PAMANTASAN NG LUNGSOD NG MAYNILACollege of Engineering and TechnologyDepartment of Chemical Engineering

SEDIMENTATION According to Brown (1950), sedimentation is the separation of a suspension into a supernatant clear fluid and a rather dense slurry containing a higher concentration of solid.According to Perry (), sedimentation is the partial separation or concentration of suspended solid particles from a liquid by gravity settling. This field may be divided into the functional operations of thickening, whose purpose is to increase the concentration of suspended solids in a feed stream; and clarification whose purpose is to remove a relatively small quantity of suspended particles and produce a clear effluent.Classification of Settleable SolidsThere are different factors to be consider in choosing the type of sedimentation to be used, some of it are particle size, viscosity, solid and solution densities as well as the characteristics of the particles within the slurry.

In this figure, it illustrates the relationship between solid concentration, interparticle cohesiveness and the type of sedimentation that may exist. Totally discrete particles include many mineral particles, salt crystals and similar substances that have little tendency to cohere. The flocculent particles generally include those smaller than 20 micrometer, metal hydroxides, chemical precipitates and most organic substances other than true colloids.Sedimentation TestsDetermination of Clarification-Zone Requirements In the treatment of solids suspensions which are in the particulate-settling regime, the usual objective will be the production of a clear effluent and test methods limited to this type of settling will be the normal sizing procedure, although the area demand for thickening should be verified.

Long-Tube MethodA transparent tube 2 to 4 m long and at least 100 mm in diameter (preferably larger), fitted with sampling taps every 200 to 300 mm, is used in this test. The tube is mounted vertically and filled with a representative sample of feed suspension. At timed intervals approximately 100-mL samples are withdrawn from successive taps, beginning with the uppermost one. The time intervals will be determined largely by the settling rate of the particles and should be chosen so that a series of at least four time intervals will produce samples that bracket the desired solids-removal target. Also, this procedure will indicate whether or not detention time is a factor in the rate of clarification. Typically, intervals may be 30 min long, the last series of samples representing the results obtainable with 2-h detention. The samples are analyzed for suspended-solids concentration by any suitable means, such as filtration through membranes or centrifugation with calibrated tubes.

Short-Tube Method

This test is suitable in cases in which detention time does not change the degree of particle flocculation and hence has no significant influence on particle-settling rates. It is also useful for hydroseparator tests where the sedimentation device is to be used for classification.

