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PART 3. EQUIPMENT


3.1 TANKS

Tanks are containers used in a hatchery for fish rearing or live-food production. Fibreglass or concrete are the more common materials used for tank construction. Fibreglass is mostly used for small (0.5 to 30 m3) cylindrical or cylindro-conical tanks whereas concrete is frequently preferred for larger (over 20 m3) square or rectangular tanks, which are mainly found in the nursery, pre-growing or broodstock sections.

Rectangular and square tanks are frequently preferred because they maximise the utilization of space and, in some countries, because they are cheaper to build. In rectangular tanks, the bottom should have a gentle slope (1 to 2 percent) towards the outlet. In square tanks, this slope should be from the sides to the centre. Such tanks are normally built in concrete. Floors and inside walls are either plastered or painted with epoxy resin.

Circular tanks are better for water circulation and self-cleaning. But they are usually more expensive and the ratio between occupied floor space and available volume is less advantageous. Their bottom is frequently slightly conical or rounded with a central outlet. They are mostly made of fibreglass with smooth gel-coated inside surfaces.

Fig. 58 - Plastic tanks

In both rectangular and circular tanks, a white or light coloured bottom is preferable to facilitate routine controls and cleaning, since larvae and sediments are easier to observe against such backgrounds.

Apart from materials and costs criteria, which mainly depend on local availability and technology, the two most important points to consider for a good choice of production tanks are the following:

3.2 FILTERS

Filters are used to remove or to separate materials like suspended solids, ammonia, chemicals, etc., from liquids or gases. In marine hatcheries, three types of filters are used for treating seawater: mechanical, biological and chemical filters.

Mechanical filters

Fig. 59 - Mechanical filter

Mechanical filters are used to remove solids from water using a porous sept, a screen or a coarse layer of sand. To design mechanical filters, it is important to analyze:

Types of mechanical filters

Another classification, useful in understanding the way mechanical filters work, is that related to the energy needed for water to pass through the filter. There are two types:

(a) Gravity filters. When the only energy available to pass the filter is gravity, filters are usually open. They work with a reduced head and water frequently reaches the filter through an open channel. Such filters are frequently used when a large amount of water with an important quantity of suspended solids has to be filtered (with particles up to 20 mm).

This type of filter has typically a large area and a minimum head loss. It carries out an absolute filtration only and it is generally equipped with an automatic cleaning system starting automatically or manually, which is used to avoid filter clogging by solids.

Fig. 60 - Drum filter and disc filter (Hydrotec catalogues)

Examples of such gravity filters are: screens or grids, drum filters, wheel filters, and disc filters.

These last three types of filter are used in the hatchery for:

- Filtering effluents at the outlet to reduce environmental impacts;
- In recirculation systems, to reduce organic matter content.

(b) Pressure filters. When a pump or a reservoir with a considerable head is placed before a filter, then the filter works under pressure. Such filters are totally enclosed in order to maintain water pressure. Head loss may be higher than with gravity filters (3 to 15 m at maximum clogging). They are used to treat small to medium quantities of water with medium loads of suspended solids. The size of the filter mesh varies (1 mm or more). For medium water quantities, such filters are frequently equipped with backwashing, whereas for small water quantities (the case of cartridge or small bag filters) they are manually cleaned. Filtration can be either absolute (normally for special cartridge filters or bag filters) or relative for all types of filters. In the case of absolute filtration, 100% of the particle equal or larger than the filter size will be stopped. In relative filtration, only a percentage of particles will be retained (normally more than 95%). All the rest will pass through the filter.

Fig. 61 - Pressurized sand filter

Examples of pressure filters are: cartridge, sand and bag filters.

Pressure filters are frequently used for seawater filtration at the inlet. They are normally installed in series in order to avoid fast clogging of the finest ones. In fact, when water for rotifers is to be filtered at 5 mm, one should not install a single mechanical filter of this filtration size directly on the pipeline as it would clog too easily. Instead, a series of three filters of 100, 50 and 10 mm respectively, should be installed upstream of the 5 mm filter. The system will work better and the total need for filtering elements will be reduced. As price of the equipment decreases as filtration size increases, costs will also be reduced.

Fig. 62 - Bag filter

Biological filters

In intensive culture systems the fish biomass per unit flow rate is high. The main limiting factors are dissolved oxygen and ammonia content, the latter being the most dangerous part of the metabolic wastes produced by fish. But water in theses systems contains also suspended solids (uneaten food, faeces), dissolved solids and other organic compounds.

Biological filters or diluters allow for the partial re-use of heated water because of their capacity to transform ammonia into less toxic nitrites and nitrates. Partial water recirculation presents several advantages, such as a reduced consumption of external water per unit of biomass stocked, reduced heating or cooling costs and a reduced impact on the environment.

The main differences between biofilters and diluters are in the quantity of new water introduced in the system in each cycle and/or the quantity of ammonia produced by the system. In the case of large stocked biomass and large productions, it is preferable to adopt biofilters while in the case of small biomass (i.e. larval rearing until day 35) it is preferable to dilute water and ammonia, instead of trying to start a biofilter that will never work properly.

Fig. 63 - Old fashioned biofilter

The biological filter is, within a re-circulation system, the more complex component, to the extent that it can be considered almost a living organism. As such it requires stable physical and chemical parameters, a permanent supply of food (ammonia) and adequate levels of oxygen.

But water recirculation tends to accumulate metabolic wastes and bacteria to an extent that can easily become dangerous. Recirculated water should be reconditioned by constant removal of metabolic wastes and bacteria from the system.

