What is settling velocity? What are the various types of settling in sedimentation tank?

Settling Velocity

Settling velocity refers to the speed at which suspended particles or solids settle downwards in a fluid under the influence of gravity. It is a critical parameter in sedimentation processes and is determined by the particle's size, shape, density, and the properties of the surrounding fluid, such as viscosity. Settling velocity is usually measured in units of length per unit time, such as meters per second (m/s) or centimeters per minute (cm/min).

Various Types of Settling in Sedimentation Tanks

In sedimentation tanks, various types of settling occur depending on the characteristics of the suspended particles and the design of the tank. The main types of settling observed in sedimentation tanks include:

1. Discrete Particle Settling

Discrete particle settling, also known as Type I settling, occurs when individual particles settle independently of each other without interacting significantly. In this type of settling, particles follow straight-line paths and settle at their terminal settling velocity, which is determined by their size, shape, and density. Discrete particle settling is typical in dilute suspensions where particles are well-separated and settle without hindrance.

2. Flocculent or Hindered Settling

Flocculent or hindered settling, also known as Type II settling, occurs when suspended particles aggregate or clump together to form larger flocs or aggregates. These flocs settle more rapidly than individual particles due to their increased size and density. Flocculent settling is often facilitated by the addition of chemicals called coagulants or flocculants, which promote particle aggregation. Hindered settling occurs when these flocs hinder the settling of other particles, leading to slower settling rates compared to discrete particle settling.

3. Zone Settling

Zone settling refers to the stratification of particles into distinct layers or zones within the sedimentation tank based on their settling velocities. In a properly designed sedimentation tank, denser particles settle to the bottom to form a sludge layer, while lighter particles remain suspended or settle more slowly. Zone settling helps to separate particles based on their size, density, and settling characteristics, resulting in effective particle removal and sedimentation.

4. Compression Settling

Compression settling occurs when settled solids undergo compaction or consolidation over time, leading to a reduction in volume and an increase in density. As particles settle and accumulate at the bottom of the tank, they are subjected to pressure from the weight of overlying solids and water. Compression settling reduces the volume of settled solids and increases the density of the sediment layer, improving sedimentation efficiency.

5. Hindered Bed Settling

Hindered bed settling occurs when settled particles experience resistance or hindrance from the compacted bed of solids at the bottom of the sedimentation tank. As particles settle through the sediment layer, they encounter previously settled solids, which impede their settling velocities. Hindered bed settling can occur in tanks with high sludge concentrations or excessive solids loading, affecting sedimentation performance and requiring periodic sludge removal or tank maintenance.

Understanding the different types of settling in sedimentation tanks is crucial for optimizing the design and operation of water treatment processes and ensuring efficient particle removal and water clarification.