Principle of settling: the sedimentation rate

Decanting is a process of separating solid particles in water, based on their density.

  • Heavy particles : They settle quickly at the bottom of the pond, because they are denser than water.
  • Light particles : If their density is close to that of water, they remain suspended and do not settle easily.

Key indicator: The ability of the sludge to settle is measured by the Sludge Volume Index (SVI). The lower this index, the easier it is for the particles to settle.

The different types of settling

  • Gravity settling : Simple and economical, it is based on the natural separation of suspended particles by gravity.
  • Centrifugal decantation : Used for finer particles, it accelerates separation by creating a centrifugal force.
  • Lamellar decantation : An optimized version, where angled plates allow for increased contact surface for faster separation.
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How does settling separate solid particles from liquids?

Settling is a physical process used to separate solid particles from liquids, based on their difference in density. This process takes place in several stages in a settling tank.

  • 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. If the particles float, other methods such as filtration or flotation are recommended.

How can particle size be reduced?

Settling is a physical process used to separate solid particles from liquids, based on their difference in density. This process takes place in several stages in a settling tank.

  • 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. If the particles float, other methods such as filtration or flotation are recommended.

Physical forces that allow settling

Sedimentation rate 1h2o3

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

  • In a longitudinal flow, the ratio of the length of the reservoir to the height of the reservoir is greater than the ratio of the water velocity to the sedimentation velocity.
  • In a vertical upward flow, the upward water velocity is lower than the sedimentation limit velocity.

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 in sanitation or for the production of drinking water.

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 flocculation and settling, is widely used in water treatment. It effectively removes suspended particles and impurities from the water.

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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. On the other hand, tertiary treatment basins mainly receive treated water containing flocs of suspended sludge (tertiary sludge).

Effluents à Décanter
Type d’EffluentDescription
Matières en suspension (MES)Les particules solides à séparer pour obtenir un surnageant le plus propre possible.
GraissesElles flottent et représentent une importante fraction de pollution, pouvant obstruer les filtres.
FilassesDes amas de cheveux, lingettes et tissus qui se mélangent avec les graisses pour former une croûte.
SableComposé de pierres variées, il peut être récupéré, nettoyé et réutilisé comme remblais.
Gros objetsDes morceaux de bois, canettes, bouteilles, à éliminer dès le début avec un dégrillage grossier.

Points clés à retenir :

  • Décantation primaire: reçoit des matières solides comme les graisses, sable, filasses, et gros objets.
  • Décantation tertiaire: principalement utilisée pour les boues tertiaires (flocs de boues en suspension).

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:

– the density of the particles to be settled
– the residence time in the settling tank
– the shape of the settling tank.

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 various 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).

Scum

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, because not all of them have a density higher 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.

Sludge

It all depends on the quality of sludge required. The size of the water settling structure, shape and volume 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.

FAQ

What is the ideal length of the decanting process?

The ideal settling time depends on several factors, such as the rate of particle sedimentation , the effluent flow rate and the size of the pond. In general, settling can last from a few hours to several days to allow for optimal separation of solids.

The sedimentation rate can be improved by flocculation, which groups particles together to make them heavier and easier to deposit.

The Sludge Volume Index (SVI) measures the ability of the sludge to separate from the water during settling. The lower this index, the better the sludge separation capacity, which improves the quality of the treated water.

Coagulants (such as ferric chloride) and flocculants are used to agglomerate fine particles, making them easier to separate during settling. This improves the efficiency of the treatment, especially for light sludge or very fine particles.

An incorrectly sized lamellar decanter can lead to reduced efficiency in separating solid particles, increasing the operational cost and affecting the quality of the treated water. The right sizing allows for the optimization of residence time, throughput, and overall treatment performance.

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