L. Váradi
Fish Culture Research Institute
Szarvas, Hungary
1. INTRODUCTION
2. WATER SUPPLY
3. THE FUNDAMENTALS OF THE DESIGN OF FLOW-THROUGH SYSTEMS
REFERENCES
In development of fish culture an increasing importance is given to the full utilization of water resources. The first step was to increase the natural yield of existing waters, followed, as a second step, by the construction and use of ponds to fulfill the requirements of industrial-type fish culture. Then came the real breakthrough, with the construction of so called flow-through systems, where not the size of the water area but the quantity of water flowing through limited the yield.
The earlier principle then had to be revised, since the water area did not limit the amount of fish produced, but the volume of water supplied did.
Fish culture in flow-through systems is a type of intensive culture where the fish are stocked densely in a long and narrow pond or tank in which there is an abundant continuous water flow. The fish are stocked in these ponds or tanks on the basis of the volume of inflowing water. They are fed a formulated pelleted food and usually this is their only source of nutrition. A continuous water flow ensures the proper oxygen supply to the fish and flushes away the metabolic wastes.
The flow-through system is a typical and traditional facility for trout culture, but other species can also be cultivated in this type of system with good results.
2.1 Conventional Flow-Through Systems
2.2 Intensive Flow-Through Systems
Among all systems of fish culture, the flow-through system depends to the highest degree upon an abundant and continuous water supply.
In a conventional flow-through system the oxygen requirement of the fish is supplied by the inflow water. The water flow rate that is needed for proper oxygen supply of the fish usually is larger than is required for flushing the metabolic wastes. Thus, in a conventional flow-through system the water flow rate should be calculated on the basis of the oxygen requirement of the fish. Recently, intensive flow-through systems have been designed in order to increase the stocking density or to decrease the water flow. In these systems the oxygen requirement of the fish is met by oxygenation of the inflow water. When the water flow rate of an intensive flow-through system is calculated, the flow rate that is needed for the flushing of the metabolic wastes becomes the critical factor.
The specific flow rate (q)is one of the basic parameters of flow-through systems that can be expressed:1/
1/ In this formula and all subsequent formulas, the notation h-1 denotes per hour, kg-1 per kg, etc.
where
Q = actual water flow (m h)
W = actual mass of fish in the tank (kg)
In the conventional flow-through system the oxygen requirement of the fish stock is ensured by the inflow water as follows:
Q . (Cs - C) = W . r
where
Cs = dissolved oxygen concentration at saturation level (when the oxygen is dissolved from the atmosphere) (gm-3)C = allowable minimum dissolved oxygen concentration (gm-3)
r = specific oxygen consumption of the fish (gh-1kg-1)
Assuming the following basic data
- temperature of inflow water is 15°C
- inflow water is saturated with oxygen (C = 10 gm-3)
- specific oxygen consumption of the fish is 0.4 gh-1kg-1
the specific flow rate can be calculated as follows:
There is another important parameter that can be used for the design of flow-through systems, the specific volume (V) of water necessary for a 1 kg weight increase of fish. This can be expressed:
where
G = specific growth rate (kg kg-1 day-1)
G can be expressed by the known equation as follows:
where
Rp = Food protein consumption of 1 kg of fish per day (kg kg day)
PER = Protein Efficiency Ratio (kg fish flesh/kg food protein)
h = daily fish mortality rate (kg kg-1 day-1)
assuming
Rp =0.01 kg kg-1day-1
PER = 2kg kg-1
h = 0.01kg kg-1day-1
Then G = RpPER - h = 0.01 . 2 - 0.01 = 0.01 kg kg-1day-1
The specific volume of water (V) can now be calculated as follows:
If the temperature of the inflow water is not 15°C but 20°C (Cs = 9 gm), the specific values are as follows:
q (20) = 2.4 m3day-1kg-1
v (20) = 240 m3kg-1
The value of specific water consumption can be decreased by supplying pure oxygen to-the inflow water. When the pressure is 100 kPa and the water temperature is 15°C, 48 gm of pure oxygen can be dissolved in clean water 
. At a water temperature of 20°C, the 
If the minimum allowable oxygen concentration is 5 gm-3 ,1 m3 of water contains 38.6 g dissolved oxygen available for fish at 20°C which is almost tenfold that contained by water with oxygen dissolved from the atmosphere:
The specific flow rate (q) at two different temperatures is the following:
q (15) = 0.223 m3 day-1kg-1
q (20) = 0.248 m3 day-1kg-1
The specific volume of water (V) is the following:
v (15) = 22.3 m3kg-1
v (20) = 24.8 m3kg-1
These data, however, should always be checked as to whether toxic metabolites accumulate at these flow rates.
Our assumption is that fish can incorporate some 30 percent of the feed protein (PPV = 0.3),the rest, 85 percent of which is ammonia, is excreted. Since 16 percent of the protein is nitrogen, fish excrete 95.2 g ammonia for each kg of feed protein consumed.
The calculation of the amount of ammonia-nitrogen excreted after feeding a unit of food protein (a) is as follows:
a = (1 - PPV) 0.85 . 0.16 (kg kg-1)
where
PPV = Productive Protein Value (kg fish protein/kg feed protein)
a = (1 - 0.3) 0.85 . 0.16 = 0.0952
Ammonia, depending on the pH and temperature of water, is present in the water in two forms:
Figure
as ammonium ion (NH4+) and as 'free' or 'un-ionized' ammonia (NH3) that is toxic to the fish. In our calculation the allowable maximum value of un-ionized ammonia concentration is 0.045 gm-3.
