Chapter 21 Aeration and Oxygenation in Aquaculture

J. Kepenyes and L. Váradi
Fish Culture Research Institute
Szarvas, Hungary

1. OXYGEN BUDGET OF FISH PONDS
2. AERATION DEVICES FOR FISH PONDS
3. UTILIZATION OF PURE OXYGEN IN INTENSIVE FISH CULTURE SYSTEMS
4. BASIC PRINCIPLES OF THE DESIGN OF FISH POND AERATORS
5. DIMENSIONING OF AERATORS
6. REFERENCES

1. OXYGEN BUDGET OF FISH PONDS

1.1 Diurnal Changes of Dissolved Oxygen Content
1.2 Factors Influencing the Dissolved Oxygen Content of Pond Water

Dissolved oxygen content of the water of fish ponds is one of the most important parameters of water quality, as the oxygen is a vital condition for all the organisms living in the water and having an aerobic type of respiration. The solubility of oxygen is influenced by several factors (e.g. air pressure, hydrostatical pressure, salt content), but in pond fish farms it is generally enough to consider only water temperature.

Table 1 shows values of oxygen saturation of water as a function of water temperature. In pond fish farming the dissolved oxygen content of the water can be expressed as the absolute quantity of oxygen in one unit of water (mg/l; ppm) or as relative oxygen saturation (%).

1.1 Diurnal Changes of Dissolved Oxygen Content

Diurnal changes of the dissolved oxygen content of pond water during a period of time t¢ ¢ - t¢ can be expressed by the following equation:

O₂¢ ¢ - O₂¢ = P - R - Y ± A

where

P = quantity of oxygen produced during the photosynthetic process of the phytoplankton, the phytobenthos and the water plants,

R = quantity of oxygen consumed during the respiration of bacterial-, phyto- and zooplankton and organisms of animals and plants,

Y = quantity of oxygen fixed by the mud of the pond bottom,

A = quantity of oxygen dissolved from the atmosphere or in case of supersaturation released to the atmosphere.

Since the most significant positive value (P) of the above formula is dependent on light conditions (the photosynthesis can take place only by utilization of solar energy), the dissolved oxygen content of the fish pond varies over a period of 24 hours.

Figure 1 shows a typical daily change of oxygen concentration of a fish pond. The photosynthesis starts gradually from the early morning hours while the dissolved oxygen content of the water is increasing. The maximum value, in several cases much higher than the saturation level, can be observed early in the afternoon.

From the late afternoon hours photosynthesis is rapidly decreasing and is stopped after dark. The dissolved oxygen content of the water is also decreasing during this period, the minimum value occurring during the summer months.

1.2 Factors Influencing the Dissolved Oxygen Content of Pond Water

1.2.1 Production of macro- and microorganisms in the water
1.2.2 Oxygen consumption by pond water
1.2.3 Oxygen consumption by the pond bottom
1.2.4 Oxygen consumption by fish
1.2.5 Natural diffusion caused by wind action
1.2.6 Artificial control of the dissolved oxygen content of water

1.2.1 Production of macro- and microorganisms in the water

Those organisms which live in the water of a fish pond and have chlorophyll, produce carbohydrates from the carbon dioxide and the water in the process of photosynthesis by utilization of solar energy, while oxygen is produced as a by-product of the process. In deeper water ponds the biggest part of the oxygen produced comes from the phytoplankton, but in shallow ponds the macrovegetation, the algae and the benthic algae have a dominating role. Among the factors influencing production of fish ponds water temperature and illumination are the most important.

Table 1 The oxygen saturation concentrations in the water as a function of water temperature at 760 torr

t °C	mg O2/l	t °C	mg O2/l	t °C	mg O2/l
0	14.65
0.5	14.45	10.5	11.14	20.5	8.93
1	14.25	11	11.00	21	8.84
1.5	14.05	11.5	10.87	21.5	8.76
2	13.86	12	10.75	22	8.67
2.5	13.68	12.5	10.62	22.5	8.58
3	13.49	13	10.60	23	8.50
3-5	13.31	13.5	10.38	23.5	8.42
4	13.13	14	10.26	24	8.33
4.5	12.96	14.5	10.15	24.5	8.25
5	12.70	15	10.03	25	8.18
5-5	12.62	15.5	9.92	25.5	8.10
6	12.46	16	9.82	26	8.02
6.5	12.30	16.5	9.71	26.5	7.95
7	12.14	17	9.61	27	7.87
7.5	11.99	17.5	9.50	27.6	7.80
8	11.84	18	9.40	28	7.72
8.5	11.70	18.5	9.30	28.5	7.05
9	11.55	18	9.21	29	7.58
9.5	11.41	19.5	9.12	29.5	7.51
10	11.27	20	9.02	30	7.44