Detention TestThis test utilizes a 1- to 4-L beaker or similar vessel. The sample is placed in the container, flocculated by suitable means if required, and allowed to settle. Small samples for suspended-solids analysis are withdrawn from a point approximately midway between liquid surface and settled solids interface, taken with sufficient care that settled solids are not resuspended. Sampling times may be at consecutively longer intervals, such as 5, 10, 20, 40, and 80 min. Laboratory Batch SedimentationFrom a simple batch sedimentation where suspended fine solids are placed in a graduated cylinder allowing the contents to be undisturbed. The time rate of decrease in height of the visible interface between s supernatant clear liquid and slurry containing the particles is the sedimentation rate. This small scale experiment must be conducted at a uniform temperature to avoid movement of fluid or convection due to density differences resulting from different temperatures. (Brown, 1950)FACTORS AFFECTING SEDIMENTATION Several factors affect the separation of settleable solids from water. Some of the more common types of factors to consider are: PARTICLE SIZE The size and type of particles to be removed have a significant effect on the operation of the sedimentation tank. Because of their density, sand or silt can be removed very easily. The velocity of the water-flow channel can be slowed to less than one foot per second, and most of the gravel and grit will be removed by simple gravitational forces. In contrast, colloidal material, small particles that stay in suspension and make the water seem cloudy, will not settle until the material is coagulated and flocculated by the addition of a chemical, such as an iron salt or aluminum sulfate. The shape of the particle also affects its settling characteristics. A round particle, for example, will settle much more readily than a particle that has ragged or irregular edges. All particles tend to have a slight electrical charge. Particles with the same charge tend to repel each other. This repelling action keeps the particles from congregating into flocs and settling. WATER TEMPERATURE Another factor to consider in the operation of a sedimentation basin is the temperature of the water being treated. When the temperature decreases, the rate of settling becomes slower. The result is that as the water cools, the detention time in the sedimentation tanks must increase. As the temperature decreases, the operator must make changes to the coagulant dosage to compensate for the decreased settling rate. In most cases temperature does not have a significant effect on treatment. A water treatment plant has the highest flow demand in the summer when the temperatures are the highest and the settling rates the best. When the water is colder, the flow in the plant is at its lowest and, in most cases; the detention time in the plant is increased so the floc has time to settle out in the sedimentation basins.CURRENTS Several types of water currents may occur in the sedimentation basin: Density currents caused by the weight of the solids in the tank, the concentration of solids and temperature of the water in the tank. Eddy currents produced by the flow of the water coming into the tank and leaving the tank. The currents can be beneficial in that they promote flocculation of the particles. However, water currents also tend to distribute the floc unevenly throughout the tank; as a result, it does not settle out at an even rate. Some of the water current problems can be reduced by the proper design of the tank. Installation of baffles helps prevent currents from short circuiting the tank. SEDIMENTATION BASIN ZONES Under ideal conditions, the sedimentation tank would be filled with the water that has been coagulated, and the floc would be allowed to settle before any additional water is added. That is not possible for most types of water treatment plants. Most sedimentation tanks are divided into these separate zones: Inlet zoneThe inlet or influent zone should provide a smooth transition from the flocculation zone and should distribute the flow uniformly across the inlet to the tank. The normal design includes baffles that gently spread the flow across the total inlet of the tank and prevent short circuiting in the tank. (Short circuiting is the term used for a situation in which part of the influent water exits the tank too quickly, sometimes by flowing across the top or along the bottom of the tank.) The baffle could include a wall across the inlet, perforated with holes across the width of the tank.Settling ZoneThe settling zone is the largest portion of the sedimentation basin. This zone provides the calm area necessary for the suspended particles to settle. Sludge ZoneThe sludge zone, located at the bottom of the tank, provides a storage area for the sludge before it is removed for additional treatment or disposal.Basin inlets should be designed to minimize high flow velocities near the bottom of the tank. If high flow velocities are allowed to enter the sludge zone, the sludge could be swept up and out of the tank. Sludge is removed for further treatment from the sludge zone by scraper or vacuum devices which move along the bottom. Outlet ZoneThe basin outlet zone or launder should provide a smooth transition from the sedimentation zone to the outlet from the tank. This area of the tank also controls the depth of water in the basin. Weirs set at the end of the tank control the overflow rate and prevent the solids from rising to the weirs and leaving the tank before they settle out. The tank needs enough weir length to control the overflow rate, which should not exceed 20,000 gallons per day per foot of weir.

SELECTION OF BASIN There are many sedimentation basin shapes. They can be rectangular, circular, and square.

Rectangular Basins Rectangular basins are commonly found in large-scale water treatment plants. Rectangular tanks are popular as they tend to have: High tolerance to shock overload Predictable performance Cost effectiveness due to lower construction cost Lower maintenance Minimal short circuiting

Circular and Square Basins Circular basins are frequently referred to as clarifiers. These basins share some of the performance advantages of the rectangular basins, but are generally more prone to short circuiting and particle removal problems. For square tanks the design engineer must be certain that some type of sludge removal equipment for the corners is installed. EquipmentThickenersA thickener concentrates suspended solids by gravity settling so that a steady-state material balance is achieved. Solids being withdrawn continuously in the underflow at the rate they are supplied in the feed.

Basic Components of a thickener1. Tank to contain a slurry2. Feed piping3. Feed well to allow the feed stream to enter a tank4. Rake mechanism to assist in moving the concentrated solids to the withdrawal points5. An underflow solids-withdrawal system6. Overflow launder

Gravity SettlersAccording to Rousseau (1987), there are two types of sedimentation or settling. Type 1 settling refers to the settling where interaction between particles are minimal. This is usually the case of settling dilute slurries. All of the particles are to settle independently, knowing this and by acquiring additional data like particle size distribution and rate of particle settling, a design of the settler can be made.

If the terminal velocity (U,) of the smallest particle to be separated is known or can be calculated, then the overflow area (A1) can be calculated from the equation

where Qc is the volumetric flow rate of the liquid.

