Sedimentation and clarification

Clarification is a process that is used to improve water quality. Clarification is the general term used to describe the way that suspended solids are separated from a liquid, in this case, raw water.

In the water treatment industry, clarification is most often carried out by sedimentation. Suspended solids settle out naturally from the raw water due to gravity. For example, many of the larger suspended solids settle out in raw water catchment areas, associated waterways and settling ponds, as well as in storage reservoirs and pre-filters.

The main aim of sedimentation is to remove any suspended solids that are heavier than water to reduce turbidity, and to reduce the load on the processes that follow.

Most of the smaller sized suspended solids are removed from the raw water at the flocculation and sedimentation stages of the water treatment process. Chemicals (coagulants and flocculants) are often used to speed these processes up.

Sedimentation process

1. After coagulation and flocculation processes

In the coagulation and flocculation processes suspended solids (colloids) form into larger floc particles that are easier to settle out. There are still impurities in the water; there is floc and colloids.

These are removed in the sedimentation and clarification processes.

2. Sedimentation

The main aim of sedimentation is to remove colloids to reduce turbidity.

Sedimentation works by stilling the water moving (reducing its velocity ) through the sedimentation basins. Still water cannot keep larger particles (floc) in suspension against gravitational forces.

Gravity helps the suspended solids that are heavier than water to settle to the bottom of the tank. After the gentle movement of the flocculation mixing section, the water moves into the relatively still sedimentation tank long enough for most of the floc to settle to the bottom.

The settled solids form sludge. Sludge can then be removed relatively easily using a variety of pumping or scraping methods.

3. Suspended Solids

After coagulation and flocculation processes, suspended solids can exist in three main states:

• In a natural form. These can be clays, silts, pathogens, and organic material
• In a modified form. These are formed as a result of coagulation and flocculation processes, such as floc
• As a precipitated impurity. When there are excessive chemicals in the water, they can precipitate out become a suspended solid. For example when a metal-based coagulant is used there could be excess aluminium, magnesium and iron.

 

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