Revision as of 10:25, 3 July 2006

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

Basics

Suspended Solids (or SS), is the 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 speed of a particle is its descending speed in clear and still water.

Applications

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 SS and allows settling colloids.

Waste Water Treatment

Sedimentation is often used as a primary stage in modern Waste Water Treament Plant, reducing the content of SS as well as the pollutant emmbedded in the SS. Due to the large amount of reagent necessary for urban water, preliminary chemical coagulation and flocculation are generally not used, remaining SS 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

Sediementation Tanks can be of different shapes, often rectangular or circular. They are sized in order to have an optimal sedimentation speed. If sedimentation speed if 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:

they provide a very large surface area onto which particles may fall and become stabilised.
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.

T he 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:

@@ Line 1: / Line 1: @@
 {{unreferenced}}
-'''Sedimentation''' is a process settling out particles suspended in water  under the influence of gravity.
+'''Sedimentation''' is a physical water treatment process settling out suspended solids in water under the influence of gravity.
+=Basics=
-Chemical [[Flocculation]] can be used to increase the rate of sedimentation if the particles are very fine and slow to settle. Thus it more often used in [[water purification]], but generally not in [[sewage treatment]].{{fact}}<!--or only used in the later stages of high-level treatment?--> The same effect is achieved in sewage treatment by the craetion of a bacterial floc during the aeration stage.
+Suspended Solids (or SS), is the 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 speed of a particle is its descending speed in clear and still water.
-Sedimentation may be carried out in vessels of various size, flowing lengthways along a rectangular sedimentation [[tank]], fed in at the rim of a circular tank or upwards through a circular tank (suspended blanket flocculation).
-A [[baffle]] may be used to slow down the entering liquid - in this way, [[turbulence]] absorbs the [[kinetic energy]] of the fast liquid and reduces the velocity. (The velocity is lower as there is a larger cross-sectional area, and the same flow rate.)  With no baffle, the entering liquid will tend to carry on in a straight line, creating a small fast current of water which may short-circuit directly to thhe outlet with little sedimentation.
+=Applications=
-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.
+==Potable Water Treatment==
-#by providing a very large surface area onto which particles may fall and become stabilised. <br>
+Sedimentation in potable water treatment follows generally a step of chemical coagulation and [[Flocculation|flocculation]], which allows grouping particles together into flocs of a bigger size. This increase the settling speed of SS and allows settling colloids.
+==Waste Water Treatment==
+Sedimentation is often used as a primary stage in modern Waste Water Treament Plant, reducing the content of SS as well as the pollutant emmbedded in the SS. Due to the large amount of reagent necessary for urban water, preliminary chemical coagulation and flocculation are generally not used, remaining SS 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=
+Sediementation Tanks can be of different shapes, often rectangular or circular. They are sized in order to have an optimal sedimentation speed. If sedimentation speed if 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:
+#they provide a very large surface area onto which particles may fall and become stabilised. <br>
 #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.
+T
-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 [[colloid]]al materials.
+he 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 [[colloid]]al materials.
 For more detail, see the descriptions of sedimentation in the relevant sections of:
@@ Line 18: / Line 33: @@
 *[[Wastewater treatment]]
-{{tech-stub}}
 [[Category:Water treatment]]