air diffusers to form air bubbles at atmos-
pheric pressure; (2) saturation of the liquid
medium with air and subsequent combina-
tion of the mixture to a vacuum to create
bubbles; and (3) saturation of air with liquid
under high pressure and subsequent release
to form bubbles.
Flocculating agents are commonly used to
pretreat wastewater prior to treatment by a
DAF unit. Treatment by DAF is widespread
because of the relatively fast passage and
because solids of nearly the same as, or lower
density than water, can be removed. This
treatment technique requires high invest-
ment and operating costs, especially for
chemical additives and sludge handling.
DAF systems maintain a concentration of
bacteria that are kept alive within the system
to biodegrade pollutants in the effluent. A
dewatering device, such as a belt filter, can be
incorporated with DAF. After floatable oils
and grease are captured, they can be chemi-
cally treated and the material conditioned,
similar to a liquid-solid separation process.
Flotation technology has also been
adapted to sludge handling and to secondary
and tertiary treatments. Food processors
with substantial quantities of grease and oil
in their wastewater, use this technique as part
of their waste treatment systems. In the past,
one problem of flotation has been the pres-
ence of a turbulent flow; however, commer-
cial high rate flotation devices that eliminate
turbulent flow are now available. The instal-
lation of lamellas (vertical baffles) can pre-
vent unfavorable currents and short-circuiting
and, with a properly designed feed well, can
improve solid/liquid separation, producing
higher underflow solid concentration in grav-
ity thickeners and better effluent quality in
gravity clarifiers.
Collected sludge from primary treatment
contains approximately 2 to 6% solids, which
should be concentrated before final disposal.
Sludge treatment and disposal costs are the
major expenses of sewage treatment if this
product is not used as a fertilizer or for some
other practical application. Some treatment
systems biodegrade most of the organic mat-
ter and create little sludge. These systems can
reduce treatment and disposal costs. If
sludge is recovered as a by-product, disposal
costs can be reduced, and the value of the
salvaged material can provide enough profit
to defray other treatment costs. Recovered
solids (sludges) can also be treated by bio-
logical oxidation methods as a means of ulti-
mate disposal.
A method developed in the past (Sofranec,
1991) utilizes a series of coagulants formed
from cornstarch to separate oil, grease, and
suspended solids from wastewater prior to its
discharge. The resultant grease and solids
recovered from the DAF can be rendered.
These starch-based coagulants are normally
added to an equalization tank prior to the
DAF system, where they can reduce the sur-
face charge on the solids and grease, allow-
ing the materials to coalesce and be removed
by DAF.
In the past, wastewater treatment has nor-
mally involved the removal of solids from liq-
uids. New equipment that utilizes a water
loop principle can filter water from behind a
chiller and flow it through a series of filters
before returning it to the chiller. In this
process, organic matter is filtered out so that
the water can be recycled. Furthermore, water
concentrates of as little as 3% of organic mat-
ter can be recycled through rendering equip-
ment such as a disk dryer to concentrate a
product into dry powder, with the vapors
directed back into the evaporative system to
be used as an energy source. The evaporative
system provides free energy.
The sludge originating from the ponds
of stabilization present another problem
because of the amount of chemicals, organic
and inorganic, from the lingering decomposi-
tion of wastewater residues. So, the cleaningWaste Product Handling 223