Figure 1. Diurnal change of the dissolved oxygen concentration in intensive fish pond

Figure 2. Relationship between the oxygen production and illumination in fertilized fish pond

The relationship between illumination and oxygen production can be seen in Figure 2. The intensity of light affects significantly the oxygen budget of pond water. If the intensity of solar radiation is high during a day without overcast, a quantity of dissolved oxygen is produced. This covers the oxygen demand of respiration during the night. In the cases when solar radiation is less intensive because of cloud cover, the oxygen production and thus the oxygen reserve for the night is less. During the night the respiration of organisms in the water does not decrease - or at least not in the same rate - and the oxygen reserve of the pond water can be exhausted. Thus the concentration of dissolved oxygen can decrease below the lethal level for fish.

This situation can be extremely dangerous at the end of a growing season in August, when the length of the days is shorter (Figure 3), the quantity of daytime production is less, and at the same time the biomass is the highest.

Studying the values of oxygen saturation at 2 p.m. and 5 a.m. throughout the growing season, it has been found that in the case when the dissolved oxygen content at 2 p.m. did not reach 10-12 mg/l, the oxygen content decreased to a dangerous level during the hours of sunrise, and depending on the length of the oxygen shortage period, it caused fish kills (Figure 4).

Transparency of the water can be decreased by over-production of phytoplankton too. The over-produced mass of phytoplankton has a self-screening effect and it can be a significant factor in intensively manured and foddered ponds. In these ponds production is limited only to the upper water layer and the production here can not cover the oxygen demand of the lower water layer where respiration is dominant.

If the light conditions are not favourable a very dangerous situation can develop (short day, cloudy sky).

After over-production of phytoplankton, production can be decreased not only because of the self-screening effect but by the disruption of nutrient equilibrium. In critical cases heavy losses of plankton can take place.

The relationship between oxygen production and water temperature in an intensive carp production pond is shown on Figure 5. Although the oxygen production of pond water will be higher and higher due to the increase of temperature, a critical situation occurs in the case of high water temperatures because the rate of oxygen consumption exceeds the rate of production.

1.2.2 Oxygen consumption by pond water

Oxygen is consumed in pond water by decomposition of organic materials in the respiration of macro-vegetation, phyto- and zooplankton and bacteria. On the basis of measurements carried out in large scale fish ponds of Hungary it was found that oxygen consumption of the water in a pond full of weeds was nearly completely the result of respiratory action of macro-vegetation.

In another case in a manured but empty fishpond the mass of phytoplankton decreased nearly to zero due to over-production of zooplankton. In this case the zooplankton was the main consumer of oxygen. In the case of a stocked fish pond on the other hand, the mass of zooplankton decreased to such a minimum level due to the consumption by fish that even the chemical destruction of the remaining zooplankton did not make any demonstrable change in oxygen consumption of the pond water.

Fish pond water is a complicated biological complex, thus its oxygen consumption is different from pond to pond under different conditions.

While oxygen production is dependent mainly on light conditions, respiration is mainly determined by water temperature and dissolved oxygen content of the water.

In an intensive channel catfish production pond a correlation has been found between oxygen consumption of pond water and water temperature as shown in Figure 6.

Determination of primary production and of oxygen consumption of pond water has been carried out by the Fish Culture Research Institute. Based on a mathematical analysis of daily oxygen curves, it was found that the intensity of respiration in the fish ponds studied has shown a significant daily rhythm, with an afternoon maximum, and that it has been in direct proportion to oxygen concentration (Figure 7).

Figure 3. Sunrise and sunset at Budapest according to Central European Standard Time

Figure 4. The oxygen concentration in the surface of a fish pond at 0500 and 1400 hours

Figure 5. The oxygen production in intensive fish pond as a function of water temperature

Figure 6. The oxygen consumption of the water of a fish pond as a function of water temperature

Figure 7. Relationship between the oxygen concentration and respiration

Figure 8. Routine oxygen consumption in representative cyprinid and salmonid species following thermal acclimation

Source: Chavin, W.: Responses of Fish to Environmental Changes, Springfield, 1973, C.C. Thomas, 459.p./

Figure 9. The oxygen consumption of some fresh water fishes

Figure 10. The oxygen consumption of different fish species as a function of the dissolved oxygen content of the water (in closed system)

1.2.3 Oxygen consumption by the pond bottom

Oxygen consumption by the pond bottom is dependent on the oxygen requirement for decomposition of organic material accumulated on the pond bottom and for vital functions of benthos. Oxygen fixation taking place in the water mud interfacial area can also not be neglected. The dissolved oxygen content of the water layer directly in contact with the mud is much lower than that of the upper layers of water.

