What is the sedimentation rate

Sedimentation is a physical water treatment process that uses gravity to remove suspended solids from water.

Solid particles set in motion by water turbulence can be removed naturally through sedimentation in a still body of water, such as ponds or lakes. The settling tanks are structures built to collect the solids carried by sedimentation. The sedimentation rate is then used to design the structures.

What you should remember

The sedimentation rate of a particle is its theoretical settling velocity in clear, stagnant water. A particle will sediment only 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.

Theoretical section

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.

We invite all mathematicians and complex calculations geeks to dig this part on Wikipedia, since we do not intend here to focus on theoretical parts.

Applications

In drinking water

Sedimentation in drinking water treatment generally follows a chemical step of coagulation and flocculation, allowing the particles to agglomerate into larger flocs. This increases the settlingrate of suspended solids and allows trapping and settling colloids.

In wastewater

The primary treatment of wastewater allows the removal of floating and suspended matter through sedimentation. The primary ponds reduce the amount of suspended solids and pollutants incorporated into them. This has a significant impact on COD and BOD, but almost no impact on nitrogen.

In small treatment plants, only the sedimentation rate will be implemented to collect these pollutants. This is for the sake of cost and simplicity of operation.

However, for large plants, the construction of huge ponds is not technically and economically viable, as the footprint would be so large. This is why manufacturers are implementing lamella clarifiers coupled with the action of coagulants and flocculants, to gain in compactness.

The settling tanks called “secondary clarifiers” remove bacterial flocs created in certain treatment methods, such as activated sludge, trickling filters, and rotating biological contactors.

Operating principle

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 mud to settle is measured by the sludge index.

How to mesure the sludge volume index (SVI)

Settling matter is measured by the accumulated visible volume at the bottom of an Imhoff cone (or a one-litre test tube) after the water has been left to stand for 30 minutes. This measuring allows us to appreciate the capacity of a mud to be decanted:

Fill one liter of well-mixed effluent into a long measuring cylinder.
Let settle for 30 minutes
Read the settling sludge volume (e. g. 440 ml)
Remix everything and measure the dry matter concentration of the effluent (e. g. 4 grams TSS / liter)
In our example, sludge volume index = 440 ml / 1 litre / 4 grams / litre = 110 ml/g

For municipal wastewater treatment plants of the activated mud type, typical values are between 80 and 120 ml/g.

Values are much higher for mud with bad clarification, which may indicate, for example, that filamentous bacteria have developed.

When the IB is high, the concentrations of COD, BOD, SS, and phosphorus at the outlet of the settling tank are higher. It will influence the overall functioning of the system.

Design

General information

The settling speed allows :

define the time required for the particles to settle in the tank
calculate its volume.

The design and operation of a settling tank is very important to ensure the best sedimentation of particles.

First, reducing the flow velocity as much as possible increases the sedimentation rate. For example, expanding the approach channel, and using a siphonic septum to break up the flow.
Secondly, by retaining the effluent for as long as possible to increase retention time.

Mechanical systems are also available to improve particle collection. For example, scum flushing systems or skimmer arms or surface scraper. It is also possible to collect settled particles (or sludge) quicker using scrapers at the bottom of the structure.

Surface area calculation of the tank

Settling ponds should be developed based on the sedimentation rate of the smallest particle to achieve 100% removal.

Actually, achieving 100% removal is not reachable in wastewater because :

the size of the particles is not homogeneous
the size of the pond would become so large that the project would not be economically viable.
As a result, an elimination rate of 80 to 90% is frequently applied.

For a low load wastewater, the sedimentation rate generally used is 0.4 m/h.

This makes it possible to calculate the required tank area. For example, for a flow rate of 14 m3/h, a surface area of 35 m² is required.

Different types of tanks

Settling ponds can be built in different ways.

Generally, large and wide ponds are used for large hydraulic loads. These same pond types with lamellae allow :

to increase the settling area
reduce the footprint of the structure. They are called lamella clarifiers.

Clarifiers are generally circular works, fed from the center. They allow sludge thickening (typically after the biological part of a treatment, to separate the sludge from the treated water).

Finally, the Imhoff tanks are also fed in a center and are designed as ascending tanks. They are fitted with an Imhoff cone to separate the sludge and store it. This is very often the case for small wastewater treatment plants. In Germany, an effective variant of this work is the three-chamber pi (or bassin multi chambre in French, dreikammergrube in German).

The sedimentation efficiency does not depend on the depth of the structure. If the upstream velocity is low enough to prevent settling particles from rising from the bottom of the tank, then the settling surface is the main conception parameter of a tank. However, the depth should not be too small.

The retention time in the tank

In a settling structure, we find four zones:

Feed: where the effluent arrives
Settling: area where particles separate from the liquid
Accumulation: where sludge is formed
Outlet: area where treated water accumulates and is discharged

To ensure that these four zones can establish themselves in the structure, it is necessary to have a adequate volume. Thus, the ideal retention time is between 30 minutes and 2 hours. The longer this retention time, the better the particle separation will be.

Let’s take our previous example with a flow rate of 14 m3/h at the inlet. If we aim for a retention time of one hour and fifteen minutes, then the volume should be: 14 x 1.25 = 17.5 m3.

For small wastewater treatment plants, it is recommended to have a retention time of at least two hours for primary settling. One hour will be sufficient in tertiary settling.