How long does sedimentation take

This can be achieved by increasing the breadth of the tank, but this would take up more land and be costly. An alternative and less costly solution would be to insert V-notches in the weir and so effectively increase the weir length, or have a suspended collection trough at the end of the tank so that the effluent could flow into the trough from either side, thus doubling the weir length.

What length of effective weir would be required for a rectangular sedimentation tank with a maximum flow of 0. In the UK, upward-flow or hopper-bottomed sludge-blanket clarifiers Figure 23c are extremely popular.

This type of tank is an inverted cone, with the flocculated water entering from the bottom of the cone. Because the cross-sectional area of the tank increases rapidly from the apex at the bottom to the base of the cone at the top the upward velocity of the water is reduced as it rises. In the tank there will, therefore, be a horizontal plane where the upward water velocity equals the average downward rate of fall of the floc.

This results in the formation of a horizontal 'blanket' of floc suspended in the water. This blanket of floc acts as a filter through which the upward flowing water must pass. Maximum use of the tank is made when the top of the floc blanket is as high as possible in the tank. When the blanket becomes too dense, it is removed by bleeding off the excess floc. These days, due to cost reasons, upward flow tanks are often flat-bottomed with internal pipe work to distribute the flow across the full area of the tank base.

Which of the following characteristics of raw water are greatly improved by coagulation, flocculation and sedimentation? Making the decision to study can be a big step, which is why you'll want a trusted University. Take a look at all Open University courses. If you are new to University-level study, we offer two introductory routes to our qualifications.

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What other factors do you think need to be known? In a settling structure, we find four zones:. To ensure that these four zones can establish themselves in the structure, it is necessary to have a adequate volume. Thus, the ideal retention time is between 30 minutes and 2 hours. The longer this retention time, the better the particle separation will be.

If we aim for a retention time of one hour and fifteen minutes, then the volume should be: 14 x 1. For small wastewater treatment plants, it is recommended to have a retention time of at least two hours for primary settling.

One hour will be sufficient in tertiary settling. Settling Sedimentation rate. What is the sedimentation rate. What you should remember. In a vertical ascending flow, the velocity of rising water is below the settling speed limit.

Theoretical section. In drinking water. In wastewater. Operating principle. How to mesure the sludge volume index SVI.

This test assesses the suitability of sludge for decantation: Fill one liter of well-mixed effluent into a long measuring cylinder. Let settle for 30 minutes Read the settling sludge volume e. General information. Surface area calculation of the tank. For a low load wastewater, the sedimentation rate generally used is 0.

Different types of tanks. The retention time in the tank. In a settling structure, we find four zones: Feed: where the effluent arrives Settling: area where particles separate from the liquid Accumulation: where sludge is formed Outlet: area where treated water accumulates and is discharged To ensure that these four zones can establish themselves in the structure, it is necessary to have a adequate volume.

A fourth type of sedimentation basin is more complex. Solids-contact clarifiers , also known as upflow solids-contact clarifiers or upflow sludge-blanket clarifiers combine coagulation, flocculation, and sedimentation within a single basin. Solids-contact clarifiers are often found in packaged plants and in cold climates where sedimentation must occur indoors. This type of clarifier is also often used in softening operations.

The sedimentation and flotation units commonly found are primary clarifiers, secondary clarifiers, combined sedimentation-digestion units, flotation units, and Imhoff tanks. The most important function of the primary clarifier is to remove as much settleable and floatable material as possible. Removal of organic settleable solids is very important because they cause a high demand for oxygen BOD in receiving waters or subsequent biological treatment units in the treatment plant.

If the horizontal velocity of water is slowed to a rate of 1. Specific gravity of water is 1. There are many factors that will influence settling characteristics in a particular clarifier. A few of the more common ones are temperature, short circuits, detention time, weir overflow rate, surface loading rate, and solids loading. In general, as water temperature increases, the settling rate of particles increases; as temperature decreases, so does the settling rate. Molecules of water react to temperature changes.

They are closer together when liquid temperature is lower; thus density increases and water becomes heavier per given volume because there is more of it in the same space. As water becomes more dense, the density difference between water and solid particles becomes less; there the particles settle more slowly. All sedimentation basins have four zones - the inlet zone, the settling zone, the sludge zone, and the outlet zone.

Each zone should provide a smooth transition between the zone before and the zone after. In addition, each zone has its own unique purpose. Zones can be seen most easily in a rectangular sedimentation basin, such as the one shown below:. In a clarifier, water typically enters the basin from the center rather than from one end and flows out to outlets located around the edges of the basin.

But the four zones can still be found within the clarifier:. The two primary purposes of the inlet zone of a sedimentation basin are to distribute the water and to control the water's velocity as it enters the basin.

In addition, inlet devices act to prevent turbulence of the water. The incoming flow in a sedimentation basin must be evenly distributed across the width of the basin to prevent short-circuiting. Short-circuiting is a problematic circumstance in which water bypasses the normal flow path through the basin and reaches the outlet in less than the normal detention time. We will discuss short-circuiting in the next lesson. In addition to preventing short-circuiting, inlets control the velocity of the incoming flow.

If the water velocity is greater than 0. Breakup of floc in the sedimentation basin will make settling much less efficient. Two types of inlets are shown below. The stilling wall , also known as a perforated baffle wall , spans the entire basin from top to bottom and from side to side. Water leaves the inlet and enters the settling zone of the sedimentation basin by flowing through the holes evenly spaced across the stilling wall.

The second type of inlet allows water to enter the basin by first flowing through the holes evenly spaced across the bottom of the channel and then by flowing under the baffle in front of the channel. The combination of channel and baffle serves to evenly distribute the incoming water. After passing through the inlet zone, water enters the settling zone where water velocity is greatly reduced. This is where the bulk of floc settling occurs and this zone will make up the largest volume of the sedimentation basin.

For optimal performance, the settling zone requires a slow, even flow of water. The settling zone may be simply a large expanse of open water. But in some cases, tube settlers and lamella plates, such as those shown below, are included in the settling zone. Tube settlers and lamella plates increase the settling efficiency and speed in sedimentation basins. Each tube or plate functions as a miniature sedimentation basin, greatly increasing the settling area.

Tube settlers and lamella plates are very useful in plants where site area is limited, in packaged plants, or to increase the capacity of shallow basins. The outlet zone controls the water flowing out of the sedimentation basin - both the amount of water leaving the basin and the location in the basin from which the outflowing water is drawn.

Like the inlet zone, the outlet zone is designed to prevent short-circuiting of water in the basin. In addition, a good outlet will ensure that only well-settled water leaves the basin and enters the filter. The outlet can also be used to control the water level in the basin. Outlets are designed to ensure that the water flowing out of the sedimentation basin has the minimum amount of floc suspended in it.

The best quality water is usually found at the very top of the sedimentation basin, so outlets are usually designed to skim this water off the sedimentation basin. A typical outlet zone begins with a baffle in front of the effluent.

This baffle prevents floating material from escaping the sedimentation basin and clogging the filters. 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 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. The 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 sedimentation basin often enough, the effective useable volume of the tank will decrease, reducing the efficiency of sedimentation.