A biofilter usually consists of a coarse particle substratum, which is submersed in a separate container and is colonized by nitrifying bacteria on its relatively large surface. These microorganisms convert highly toxic ammonia to less toxic nitrites and nitrates.

The efficiency of bacterial nitrification is related to:

Fig 64 - Different kind of substrate: bio-rings and bio-sphere

The efficiency of a biological filter, defined as the ratio between the organic load produced by cultured fish and the organic load removed by the filter, is heavily influenced by values, or sudden variations in values of: temperature, salinity, pH, oxygen, alkalinity, hydraulic flow, light intensity and by the concentration of ammonia and nitrites. Moreover, it is extremely important to feed the filter in a regular way avoiding peaks of ammonia and to manage therefore the entire system in relation to the filter performance.

In essence, for any type of filter used, the functioning of a biological filter is based on creating favourable conditions for the development of autotrophic and aerobic bacterial colonies that could set a nitrifying process. This means to oxydize the ammonia to nitrites (Nitrosomonas sp., equation 1) and then to transform also by oxydation the nitrites into nitrates (Nitrobacter sp., equation 2).

NH4- + 1.5 O2 ® NO2- + 2H- + H2O (equation 1)

NO2- + 0.5 O2 ® NO3- (equation 2)

As nitrites are unstable and toxic for fish their concentration should not exceed 0.5mg/l. The haemoglobin in contact with nitrites forms a compound called metahaemoglobin.

The nitrates, less toxic than ammonia and nitrites can be accumulated in the hydraulic system to concentrations two orders of magnitude higher (it should be also possible to reach concentrations of 100 mg/l). Then, they can be eliminated through a dilution process, or else through a denitrification process.[1]

Fig. 65 - Pressure biofilter and plate heat exchanger

The growth of nitrifying bacteria colonies depends, in addition to optimal environmental conditions, on the availability of surface on which to grow. This surface is defined as specific surface area, and it is usually indicated as m2 (of surface) on m3 (of volume of filtering material) and it is one of the fundamental parameters in the evaluation of the quality of a biological filter. The total surface of a filtering bed is also determined by the need to leave sufficient space between the particles of the substrate for an adequate water circulation and to reduce the risk of clogging. This void ratio in the substrate is generally related to the specific surface area desired. In practice an efficient filter has a void ratio of about 90%.

However, it should be borne in mind that in terms of surface the objective of the manager is to devote as much surface as possible to the rearing tanks. With this in mind the size of biological filters has been progressively reduced creating compact structures requiring less space.

The selection of substrates on which bacteria could form colonies has led to a reduction of the size of biological filters. From the initial substrates formed by gravel or shells, used in submerged or trickling filters, now inert fibre cushions, similar to those used in the filters or air conditioners, or small moulded pieces of plastic for packing purposes or else blocks of undulated PVC soldered sheets ("structured packings") are utilized. The continuous search for ideal substrates is oriented towards materials offering the highest surface/volume ratio, limited weight, strong mechanical resistance and limited clogging characteristics in addition to being cheap and easy to maintain.

Several typologies of biological filters exist. The ones more commonly utilized in aquaculture are:

(a) Submerged filters

Submerged filters are so-called because the filtering substrate is constantly under water (Fig. 63). They are usually crossed by a vertical flow of water ("upflow" or "downflow" filters) or, more rarely by a horizontal flow. The main advantages of these filters consist in the possibility of modifying the duration of the contact between water and bacteria colonizing the substrate, simply by changing the speed of the water flow. A limit in this case is the fact that all the oxygen required by nitrifying bacteria is provided through the water. Too slow flows would lead to insufficient oxygen supply, limiting the nitrification process. Too slow flow could also favour clogging and the appearance of preferential directions inside the filter. In this case, this creates areas in the filter with a too fast water flow and also other areas where organic sediments accumulate and decompose in anaerobic conditions.

(b) Trickling filters

In a trickling filter, the water volume that crosses it is a fraction of the volume of the filter. The water is distributed on the substrate through water jets so that it can cover all the surface of the filter and thus favour the creation of a homogeneous layer of bacterial film. The bacterial film, which is not constantly submerged, is exposed to the air. In this type of filter, water is well oxygenated, thus favouring the process of oxidation of ammonia and nitrites, the removal of undesirable gases (CO2, H2S, N2), and preventing risks of filter clogging.

Fig. 66 - Trickling filter

(c) Rotating biological contactors

The biodrum or biodiscs use the same principle of the trickling filters. The difference is that during the rotation of the filter, the bacterial film fixed on the drum or discs is submerged in water for half of the time of the rotation and also exposed to the air for the other half. The speed of rotation of the drum or discs is selected in such a way as to avoid oxygen depletion on the bacterial film while it is under water, and at the same time preventing excessive dehydration while it is exposed to the air. In these two types of filter (biodrum and biodisc) the main drawback is that the time of contact between water and bacterial film can be increased only by increasing the surface of the filter and thus its size.

(d) Pressure filters

These filters can be considered a special form of submerged filters (Fig. 65). The filter is made with a closed container, which is crossed from bottom to top by a flow of water under pressure. Inside the container you find packs of plastic on which the bacterial film grows. These plastic pieces are maintained in constant suspension by the water flow. In this way the bacterial film receives sufficient oxygen and the risk of filter clogging due to sediments in the water is minimized. A particular type of these filters is the fluidized bed, in which the filtering substrate is made with very small particles (e.g. sand) with a very large surface/volume ratio. The bed is maintained in constant suspension by the water flow which, however, has to be controlled to avoid the risk of draining the substrate that forms the fluidized bed. Also in this case the risk of clogging is minimal.

