when the surface of the disc is rotated above
the water and by a thin film of water, which
is aerated as it adheres to the disc’s sur-
face and rises out of the water. Acceleration
in rotational speed increases the dissolved
oxygen in the tank. The biofilm undergoes
sloughing as in trickling filters, and these
solids must be settled and removed. Although
this treatment process is considered to be sec-
ondary treatment, primary sedimentation
may be eliminated if the wastewater sus-
pended solids are not unusually high (greater
than 240 mg/L).
Magnetic Separation
This secondary physical treatment method
has applications for tertiary use. The organic
waste solids in suspension are chemically
treated with magnetite (Fe 3 O 4 ). Alum or
ferric chloride coagulation flocculation is
performed, and the coagulated particles sub-
sequently contain magnetite. This process
consists of a chamber containing a stainless
steel wool matrix located in a magnetic field.
The magnetized coagulated particles in
wastewater suspension are passed through
the chamber and adhere to the stainless steel
wool in the magnetic field. The collected
organic waste is removed through reducing
the magnetic field to zero and washing out
the waste solids. This process was developed
in Australia and has seen limited applica-
tions in North America.
Tertiary Treatment
Tertiary treatment processes for waste-
water, which are collectively known as
advanced wastewater treatment, are incorpo-
rated to improve the quality of waste treat-
ment effluents to meet NPDES guidelines.
Tertiary waste treatment is applied to food-
processing wastewaters to remove pollutants
of food-processing, such as colors, odors,
brines, and flavoring compounds. Some of
the processes for tertiary treatment of
municipal waste treatment are frequently
used as a primary waste treatment for certain
food processors.Physical Separation
Sand filters and microstrainers have been
developed for tertiary wastewater treatment
and purification. Both of these physical sep-
aration methods remove suspended solids
down to the micrometer particle range.
The microstraineris a rotating cylinder
covered by a screening material (usually fine
mesh nylon or metal fabric) housed in a hor-
izontal position in an open tank. Wastewater
enters the inside of the cylinder and is fil-
tered through the screen. As the cylinder
rotates slowly, an exposed section above the
wastewater surface is backwashed to clean
the screen and to collect the solids into a sep-
arate channel. Particle removal by microstain-
ing is a function of screen pore size, which
normally ranges from 20 to 65 μm. This is a
relatively low-cost method of tertiary treat-
ment because the screens are self-cleaning,
and operating and maintenance costs are
low. The effectiveness of this treatment
method is limited by partial screen clogging,
with a resultant decrease in the life of the
screen. Also, microorganisms can grow in
secondary water inside the cylinder, causing
slime formation on the screen. Ultraviolet
light or chlorination treatment has been used
to reduce slime formation.
Therapid sand filterandmixed media and
continuous countercurrent filtration are fre-
quently used in tertiary wastewater treat-
ment. This treatment method requires
underdrains for removal of clarified liquids
and a system for recovering collected solids.
Automatic backwash mechanisms are avail-
able to enable self-cleaning of filters.Physical-Chemical Separation
Food-processing wastewaters contain a
substantial amount of dissolved solids that