secondary flotation clarification) is the final step of secondary treatment, the
secondary effluent is also the secondary clarification effluent.
Secondary flotation clarification: It is a unit process or unit operation for the
removal of the bio-oxidation process generated activated sludge and/or the
chemical precipitation/coagulation process generated chemical sludge using a
secondary flotation clarifier.
Secondary flotation clarifier: A dissolved air flotation (DAF) reactor is used to
float biological sludge (activated sludge) from biologically oxidized wastewater
(such as aeration tank effluent) and/or chemical sludge from chemically coagulated/flocculated wastewater (such as flocculator effluent) by decreasing the
sludge’s apparent density. DAF consists of saturating a portion or all of the
wastewater feed or a portion of recycled effluent with air at a pressure of
25–90 lb/square inch (gage). The pressurized wastewater is held at this pressure
for 0.5–3 minutes in a retention tank and then released to atmospheric pressure to
the flotation chamber. The sudden reduction in pressure results in the release of
microscopic air bubbles which attach themselves to suspended solids and other
particles in the wastewater in the flotation chamber. This results in agglomeration
which, due to the entrained air, has greatly increased vertical rise rates of about
0.5–2 ft/min. The floated materials rise to the surface to form a froth layer (float).
Specially designed scrapers or other skimming devices continuously remove the
froth (or float). The retention time in the flotation chambers is usually about
20–60 minutes for rectangular flotation clarifier and about 3–15 minutes for
circular flotation clarifier using zero-horizontal velocity design. The effectiveness
of dissolved air flotation depends upon the attachment of bubbles to the
suspended solids which are to be removed from the waste stream. The attraction
between the air bubble and particle is primarily a result of the particle surface
charge and bubble size distribution. The more uniform the distribution of water
and micro-bubbles, the shallower the flotation clarifier can be. Generally, the
depth of effective flotation units is between 3 and 9 feet.
Secondary membrane clarification (or secondary membrane separation or
secondary membrane filtration): A water-solid separation/clarification process
uses membrane device instead of conventional sedimentation clarification in the
secondary wastewater treatment step.
Secondary sedimentation clarification: It is a unit process or unit operation for the
removal of the bio-oxidation process generated activated sludge and/or the
chemical precipitation/coagulation process generated chemical sludge using a
secondary sedimentation clarifier.
Secondary sedimentation clarifier: It is a tank used to settle the chemical precipitation/coagulation process generated chemical sludge and/or the bio-oxidation
process generated activated sludge for removing dissolved organic/inorganic
substances from wastewater. The main objectives of a secondary sedimentation
clarifier are removal of settleable chemical and/or biological sludge solids by
settling them to the clarifier bottom. In a rectangular sedimentation clarifier, the
wastewater flows from one end to the other, and the settled sludge is moved to a
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155
secondary effluent is also the secondary clarification effluent.
Secondary flotation clarification: It is a unit process or unit operation for the
removal of the bio-oxidation process generated activated sludge and/or the
chemical precipitation/coagulation process generated chemical sludge using a
secondary flotation clarifier.
Secondary flotation clarifier: A dissolved air flotation (DAF) reactor is used to
float biological sludge (activated sludge) from biologically oxidized wastewater
(such as aeration tank effluent) and/or chemical sludge from chemically coagulated/flocculated wastewater (such as flocculator effluent) by decreasing the
sludge’s apparent density. DAF consists of saturating a portion or all of the
wastewater feed or a portion of recycled effluent with air at a pressure of
25–90 lb/square inch (gage). The pressurized wastewater is held at this pressure
for 0.5–3 minutes in a retention tank and then released to atmospheric pressure to
the flotation chamber. The sudden reduction in pressure results in the release of
microscopic air bubbles which attach themselves to suspended solids and other
particles in the wastewater in the flotation chamber. This results in agglomeration
which, due to the entrained air, has greatly increased vertical rise rates of about
0.5–2 ft/min. The floated materials rise to the surface to form a froth layer (float).
Specially designed scrapers or other skimming devices continuously remove the
froth (or float). The retention time in the flotation chambers is usually about
20–60 minutes for rectangular flotation clarifier and about 3–15 minutes for
circular flotation clarifier using zero-horizontal velocity design. The effectiveness
of dissolved air flotation depends upon the attachment of bubbles to the
suspended solids which are to be removed from the waste stream. The attraction
between the air bubble and particle is primarily a result of the particle surface
charge and bubble size distribution. The more uniform the distribution of water
and micro-bubbles, the shallower the flotation clarifier can be. Generally, the
depth of effective flotation units is between 3 and 9 feet.
Secondary membrane clarification (or secondary membrane separation or
secondary membrane filtration): A water-solid separation/clarification process
uses membrane device instead of conventional sedimentation clarification in the
secondary wastewater treatment step.
Secondary sedimentation clarification: It is a unit process or unit operation for the
removal of the bio-oxidation process generated activated sludge and/or the
chemical precipitation/coagulation process generated chemical sludge using a
secondary sedimentation clarifier.
Secondary sedimentation clarifier: It is a tank used to settle the chemical precipitation/coagulation process generated chemical sludge and/or the bio-oxidation
process generated activated sludge for removing dissolved organic/inorganic
substances from wastewater. The main objectives of a secondary sedimentation
clarifier are removal of settleable chemical and/or biological sludge solids by
settling them to the clarifier bottom. In a rectangular sedimentation clarifier, the
wastewater flows from one end to the other, and the settled sludge is moved to a
3 Biological Processes
155