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The water storage tank is normally elevated to effectively distribute water by gravity to the hatchery. The water storage tank capacity should be at least 20% of the larval rearing tanks. Storage tanks are normally constructed out of reinforced concrete to withstand the water pressure (Kungvankij et al., 1986).
When the water is turbid, installation of a filtering screen and sand filter unit becomes necessary. The filter chamber may be constructed adjoining the holding tank.
There are two types of filter systems:
- gravity filter where water is pumped into the f ilter chamber over the surface of the filter bed and allowed to pass through the filter material by gravity to the holding chamber which is located under the filter chamber
- reversing filter where water is pumped directly to the space under the filter chamber and pumped upward through the filter to the surface and on to the holding tank. In both systems, the filter chamber usually contains either white sand, charcoal, gravel, or all the three as filter material.
The advantage of the reverse system is that water passes slowly through the filter material and the whole surface area of the f ilter is utilized. It is easy to backwash by spraying water from the surface of the filter and the detritus underneath are easily washed out. On the other hand, gravity filter method allows water to pass through the water chamber too fast and do not f ully utilize the surface area of the filter unless it is provided with a pipe spray water over the whole surface area. The disadvantage of the gravity filter system is that the filter is easily blocked with detritus after a few days of operation resulting in turbid water and difficulty in back-washing (Kungvankij et al., 1986).
Inland hatcheries that do not have access to seawater, store concentrated seawater in reservoir tanks. This concentrated seawater is transported from solar salt operation to the hatchery (Kungvankij et al., 1986).
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