Principles of Food Sanitation

attention is required once a proper operation
is established. Several food processors use
oxidation ditches for wastewater treatment.
There is a current interest in the use of the
total barrier oxidation ditch (TBOD) design
for treating municipal food processing and
industrial wastewater. TBOD biologically
purifies water as it mixes oxygen with waste
particles and permits the bacteria to feed on
these pollutants. The system can achieve
high oxygen transfer efficiency at a single
point along the ditch, which allows for effec-
tive process control and design flexibility. A
constant, powerful flow of wastewater is
then maintained, preventing settling of the
biomass at the bottom of the ditch reactor.
The aeration and pumping unit consists of
submerged, turbine draft tube aerators that
transfer oxygen into the mixed liquid.

Land Application

The two types of land application tech-
niques that are the most efficient are infiltra-
tionandoverland flow. With land application
techniques, the pollutants can harm vegeta-
tion, soil, and surface and ground waters if
not properly operated. However, both of
these treatment techniques can effectively
remove organic carbon from high-strength
wastewater. Pollutant removal efficiencies of
approximately 98% for the infiltration flow
system and 84% for the overland flow system
can be attained. The advantage of higher
efficiency obtained with an infiltration sys-
tem is offset by its more expensive and com-
plex distribution system. Less pollution of
potable ground water supplies is usually
experienced with the overland flow system.
Although land application has been a
standby in the past for discharge of some
food processing wastes, this approach is now
limited. Hydraulic loads that are high may
necessitate an unreasonably large amount of
land. Runoff and proper utilization of nutri-
ents can restrict the vegetation. Buildup of

minerals and other materials in the soil has

the potential for long-term liability for

residues possibly as yet undiscovered (Rush-

ing, 1992).

Rotating Biological Contactor

The rotating biological contactor (RBC) is

an attached growth type of biological treat-

ment system similar in concept to the trickling

filters. Initial costs of this equipment are high,

but operating costs and space requirements are

moderate. This system consists of a number of

large-diameter (approximately 3 m) and light-

weight discs that are mounted 2 to 3 cm apart

(to prevent bridging between the growths) on

a horizontal shaft (in groups or packs, with

baffles between each group to minimize surg-

ing or short-circuiting) to form an RBC unit

(Figure 12–1). The discs are partially (30 to

40%) immersed and rotate slowly (0.5 to

10 rpm) as wastewater passes through a hori-

zontal open tank, which usually has a semicir-

cular bottom to fit the contour of the discs.

The RBC unit functions by attachment of

microorganisms to the surface of the discs

and grows by assimilating nutrients from the

wastewater. Aeration is achieved through

direct exposure of microorganisms to air

Waste Product Handling 229

Figure 12–1Rotating biological contactors used to

remove ammonia from water of an aquaculture pro-

duction operation.