wastewater feed, or a portion of recycled effluent with air at a pressure of 25 to
90 lb./square inch (gage). The pressurized wastewater is held at this pressure for
0.5 to 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 to 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 microbubbles, 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
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
hopper at on end, either by scrapers called “flights” set on parallel chains, or by a
single bottom scraper set on a traveling bridge. In a circular sedimentation
clarifier, the wastewater usually enters in the middle and flows toward the outside
edge. Settled sludge is pushed to the hopper that is in the middle of the circular
clarifier’s tank bottom.
Secondary treatment It is a wastewater treatment step after primary treatment
(either primary sedimentation clarification or primary flotation clarification).
Secondary treatment may be either biological treatment (such as activated sludge
aeration plus secondary clarification) or physicochemical treatment (such as
chemical precipitation/coagulation plus secondary clarification) mainly for
removing dissolved organic/inorganic pollutants from primary effluent.
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M.-H. S. Wang and L. K. Wang
90 lb./square inch (gage). The pressurized wastewater is held at this pressure for
0.5 to 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 to 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 microbubbles, 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
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
hopper at on end, either by scrapers called “flights” set on parallel chains, or by a
single bottom scraper set on a traveling bridge. In a circular sedimentation
clarifier, the wastewater usually enters in the middle and flows toward the outside
edge. Settled sludge is pushed to the hopper that is in the middle of the circular
clarifier’s tank bottom.
Secondary treatment It is a wastewater treatment step after primary treatment
(either primary sedimentation clarification or primary flotation clarification).
Secondary treatment may be either biological treatment (such as activated sludge
aeration plus secondary clarification) or physicochemical treatment (such as
chemical precipitation/coagulation plus secondary clarification) mainly for
removing dissolved organic/inorganic pollutants from primary effluent.
480
M.-H. S. Wang and L. K. Wang