Page 176 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Sedimentation 131

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Dynamic viscosity: Defined as the coefficient, m, in the rela- (998.2040 kg=m )¼ 1.007 10 6 m =s. See also
tion, t ¼ m(dv=dy), where m ¼ dynamic viscosity dynamic viscosity.
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(Ns=m ), t ¼ shear stress in laminar fluid flow Loading rate: Although used frequently, this term should be
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(N=m ), (dv=dy) ¼ velocity gradient (s ). used with full designation such as hydraulic loading
Final clarifier: The final settling basin in biological treatment rate, surface loading rate, or solids loading rate with
for the separation of biological floc suspension units specified.
before discharge of the clarified water, often called Outlet zone: The zone of a horizontal flow settling basin that
secondary settling basin. is dedicated to the outlet flow and is not counted as
Floc: A particle comprised of aggregation of other like par- being effective in settling.
ticles. Examples include biological floc, alum floc, Overflow launders: The weirs of a settling basin over which
and iron floc. the effluent flow is discharged.
Flocculent settling: Settling of flocculent particles, i.e., those Overflow rate (OR): See surface overflow rate or surface
that grow in size. overflow velocity, v o .
Flow: Defined as volume per unit time that passes a given Overflow velocity (v o ): See surface overflow rate or surface
plane and often is called ‘‘flow rate.’’ overflow velocity, v o .
Flow net: Configuration of streamlines and potential lines Plain sedimentation: A settling process in water treatment
that define a flow field between a given source and intended to settle mineral sediments in natural
sink and boundaries. waters, usually rivers. If used, plain sedimentation
Flux: Volume, mass, or particle numbers transferred per unit would be after intake screening and helps to reduce
time across a given area that is normal to the velocity the load on other processes. The process may be
vector of the volume, mass, or particles undergoing most efficacious during spring runoff, but may be
transport. The particles may be discrete, molecules, required during the entire year for those rivers that
ions, or photons. The term is more general, however, are inherently sediment laden.
and may apply to a field (gravitational, electric, or Plate settler: An array of plates inclined at an angle of 558 or
magnetic) or to energy. 608. The suspension flows between the plates and by
Flux density: Flux per unit area—the area being normal to gravity is carried to the surface of the lower plate. As
the velocity vector. Hydraulic loading rate and solids the solids build up on the lower plate, the gravita-
loading rate are examples. tional vector parallel to the plate at some point
Fractal: Ill-defined geometry of something, e.g., a particle. exceeds the shear resistance and the mass slides
A floc particle is an example of a fractal. down to a solids collection zone. This usually occurs
in a random fashion as ‘‘patches’’ of solids may be
Hydraulic loading rate (HLR): By definition, HLR ¼
Q(plate)=A(cross section), in which A(cross section) observed sliding down the plate at random locations.
is the area normal to the orientation of the flow Primary clarifier: The first settling basin in wastewater treat-
direction of the individual plates or tubes. ment that separates settleable solids before biological
Hindered settling: Settling in which the flow field of the treatment.
settling particles are mutually affected by the others. Relationship between HLR and HLR(horiz plane): The
Horizontal flow basin: A traditional basin in which flow HLR(horiz plane) is the flow, Q(plate), divided by
enters one end of a basin and leaves the other, the cross-sectional area of an individual plate, or
leaving as a rule by means of overflow weirs. tube, projected on the horizontal plane, i.e., A(horiz
Ideal basin: As defined by Camp (1946), an ideal basin has plane). For a plate, the A(horiz plane) ¼ w(plate)
uniform velocity in all parts of the settling zone, and d=sin u. Therefore, HLR(horiz plane) ¼ HLR sin u.
thus each molecule of water remains in the basin for The HLR(horiz plane) has been used by regulatory
a time equal to its detention time, the suspended agencies (Colorado Health Department, 1987, p. 32)
particles are uniformly distributed at the inlet, and a and by those in the industry using tube settlers.
particle that reaches the bottom is removed from the Rim-flow clarifier: A clarifier in which the influent flow is
suspension. from a peripheral feed inlet.
Ideal flow: Flow that is frictionless and for which a flow net Sedimentation: Synonymous with settling.
is applicable. Settling: Unit process for separation of a suspension by grav-
Inlet zone: The zone of turbulence dissipation at the entrance ity settling of particles.
to a horizontal flow settling basin, which is not Settling basin: As defined by Camp, a settling basin has four
effective in settling. zones: inlet, settling, outlet, and sludge collection.
Kinematic viscosity: Calculated as the dynamic viscosity of Settling zone: The ‘‘effective’’ zone of a clarifier in which
a fluid divided by its density, i.e., n ¼ m=r, where settling occurs; the term was derived from the
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n ¼ kinematic viscosity (m =s), r ¼ density of ‘‘ideal’’ basin.
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fluid (kg=m ). Units conversion is (m =s) ¼ Shield’s equation: An empirical mathematical relationship
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(N s=m ) (m =kg) (kg m=Ns ). Example calcula- that relates particle characteristics to the critical
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tion for T ¼ 208C: n ¼ m=r ¼ (1.005 10 3 Ns=m )= channel velocity for scour.