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Similar to most food industries, fish processing
operations produce waste in a solid (fish
carcasses, viscera, skin, heads) or liquid form
(washing and cleaning water discharge, bloodwater
from drained fish storage tanks, brine). This waste
must be stored so as to prevent the contamination
to the processing environment, and should be
disposed of in a manner that is not detrimental to
the receiving environment. The magnitude of the
problem of waste management in the fish industry
depends on the waste volume, its polluting charge,
rate of discharge and the assimilatory capacity of
the receiving medium.
In many countries, solid waste is recycled into
fish meal plants or treated along with the
municipal waste, whereas liquid waste is disposed
of through the municipal sewage system or directly
into a waterbody. In the latter case, care must be
exercised to ensure that the receiving waterbody
can degrade the biological and chemical
constituents of the waste in a manner that is not
detrimental to the aquatic fauna and flora.
Designing appropriate measures to dispose of
liquid waste from fish processing operations
requires assessment. This is done through the
evaluation of various physicochemical and
biological parameters of which the most important
ones are: solid content, pH, temperature, odor,
organic matter, biochemical oxygen demand or BOD,
chemical oxygen demand or COD, oil and grease
content, nitrogen and phosphorous content.
Primary and secondary waste treatments
Depending on the effluent polluting capacity and
nature, one or several treatments can be
considered. These treatments are classified into
primary and secondary treatments.
Primary treatments include operations
designed to remove floatable and settling solids.
They include screening, sedimentation, and
flotation to remove oil and grease and other
suspended solids.
Secondary treatments comprise biological
and physicochemical treatments. In biological
treatments, the organic polluting matter is
degraded by micro-organisms, which metabolise it
into energy and biomass. These microorganisms can
be aerobic or anaerobic. The most used aerobic
processes are activated sludge system, aerated
lagoons, trickling filters or bacterial beds and
the rotating biological contractors. In anaerobic
processes, the anaerobic microorganisms digest the
organic matter in tanks to produce gases (mainly
methane and CO2) and biomass. Anaerobic digesters
are sometimes heated, using part of the methane
produced, to maintain a temperature of 30 to
35°C.
In the physicochemical treatments, also called
coagulation-flocculation, a chemical substance is
added to the effluent to reduce the surface charges
responsible for particle repulsions in a colloidal
suspension, thus reducing the forces that keep its
particles apart. This reduction in charge causes
flocculation (agglomeration) and particles of
larger sizes are settled and clarified effluent is
obtained.
The sludge produced by primary and secondary
treatments is further processed in digesting tanks
through anaerobic processes or sprayed over land as
a fertilizer. In the latter case, care must be
exercised to ensure that the sludge is freed of its
pathogens.
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