Sedimentation tank pdf

After the baffle comes the effluent structure, which usually consists of a launder, weirs, and effluent piping. A typical effluent structure is shown below:. The primary component of the effluent structure is the effluent launder , a trough which collects the water flowing out of the sedimentation basin and directs it to the effluent piping. The sides of a launder typically have weirs attached.

Weirs are walls preventing water from flowing uncontrolled into the launder. The weirs serve to skim the water evenly off the tank. A weir usually has notches, holes or slits along its length.

These holes allow water to flow into the weir. The most common type of hole is the V-shaped notch shown on the picture above, which allows only the top inch or so of water to flow out of the sedimentation basin.

Conversely, the weir may have slits cut vertically along its length, an arrangement which allows for more variation of operational water level in the sedimentation basin. Water flows over or through the holes in the weirs and into the launder. Then the launder channels the water to the outlet, or effluent, pipe.

This pipe carries water away from the sedimentation basin and to the next step in the treatment process, filtration. The effluent structure may be located at the end of a rectangular sedimentation basin or around the edges of a circular clarifier. Alternatively, the effluent may consist of finger weirs , an arrangement of launders which extend out into the settling basin as shown below.

The sludge zone is found across the bottom of the sedimentation basin where the sludge collects temporarily. Velocity in this zone should be very slow to prevent resuspension of sludge. A drain at the bottom of the basin allows the sludge to be easily removed from the tank. Thank tank bottom should slope toward the drains to further facilitate sludge removal. In some plants, sludge removal is achieved continuously using automated equipment.

In other plants, sludge must be removed manually. If removed manually, the basin should be cleaned at least twice per year, or more often if excessive sludge buildup occurs. It is best to clean the sedimentation basin when water demand is low, usually in April and October. Many plants have at least two sedimentation basins so that water can continue to be treated while one basin is being cleaned, maintained, and inspected.

If sludge is not removed from the basin often enough, the effective useable volume of the tank will decrease, reducing the efficiency of sedimentation. In addition, the sludge built up on the bottom of the tank may become septic , meaning that it has begun to decay anaerobically. Septic sludge may cause taste and odor problems or may float to the top of the water and become scum. Sludge may also become resuspended in the water and be carried over to the filters.

The final sedimentation basin is similar in structure to a conventional primary sedimentation basin. However, it has the provision for adding some chemicals, such as polymer, into the water. This basin provides another step for turbidity removal by the further sedimentation of any carryover turbidity in the effluent of the primary sedimentation basin.

Because there is either none or very little floc in the water, a small dose 0. Generally, the final basin has a very small amount of sludge. Final basin effluent is, usually, disinfected with a small amount of chlorine for controlling the biological growth in the filter media. Water from these basins is crystal clear with turbidity less than 1 NTU. It flows to the filters for the final removal of turbidity.

The size, shape, and density of the floc entering the sedimentation basin will all influence how well the floc settles out of the water. Floc which is too small or too large, is irregularly shaped, or has a low density will not tend to settle out in the sedimentation basin. Previously formed floc will disintegrate if the water velocity is too high, if there are sharp bends in the pipe at the inlet. Another major cause of inefficiency in the sedimentation basin is short-circuiting, which occurs when water bypasses the normal flow path through the basin and reaches the outlet in less than the normal detention time.

The picture below shows a basin in which the water is flowing primarily through the left half of the basin. Flowing water is shown as green blobs.

An efficient sedimentation basin would have water flowing through the entire basin, rather than through just one area. When water in the sedimentation basin short-circuits, the floc does not have enough time to settle out of the water, influencing the economy of the plant and the quality of the treated water. Short-circuiting in a sedimentation basin can be detected in a variety of ways. If areas of water in the basin do not appear to be circulating, or if sludge buildup on the bottom of the basin is uneven, then tests may be called for.

Floats or dyes can be released at the inlet of the basin to determine currents. A variety of factors can cause short-circuiting in a sedimentation basin.

Basin shape and design, along with design of the inlet and outlet, can cause short-circuiting. You may remember from the last lesson that a long, thin sedimentation basin is less likely to short-circuit than is a short broad one.

Uneven distribution of flow either at the inlet or outlet can also cause short-circuiting. If the weir at the outlet is not level or if some of the notches clog, flow will be uneven and will cause short-circuiting. In addition to the design of the basin, characteristics of the water can also cause short-circuiting. Differences of temperature can cause stratification of the water - separation of water into bands of different temperature. Incoming water will tend to flow through the band of water which corresponds to its own temperature, and will not spread throughout the rest of the basin.

We should note that temperature can cause other problems with sedimentation as well. Cold water prevents floc from settling, so that longer settling times or larger doses of coagulant chemicals are needed. Gases in the water may cause floating scum, which can carry over into the filters.

Sprinkling water on the on the scum may cause the scum to settle, but it is usually a better practice to find and fix the source of the problem. Gases in the sedimentation basin are usually caused by water being introduced in the pump or by a leak in the raw water line. Another sedimentation basin problem is algal growth. If sedimentation basins have sufficient sunlight, algae will grow on the walls of the basin.

These algae can break loose and clog the filter. Algae are best treated with shock chlorination , a method of feeding ppm of chlorine into the raw water or of sprinkling HTH around the basin walls just before the plant is shut down for a few hours. The chlorine will kill the algae while the chlorinated water sits in the tank.