As for the submerged filter, the oxygen supply in pressure filters is provided through the water that passes through the filter. The water flow is usually enough to grant optimal conditions if the water that enters the filter has an oxygen level close to or above saturation.

Pressure filters are often used in combination with more traditional biological filters, such as trickling filters, as their main function is to reduce suspended solids given their efficiency in sedimenting organic matter ("bioflocs") in suspension.

It should also be borne in mind that clogging problems of biofilters are in large part resolved by adding mechanical filters upstream of the biological filter. An efficient reduction of particulated organic matter contributes to the reduction of heterotrophic bacteria which, having a faster growth rate than the nitrifying bacteria, could compete successfully both for oxygen and substrate.

How to calculate a biological filter

With submerged substrate and a specific inoculum of Nitrosomonas and Nitrobacter

- Standard reduction

J. Petit has established the following formula:

with

NH4+ in g/m3



K in g/m3/h



U in m/h



H in m

- is the reduction without limiting factors

- 1.08 represents an average growth rate for bacteria. Theoretically the growth of Nitrosomonas is slower and sets the speed of the whole system.

This formula is valid when. NH4+ ³ 0.5 g/m3. In fact this ammonia concentration would not represent a limiting factor

- Definition of K

with which depends on the strain and

e x a which depends on the type of substrate

- 1.28

Correction factor to obtain the results in mg/l

- Y(%)

Cell performance g MVS/gN (bacteria rate/g of eliminated nitrogen) (according to the various authors, values range between 6 and 17%)

- B (g/m2)

bacteria per m2 (average value: 0.6)

- m1 (mg/mg/h)

growth rate at 10°C (mg of bacteria appearing/mg of bacteria/h) (average value: 11.10-3)

- e (%)

coefficient of emptiness of material (from 0.4 to 0.9)

- a(m2/m3)

usable surface (from 100 to 500) a can be different from the data provided by the supplier as a function of the effective colonization of the surface)

The values found in the literature are based on studies on farms or pilot plants and provide the following K values:

- expanded clay:

10-20 in freshwater


(a = 360)

3-9 in seawater

- plastic (type bio-balls):

14 a 28


(a = 225)


- Corrections

The Petit formula was derived for trout farms (fresh and cold water). Several parameters have an influence on the calculation of the filter:

(a) Impact of the NH4+ concentration at the input

Real reduction = NH4+ x FN

with

N = concentration at the input


Y = temperature °C

The concentration of dissolved ammonia has an influence on bacterial growth. When this concentration is low, the bacterial growth will also be low and the correction factor will also lower the effective reduction. On the contrary if the concentration is high, the correction factor will have a minimal impact.

(b) Impact of the oxygen levels

DO is the oxygen concentration in the center of the filter
It has to be borne in mind that for the nitrification reaction 4.3mg O2 per 1 mg of N-NH4+ are required. is the oxygen saturation coefficient that corresponds to a reaction speed equal to half of the maximum speed of bacterial growth. According to various authors this values ranges from 0.3 to 0.5.

Therefore,

The Petit general formula applies with non limiting oxygen levels. With the correction factor, it would be possible to utilize the formula when the oxygen levels are not at saturation. However, it is important to bear in mind that nitrification:

(c) pH impact

The pH variation has some influence on the kinetics of nitrification. In the literature several pH levels are reported. The optimal pH is not the same for Nitrosomonas and Nitrobacter. The optimal values range from 7.5 to 8.3. Nitrification also reduces alkalinity in the water and this process of acidification is mainly due to:

Until water has sufficient buffer capacity pH remains relatively stable. Otherwise the H+ ions do accumulate according to the nitrification equation:

NH4+ + 2O2 ® NO3- + H2O +2H+

The ion H+ is neutralized by bicarbonates present in the water according to the following equation:

H+ + HCO3- ® CO2 +H2O

This decrease in the level of bicarbonate ions which are transformed in carbon dioxide results in a reduction of the pH level.

To neutralize 1 mg of NH3-N a total of 7.13 mg of HCO3- are required

Real reduction = NH4+ x FpH

With FpH = 1 - 0.83 (7.2 - pH)

Fig 67 - Bacteria on filaments

(d) Salinity impact

Salinity can inhibit nitrification. In fact, in seawater the quantity of nitrites and nitrates produced is higher than in freshwater and their oxidation is slower. Moreover, oxygen saturation also decreases when salinity increases (with similar temperature and pressure). The literature indicates a difference of 15% between freshwater and seawater Fs = 1 - 0.15 = 0.85

(e) Final reduction

Chemical filters

Chemical filters are seldom used in aquaculture, except for scientific work or test. These filters require reagents and are used with small quantities of water. However, and with increased frequency, due to the expansion of closed circuits, chemical or biological additives are used to correct water parameters, mainly pH. These additives (in powder or liquid form) area added to the circuits with the help of industrial dosing dispensers, driven by a probe.

3.3 SETTLEMENT TANKS AND OTHER SETTLEMENT DEVICES

Another way to separate suspended solids from water consists in the physical separation of the solids on the basis of the difference in specific weight. Therefore, with this method, it is very difficult to separate solids with a specific weight similar to that of water and separation is impossible for floating solids.

