Sedimentation

Sedimentation is a natural or engineered process. In this process, solid particles are suspended in a fluid, mostly water. These particles settle under the effect of gravity. Sedimentation is used in various fields, such as water treatment and geology.

In this process, gravity acts on particles, due to which they sink to the bottom, and form a sediment layer. The process is affected by many factors. It can be particle size, shape and many more. Sedimentation is important for the separation of solids from liquids, water clarification and in various industrial and environmental processes.

Principle of Sedimentation

Sedimentation is based on the principle that denser particles settle faster than lighter ones when subjected to gravity. Particles are pulled downward due to the force of gravity. Factors like particle size, shape, and fluid viscosity influence their settling speed.

Process of Sedimentation

The process of sedimentation is a natural phenomenon powered by gravity. In this process, solid particles are suspended in a fluid, settling down over time.

• Initiation of Sedimentation

This process begins when water holds suspended particles. These particles can vary in size, density, and composition. Gravitational force influences the sedimentation acting on the particles. The viscosity of the fluid also influences Sedimentation.

• Forces at Play

Gravity is the primary force controlling sedimentation. The denser the particles, the faster they will settle. Particles are pulled downward due to the force of gravity, which causes them to move through the fluid until they encounter resistance.

• Particle Characteristics

The rate of sedimentation is affected by the size and shape of particles. Larger and denser particles settle more rapidly than smaller and lighter ones.

• Enhancement of Sedimentation

Sedimentation can be enhanced in various applications to speed up the settling process. Coagulants and flocculants are added to the fluid for the promotion of the aggregation of particles. These chemicals neutralise charges on particle surfaces.

• Containers for Sedimentation

Sedimentation commonly takes place in containers known as sedimentation tanks, clarifiers, or settling basins. These tanks come in different designs, such as rectangular, circular, or up-flow configurations, depending on the specific needs of the application. The choice of tank design is influenced by factors like available space, required capacity, and the characteristics of the particles being removed.

• Separation of Phases

A layer known as sediment or sludge is formed at the bottom of the tank. The clear liquid remaining above, known as supernatant, is separated from the settled particles. This clarified liquid can be further processed or distributed depending on the intended use.

Sedimentation process by an experiment

To demonstrate sedimentation, a simple experiment can be conducted using water and different types of particles.

Materials

1. Clear glass or plastic containers
2. Water
3. Various particles (sand, soil, small pebbles)
4. Stirring stick

Procedure

1. Fill the container with water, leaving some space at the top.
2. Add a layer of sand, soil, or small pebbles to the water.
3. Stir the mixture gently to suspend the particles in the water.
4. Observe what happens over time.

Explanation

• Sedimentation occurs as heavier particles settle to the bottom due to gravity.
• The experiment illustrates how different-sized particles settle at different rates.
• It helps visualise the natural process of sedimentation in bodies of water.

Types of Sedimentation Tanks

Sedimentation tanks are also known as clarifiers or settling tanks. They are designed to facilitate the separation of solid particles from liquids. There are various types of sedimentation tanks, including:

• Rectangular Sedimentation Tank

These tanks are used in smaller facilities. They have a rectangular shape and are more space-efficient.

• Circular Sedimentation Tank

They are used in larger water treatment plants. These tanks have a circular design, which helps in the efficient removal of suspended solids.

• Upflow Sedimentation Tank

Water is introduced at the bottom of these tanks, and sedimentation occurs as particles rise to the surface.

Sediment Filter

A sediment filter is a device that is designed to remove suspended particles from liquids. It is commonly used in water treatment systems. It ensures that water is free from visible impurities. Sediment filters can be made from various materials, like paper, fabric, or ceramic. They work by physically trapping particles as the water passes through.

Types of Sedimentation

• Plain Sedimentation

It is a fundamental process in water treatment involving settling suspended particles in water due to gravity. In this method, water is allowed to stand in a rest state, allowing larger and denser particles to settle at the bottom. This process removes impurities like sand, silt, and other particulate matter and results in clearer water. However, plain sedimentation may not be sufficient for treating water with finer particles or colloidal substances.

• Sedimentation using Clarifier and Contact Tank

Clarifiers and contact tanks are employed to enhance the process of sedimentation. A clarifier is a settling tank equipped with mechanisms to improve the settling efficiency. In a clarifier, water is introduced slowly. Flow control devices distribute the water evenly and it promotes particle settling. The settled particles are then removed and clarified water is collected.

• Chemically Assisted Sedimentation

This involves the addition of coagulants and flocculants to the water to increase particle aggregation and settling. Coagulants, like aluminium sulfate or ferric chloride, neutralise the charges on suspended particles and promote their collision and formation of larger flocs. Flocculants, such as polymers, aid in binding. These flocs together create heavier and faster-settling particles. The combination of chemical agents and sedimentation tanks accelerates the removal of impurities from water.

This method is effective in treating water having high turbidity or colloidal particles. Coagulants neutralise the charges on these particles and allow them to clump together. While flocculants aid in forming larger, settleable flocs.

FAQs on Sedimentation

What is sedimentation?

Sedimentation is the process of settling down of heavier insoluble particles at the bottom of the liquid. These particles that settle down are called sediments.

What is the principle of sedimentation?

The principle of sedimentation is that the denser particles settle faster than lighter ones when subjected to gravity.

Name three types of sedimentation tanks?

Rectangular Sedimentation Tank, Circular Sedimentation Tank, Upflow Sedimentation Tank