run length. The head loss at which filtration is interrupted for cleaning is called
the terminal head loss, and this head loss is maximized by the judicious choice of
media sizes. Dual media filtration involves the use of both sand and anthracite as
filter media, with anthracite being placed on top of the sand. Gravity filters
operate either by using the available head from the previous treatment unit or
by pumping to a flow split box after which the wastewater flows by gravity to the
filter cells. Pressure filters utilize pumping to increase the available head. A filter
unit generally consists of a containing vessel; the filter media; structures to
support the media; distribution and collection devices for filter influent, effluent,
and backwash water flows; supplemental cleaning devices; and necessary controls for flows, water levels, and backwash sequencing. Backwash sequences can
include air scour or surface wash steps. Backwash water can be stored separately
or in chambers that are integral parts of the filter unit. Backwash water can be
pumped through the unit or can be supplied through gravity head tanks.
Flotation thickening: In a dissolved air flotation (DAF) sludge thickening process,
air is introduced into liquid sludge that is being held at an elevated pressure.
When the sludge is depressurized, the dissolved air is released as finely divided
air bubbles carrying the solids to the top, where they are compacted and removed.
Gravity thickening: Its tank design is similar to a conventional sedimentation tank.
Dilute sludge is fed to a center feed well. The feed sludge is allowed to settle and
compact, and the thickened sludge is withdrawn.
Grinding: It is a unit operation for reducing the particle size of objects or debris in
the influent wastewater, also termed shredding or comminuting. These devices
may be installed with a screen directly in the wastewater flow or separately out of
the wastewater flow, with the shredded particles returned to the flow downstream
of the screen. Only those shredding and grinding devices that are installed directly
in the influent channel are termed comminuting devices.
Grit chamber: It is a grit removal device that is designed to allow the settling out of
this material. Grit removal is an important process for several reasons: (1) to
prevent cementing effects at the bottom of sludge digesters and primary clarification tanks; (2) to reduce the potential for clogging of pipes and sludge hoppers;
(3) to protect moving mechanical equipment and pumps from unnecessary wear
and abrasion; (4) to reduce accumulations of materials in aeration tanks and
sludge digesters which would result in a loss of usable volume; and (5) to reduce
accumulations at the bases of mechanical screens. There are two types of grit
chambers. The velocity controlled grit chambers limit the velocity in the rectangular channels to a maximum of 1 foot per second (fps). This velocity is low
enough to allow the grit to settle but fast enough to maintain a majority of the
organic material in suspension. The aerated grit chambers are normally sized on
the basis of both detention time and volume of air. Typically, the detention time is
in the range of 2–5 minutes, and the air flow is in the range of 0.04–0.06 cu
ft/gallon of wastewater. The constant head type of system is normally designed
using an overflow rate of 15,000 gallons per day per square foot and a 1 minute
detention time at peak day flows.
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L. K. Wang et al.
the terminal head loss, and this head loss is maximized by the judicious choice of
media sizes. Dual media filtration involves the use of both sand and anthracite as
filter media, with anthracite being placed on top of the sand. Gravity filters
operate either by using the available head from the previous treatment unit or
by pumping to a flow split box after which the wastewater flows by gravity to the
filter cells. Pressure filters utilize pumping to increase the available head. A filter
unit generally consists of a containing vessel; the filter media; structures to
support the media; distribution and collection devices for filter influent, effluent,
and backwash water flows; supplemental cleaning devices; and necessary controls for flows, water levels, and backwash sequencing. Backwash sequences can
include air scour or surface wash steps. Backwash water can be stored separately
or in chambers that are integral parts of the filter unit. Backwash water can be
pumped through the unit or can be supplied through gravity head tanks.
Flotation thickening: In a dissolved air flotation (DAF) sludge thickening process,
air is introduced into liquid sludge that is being held at an elevated pressure.
When the sludge is depressurized, the dissolved air is released as finely divided
air bubbles carrying the solids to the top, where they are compacted and removed.
Gravity thickening: Its tank design is similar to a conventional sedimentation tank.
Dilute sludge is fed to a center feed well. The feed sludge is allowed to settle and
compact, and the thickened sludge is withdrawn.
Grinding: It is a unit operation for reducing the particle size of objects or debris in
the influent wastewater, also termed shredding or comminuting. These devices
may be installed with a screen directly in the wastewater flow or separately out of
the wastewater flow, with the shredded particles returned to the flow downstream
of the screen. Only those shredding and grinding devices that are installed directly
in the influent channel are termed comminuting devices.
Grit chamber: It is a grit removal device that is designed to allow the settling out of
this material. Grit removal is an important process for several reasons: (1) to
prevent cementing effects at the bottom of sludge digesters and primary clarification tanks; (2) to reduce the potential for clogging of pipes and sludge hoppers;
(3) to protect moving mechanical equipment and pumps from unnecessary wear
and abrasion; (4) to reduce accumulations of materials in aeration tanks and
sludge digesters which would result in a loss of usable volume; and (5) to reduce
accumulations at the bases of mechanical screens. There are two types of grit
chambers. The velocity controlled grit chambers limit the velocity in the rectangular channels to a maximum of 1 foot per second (fps). This velocity is low
enough to allow the grit to settle but fast enough to maintain a majority of the
organic material in suspension. The aerated grit chambers are normally sized on
the basis of both detention time and volume of air. Typically, the detention time is
in the range of 2–5 minutes, and the air flow is in the range of 0.04–0.06 cu
ft/gallon of wastewater. The constant head type of system is normally designed
using an overflow rate of 15,000 gallons per day per square foot and a 1 minute
detention time at peak day flows.
146
L. K. Wang et al.