All you need to know about settling tanks



In water treatment, settling (décantation en français) is an operation that removes suspended particles from the water to be treated. It is a physical process that consists to separate particles of higher density than water from the liquid in which they are found. These particles are recovered at the bottom of the tank. In a wastewater treatment plant, we are talking about primary sludge when there is a primary settling tank upstream of the biological treatment. We are talking about tertiary sludge for all sludge that is recovered in a tertiary treatment structure, such as a clarifier for example.



Incoming elements (inlets)

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).

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.


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



Elements that emerge from it (outlet)

The supernatant

The supernatant is the fraction of liquid that is recovered in the upper part of the settling tank: it is the purified part of the liquid. Its quality depends on various parameters, among others:

  • the 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, there are different possibilities to improve the sedimentation. By adding lamellae to this, the settling surface is improved. This is called lamellar settling tank. By adding an Imhoff cone and a larger bottom, we will have an Imhoff tank, ideal for reducing sludge and storing it. In Germany, the most common method is the three chamber pit (or bassin multi chambres in French, 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 sludges 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 sludge can no longer be removed and ends up somehow in the river.

Sizing and costs

Sludge storage

Another important design parameter is the sludge storage time. This has a direct impact on OPEX (operational expenditure) and CAPEX (capital expenditure). The larger the sludge storage capacity of a settling tank, the more sludge it will be stored in, and the lower the discharge frequency will be. In fact, the more you invest initially (CAPEX), the more you reduce OPEXs afterward. As in any project, it is better to have a long-term vision and estimate the overall costs.

Some settling equipment, such as lamellar settling tanks, are not designed to store sludge, while others such as the Imhoff tank are specifically designed to store and concentrate the sludge. Usually, all structures used to store sludge are made of concrete.

The concentration of sludge depends on its retention time. However, this increase in concentration is not linear. After a certain period of time, usually 6 months, the concentration does no longer vary. However, designing generously a settling system for sludge storage can increase the sludge concentration by 50%! This reduces the sludge volume and operating costs!

Finally, after a certain plant size, the sludge volume produced is so large that other extraction technologies such as belt filters or centrifugation have to be considered.

Coagulants and flocculants

To improve the decantability of particles to be separated and thus to improve efficiency, it is possible to add coagulant and flocculant chemicals. The heavier a molecule is, the better its decantation. Let’s consider sand, for example, it will decant very easily! On the other hand, very light particles such as very aerated sludge, for example, will settle very slowly. In these cases, these famous coagulants and flocculants are used to increase the mass of particles and thus their decantability. Some processes use microsand in addition to the coagulants and flocculants action.

In a sewage treatment plant, the most commonly used coagulant is ferric chloride: FeCl3. It is an orange-red liquid, very effective and cheaper than other coagulants that can be found.

Regarding flocculants, there are many different types of them. Each type of sludge has a flocculant with which it reacts the most effectively. It is necessary to test several products to identify the ideal candidate, by carrying out jar tests.

In fact, in the wastewater treatment plant operating budget, coagulants and flocculants constitute a major expense category.

Other parameters

Finally, several other elements come into consideration. For example: BOD5 load, peak flow rate, buffer volume, sedimentation volume, impact of FeCl3 and denitrification… All these parameters influence the calculation of a settling system. If you would like to know more about the size of a small wastewater treatment plant, download my white paper on settling for free.