Answer to Question #184537 in Civil and Environmental Engineering for Fatima omar

Question

Design and operation the different types of settling

Answer 1

Type 1- Discreet Settling

In discrete settling, particles settle as individual entities, and there is no significant interaction with neighbouring particles. Discrete particles have little tendency to flocculate or coalesce upon contact with each other and hence they do not change their size, shape or mass during settling. 

Type 2- Flocculent Settling

In flocculent settling, particles flocculate or coalesce during settling. By flocculation or coalescing, the particles increase in mass and thus settle at a faster rate. The degree of flocculation depends on the contact opportunities which in turn are affected by the surface overflow rate, the depth of the basin, the concentration of the particles, the range of particle sizes and the velocity gradient in the system. The removal of organic suspended solids from raw or untreated sewage in primary settling tanks, settling of chemical floes in settling tanks and of bioflocs in the upper portion of secondary settling tanks are the examples of flocculent settling.

Type 3- Hindered or Zone Settling

When concentration of flocculent particles in in intermediate range, they are close enough together so that inter-particle forces are sufficient to hinder the settling of neighbouring particles resulting in hindered settling. The particles maintain their relative positions with respect to each other and the whole mass of particles settles as a unit or zone.

This type of settling is applicable to concentrated suspensions such as are found in secondary settling tanks used in conjunction with biological treatment units such as trickling filters and activated sludge units. In the hindered settling zone, the concentration of particles increases from top to bottom leading to thickening of sludge.

Such secondary clarifiers where zone settling occurs are designed on the basis of solids loading or solid flux and checked for surface overflow rate, both of which can be determined by conducting settling column analysis.

Type 4- Compression Settling

This refers to settling in which the concentration of particles is so high that particles are in physical contact with each other resulting in the formation of a structure with lower layers supporting the weight of upper layers. Consequently further settling occurs due to compression of the whole structure of particles and accompanied by squeezing out of water from the pores between the solid particles.

Compression takes place from the weight of particles which are constantly being added to the structure by sedimentation from the supernatant liquid. Compression settling usually occurs in the lower layers of a deep sludge mass, such as in the bottom of secondary settling tanks following biological treatment by trickling filters and activated sludge process, and in tanks used for thickening of sludge.