Settlement tanks

The simplest settlement tank is just a large reservoir through which the outlet channel flows. As the water enters the reservoir its speed is drastically reduced and the solid’s energy decreases. Every kind of solid (faeces, wasted pellets, etc.) has its own sedimentation speed; the higher the speed, the more effective is the separation of the solid from effluent water.

Settlement devices use the same basic principle but they increase the efficiency by adding various kinds of obstacles in order to decrease the solid’s energy as quickly as possible.

Settlement is actually used for two main purposes:

- separation of large sediments such as sand before the main pumping station, and
- settlement of organic wastes at marine hatchery outlets when simple engineering is all that is needed and a large outdoor area is available.

Fig. 68 - Settlement pond

When a decision is taken to use sedimentation to separate solids, the sedimentation process takes place usually in specific ponds in which the water circulation must be slow and possibly laminar, avoiding the creation of turbulences. In the sedimentation tank, four different areas can be recognized;

1) The area when the effluents enter, usually with high turbulence.
2) The sedimentation tank, with slow and laminar flow, where particles sediment.
3) The area where mud is deposited.
4) The outflow area, which is a transition area between the sedimentation tank and the outlet where the flow speed and turbulence increase again.

The general formula for the sedimentation process for particles in a static liquid, with F as the acceleration force determining the sedimentation speed, is:

F1= (rp rf) gV

where: F1 = acceleration force

rp = particle density
rf = liquid density
g = gravity acceleration
V = particle volume

The factors involved are therefore; the difference in density between particle and liquid, the force of gravity and the volume of the particle. In a laminar horizontal flow, the sedimentation speed of a particle is found by adding the vectors representing the vertical sedimentation speed (vs) and the velocity of the horizontal flow (vh).

Fig. 69 - Concrete settlement tank

The F1 acceleration becomes small when the volume of the particle decreases and when the difference between its density and that of the fluid in which it moves is small. The permanence in water of organic particles favours also their hydration, thus reducing the difference rp - rf. In these conditions, the sedimentation efficiency can be increased by reducing the velocity of the horizontal flow, which for a given quantity of water to be treated, will be achieved only by increasing the sedimentation surface. This is the main limitation for the utilization of sedimentation basins in circuits which have to treat large quantities of water.

Cyclonic and laminar sedimentation chambers

Different models of sedimentation chambers have been designed in order to reduce the sedimentation time and therefore the size of the sedimentation basins. The cylindro-conical decanters were designed to make better use of the force of gravity, by reducing to zero the horizontal flow. Water enters from the bottom and exits by overflow. The suspended particles sediment vertically, in the opposite direction to the water flow. Obviously, to maintain a low speed of the mounting water and to limit their size, cylindro-conical decanters can be utilized only for situations in which modest flows have to be treated.

The laminar sedimentation tanks base their better efficiency on the presence of obstacles (baffles) inside the sedimentation tanks, which are placed to absorb part of the energy of the moving solids allowing a faster sedimentation. The laminar sedimentation tanks require, however, complex cleaning procedures which are not always justified by the limited reduction in the size of the sedimentation tank.

Another example is the cyclonic sedimentation chamber (Hydroclone). These are circular tanks with conical bottom that utilize the centrifugal force to separate the sediment from the fluid. The rotation of the entire volume of water to be treated is induced by the tangential entrance of the water in the tank. The spiral flow in the tank forms a depression in the water, like a cylinder full of air, at about two thirds of the diameter of the tank. Clean water goes upwards in the inner part of the spiral and drains in the upper part of the tank. The solids which sediment against the wall and part of the water leave the tank through the bottom. The efficiency of these sedimentation tanks improves when the diameter increases. They are, however, seldom used due to their high cost, the need to operate a pump continuously to maintain the circular water flow, their limited flexibility and their size, which is large when large volumes of water have to be treated.

The use of sedimentation tanks in aquaculture is limited by the large areas required for the treatment of large amounts of water. Other disadvantages are the variability in their efficiency and the need to stop the operation periodically to clean the tanks. Where possible sedimentation has been replaced by mechanical filters.

3.4 WATER STERILIZERS

The most common treatments for water sterilization are UV and ozone. Treating the water with chemicals such as formaldehide or hypochlorite is normally avoided in close circuits to avoid damaging fish, algae or rotifers.

Blowing air or pure oxygen between two electrodes with a high voltage current produces ozone. The spark generates ozone, which is an allotropic form of oxygen. This gas must be produced locally and used immediately since it is highly unstable and reverts to normal oxygen molecules quickly. Ozone is a powerful oxidant, whose efficiency depends on the dosage and on the time in which it is in contact with the substances or organisms it has to oxidise. Mixed with water, it oxidizes organic matter and interacts with bacteria and viruses. Ozone also works as an oxidant on compounds and inorganic elements such as iron or manganese, and generates insoluble oxides.

Its use as a sterilizer has to be carefully evaluated because of the high toxicity for workers. Its residues, even in low concentration, can be dangerous for farmed fish. The presence of ozone, even in small quantities in the rearing tanks must be avoided. Although theoretically very interesting, ozone treatment is expensive because sophisticated equipment is required to measure the residual levels in out flowing water. As it is dangerous for the workers due to its oxidative characteristics, UV treatment for sterilization of water in the hatcheries is much more common. This section will deal only with UV treatment.

UV lamps

One of the most effective ways to drastically reduce bacterial growth inside a semi-closed system or to eliminate pathogens from raw seawater is to use UV sterilizers.

