1.2.2 Seawater system
The need for a supply of high quality seawater was previously discussed. It is important to ensure that the seawater source and system to pump and treat it is located conveniently close to the hatchery and optimum use made of it to keep capital and operating costs to a minimum.
The hatchery should be located as close to sea level as possible to avoid lifting water. Intakes for the seawater should be as short as possible and conveniently located so they can be serviced and maintained with minimum effort. Intakes for the salt water should be located at depth to avoid fluctuations in temperature and salinity and also to reduce the number of organisms and amount of detritus that will enter the system. In temperate areas, intakes should be located below any thermocline that occurs in summer to reduce temperature variability. In areas where periods of heavy rain occur, the intakes should be deep enough to avoid sudden fluctuations in salinity and heavy siltation that may occur with the rains. Intakes at depth avoid major plankton blooms, some of which may be harmful to bivalve larvae and also greatly diminishes the number of fouling organisms entering the system. Fouling organisms can settle in pipes and greatly reduce water flow into the hatchery. Many of the above sources of variability can be avoided by accessing seawater from drilled wells. This possibility should be investigated before any other solution is considered.
Size of pumps and the diameter of the pipes required will depend on the scale of the operation and the volumes of seawater required to meet all aspects of production. Pumps are available through commercial outlets and the type and size of pump required can be determined after discussions with dealers. It is important to ensure that surfaces that come into contact with the seawater are non toxic. Most plastics, cast iron and certain grades of stainless steel are suitable. Pumps that contain mild steel or brass components should be avoided.
Seawater pumped directly from the ocean is first passed through sand filters that filter out most particulate material greater than 20-40 ?m in size (Figure 4). A well maintained sand filter will remove the major portion of detritus and organisms from the water that may interfere with bivalve larvae. It also eliminates many of the fouling organisms that could settle and grow in pipes in the hatchery. They not only can cause problems with water flow but when they die they can produce anaerobic conditions that can be toxic to bivalve larvae. They may also harbour and eliminate bacteria that can be deleterious to larvae. Sand filters are commercially available and are the same or similar to those used to filter water in swimming pools. A series of two or more such filters are generally installed and they are regularly back-flushed to avoid clogging of the filter media. Other types of filters may be used depending on personal preference and cost considerations. Self-cleaning, rotating drum filters offer an alternative to remove larger particulate material and large surface area cartridge or bag filters are available and are extremely effective in removing smaller sized particulates.
Another method to obtain seawater for a hatchery is to pump it from seawater wells. This has become the preferred method for hatcheries to obtain their water supply in recent years. A well is dug or drilled close to the hatchery and is deep enough to provide a sufficient supply of seawater for the hatchery. Water from such wells is of high quality and generally has a constant temperature and salinity. It has already been filtered through sedimentary or porous rock, contains little detritus and few, if any, fouling organisms. Water abstracted in this way requires little if any further filtration. Constructing seawater wells can be expensive initially but the high capital cost is offset by reduced operating costs.
Figure 4: A diagram of the various stages of seawater treatment for hatchery usage from the intake pipes (IL) to the points at which water is used in the different aspects of the operation (1 to 5). Key: P – seawater pumps; SF – sand filters (photograph C) or alternatively self-cleaning drum filters (photograph A); ST – to storage tanks (if required); CF – cartridge filters of 20 ?m and 10 ?m; CU – seawater chilling unit (if required); HU – seawater heating unit (if required – photograph B); FF – final filtration (5 ?m and 1 or 2 ?m - photograph D); UV – ultra-violet light disinfecting units (if required).
A guide to typical usages (treatment levels vary from hatchery to hatchery):
1 – Unheated, sand-filtered water for broodstock and larger juveniles (if water requires to be heated, then 3).
2 – Chilled seawater filtered to 10 ?m for spawning broodstock or for large-scale algal culture of hardy species. Chilled (or ambient temperature) seawater is often mixed with heated seawater to provide intermediate temperatures for a variety of purposes.
3 – Heated seawater filtered to 10 ?m for conditioning and spawning broodstock and for growing larger spat. Some hatcheries have a separate heating system for either unfiltered or sand-filtered seawater for broodstock conditioning.
