Purpose Of  Sedimentation

Sedimentation is a unit operation to settle out the suspended particles and flocs in water. Sedimentation water treatment is achieved by lowering the velocity of the water below the suspension velocity to settle out suspended particles by gravity. The process is also known as settling or clarification.

Most water treatment plants include sedimentation in their treatment processes. However, sedimentation may not be necessary in low turbidity water of less than 20 NTU. In this case, coagulation and flocculation are used to produce pinpoint (very small) flocs which are removed from the water in the filters.

The sedimentation generally follows coagulation and flocculation and precedes water treatment filtration. This type of sedimentation commonly known as clarification requires pretreatment with chemical addition (in the coagulation/ flocculation process) and removes the resulting floc from the water. Clarification in the treatment process should remove 90% of the suspended particles from the water, including bacteria and reduce the load on the filters.

Sedimentation can also occur as part of the pretreatment process, where it is known as pre sedimentation which can also be called as plain sedimentation because the process depends merely on gravity without addition of chemical coagulants. Without coagulation and flocculation, plain sedimentation can remove only coarse suspended matter (such as sand, grit etc.) which will settle rapidly out of the water. This type of sedimentation typically takes place in a reservoir, grit basin, or sand trap at the beginning of the treatment process. So plain sedimentation will reduce the load on the rapid mix basin, flocculation basin and sedimentation chamber, as well as reducing the volume of coagulant chemicals required to treat the water. In addition, plain sedimentation basins are useful because raw water entering the plant from a plain sedimentation tank is usually more uniform in quality than water entering the plant without such a holding basin. Impounding storage reservoirs are generally serving as plain sedimentation tanks

Types Of  Sedimentation

In general, there are four types of sedimentation.

• Type I Sedimentation, known as discrete settling, describes the sedimentation of low concentrations of particles that settles as individual entities. Example of Type I settling in water treatment plants is the settling of silt, grit etc., from river water before applying to the slow sand filters.

• Type II sedimentation, known as flocculant settling, describes sedimentation of larger concentrations of solids that agglomerate as they settle. Sedimentation of coagulated surface waters prior to rapid sand filters is an example of flocculant settling. This type of settling is known as the clarification.

• Type III sedimentation, known as hindered settling or zone settling, describes sedimentation of a suspension with solids concentration sufficiently high to cause the particles to settle as a mass. An example of the hindered settling is the upper portion of the sludge blanket in sludge thickeners.

• Type IV sedimentation, known as compression settling, describes sedimentation of suspension with solids concentration so high that the particles are in contact with one another and further sedimentation can occur only by compression of the mass. The lower portion of a sludge thickener is an example of compression settling.