UV light can be divided according to its wavelength in three types:

extreme UV

100 - 190 nm

far UV

190 - 300 nm

near UV

300 - 380 nm

UV is high energy radiation and it affects marine micro-organisms starting from 190nm up to 380nm. The most important effect on bacteria or viruses consists in the modification of nucleic acids of those micro-organisms thanks to the absorption of energy by the nucleotides and the consequent modification. Very effective DNA damage produced by the UV radiation is the production of thymine dimers occurring by linking two adjacent thymine molecules. This stops DNA replication and therefore the reproduction of the micro-organism.

Sometimes micro-organisms are able to repair some of the damage caused by UV. This capacity is called reactivation and can happen in a dark environment and/or in presence of light. This is why the choice of UV is critical for the engineers who have to choose the proper lamp with the correct energy.

UV equipment consists of a pipe or a cylindrical chamber containing one or more quartz tubes (permeable to UV), producing ultra-violet radiation. Water flowing through the pipe/chamber is exposed to UV-C radiation produced by special lamps. The most effective UV radiations are UV-C and UV-B with a wavelength range of 200 to 300nm. The highest bactericidal efficiency is obtained at 240 to 275nm.

UV-sterilizer chambers are made of different materials such as specific plastic or stainless steel. The most convenient solution in terms of maintenance and cost is the use of plastic chambers, which are usually cheaper than stainless steel, but as mechanical performance and safety are far from those of stainless steel, many hatchery technicians still prefer the latter.

UV chambers should be equipped with: a UV meter to indicate UV radiation intensity (in percentage), a UV alarm for low intensities, a water sensor to indicate the presence of water and to protect the lamps from overheating, as well as a counter to record time when the UV-lamp is being used, as the lamps have to be changed periodically.

Fig. 70 - UV lamp

In order to damage the DNA of microorganisms present in the water through the action of high-energy UV-C radiation, two types of UV lamps are used; low/medium and high-intensity lamps. The former contains mercury vapours at low pressure (max. 3 millibars) while in the latter the pressure of the mercury vapours reaches 1 to 3 bars.

Which type of UV lamps to choose

Low-intensity lamps range from 20 to 80 watts and the major output wavelength is 253.7nm, giving maximum efficiency at 15°C. High-intensity lamps range from 1.5 to 4 kW. Their output spectrum is broader but their UV-C efficiency is lower. In terms of electrical consumption, low-intensity lamps are much more cost effective than high-intensity lamps, but there are certain advantages in choosing equipment with high-intensity lamps:

The best solution seems to be low/medium-intensity equipment for small flow systems (with less than 30 to 100 m3/h) or for intermittent use. On the contrary, medium high-intensity equipment should be preferred for large water flows or for dirty water.

Since quite often the water circulating in the system is not perfectly clean, (semi-closed system, reduced filtration of suspended solids, presence of colloids, etc.), it would be very useful to have inside the sterilizer some device to clean the external walls of the quartz lamps when they are switched on. This would avoid wasting time and interrupting the treatment and would increase the effectiveness of the lamps. However, manual cleaning often remains the most effective and common solution.

Radiation intensity or dosage is expressed in millijoule/cm2; one millijoule (mJ) corresponding to one milliwatt (mW) per second. If lamp output is 10 mW/cm2 and residence time of water inside the sterilization chamber is three seconds, total UV-C dosage applied equals 30 mJ/cm2.

The dosage of UV-C radiation needed to kill at least 90 percent (log 1) of the population present in water, is well known for many organisms. This dosage is called the D10 for the organism considered because its survival will be at the most 10 percent. D10 (in mJ/cm2) values for various organisms are listed in Annex 10.

For example, the D10 of Escherichia coli (wild isolate) ranges from 3 to 10 mJ/cm2. To find the theoretical energy necessary to kill 99.9 percent (log 4) of its population, multiply the D10 (example 6) by the log requested (6 x 4 = 24 mJ/cm2). As a general rule, an output of 40 mJ/cm2, at the end of the UV lamp life span, is considered safe.

The purpose of using a UV sterilizer is not always to exterminate all bacteria present in the water, as the energy required to achieve this would be excessive. In fact, UV equipment is used mostly in closed recirculation systems to maintain bacterial populations below dangerous levels.

In addition to the power of a UV lamp, it is also necessary to know the useful life span (usually 2 500-10 000 hours). During this period the UV dosage of the lamp will progressively decrease until it reaches a value close to 50-60% of the original dosage which is considered the end of its life span. Usually the manufacturers indicate this as a value in milliJoule/cm2 at the end of the lamp life span.

It must be remembered that the dosage is defined as intensity by the time of irradiation:

D = E x t

where:

D = dosage (milliJoule/cm2)


E = radiation intensity (milliWatt/cm2)


t = radiation time

The efficiency of the sterilization using UV radiation is strongly conditioned by the way in which this radiation is transmitted in the water (transmittance). The transmittance can be drastically reduced by the presence of suspended solids.

Considering as 1 the maximum value of transmittance (meaning that the dosage of the lamp passes through the liquid and reaches the opposite side of the radiation chamber unaltered), in normal conditions in recirculation water in a hatchery this index decreases to 0.75-0.80. For this reason all UV equipment should have a filtration system upstream to reduce suspended solids.

The efficacy of the radiation is also closely related to the thickness of the water mass that has to the radiated. Sterilizers are, in fact, built with a radiation chamber that contain the lamps inside one or more quartz tubes. In this way, water circulates around the lamp with a predetermined thickness. The number of lamps and tubes determines the volume of water than can be treated by the sterilizer.

