11)

H 3 CDpw

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