According to some studies the 24 hour-long oxygen consumption of the pond bottom is 1-3 grams per m². Calculating with 1 m water depth, the 24 hour-long oxygen consumption of the pond bottom changes between 1 and 3 mg/l.

1.2.4 Oxygen consumption by fish

The different fish species have become adapted to different living conditions during their evolution. Generally all those fish species which live in fast flowing and oxygen rich streams (e.g. Salmonids) need high quantities of oxygen, and only a small decrease of dissolved oxygen can cause losses. At the same time the species which became accustomed to slow motion of the water or stagnant water (e.g. several Cyprinids) need less oxygen and are able to tolerate short periods of oxygen deficiency.

Some fish species are also known, mainly in the tropics, to utilize the oxygen of the atmosphere by cutaneous respiration, intestinal and swim-bladder respiration or by a respiratory organ similar to lungs.

Oxygen consumption by fish is generally given in mg/body weight and kg/hour. Standard oxygen consumption means the quantity of oxygen consumed by the fish without swimming and feeding (quantity of oxygen necessary for subsistence). Standard oxygen consumption is not dependent on oxygen saturation of the water but it is significantly influenced by water temperature.

Figure 8 shows standard oxygen consumption of some fish species as a function of water temperature on the basis of the data of several authors. In the practice of fish culture the active oxygen consumption (the oxygen consumed by swimming and feeding fish) can be used.

Active oxygen consumption by fish has been analyzed very rarely till now because of the technical difficulties of measuring, and available data can not be compared because of inaccurate or imperfect description of the circumstances of measurement.

Table 2, showing oxygen consumption of different fish species, gives the size of fish and an idea of the motion where they have been cited. From Table 3 it is obvious that oxygen consumption by fish can significantly change at a given water temperature depending on the activity of fish. Table 2 shows that in identical conditions oxygen consumption by smaller size fish is higher.

Figure 9 shows changes of oxygen consumption by common carp, grass carp, silver and bighead carp against body weight.

Active oxygen consumption by fish has a strict connection with oxygen saturation of the water. Figure 10 shows the correlation between oxygen saturation and oxygen consumption by fish, for seven fish species. Some scientists are of the opinion that fish are able to sense a decrease of dissolved oxygen content of the water before it causes troubles in their respiration and therefore they look for water of higher oxygen content. If this is not successful and the dissolved oxygen content of the water decreases further, the respiratory motion of the fish is increased as long as the oxygen supply to respiratory muscles permits. This reaches a limit when further acceleration is hindered by lack of oxygen. Fish are tolerant till this level. If the dissolved oxygen content of the water decreases further, the pulsation of the heart slows down and the fish dies after a time depending on the resistance characteristic of the species. Lethal oxygen concentration is the dissolved oxygen content that can not be tolerated by the given fish species for a given period of time.

Table 2 The oxygen consumption of several fish species (Source: J. Oláh, Manuscript, Fish Culture Research Institute, 1979)

Fish species	Size (g)	Feeding	Swimming	Temperature	Oxygen consumption mg/kg . day	Authors
Cyprinus carpio	806	+	-	12	1 921	Nakanishi, Itazawa, 1974
Cyprinus carpio	-	-	-	20	1 200	Houston, 1973
Cyprinus gibelio	-	-	-	20	3 360	Houston, 1973
Hypophthalmichthys molitrix	15	-	-	20	4 580	Muhamedova, 1977
Hypophthalmichthys molitrix	240	+	+	23	5 947	Vetskanov, 1975
Hypophthalmichthys molitrix	2	+	+	15	13 899	Vetskanov, 1975
Lepomis gibbosus	-	+	-	20		Roberts, 1973
Anguilla anguilla	106	+	-	18	924	Jedryczkowski and Fischer, 1973
Anguilla anguilla	106	+	-	18	1 430	Jedryczkowski and Fischer, 1973
Anguilla japonica	261	+	-	11	823	Nakanishi and Itazawa, 1974
Ictalurus punctatus	100	-	+	26	9 600	Andrews and Matsuda, 1975
Ictalurus punctatus	100	+	+	26	14 600	Andrews and Matsuda, 1975
Brevocrtis tyrannus	74	+	+	20	5 520	Hettler, 1976
Spicara smaris	50	-	-	20	4 804	Muravskaya and Belokopitin, 1975
Spicara smeris	50	-	+	20	8 854	Muravskaya and Belokopitin, 1975
Lutjanus campechanus	316	-	-	18	2 688	Moore, 1973
Rhomboplites aurorubens	354	-	-	18	4 080	Moore, 1973
Lutjanus campechanus	316	-	+	18	6 960	Moore, 1973
Rhomboplites aurorubens	354	-	+	18	7 080	Moore, 1973
Lutjanus apodus	-	-	-	20	7 536	Scholander et al., 1953
Haemulon boneriense	-	-	-	20	4 776	Scholander et al., 1953
Scarus croicensis	-	-	-	20	6 840	Scholander et all. , 1953
Cynoglossus brevis	-	-	-	20	2 400	Edwards et al., 1970, 1971
Cynoglossus puncticeps	-	-	-	20	1 512	Edwards et al., 1970, 1971
Halophryne dussumeri	-	-	-	20	768	Edwards et al., 1970, 1971
Lagodon rhomboides	-	-	-	20	2 736	Wohlschlag et al., 1968
Porichthys porosissimius	-		-	20	960	Moore, 1971
Salmo gairdneri	73	+	-	11	2 917	Nakanishi and Itazawa, 1974
Salmo gairdneri	-	-	-	15	3 600	Houston, 1973
Salvenilus fontinalis		-	-	15	2 640	Houston, 1973
Oncorhynchus nerka	-	+	-	20	10 800	Davis, 1975
Oncorhynchus nerka	-	+	+	20	15 000	Davis, 1975