The UV dosage is also influenced by other factors such as the variation of the water flow inside the radiation chamber or the temperature of the water to be treated.

Selection of UV sterilizers

For proper selection of UV sterilizers, the following parameters should be given to the UV supplier:

The size of each sterilizer should be calculated separately.

3.5 OXYGENATORS AND AERATORS

Increasing dissolved oxygen content of water

Fish metabolism is based on respiration; a physiological process in which the energy required by the organism is produced through oxidation of organic matter. Fish obtain the oxygen required for this process from the water.

Thus the oxygen dissolved in the rearing tank water is constantly being used by fish. As modern hatchery practices are characterised by high fish densities in confined water volumes, oxygen content in water has to be kept under close control and as far as possible be regulated. Since water temperature in a hatchery is usually fairly stable, oxygen regulation is needed due to the demand created by fish metabolism.

Fig. 71 - Pump set

Oxygen consumption in fish is related to many factors, such as the species, the body size, activity (rest, forced swimming, feeding), temperature, feeding and water quality. In general, oxygen consumption for a given fish species reaches a peak during feeding or full activity (swimming), and when dissolved oxygen is high and when temperature increases. When size is small (larvae, fry) oxygen consumption is higher in relation to the biomass. For example, a 4 g seabass has an oxygen consumption of 863 mg O2/h/kg during feeding.

In general, there are three ways to match the increasing oxygen needs of fish:

a) by renewing water more often and thus introducing more dissolved oxygen. To a certain extent, increasing water renewal is the most common method of adding oxygen to the rearing tanks. The growth of larvae and fry is associated with a regular increase of water exchange in the rearing tanks. This increase not only provides more oxygen to the fish, but it also helps to eliminate feed residues and metabolic byproducts. The limiting factor remains the cost of supplying a large quantity of water (intake facilities, pumps, pumping station, electricity generator set, distribution facilities, etc.) compared with the cost of introducing oxygen by other means.

b) by adding atmospheric oxygen to the water (aeration). Before the introduction of liquid oxygen this was the most common method of adding oxygen to water. But the oxygen content of atmospheric air is low (21 percent) and a relatively high volume of air is needed to add a small quantity of oxygen to the water. There is also a limit to the quantity of oxygen that can be dissolved in seawater. Temperature and atmospheric pressure determine the saturation threshold. On the basis of normal temperature and salinity ranges in hatcheries, and at atmospheric pressure, the quantity of additional oxygen that can be added by aeration is limited and cannot go above saturation values.

c) by adding pure oxygen to the water (oxygenation).This method brings rearing water into contact with pure gaseous oxygen. Since the oxygen content is higher in the gaseous phase than in water, there is a tendency for oxygen to dissolve following this concentration gradient. If the entire process takes place under a pressure level higher than atmospheric pressure and if pure oxygen is used, the result is an even higher and quicker dissolution of oxygen, into the rearing water. If instead of pure oxygen, air under pressure was used, we would have a problem due to the supersaturation of nitrogen, which would become dangerous. This is why only pure oxygen is used to oxygenate water under pressure.

These three ways can normally be found in marine hatcheries, either separately or in combination.

Fig. 72 - Air injector


Fig. 73 - Liquid oxygen reservoir and oxygen bottles

Improving oxygen transfer into water

Gas transfer into water is regulated by the difference between the partial gas pressures in the atmosphere and in the water. If a pressure gradient is present, the gas will follow this gradient.Therefore if the partial pressure of oxygen in the air is higher than that in the water, oxygen will dissolve into the latter. Since this process tends to reach an equilibrium, it is of great importance to keep this difference of pressure positive, to transfer oxygen continuously to the water. To achieve this, both the liquid phase (water flow) and the gaseous phase (gas flow) should be renewed continuously. Therefore it is necessary to keep a continuous flow of water in the proximity of the source of oxygen.

Fig. 74 - Oxygen generator

Apart from water temperature and salinity that we will consider constant, three main factors control oxygen dissolution into water: pressure, exchange surface, and contact time.

Air and oxygen diffusers

Diffusers are devices to mix air or pure oxygen, as bubbles of various sizes, with the rearing water. Diffusers can be made with simple perforated PVC pipes, with a series of holes of suitable diameter (typically pipes of 3/4, 1 and 1 1/4 inches diameter and holes of 2 to 5 mm diameter), or with any porous material having coarse to very fine holes (porous stones, porous rubber hoses, porous steel tubes, porous soft wood).

The porous material or the perforated pipe is connected to a source of compressed air or pure oxygen, and they are placed in the tank to be aerated/oxygenated. Since, as indicated above, the transfer of a gas into water is also a function of the exchange surface and contact time, the more efficient diffusers will be those producing the finest bubbles rising slowly into the water.



Fig. 75 - Air diffusers

Porous stones connected to a compressed air line are usually placed into various culture tanks for live-food production, egg incubation, larval rearing and fry weaning. The bubbles they produce help in maintaining the still passive yolk-sac larvae and the first-feeding larvae afloat, and in homogenizing the rearing medium (rotifers, microalgae). When an additional quantity of oxygen is required (for example, when there is a temporary failure of the water supply system or when there is a temporary high density in the rearing tanks), one or more porous stones can be placed in the rearing tanks, connected to a separate compressed oxygen line. The porous material of the diffusers should be periodically cleaned, since small particles (algae, food residues, faeces) in the rearing water can easily clog the pores and reduce (or even block) the air/oxygen flow.

Injection of pure oxygen using a submersible pump

To dissolve pure oxygen in rearing water, an easy and quickly assembled device would be an oxygen supply pipe placed under the intake of a submersible pump.