4 – Chilled water filtered to 1 ?m and either UV-disinfected or not for algal culture. 5 – Heated water filtered to 1 ?m and either UV-disinfected or not for larval culture.
After filtration, all or part of the seawater may be pumped to a storage tank that may be made of either concrete or fibreglass. Use of a storage tank may be a matter of preference and many hatcheries do not have them. They are useful when water can only be obtained at a particular time, e.g. at high tide. Sometimes this method is used in areas where electrical power is unreliable to ensure a supply of seawater is always available. Sufficient water is pumped into the storage tank so it can supply the hatchery until the tank can be refilled. The tank is located at height so that the effect of gravity maintains a sufficient water flow through the hatchery. In other hatcheries, the salt water system is a flow-through system and water is pumped continuously through the hatchery for use where it is needed and then is discharged to waste. Recently, many hatcheries have installed recirculating or partial recirculating systems to reduce operating costs. This is particularly true if seawater is in short supply or if it has been heated or chilled. Recirculated water may be passed over biologically activated filters to remove metabolic wastes of the animals and held before it is reused. If the water has been heated or chilled it may be passed through heat exchangers to partially heat or chill incoming water and thus reduce energy costs.
All piping must be non-toxic, usually PVC (polyvinylchloride) schedule 40 or 80, although ABS or polyethylene pipes and fittings are also sometimes used as alternatives. The diameter of the pipes depends on water demand. In most hatcheries the main distribution lines within the hatchery are 50 mm diameter or less although the main intake pipes may be up to 15 cm diameter. The piping should be well supported and high enough off the ground so that it is out of the way but readily accessible for cleaning. Valves and outlets should be conveniently located. If the water is sufficiently filtered there should be little need to clean the lines frequently. Cleaning may be required periodically, hence, it is important to have clean-out ports or screw unions located conveniently so that the lines can be easily cleaned in situ or quickly dismantled for more thorough cleaning.
In most hatcheries in temperate areas there needs to be the capability to heat and sometimes to chill part of the seawater supply. There are commercial units available for this purpose and discussions and calculations on required capacity with dealers will ensure that an adequate supply at the required temperatures is available. Again, it is essential to ensure that surfaces of such units coming in contact with the seawater are non-toxic to bivalve larvae. Most commercially available heat exchange units utilize titanium as the heat transfer surface and this material is preferred by most hatcheries.
Hatchery managers may wish to sterilize (or more correctly, disinfect) all or part of the seawater before use, particularly if disease problems arise. Seawater can be sterilized with either UV (ultra-violet) light or ozone. Commercial units are available and simple calculations will determine the size of unit that is required. Commercial units are normally rated for their performance in sterilizing freshwater. In seawater situations where organic loadings and turbidity caused by colloidal materials are frequently higher than for freshwater, it is recommended that such units are used at half (or less) of the recommended flow rate for satisfactory performance. If UV-light sterilization is used, the water must be filtered to about 1 ?m prior to sterilization since UV-light is readily absorbed by particles in the water reducing the efficiency of the unit. Filtration can easily be incorporated into a UV unit and many available units have both filters and the UV lamps combined.
Government regulations may exist in some areas that control the discharge of effluent from a hatchery. Before constructing a hatchery, government regulations controlling discharge of effluents should be reviewed and if they exist they must be followed.
Large floor drains sunk into the floors of wet areas are essential and should be located conveniently throughout the hatchery. Periodically large volumes of water must be discharged, e.g. when emptying tanks, and the drains must be able to handle such discharges.
Some hatcheries may wish to breed exotic species or strains or races of a species that do not occur locally. Depending on government regulations, this may entail installation of a quarantine facility to ensure that pests, parasites and diseases are not introduced with the exotic species or larvae accidentally escape into the natural environment. This will require a separate drainage system in the area of the hatchery designated for quarantine that empties into special holding tanks where the effluent can be sterilized with a strong hypochlorite solution. The sterilized water is then treated with thiosulphate to neutralize any residual chlorine before it is discharged back into the environment. Quarantine facilities may require a separate room to hold, condition and spawn adults. Drains from this room will also empty into the quarantine treatment tanks.