Types of plants, activated sludge plants
In a tank, with aerobic conditions half of the time (and in the other half for example
anoxic conditions), the aerobic sludge mass will be 50 per cent of the total sludge
mass, that is 0.5 · V · X.
4.3. Types of plants, activated sludge plants
The physical design of activated sludge plants may vary considerably. Many designs
and depths can be used. The design is normally determined on the basis of economy. Irrespective of the design, a plant must consist of two elements:
-aeration tank, with a certain volume, V2, and a certain sludge concentration,
X?.t
- settling tank from where treated wastewater is discharged at the top (concentrations C3 and X3) and concentrated return sludge is withdrawn from
the bottom (concentrations C4 and X4).
In certain plant designs the two elements have physically been constructed as one
element with two functions, see for example Section 4.3.2 concerning oxidation
ditches.
4.3.1 Activated sludge with recycle
Fig 4.5 shows examples of activated sludge plants with separate settling tanks. The
aeration tank (type a) may be designed as a rectangular or a square tank, or it may
be an oxidation ditch or a pond (lined with plastics or placed directly in the
ground/in clay).
In type b the tank is long, narrow and with plug flow whereas the aeration tanks in
type c (Fig 4.5) are ideally mixed tanks and they are usually rectangular.
The plant type with step feeding (type d) normally has an oblong aeration tank with
a type of plug flow (resembling a contact stabilization plant, see under contact
stabilization plants) whereas the selector system (type e) consists of 1-3 small ideally
mixed tanks followed by a large ideally mixed tank which may be rectangular,
square, an oxidation ditch, or a pond.
Irrespective of the physical design, sufficient stirring/ turbulence is required to
ensure that the sludge is kept suspended, and that it gets into contact with the
influent wastewater. This is usually achieved through aeration, but the demand for
air (oxygen demand) can be so insignificant that the air supply cannot support the
mixing, or the aeration system may be designed in such a way that a good mixing
is not ensured.
126
In a tank, with aerobic conditions half of the time (and in the other half for example
anoxic conditions), the aerobic sludge mass will be 50 per cent of the total sludge
mass, that is 0.5 · V · X.
4.3. Types of plants, activated sludge plants
The physical design of activated sludge plants may vary considerably. Many designs
and depths can be used. The design is normally determined on the basis of economy. Irrespective of the design, a plant must consist of two elements:
-aeration tank, with a certain volume, V2, and a certain sludge concentration,
X?.t
- settling tank from where treated wastewater is discharged at the top (concentrations C3 and X3) and concentrated return sludge is withdrawn from
the bottom (concentrations C4 and X4).
In certain plant designs the two elements have physically been constructed as one
element with two functions, see for example Section 4.3.2 concerning oxidation
ditches.
4.3.1 Activated sludge with recycle
Fig 4.5 shows examples of activated sludge plants with separate settling tanks. The
aeration tank (type a) may be designed as a rectangular or a square tank, or it may
be an oxidation ditch or a pond (lined with plastics or placed directly in the
ground/in clay).
In type b the tank is long, narrow and with plug flow whereas the aeration tanks in
type c (Fig 4.5) are ideally mixed tanks and they are usually rectangular.
The plant type with step feeding (type d) normally has an oblong aeration tank with
a type of plug flow (resembling a contact stabilization plant, see under contact
stabilization plants) whereas the selector system (type e) consists of 1-3 small ideally
mixed tanks followed by a large ideally mixed tank which may be rectangular,
square, an oxidation ditch, or a pond.
Irrespective of the physical design, sufficient stirring/ turbulence is required to
ensure that the sludge is kept suspended, and that it gets into contact with the
influent wastewater. This is usually achieved through aeration, but the demand for
air (oxygen demand) can be so insignificant that the air supply cannot support the
mixing, or the aeration system may be designed in such a way that a good mixing
is not ensured.
126