Sedimentation is the process of allowing suspended solids in water to settle out through the effect of gravitational force. This process usually follows coagulation and flocculation to let colloidal particles (very fine solids) in raw water form larger particles or flocs first. Sedimentation, also called settling or clarification, precedes the filtration process.

In a purification system, water carried over from the flocculation basin enters the sedimentation tank or settling basin, where the flow of water is very slow. The water velocity is significantly decreased at this point so that it can no longer support the transport of suspended particles, thus leaving gravity to act on them. When this happens, the flocs settle at the bottom of the tank.

The sediments or solids that settle out during sedimentation are collectively referred to as sludge. Some settling tanks are fitted with equipment that facilitates the automatic and continuous removal of sludge. Otherwise, tank operators would have to manually remove this.


Factors Affecting Sedimentation

Particle Characteristics

The type of solid particles present in the water is one of the major factors that affect how a sedimentation tank is operated. Bigger and heavier solids such as sand, silt, and grit are removed with little effort. A water-flow velocity of less than one foot per second should be adequate to remove these particles from the water.

In cases where coarse suspended matter such as those mentioned above make up majority of the particles present in the source water, sedimentation can be done as part of the pretreatment process. This process is called presedimentation, and is done to reduce the load on the coagulation/flocculation basin.

However, when there is high turbidity and electrically-charged colloidal particles abound, chemical coagulants need to be added first and the particles allowed to flocculate, before sedimentation can take place.

Aside from size and density, the shape of the particle also has an effect on the clarification process. Circular and flat solids will settle more easily than those with ragged edges.


Within the settling tank, there may be various types of currents present which can have an impact on sedimentation, and the two most common ones are density and eddy currents.

Density currents are brought about by the different elements that have come together in the tank: the concentration of the solid particles, the individual and collective weight of these particles, and the water temperature. Eddy currents on the other hand, are caused by the flow of the water entering and exiting the tank.

One or both of these current types can have a positive effect on the process as they can aid in bringing the particles together and promote the formation of flocs. But then again, currents may also cause solid matter to spread out rather haphazardly in the water. When this happens, the flocs also settle at an uneven rate.

Basin Retention Time

As a general rule, water must remain in the sedimentation basin long enough for the particles to settle, and then be removed. Smaller particles settle more slowly than bigger ones and therefore, the waters carrying these require longer retention times and larger tanks. On the average, water stays in the settling tank for at least four hours.

Basin Depth

The size and depth of the sedimentation tank or basin is also key to the efficiency of the operation. Large particles are known to sweep along and combine with smaller ones as they settle and because of this, a deep basin will promote settling better than a shallow basin will. The further the flocs need to travel down, the more smaller particles they can integrate with along the way.

Water Temperature

The temperature of the water in the tank is another factor to contend with in sedimentation. With a higher temperature normally comes a higher rate of settling, and consequently, particle settling slows down as temperature goes down.

A decreased rate of settling can have an effect in the other steps of the process. For instance, the retention time in the sedimentation tank will need to be longer when settling decelerates. In addition, when settling slows down, addition of coagulants may be needed to achieve about the same amount of sludge despite the decreased settling rate.


The Sedimentation Basin

Basin Shapes

There are various types of sedimentation basins that can be used for this water purification process. The settling basin should ideally be located close to the flocculation basin to prevent the breakup of floc as the water flows from the first basin to the next.

Sedimentation basins may be:

Rectangular. The simplest and most common design used today, a rectangular basin allows water to flow through horizontally from one end of the tank to the other. While rectangle-shaped tanks require a sizeable land area, these basins are popular because they are ideal for large-scale water treatment plants. This type is also the most cost-effective to construct and maintain, and the least likely to short circuit.

As a side note, short-circuiting in sedimentation is a tricky situation wherein the water does not follow the normal flow in the tank and proceeds to the outlet, thereby spending less time in the basin than what is normally required.

Double-deck Rectangular. As described by its name, a double-deck rectangular settling basin is essentially made up of two rectangular tanks placed atop each other. Water from the flocculation tank enters the bottom basin and then slowly flows through and goes up to the basin on top where the exit is located. One advantage of the double-deck basin is that it conserves space while still providing ample area for particle settling to take place. However, it is also more expensive to maintain than a rectangular basin.

● Square or circular. Better known as clarifiers, square and circular sedimentation basins have inlets located at the center where the water flows in, and outlets at the edges where the treated water exits. Clarifiers are known to be prone to short-circuiting and sludge removal problems.

Basin Zones

The sedimentation basin is divided into four basic zones: inlet zone, settling zone, sludge zone, and outlet zone.

Inlet Zone. The water enters the inlet zone of the sedimentation basin from the flocculation tank. From this point on, the water should be evenly distributed throughout the basin and its speed of flow controlled. Water velocity should be no more than 0.5 feet per second otherwise, the flocs could disintegrate back into smaller particles. Even distribution and speed control is usually done with the aid of baffles that gradually spreads the water across the area. Baffles also help prevent short circuiting.

Settling Zone. Easily having the largest area of the sedimentation basin, the settling zone is where the solid particles actually descend to the bottom and settle out. The velocity of water here is significantly decreased to allow gravity to exert the needed pressure on the suspended flocs. Some settling zones simply consist of a wide expanse of area while others are equipped with tube settlers and lamella plates to facilitate optimal efficiency in the settling process.

Sludge Zone. The bottom portion of the basin is designated as the sludge zone. This area serves as a holding space for the sediments or sludge that have settled out from the water until such time that these are taken away for proper disposal. As in the settling zone, the velocity of flow in the sludge area should be at a minimum to prevent the settled particles from being swept up and re-dispersed into the water.

The amount of sludge typically generated may be about 3 to 5 percent of the total volume of water. The regular removal of sludge from the basin is crucial to the efficient operation of the basin, and this is why most basins have a drain at the bottom for easy sludge removal.

Outlet Zone. When most particles have already settled at the bottom, the now-treated water proceeds to the outlet zone. The outlet serves as the control point for the depth of water that the basin contains and ensures that only clarified water exits the basin and enters the filtration area. Since the clearest water is usually that on the uppermost part of the basin, outlets are designed to evenly skim off this water.

To achieve the ideal depth of water and prevent solids from flowing out before they can settle, weirs, which are like walls that have notches or slits along its length, are installed at the end of the tank.

While most large-scale water purification plants include a sedimentation basin, the process may not be necessary when treating water with relatively low turbidity. In such cases, filtration follows the coagulation and flocculation processes.


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