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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