A few other factors can also influence sedimentation basin efficiency. Intermittent operation of the basin can cause settling problems. Also, design problems such as excessive surface loading or weir loading can cause problems. We will discuss surface and weir loading in the second half of this lesson. As mentioned earlier, flocs are formed via the coagulation and flocculation treatment processes.

The floc, comprised of organic and inorganic matter, must be removed prior to filtration in order to reduce the turbidity load on the filters. Conventional sedimentation requires the flocculated water to move slowly through a sedimentation tank with a minimum of turbulence at the entry and exit points and a minimum of hydraulic short-circuiting.

The weight of the solids at the slow water velocity will allow the solids to settle by gravity to the bottom of the tank prior to the end of the basin. The mass of collected solids is referred to as sludge. The U. In addition, the combined filter effluent must never exceed 1 NTU.

Sedimentation, the solid-liquid separation by gravity, is one of the most basic processes of water treatment. The two common tank shapes of sedimentation tanks are rectangular and circular. There are certain calculations that need to be performed to ensure that sedimentation is properly occuring, including tank volume, detention time, surface overflow rate, mean flow velocity, weir overflow rate, percent settled biosolids and determining lime dosage.

A sedimentation basin is 25 ft wide by 80 ft long and contains water to a depth of 14 ft. What is the volume of water in the basin, in gallons? A clarifier has a diameter of 25 ft and contains water to a depth of 18 ft. Detention time for clarifiers varies from 1 to 3 hours.

There are a couple of different equations used to determine detention time, depending on the shape of the tank:. You'll notice the detentino time equations for rectangular and circular bains basically combine the volume and detention time in one equation.

A sedimentation tank has a volume of , gal. A sedimentation basin is 60 ft long by 22 ft wide and has water to a depth of 10 ft. If the flow to the basin is 1,, gpd, what is the sedimentation basin detention time? The surface overflow rate is used to determine loading on sedimentation basins and circular clarifiers.

Surface overflow rate only measures the water overflowing the process plant flow only. Surface overflow rate calculations do not include recirculated flows and is determined with the following equation:. A circular clarifier has a diameter of 80 ft.

A sedimentation basin 70 ft by 25 ft receives a flow of gpm. The measure of average velocity of the water as it travels through a rectangular sedimentation basin is known as mean flow velocity and is calclated by the following equation:. A sedimentation basin is 60 ft long by 18 ft wide and has water to a depth of 12 ft. Weir loading rate weir overflow rate is the amount of water leaving the settling tank per linear foot of weir.

The result of this calculation can be compared with design. Typically, the weir loading rate is a measure of the flow in gallons per minute gpm over each foot of weir. The weir loading rate is determined using the following equation:. A circular clarifier receives a flow of 3. The percent settled biosolids test volume over volume test is conducted by collecting a mL slurry sample from the solids contact unit and allowing it to settle for 10 iminutes.

After 10 minutes, the volume of settled biosolids at the bottom of the mL graduated cylinder is measured and recorded. Weirs shall be adjustable, and at least equivalent in length to the perimeter of the tank. However, peripheral weirs are not acceptable as they tend to cause excessive short-circuiting.

Weir Overflow Rates. Large weir overflow rates result in excessive velocities at the outlet. These velocities extend backward into the settling zone, causing particles and flocs to be drawn into the outlet. It may be necessary to provide special inboard weir designs as shown to lower the weir overflow rates.

Circular Basins. Settling Operations. Design Details. Clarified supernatant leaving the top of the sedimentation tank overflow. Concentrated sludge leaving the bottom of the sedimentation tank underflow. To remove coarse dispersed phase. To remove coagulated and flocculated impurities. To remove precipitated impurities after chemical treatment. Suspended solids present in water having specific gravity greater than that of water tend to settle down by gravity as soon as the turbulence is retarded by offering storage.

Basin in which the flow is retarded is called settling tank. Theoretical average time for which the water is detained in the settling tank is called the detention period. Size, shape and specific gravity of the particles do not change with time. Once the motion has been initiated, a third force is created due to viscous friction. Intermediate values indicate transitional flow. Sedimentation tanks may function either intermittently or continuously.

The intermittent tanks also called quiescent type tanks are those which store water for a certain period and keep it in complete rest.

In a continuous flow type tank, the flow velocity is only reduced and the water is not brought to complete rest as is done in an intermittent type.

Settling basins may be either long rectangular or circular in plan. Long narrow rectangular tanks with horizontal flow are generally preferred to the circular tanks with radial or spiral flow. Long rectangular basins are hydraulically more stable, and flow control for large volumes is easier with this configuration. A typical long rectangular tank have length ranging from 2 to 4 times their width. The bottom is slightly sloped to facilitate sludge scraping.

A slow moving mechanical sludge scraper continuously pulls the settled material into a sludge hopper from where it is pumped out periodically. Circular Basins Circular settling basins have the same functional zones as the long rectangular basin, but the flow regime is different.

When the flow enters at the center and is baffled to flow radially towards the perimeter, the horizontal velocity of the water is continuously decreasing as the distance from the center increases. Thus, the particle path in a circular basin is a parabola as opposed to the straight line path in the long rectangular tank. Sludge removal mechanisms in circular tanks are simpler and require less maintenance. Assume that a settling column is suspended in the flow of the settling zone and that the column travels with the flow across the settling zone.

Consider the particle in the batch analysis for type-1 settling which was initially at the surface and settled through the depth of the column Z 0 , in the time t 0.