passing through with solids constantly
removed from the screen) mode of operation
and can vary widely in mechanical action
and in mesh size. Mesh sizes used in pre-
treatment range from approximately 12.5
mm in diameter for a static screen to approx-
imately 0.15 mm in diameter for high-speed
circular vibratory polishing screens. Screens
are sometimes used in combination (e.g.,
prescreen polish screen) to attain the desired
efficiency of solids removal.
Skimming
This process is frequently incorporated if
large, floatable solids are present. These
solids are collected and transferred into
some disposal unit or preceding equipment.
Lime and FeCl 3 , or a selected polymer may
be added to enhance separation of solids,
and paddle flocculation may follow to assist
with the coagulation of these solids.
Primary Treatment
The principal purpose of primary treatment
is to remove particles from the wastewater.
Sedimentation and flotation techniques are
used.
Sedimentation
Sedimentation is the most common pri-
mary treatment technique used to remove
solids from wastewater influent because
most sewage contains a substantial amount
of readily settleable solid material. As much
as 40 to 60% of the solids, or approximately
25 to 35% of the BOD 5 load, can be removed
by pretreatment screening and primary sedi-
mentation. Some of the solids removed are
refractory (inert) and are not measured by
the BOD test.
A rectangular settling tank or a circular
tank clarifier is most frequently used in pri-
mary treatment. Many settling tanks incor-
porate slowly rotating collectors with
attached flights (paddles) that scrape settled
sludge from the bottom of the tank and skim
floating scum from the surface.
Design of a sedimentation system should
incorporate sizing of the detention vessel
and provide a quiescent state for the raw
wastewater. Temperature variation of the
wastewater also affects sedimentationbecause
of the development of heat convection cur-
rents and the potential interference with
marginal setting participles. Grease removal
is accomplished during this pretreatment
process through elimination of the surface
scum.Flotation
In this treatment process, oil, grease, and
other suspended matter are removed from
wastewater. A primary reason that flotation
is used in the food industry is that it is effec-
tive in removing oil from wastewater.
Dissolved air flotation (DAF) removes
suspended matter from wastewater by using
small air bubbles. Flocculants and polymers
are added to the wastewater to separate
grease, oils, and fats from the water. When
discrete particles attach to tiny air bubbles,
the specific gravity of the aggregate particle
becomes less than that of water. The particle
separates from the carrying liquid in an
upward movement by attaching to the air
bubble. The particles are then floated for
removal from the wastewater. Also, this pre-
treatment process involves contact of the raw
wastewater with a recycled, clarified effluent
that has been pressurized through air injec-
tion in a pressure tank. The combined flow
stream enters the clarification vessel, and the
release of pressure causes tiny air bubbles to
form, which move up to the surface of the
water, carrying the suspended particles with
them.
Air bubbles, which incorporate the flota-
tion principle by removal of oil and sus-
pended particles, can be created in the
wastewater by (1) use of rotating impellers or