As some species of molluscs, especially oysters, are eaten raw, there are strict regulations in many countries relating to bacterial concentration in the products or in the environment in which they are grown. If the median total coliform concentration exceeds 70 per 100ml, it is mandatory to depurate them before sale. Shell-fish can concentrate pathogenic organisms from sewage-polluted water. Viral pathogens, especially those causing infectious hepatitis, occur in many areas and it is often difficult to monitor them in the environment or in the shellfish. So depuration is necessary to reduce public health hazards caused by the consumption of contaminated shellfish.
Holding the animals in clean water helps them to cleanse themselves of bacteria. The three methods of depuration commonly practised consist of treatment with chlorine, ozone or ultra-violet (UV) rays. Cleansing or depuration plants are located in areas with a convenient supply of unpolluted sea water. Different systems are used to bring in the harvests and transfer them to treatment tanks. The shellfish may be spread in shallow tanks or placed in shallow baskets (about 8cm deep) made of wood with galvanized wire mesh, plasticcoated expanded metal or plasticcoated wire mesh. The dimensions of the tanks differ very considerably according to the quantity of oysters or other shellfish to be depurated. They are made of cement concrete, with or without epoxy lining, marine plywood coated with fibreglass or of moulded fibreglass. The quality of the water and its proper circulation over the tank are very important. Cascading or splashing of water is the common practice, even though a timed air bubbling system has been incorporated in some. In order to maintain the dissolved oxygen level above 5mg/l, a flow rate of 105l/min per m3 is generally needed. It is advisable to maintain the same salinity as in the harvest site, but it should not vary more by than 20 per cent. Significant reductions in purification rates have been observed inCrassostrea virginica at salinities below 7ppt.
The traditional method of cleansing the shell-fish of microbes is by the use of chlorine. Initially, about 3ppm of chlorine is added to the sea water at the time of tank filling. This excess dose makes them close their shells and enables easy cleansing of the shell. As sea water continues to flow into the tank, the concentration of chlorine decreases, resulting in a 10- to 20-fold dilution. There are reservations about the use of chlorine because of the effects of chlorine residue on the shellfish tissue. Dechlorination with sodium thiosulphate has been used in the past to remove residue, but this practice is not very common now. Many producers contend that chlorinetreated oysters retain the
original flavour better than the ones treated with ozone.
Depuration with ozone treatment is commonly practised in France, Spain and Japan. Being a strong oxidizing agent, ozone can kill bacteria and viruses rapidly and therefore the time taken for purification is less. It dissipates into dissolved oxygen in water and, unlike chlorine, leaves no residue. In waters containing 2000–5000 coliform bacteria per litre, 1.50– 2.10g/m3 ozone is required to sterilize the water; for a bacterial concentration of 250–1000 per litre, the requirement is about 0.75–1.15g/m3. It is also very effective in reducing viruses. Ozone bubbled through the water purifies the water and the shell and meat of oysters. The meat is reported to be cleansed in about two to three hours.
Ultra violet rays are used for disinfecting sea water for depuration in countries such as the UK, USA and to a limited extent Japan. The water can either flow through the tank or circulate. Honma (1971) described systems in which disinfected water falls in a shower on the tank and the wastes are collected at the bottom and drained out. About 300–400 oysters can be cleansed per m3 in about 10 hours. Other designs of tanks based on different methods of delivery of disinfected water, enabling greater circulation and easy draining, have also been used. The effective penetration of UV light is limited by turbidity, as well as the depth of water. When the turbidity is high, better disinfection of viruses can be achieved by reducing the flow rate and thus increasing exposure time.