The pump propeller generates and mixes a great number of very fine gas bubbles and the higher pressure created during the suction allows for a quick and abundant dissolution of oxygen in the water. When these systems are used, the pump should be made of plastic material, since oxygen can cause corrosion in the propeller chamber of cast-iron pumps. Cavitation should not be a problem when using this device, since the oxygen micro-bubbles in the propeller chamber do not implode, as air bubbles would do. This system has been tested to have an efficiency of 60 to 80 percent for oxygen transfer.

This device can be easily assembled in situations in which a pump is continuously working to supply rearing water (for example, in recirculation or flow-through systems) or in the compensation line going to the biofilter. Advantages of this system are that the whole mass of flowing water is enriched with oxygen, that no modification of the water distribution system is needed (at least if the inlet pipe in the tank is lower than the water surface) and that the response time is very short (only a few minutes are required to oversaturate 2 to 4 m3 of water).

Injection of oxygen into a pipeline

As a simplification of the system described above, oxygen injection can take place directly into the water distribution pipeline, after any priming device, such as a pump or header tank.

However, this system creates relatively coarse bubbles in the water pipes. These bubbles, even when using a porous diffuser, tend to be unstable and to become larger, thus reducing exchange surface and contact time. Complete dissolution of oxygen is also hampered if the injection point is too close to the water inlet of a tank, since the gas will not have enough time to dissolve.

This system can be used with pipelines distributing water by gravity (taking advantage of the pressure provided by an elevated distribution reservoir), as well as with a pressurized water distribution line. Since each tank can be supplied with oxygen individually, by having different injection points, this system increases the flexibility of the oxygenation system. For further refinement of the system, oxygen injection for the individual tanks can be automated using a solenoid valve at injection point.

Pressurized mixers

Fig. 76 - Mixing chambers

In this system a mixing barrel, with the shape of a cylinder or a double cone, is connected to the water distribution line, after the priming pump. Part of the rearing water is pumped into the barrel and is mixed with pure oxygen. The gas is injected just below a special perforated plate through which water must pass or is injected in the lower part of the bi-conical barrel. An almost complete dissolution of oxygen can be achieved with this equipment.

This device needs to be supplied with water under pressure (best from 1 to 2 bars) and it is connected to a compressed oxygen line. A thin oxygen-filled chamber is formed under a perforated plate. Since the diameter of the barrel is greater than that of the distribution line, the water entering the barrel loses velocity, stabilizing its gaseous phase. Water under pressure is forced through the perforated plate and through the oxygen chamber in the form of multiple jets. These jets suck the oxygen bubbles from the gas chamber into the water under it, forming a cloud of fine bubbles. As these bubbles have a rising velocity greater than the water velocity in the mixing barrel, they remain trapped inside it long enough to complete the oxygen transfer into the water. In salt water, a few very fine bubbles may leave the barrel with the outgoing water, but owing to their small size they become almost completely dissolved in the water distribution line (due to the long contact time and great exchange surface).

The efficiency of an aeration/oxygenation device is measured as the quantity of oxygen dissolved in relation to the quantity of oxygen utilized. Pressure mixers of this kind have an efficiency greater than 80 percent under the operating conditions encountered in a hatchery.

This type of device is used to hyper-oxygenate part of the rearing water or the make-up water of the bio-filters. Dissolved oxygen content can reach as much as four to five times the saturation level. This hyper-oxygenated water is then mixed with the rest of the rearing water, either in the main distribution line (centralized oxygenation) or at each tank inlet (individual oxygenation). The system can be automated by using a solenoid valve linked to a remote oxygen monitoring probe.

Estimating oxygen requirements in tanks

In the larval rearing section, dissolved oxygen content is always kept at saturation level, but this would not be sufficient in the weaning and pregrowing sections in which the higher biomass and the distribution of compounded feeds (crumbs or pellets) will increase the oxygen demand.

Fig. 77 - Bicone oxygenator

A widely-used formula can be used to calculate the hourly oxygen requirements (DOn) for each tank or for the entire system. It relates the oxygen required to metabolize one kilogram of feed (Iox) to the maximum fish biomass present and to its daily feeding rate as follows:

B x DFR = DTF

where:

B

is the maximum fish biomass to be present in tank/system, in kg

DFR

is the daily feeding rate, in percent

DTF

is the daily total feed quantity distributed, in kg

and therefore

DOn = (DTF x lox) ÷ 24

where:

Iox

is the oxidation index equal to 0.4 kg O2 per kg feed

24

is the number of hours per day

DOn

is the hourly oxygen consumption for a certain fish biomass, in kg O2

Of course, this formula will give only the theoretical amount of oxygen consumption per hour as an average during the day and without considering the way it dissolves in water. The real oxygen needs in fact will have to cope also with the efficiency of dissolution and the metabolic phase (rest or feeding phase) of the fish population during the specific time.

Example

If we consider a daily consumption of 10 kg of feed and a system efficiency of around 40% the total consumption of gas (O2) will be:

10 x 0.4 = 4 Kg of dissolved oxygen

and thus the requirements on the base of the efficiency of dissolution would be 4 /0.4 = 10 Kg of oxygen consumed per day. As indicated above, this oxygen consumption will not be regular and constant during the day but will be maximal after each feeding period and minimal during the night.

