Sedimentation (water treatment)

Sedimentation is a physical water treatment process used to settle out suspended solids in water under the influence of gravity.

Basics
Suspended solids (or SS), is the mass of dry solids retained by a filter of a given porosity related to the volume of the water sample. This includes particles of a size not lower than 10μm.

Colloids are particles of a size between 0.5nm and 100nm. Due to electrostatic forces balancing the gravity, they are not likely to settle naturally.

The limit sedimentation velocity of a particle is its theoretical descending speed in clear and still water. In the theory of settling, a particle will settle only if :
 * 1) in a vertical ascending flow, the ascending water velocity is lower than the limit sedimentation velocity
 * 2) in a longitudinal flow, the ratio of the length of the tank to the height of the particle is higher than the ratio of the water velocity to the limit sedimentation velocity.

Four types of Sedimentation exist Type 1 - Dilute, non-flocculent, free-settling (Every particle settles independently) Type 2 - Dilute, flocculent (Particles can flocculate as they settle) Type 3 - Concentrated Suspensions, Zone Settling (Sludge Thickening) Type 4 - Concentrated Suspensions, Compression (Sludge Thickening)

Potable Water Treatment
Sedimentation in potable water treatment follows generally a step of chemical coagulation and flocculation, which allows grouping particles together into flocs of a bigger size. This increase the settling speed of suspended solids and allows settling colloids.

Waste Water Treatment
Sedimentation is often used as a primary stage in modern Waste Water Treatment Plant, reducing the content of suspended solids as well as the pollutant embedded in the suspended solids. Due to the large amount of reagent necessary for urban water, preliminary chemical coagulation and flocculation are generally not used, remaining suspended solids being reduced by following stages of the process. Though they can be used for building compact plant or for further polishing of the treated water.

In the Activated Sludge process, flocs being created through biological activity are collected in sedimentation tanks, generally referred to as Secondary Sedimentation Tanks.

Technology
Sedimentation Tanks can be of different shapes, often rectangular or circular. They are sized in order to have an optimal sedimentation speed. If sedimentation speed is too high, most particle will not have time and will be carried with the treated water. If too low, the tanks will be of an excessive size. As turbulence is a damaging factor leading settled particles to go back in suspension, several devices are used to ensure a quiet flow, such as carefully designed water inlet with baffles.

Sedimentation may be made more efficient by the use of stacks of flat pieces that slope slightly upwards in the direction of flow (lamellar separators). They are parallel and separated by a small distance. These work in two ways:
 * 1) they provide a very large surface area onto which particles may fall and become stabilized.
 * 2) because flow is temporarily accelerated between the plates and then immediately slows down, this helps to aggregate very fine particles that can settle as the flow exits the plates.

The use of lamellar separators may allow the sedimentation tank to be smaller and may enable finer particles to be separated. Typically such structures are used to difficult to treat waters especially those containing colloidal materials.

For more detail, see the descriptions of sedimentation in the relevant sections of:
 * Water purification
 * Wastewater treatment