The depth of the liquid should be great enough to avoid suspending effects of turbulent liquid flow. Rousseau (1987) stated that the fluid velocity being about less than 20 times of the terminal velocity will prevent the re-suspension of the particles. Since this is the case; for calculation purposes, Rousseau (1987) indicated that the flow velocity should be 10 times of the terminal velocity of the particle resulting for the flow area (AF)to be given as

If the height of the basin is one-third the width,

Example 10 /tim particles of specific gravity = 3, the terminal velocity is 0.033 ft/s. To separate these particles and all larger particles from a water stream of 100 ftVmin (833 gal/min), the basin dimensions are calculated as follows:

In actual practice the effects of turbulence and other nonuniform flow characteristics would necessitate making the basin at least 19 ft long. (Rousseau, 1987)

Another type of sedimentation; Type II sedimentation, is seen in thick and lumpy slurries where the solid particles are settling as a mass. Because of this, systems are designed by controlling the thickening capability of the basin along with sufficient overflow area in order to clarify the overflow of liquid. Settlers of this type are referred to as thickeners. The construction of thickeners may be of a rectangular basin but usually it is of circular cross-section. In the rectangular basin, solids are normally removed by a traveling syphon that moves longitudinally back and forth along the basin. In the circular design a raking mechanism is used to convey the settled solids slowly to the center of the basin where, as with the rectangular basin, a syphon is used for their removal. (Rousseau, 1987)

The figure below depicts a circular thickener. The feed is introduced below the liquid surface and above the sludge blanket. Density differences between the feed, clarified liquid, and settled solids cause the feed to spread laterally or radially from the feed point, producing the effect of feeding the basin uniformly across the area just above the sludge layer.

Solid-Liquid SeparationUp to date, there are already much inventions of equipment used to separate solids that are finely dispersed and divided within a liquid. Given below is a table showing the different classification of mechanical processes employed in solid-liquid separation. (Couper, Penney, Fair, & Walas, 2005)

SettlingSettling can be done letting the particles settle due to gravitational force of by employing centrifugal force. Other means can also be used like flotation and magnetic separation. Sedimentation behavior is important to be known in order to design the appropriate equipment. The figure below illustrates typical sedimentation progress. (Couper et al., 2005)

According to Couper et al. (2005), it is more economical to concentrate dilute slurries when dealt on a large scale basis. To make this happen, the slurries are made to undergo sedimentation with the use of sedimentation tanks or thickeners within a prescribed period of time. Typical designs of thickeners are shown next.

The illustration above shows a typical thickener design employed for slurry concentration on a large scale. The slurry is introduced at the top center via feed launder; the clear liquid overflows the top edge in the overflow launder, whereas the solids settle out and are moved gradually towards the center with slowly rotating rakes towards the discharge port at the bottom center. The concentrated slurry then is suitable for filtration or other further processing. (Couper et al., 2005)

Another design of thickener is shown above. It is a deep cone thickener which was developed for the National Coal Board in UK. The unit is about 10ft in diameter and the impellers have a rate of rotation of 2rpm. Flow rate is about 70m3 per sec with solids content of 6wt%. It can concentrate up to 25-35 wt%. (Suarovsky, 1981).

(Perry's Chemical Engineers Handbook, McGraw-Hill, New York, 1963,pp. 19.49,19.52).

Clarifiers are similar devices, primarily for recovering clear liquids from dilute suspensions. Sedimentation rates can also be improved with the use of flocculating agents. Some of them are listed in the next page. (Couper et al., 2005)

Designing a Rectangular Sedimentation Tank

IntroductionDesigning a rectangular sedimentation tank is similar in many ways to designing a flocculation chamber. However, water in a sedimentation basin is not agitated, so the velocity gradient is not a factor in the calculations. Instead, two additional characteristics are important in designing a sedimentation basin.

Theoverflow rate(also known as thesurface loadingor thesurface overflow rate) is equal to the settling velocity of the smallest particle which the basin will remove. Surface loading is calculated by dividing the flow by the surface area of the tank. Overflow rate should usually be less than 1,000 gal/day-ft.2

The weir loading is another important factor in sedimentation basin efficiency.Weir loading, also known asweir overflow rate, is the number of gallons of water passing over a foot of weir per day. The standard weir overflow rate is 10,000 to 14,000 gpd/ft and should be less than 20,000 gpd/ft. Longer weirs allow more water to flow out of the sedimentation basin without exceeding the recommended water velocity.SpecificationsThe sedimentation basin we will design in this lesson will be a rectangular sedimentation basin with the following specifications:

Rectangular basin Depth: 7-16 ft Width: 10-50 ft Length: 4width Influent baffle to reduce flow momentum Slope of bottom toward sludge hopper >1% Continuous sludge removal with a scraper velocity