Working method of the settling process: the sedimentation rate

Removal of suspended solids by sedimentation depends on particle size and density. What’s more, particles suspended in a settling tank can remain in suspension if they have a density similar to water. In contrast, very dense particles can settle in the same structure. In a wastewater treatment plant, a sludge’s capacity for settling is measured by its 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. And in the water sector, we have solid-liquid settling, which is widely used.

This method involves resting a solid-liquid mixture for a defined period of time. Solid particles will settle to the bottom, forming a distinct layer. Its process is based on the separation of two liquids of different densities. A centrifugal separator or gravity separator is used for this purpose. However, remember that the effluent is often composed of 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 suspended in a liquid and left to settle for a while.
  • 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. Particles 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. It shows that the speed at which a particle falls is proportional to the square of its size. It also depends on the difference in density 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 components, such as phosphorus, 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 wastewater treatment plant, everything depends on the location of the settling tank and the treatment purposes. For primary settling, it will receive filings, sand, grease, primary sludge (earth) and large objects. In contrast, tertiary treatment tanks receive essentially treated water containing suspended sludge flocs (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 bigger the settling tank, the better the settling, since residence time is one of the main parameters for settling.

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.
  • Adding a settling cone improves the separation process. With deeper bottoms, the clarifier digester is perfect for storing and reducing sludge. In Germany, the most common method is the multi-chamber pit (or three-chamber pit in English, dreikammergrube in German).


Grease is always present in the settling tank of a wastewater treatment plant. These particles float on the surface of the clarifier.

In a clarifier, sludge floats to the surface due to denitrification. And the supernatant often contains particles, as not all of them have a density greater than water. What’s more, not all particles are trapped during settling.. These are called floaters. Floats must be retained to optimize the settling operation. They interfere with the proper operation of a wastewater treatment plant.

To keep these particles away, our engineers use physical barriers to prevent them from crossing the settling tank. For example, in small wastewater treatment plants, settling digesters or multi-chamber tanks use a pipe. Lamella clarifiers are equipped with a siphonic septum spout. Clarifiers use a surface scraping system to collect floating matter.


It all depends on the quality of sludge required. The size, configuration and volume of the water settling structure will impact its properties. Some structures are only designed to collect and extract water, while others provide storage facilities. In all cases, the mud must be removed regularly. When the storage facility is overloaded, the sludge can no longer be disposed of. They end up in the river one way or another.

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