3.6 OXYGEN MONITORING AND REGULATING SYSTEM

Control systems

As already mentioned, dissolved oxygen values can vary rapidly and dangerously in a hatchery. The high stocking densities found in hatcheries and the need to adopt sometimes close-circuit systems to better manage water quality, require the constant monitoring of a number of rearing parameters. Oxygen level is the most important.

In hatcheries using oxygen injection in tanks and pipeline systems, maintaining optimal oxygen levels by regulating oxygen supply is important to avoid unnecessarily high costs of operation.

Considerable progress has been made in the last twenty years in oxygen monitoring systems and in the automation of aeration and use of oxygen. This progress has allowed an improvement in the reliability of systems as well as in the economy of the operation.

Fig. 78 - Fry biomass

Measuring dissolved oxygen

Oxygen level measurements in farms and hatcheries is carried out using highly reliable instruments that work as potentiometers, measuring the difference of potential between two electrodes. This difference is affected by the quantity of dissolved oxygen present in the water.

The probe has usually two electrodes, a silver one (normally the anode) and the other made of platinum, rhodium or other valuable metal that normally works as a cathode. A difference of potential is created between the two electrodes, usually with a battery. The chamber in which the two electrodes are placed is filled with an electrolyte and is closed at one end by a Teflon or polypropylene membrane which is oxygen permeable. The oxygen dissolved in water passes through the membrane and reacts with the electrolyte, creating a current with a voltage in the order of 500-800 millivolts which is proportional to the oxygen concentration. This current is read by a micro-voltmeter and it is displayed directly as oxygen concentration in mg/l.

Fig. 79 - Oxygen probe

The more recent oxygen meters can also compensate automatically for measurement variations in the oxygen concentration due to temperature variations, and can be calibrated for readings at different salinities and altitude. The oxygen readings are normally indicated as mg/l and as a percentage of saturation levels. In recent years the manufacturers have evolved from oxygen meters for laboratory use to field probes which are protected against possible damage, are impermeable and have limited maintenance requirements.

Oxygen supply management

The placement of probes in critical points of the hatchery (individual tanks, collection points, etc.) allows a continuous control in real time of oxygen values. With such a configuration of the oxygen control system, it is possible to automatize oxygen management.

The probes can be connected to analogical management systems. After determining minimum and maximum values, thresholds which should not be trespassed and optimal values to be maintained, the system, can, on the basis of the readings of the probes, switch on and off aerators and oxygenators.

Fig. 80 - Oxygen monitoring system

The use of a digital system operating the monitoring systems as well as the various switches regulating oxygen supply, allows additional possibilities. The probes are read by a central controller and the readings are transferred to a PC which, in addition to storing this information, is programmed to manage the system on the basis of several variables. As an example, oxygen levels and supply can be correlated to temperature variations or to the stocking densities in the tanks.

The computer could manage at the same time programmes for automatic feeders which will be related to the oxygen values. Thus the system will be able to control dissolved oxygen values before, during and after feed distribution, and through the control of the oxygen supply solenoids, will be able to keep optimal rearing conditions.

In case of anomalous situations in the oxygen supply, the automatic management system, in addition to warning the operator with sound or light alarms, will be able to decide autonomously on suspending feed distribution.

3.7 WATER TEMPERATURE CONDITIONING

Hatchery water temperature normally does not follow the natural cycle outside the hatchery because temperature is one of the main important factors conditioning larval rearing periods. When most of the work concentrates in the winter period, temperature conditioning means heating the water in the tanks or in the hatchery as a whole. Generally the systems employed are adaptation of heating devices used for central heating or air heating. When individual tanks are heated electrical resistances and temperature sensors are employed to reach the desired temperature in the tanks. The following figures show examples of heat exchangers, heat pumps and resistances and sensors utilized for temperature increase and control in hatcheries.

Fig. 81 - Heat exchanger


Fig 82 - Heat pumps


Fig. 83 - Tanks equipped with titanium heaters and thermostats


Fig. 84 - Internal view of tanks in Fig. 83: the heater and the thermostat probe

3.8 AUXILIARY EQUIPMENT FOR FRY MANAGEMENT

For large and medium size hatcheries, in order to maintain properly large numbers of fry and at the same time to reduce the personnel requirements, some auxiliary equipment can be installed: for example feeding systems and fish graders.

Fig. 85 - Air-driven feeding bins

Feeding equipment has developed rapidly during the last ten years, evolving from the small belt feeders to the more sophisticated automatic feeding systems that are PC driven.

The figures below show new and old feeding equipment used for larval rearing.

Another problem encountered mainly in the nursery stages in many hatcheries is the difference in growth performance between larvae and fry which, in the case of larvae of the marine species reared, leads to mortality due to cannibalism. It is the old adage; "big fish eat small fish", which is totally undesirable in a hatchery. In addition, large differences in size imply different consumption patterns and an additional competition for food in which the winners are the larger fish. The solution is to reduce losses due to cannibalism. It is best to grade the fish as often as possible to reduce competition for food wich, in turn, leads to greater size differences.

Fig. 86 - Traditional belt feeder

Procedures for grading have been explained in the first volume and figures of traditional and manual fry graders have been provided. Below the reader can find figures showing automatic grading and sorting machines which facilitate considerably the work of the hatchery operators while at the same time minimizing handling of fry. This is always undesirable when dealing with large quantities of fry. There is also an illustration of a manual grader.

Fig. 87 - Fry grader under working conditions


Fig. 88 - Manual grading for very small fry



[1] Environmental and public health considerations have led to a limit of maximum permissible nitrate values (even if they are relatively less toxic) for effluents at 11.6mg/l. (EU directive).

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