Working method of the settling process: the sedimentation rate

Removal of suspended solids by sedimentation depends on particle size and density. Suspended particles passing through a settling tank may remain suspended if their density is similar to the density of water while very dense particles passing through the same structure can settle. In wastewater treatment plants, the ability of a sludge to settle is measured by the sludge index.

The different types of settling

There are several types of clarifiers used in different industrial fields. Settling is a physical separation process used to separate liquids of different densities or solids suspended in a liquid. In the water sector, solid-liquid settling is the most common method.

This method requires a solid-liquid mixture to settle for a while, allowing the solid parts to settle to the bottom, forming a separate layer. Another method is the liquid-liquid settling, which consists of separating two liquids of different densities by using a centrifugal separator or a gravity separator. However, remember that the effluent to be treated very often contains a floating fraction. It also needs to be extracted separately.

How does settling separate solid particles from liquids?

Settling is a physical process used to separate solid particles from liquids. It depends on the difference in density between the two phases, the solid particles being heavier than the liquid in which they are suspended. The settling process is carried out in several stages.

  • Initially, the solid particles are mixed in suspension in a liquid and then the mixture is allowed to stand for a period of time.
  • During this time, the solid particles lose their kinetic energy, settle to the bottom of the container and form a separate layer.
  • The purpose is that the particles lose their dynamic energy in order to be easily collected at the bottom of the structure.

It is important to note that settling cannot separate solid particles of similar density to the liquid. In this case, filtration is the best option. In case the particles float, it is better to choose flotation.

Physical forces that allow settling

The sedimentation rate of a particle is its theoretical downward speed in clear, standing water. A particle will only settle if:

  • In a longitudinal flow, the length/height ratio of a tank is greater than the water velocity/sedimentation velocity ratio.
  • In a vertical ascending flow, the velocity of rising water is below the settling speed limit.

The settling follows what is known as Stokes’ law. This law shows that the falling velocity of a particle is proportional to the square of the particle size and the density difference between the particle and the liquid.

Therefore, the increase in particle diameter significantly increases sedimentation. This is why flocculation is a widely used technique in water treatment, whether for sanitation or for drinking water production.

There are physical forces that also play a role in settling such as viscosity and particle size.

Increase the sedimentation rate through coagulation flocculation

Coagulation flocculation is a process used to increase the sedimentation rate of solid particles suspended in a liquid. The process involves adding chemicals, called coagulants, to make the solid particles stick together and form flakes. These flocs are heavier and easier to separate from liquids by settling. It allows the particles to be heavier and therefore limits the size of the settling works. In addition, some elements such as phosphorus, for example, are precipitated by the addition of chemicals. This means that they do not end up in the supernatant. Clariflocculation, a combination of coagulation and flocculation with decantation, is widely used in water treatment. It effectively removes suspended particles and impurities from the water.

Which substances are present in the effluent to be settled?

In a treatment plant, everything depends on where the settling tank is placed and what its treatment objectives will be. In the case of primary settling tanks, this one will receive oakum, sand, grease, primary sludge (soil) and large objects. On the other hand, tertiary treatment tanks receive mainly treated water containing flocs of suspended sludge (tertiary sludge).

Suspended solids (SS)

Whether they are primary or tertiary sludge, they must be separated from the water to be treated in order to obtain the purest possible supernatant.

Fats and greases

They’re floating! Yes, I swear to you! They also represent a significant fraction of the carbonaceous pollution collected by a wastewater treatment plant. They can also easily clog filtration installations. It is important to separate them as quickly as possible from the water to be treated.


Oakums consist of an accumulation of hair, wipes and other cloths that arrive through the network. In a settling tank, part of it mixes with the grease and forms a crust that floats on the surface of the clarifier. This crust can measure up to several centimetres thick. Many pumps are clogged with oakum The other part settles with the sand.

Sand and grit

It also arrives through the network. It is made up of grits of various sizes. If it is properly collected, cleaned and screened, it can be used as an embankment for example.

Large objects

Large pieces of wood, bottles, cans… It is recommended to remove it at the beginning of the treatment. A coarse screening structure is more than sufficient.

What is collected at the clarifier outlet?

The floating substance

The supernatant is the fraction of liquid that is recovered in the upper part of the settling tank: it is the clarified part of the liquid. The quality of the product depends on various parameters, including: – density of the particles to be settled – the retention time in the settling tank – the shape of the settling structure. The larger the decanter, the better the sedimentation since one of the main parameters of settling is the retention time. In addition, to improve water settling, there are different possibilities.
  • By adding lamellae to the lamella, the settling surface is improved. This is called a lamella clarifier.
  • By adding a settling cone and a larger bottom, we will have a decanter digester, perfect to reduce sludge and store it. In Germany, the most common method is the multi-chamber pit (or three-chamber pit in English, dreikammergrube in German).


In a wastewater treatment plant settling tank, we systematically find grease. They float on the surface of the settling tank. In a clarifier, a fraction of the sludge floats on the surface of the tank due to denitrification. Very often, the supernatant still contains these particles, since all the particles do not have a density higher than the water and above all, not all of them will be trapped during the settling process. These are called scum. All these scum should be retained to improve the overall performance of the settling process because they negatively impact the efficiency of a treatment plant.

To retain these scum, manufacturers use physical barriers to prevent them from crossing the settling tank. For example, in small wastewater treatment plants, Imhoff tanks or three chamber pits use a tee pipe. Lamellar clarifiers are equipped with a collection channel which is fitted with a scum baffle. Clarifiers are equipped with a surface scraping system (a skimmer) to collect the scum.


Depending on the desired sludge quality, the size of the settling system, its shape and volume will impact sludge properties. Some structures are only used for collecting and extracting them, while others are also used for storing them. In any case, the sludge must be discharged regularly. In fact, once the storage facility has reached saturation, the mud can no longer be removed and ends up somehow in the river.

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