Calculating the tolerable total ammonia-nitrogen concentration (C) at different pH and temperature values yields the following:
CN/20°C, pH 7.01/ = 10.71 gm-3
CN/l5°C, pH 7.5/ = 4.75 gm-3
CN/20°C, pH 7.5/ = 3.43 gm-3
CN/25°C, pH 7.5/ = 2.40 gm-3
CN = ammonia-nitrogen concentration (gm-3)(NH3 - N) + (NH4+ - N)
For the calculation of CN, Figure 1 and Table 1 should be used.
In intensive flow-through systems an adequate water flow is needed in order to flush the metabolic wastes, first of all the ammonia.
The amount of ammonia + ammonium ion that is excreted by a certain mass of fish (W) during a day can be expressed as follows:
1000 . W . a . Rp (g day-1)
where
W = mass of fish in a tank (kg)
a = amount of ammonia-nitrogen excreted after feeding a unit of food protein (kg kg-1)
Rp = fish feed protein consumed by a unit mass of fish in one day (kg kg-lday-1)
The amount of ammonia-nitrogen that can be flushed by the water flow (Q) in a day, with a given maximum tolerable ammonia-nitrogen concentration (CN) can be expressed as follows:
24 . Q . CN (g day-1)
where
Q = water flow (m3 hour-1)
CN = ammonia-nitrogen concentration (gm-3)
thus,
1 000 . W . a . Rp = 24 . Q . CN
Dividing both sides of the equation by W and using the formula:
q = Q . W , we get an equation as follows,
In our calculation:
Thus, the specific flow rate (g) at different pH and temperature can be calculated as follows:
q (15°C, pH 7.5) == 0.200 m3day-1kg-1
q (20°C, pH 7.5) == 0.278 m3day-1kg-1
The specific volume of water can be calculated as follows:
V (20°C, pH 7.5) == 27.8 m3kg-1
Figure 1. Diagram of ammonia equilibrium pH and temperature
Table 1 Percent NH in Aqueous Ammonia Solutions for 0-30°C and pH 6-10
Source: Emerson et al., Ammonia equilibrium pH and temperature, J. Fish. Res. Board, Can., Vol. 32/12/1975
The values of the specific flow rate and the specific volume of water at different pH and water temperatures are shown in Table 2. It can be seen in the table that at 15°C the specific flow rate needed for the proper oxygen supply is higher than that needed for the flushing of the metabolic wastes. At higher temperatures the ammonia removal by flushing becomes the decisive factor. It also turns out from the table that the water requirement of the system is two times higher at 20°C than at 15°C.
Since flow-through systems are dependent upon large quantities of water, the design of the pumping system is critical in their design. The required quantity of water can be determined according to the calculations given below.
In a raceway with an inflow at one end, the mean water velocity (v) can be expressed as follows:
where
b = tank breadth (m)
d = tank water depth (m)
L = tank length (m)
S = actual fish stocking density (kgm-3)
q = actual specific flow rate (m3kg-1h-1)
In this equation the 'q' can be calculated and it is desired to maintain a constant value.
The 'S' is increasing during the growing period from an initial stocking density up to a maximum one. Its value can be calculated based on the growth curve. The 'v' is desired to be kept in a certain range.
The water velocity should be kept below a certain value in order to avoid stress and energy waste of the fish and wash-away of the food particles. On the other hand a certain minimum water velocity should be exceeded in order to keep the waste materials in suspension.
Taking into account the above criteria the geometric dimensions of the raceway can be calculated, assuming constant water depth during the growing period. The water depth however might be varied and thus it is possible to maintain a more or less constant water velocity throughout the growing period.
The tanks of a flow-through system usually are rectangular reinforced concrete raceways, but large size outdoor raceways can be made of earth with plastic covered inner surface. Smaller size indoor raceways can be built out of concrete, plastic, metal or wood.
Table 2 Water Requirement of the Flow-Through Systems
The tanks should be equipped with properly designed water control structures and it is advisable to install a safety device as well that gives an alarm signal when the water flow (or the water level) decreases below a certain value.
Although the conventional flow-through system is based upon the oxygen supply from the inflow water, in extreme cases oxygen depletion can occur. Then aeration is needed. Therefore emergency aerators with high efficiency should be provided during the design of the farm equipment.
In flow-through systems, intensive feeding is based on complete pelleted food that should be given to the fish regularly and in proper doses. This can be ensured by different automatic feeders first of all, but self moving food dispensers or demand feeders can also be used.
Regular grading is an essential work in these intensive systems, because the divergence in the growth rate has a disadvantageous effect on the growth of the smaller fish. Special mechanized graders are available for this purpose that handle the fish gently; however, the frequency of the grading should be minimized.
Because of the special conditions of these systems (high stocking density, concrete tank, etc.) the harvest of the fish can be mechanized as well.
Different fish pumps have been developed for this purpose.
Emerson, K et al., 1975, Aqueous ammonia equilibrium calculations: effect of pH and temperature. J. Fish. Res. Board Can., 32(12):2379-83
Kepenyes, J. and A. Ruttkay, 1983, Water requirement of fish production. In International Conference on Water management and production potential in agriculture. Szarvas, Hungary, pp. 90-100
Kerr, N.M., 1981, Design of equipment and selection of material - an engineer's assessment. Schr. Bundesforschungsanst. Fisch. Hamb.. (16/l7) vol. 2:515-21
Kramer, Chin and Mayo Inc., 1972, A study for development of fish hatchery water treatment systems. Prepared for Walla Walla District Corps of Engineers.
