What is the sedimentation rate

Sedimentation is a physical water treatment process that uses gravity to remove suspended solids from the water. Solid particles carried by turbulent water can be removed naturally by sedimentation in a calm zone 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 size 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 (boring)

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 settling rate of suspended solids and allows trapping and settling colloids.

In wastewater

The wastewater primary treatment process removes scum and suspended solids through sedimentation. Primary tanks reduce the content of suspended solids and the pollutants incorporated into them. This has a direct impact on COD and BOD, but almost none on nitrogen. In small wastewater treatment plants, due to concerns and costs and operational simplicity, only the sedimentation rate will be used to collect these pollutants. On the other hand, on larger plants, the construction of huge basins does not present any technical and economic viability, because the footprint is so important. This is why manufacturers use lamellar settling tanks, coupled with the action of coagulants and flocculants, to improve 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 a sewage treatment plant, the sludge’s suitability for settling is measured using the sludge volume index.

How to mesure the sludge volume index (SVI)

The settling matters are measured thanks to the visible volume accumulated at the bottom of an Imhoff cone (or a 1-litre measuring cylinder) after a 30 minutes settling period. This test assesses the suitability of sludge for decantation:

• 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

Typical values for activated sludge wastewater treatment plants in municipalities are between 80 and 120 ml/g. The values are much higher for sludge with poor settling properties, which may mean, for example, that filamentous bacteria have been developing. When the SVI is elevated, concentrations of COD, BOD, TSS, and phosphorus at the outlet of the settling system are higher. This will impact the overall performance of the wastewater treatment plant.

Sizing

General information

The settling speed defines the residence time required for particles to settle in the tank, and thus to calculate the tank volume. The design and operation of a settling tank are very important to ensure the best possible particles sedimentation.  First, reducing the flow velocity as much as possible increases the sedimentation rate. For example, by widening the entrance channel, and using a baffle to break 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

The settling tanks must be designed according to the sedimentation rate of the smallest particle to achieve 100% elimination. In reality, achieving 100% elimination is not realistic in wastewater because the particles size are not homogeneous, and because the size of the tank would be so large that it would not be an economically viable project. 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 tanks can be designed in different ways. Large and wide structures are usually used for high hydraulic loads. These same types of tanks, equipped with lamellae, increase the settling area and thus reduce the footprint of the structure. We’re talking about 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 design 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.

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