Precession The wobble over thousands of years of the tilt of the Earth’s axis with
respect to the plane of the solar system.
Precipitate An insoluble, finely divided substance, which is a product of a chemical
reaction, or a natural hydrological reaction.
Precipitation Any and all forms of water, whether liquid or solid, that fall from the
atmosphere and reach the Earth’s surface. A day with measurable precipitation is
a day when the water equivalent of the precipitation is equal to or greater than
0.2 mm.
Precision The degree to which a set of observations or measurements of the same
property, usually obtained under similar conditions, conform to themselves; a
data quality indicator. Precision is usually expressed as standard deviation,
variance, or range, in either absolute or relative terms.
Preliminary effluent The effluent from a preliminary treatment system (i.e., bar
screen, comminutor, and grit chamber) by which most of large objects, such as
rocks, logs and cans, grit, etc., in raw wastewater have been removed.
Preliminary treatment It is the first treatment step, or preliminary step, of either a
conventional wastewater treatment system or an independent physicochemical
treatment system. Preliminary treatment consists of bar screen, comminutor, and
grit chamber mainly for removing large objects, such as rocks, logs and cans, grit,
etc., from raw wastewater. Comminutor is an option depending on the nature and
characteristics of raw wastewater.
Prevention Actions taken to avoid an incident or to intervene to stop an incident
from occurring.
Primary effluent The effluent from a primary treatment system (either primary
sedimentation clarification or primary flotation clarification) by which most of
total suspended solids in wastewater have been removed.
Primary flotation clarification A dissolved air flotation clarification process,
which is used for primary clarification of wastewater with a main purpose of
total suspended solids removal.
Primary flotation clarification It is a unit process or unit operation for removal of
mainly total suspended solids (settleable solids and floatable solids) from
screened wastewater using a primary flotation clarifier.
Primary flotation clarifier A dissolved air flotation (DAF) reactor is used to float
total suspended solids (TSSs) from screened wastewater by decreasing their
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 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 TSS and oil particles in the
wastewater in the flotation chamber. This results in agglomeration, which, due to
the entrained air, have 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
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M.-H. S. Wang and L. K. Wang
respect to the plane of the solar system.
Precipitate An insoluble, finely divided substance, which is a product of a chemical
reaction, or a natural hydrological reaction.
Precipitation Any and all forms of water, whether liquid or solid, that fall from the
atmosphere and reach the Earth’s surface. A day with measurable precipitation is
a day when the water equivalent of the precipitation is equal to or greater than
0.2 mm.
Precision The degree to which a set of observations or measurements of the same
property, usually obtained under similar conditions, conform to themselves; a
data quality indicator. Precision is usually expressed as standard deviation,
variance, or range, in either absolute or relative terms.
Preliminary effluent The effluent from a preliminary treatment system (i.e., bar
screen, comminutor, and grit chamber) by which most of large objects, such as
rocks, logs and cans, grit, etc., in raw wastewater have been removed.
Preliminary treatment It is the first treatment step, or preliminary step, of either a
conventional wastewater treatment system or an independent physicochemical
treatment system. Preliminary treatment consists of bar screen, comminutor, and
grit chamber mainly for removing large objects, such as rocks, logs and cans, grit,
etc., from raw wastewater. Comminutor is an option depending on the nature and
characteristics of raw wastewater.
Prevention Actions taken to avoid an incident or to intervene to stop an incident
from occurring.
Primary effluent The effluent from a primary treatment system (either primary
sedimentation clarification or primary flotation clarification) by which most of
total suspended solids in wastewater have been removed.
Primary flotation clarification A dissolved air flotation clarification process,
which is used for primary clarification of wastewater with a main purpose of
total suspended solids removal.
Primary flotation clarification It is a unit process or unit operation for removal of
mainly total suspended solids (settleable solids and floatable solids) from
screened wastewater using a primary flotation clarifier.
Primary flotation clarifier A dissolved air flotation (DAF) reactor is used to float
total suspended solids (TSSs) from screened wastewater by decreasing their
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 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 TSS and oil particles in the
wastewater in the flotation chamber. This results in agglomeration, which, due to
the entrained air, have 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
474
M.-H. S. Wang and L. K. Wang