Water flows up through slanted tubes or along slanted plates. Flow settles out in the tubes or plates and drifts back down into the lower portions of the sedimentation basin. Clarified water passes through the tubes or between the plates and then flows out of the basin.
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 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.
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In: StatPearls [Internet]. In this Page. Learn More About sedimentation. Share sedimentation Post the Definition of sedimentation to Facebook Share the Definition of sedimentation on Twitter. Time Traveler for sedimentation The first known use of sedimentation was in See more words from the same year. Style: MLA. English Language Learners Definition of sedimentation. Medical Definition of sedimentation. More from Merriam-Webster on sedimentation Britannica.
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