Disinfection
Introduction
Disinfection can be described as the reduction of
micro-organisms such as bacteria, viruses, fungi and parasites to a desired
concentration. This is not the same as sterilization where all micro-organisms
are eliminated. The aim of disinfection of water in fish farming is to reduce
to an acceptable level the risk of transfer of infectious disease from the
water to the fish. When disinfecting water for fish farming, selec-tive inactivation
of fish pathogenic micro-organisms is required in addition to overall reduction
in the total number of micro-organisms. Pathogenic micro-organisms infect the
fish and cause disease. Trans-missions of infectious diseases is possible in
two ways, horizontal and vertical. Horizontal transmission includes direct or
indirect contact between individuals or populations. Direct contact occurs
between individuals or urine or faeces, while indirect contact occurs through
contact with water, equipment and personnel with pathogens. Vertical trans-port
includes transmission from one generation to the next through roe or milt, for
example.
Disinfection can be performed in different situations in
aquaculture. Water, equipment, buildings and effluent can all be disinfected.
Equipment includes tanks, nets, pipes and shoes. Disinfection of buildings
includes, for instance, disinfection of the hatchery after seasonal use.
Effluent may include sludge and by-products. Disinfection of water actually
occurs at several places in an aquaculture plant. Usually the inlet water is
disinfected, whether it is seawater or freshwater. At the larval stage it is
particularly important to reduce the number of micro-organisms because larvaes
are more vulnerable to
infections. In a water re-use plant, the water may also be
disinfected before being used again to avoid increasing the micro-organism
burden. The outlet water may also be disinfected to avoid transfer of
micro-organisms to fish species in the recipient water body.
There are several methods for disinfecting water and a number of
general textbooks are available (for example, refs 1–4). Disinfectants can be
sepa-rated into chemical agents and non-chemical agents.1
Alternatively, a four-group classification can be used: 1, chemical agents; 2,
physical agents; 3, mechanical agents; 4, radiation.4 The first
group includes chlorine and its compounds, bromine, iodine, ozone, phenol and
phenolic compounds, alcohols, heavy metals and related compounds, soaps and
synthetic detergents, quaternary ammo-nium compounds, hydrogen peroxide and
various alkalis and acids. The second group includes heating and the use of
sunlight, especially the ultraviolet (UV) end of the spectrum. The third group
includes particle separation; although particle separation is the main
objective, there will also be a reduction in the number of micro-organisms
because many are attached to particles. Larger parasites such as CostiaandGyrodactyluswill also be removed witha particle filter with small
(20 μm) mesh size. To
the fourth group belong different types of radiation including electromagnetic,
acoustic and particle. For example, gamma rays are used to disinfect and also
sterilize water and food, although this method is expensive.
Many of the chemical agents employed oxidize the organic
materials, including the micro-organisms. The oxidizing potential indicates how
effective the agent is likely to be: ozone has the highest potential, while
bromine and iodine have the lowest potentials.
Regardless of the method chosen for disinfection the quality of
the water to be disinfected is of major importance. Pure inlet water is much
simpler to dis-infect than outlet water because the latter contains more
particles. Turbid water and water with a high content of organic substances,
such as re-use water, are also more difficult to disinfect and therefore not so
commonly disinfected. Before disinfecting contaminated water, it is essential
to carry out some kind of pre-treatment, normally comprising removal of
particles.
For disinfection of water supplies to aquaculture facilities, UV
light and ozone are most often used. Later in this chapter there is a survey of
methods employed, with emphasis on UV light and ozone.
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