Table 3 The oxygen consumption (ing/kg/hour) of 100 g rainbow trout (Salmo gairdneri) as a function of water temperature and fish activity (the dissolved oxygen concentration in the water is 80-90%)

Water temperature (°C)	5	10	15	20	25
Reduced metabolism	20	22	72	90	138
Natural conditions without stress effects	100	180	250	280	-
Active metabolism (intensive feeding)	-	-	472	360	-
Forced swimming	-	480	580	544	478

Figure 11 shows that there were no significant losses of fish in an intensively manured fish pond with 0.3 mg/l dissolved oxygen content if the period of oxygen shortage was brief. But more than ten hours of oxygen shortage caused serious losses of fish.

On the basis of investigations on intensive pond culture of common carp in Central Europe, the probability of oxygen deficiency (less than 1 mg/l dissolved oxygen) is nearly continuous from the beginning of July till the beginning of September, but losses caused by oxygen deficiency occur generally in August. This can be explained by the fact that in July the period of oxygen deficiency is only 1-2 hours but in August sometimes twelve hours of oxygen deficiency can occur.

In the practice of pond fish farming oxygen deficiency has been regarded as dangerous mainly because of mass losses of fish. But recent investigations have shown that decreased oxygen saturation can have serious effects on the economy of a fish farm as well.

Increase of toxic effect of different toxic materials can not be neglected in water bodies with low oxygen supply. The low oxygen content disadvantageously influences both the food intake and the utilization of food. Investigations have shown that lower than 25% oxygen saturation occurring before sunrise has a disadvantageous effect on fish-growing.

Keeping the dissolved oxygen content of the pond water nearly at the saturation level makes it possible not only to avoid mass losses of fish but ensures better conversion rates and higher yields in intensive culture.

1.2.5 Natural diffusion caused by wind action

Diffusion is caused by partial differential pressure of oxygen in the air and in the water.

As the oxygen content of the air is considered to be constant, the rate of diffusion is determined by oxygen saturation of the water layer interfacing with the air. In the case of stagnant water the uppermost layer of water becomes saturated quickly and the convection current of oxygen into the water slows down. The mass transfer rate which determines the rate of diffusion (g/O₂/m²/hour) varies in a wide range. It can be seen from a collection of the data of several authors that the mass transfer rate has varied between 0.03-5.0 g/m²/hour depending on the circumstances (Table 4).

In fish ponds the rate of diffusion is dependent on the mixing of water layers caused mainly by wind action. The oxygen intake from the atmosphere by diffusion is 1.5 g/m²/day in small ponds and 4.8 g/m²/day in big ponds, where the wind action is stronger.

Depending on oxygen saturation of the water, diffusion can show a reversed direction. In case of over-saturation of the water oxygen diffuses to the atmosphere. Diffusion is assisted by the mixing of water caused by wind action in this case.

It is well known that wind action has a good effect on the water quality of fish ponds through increased diffusion. In the case of construction of new fish ponds the prevailing wind direction has to be taken into account in order to utilize natural diffusion.

In intensive fish ponds where the oxygen budget of the pond water can be regulated artificially, diffusion by wind action has less importance.

1.2.6 Artificial control of the dissolved oxygen content of water

Artificial control of the oxygen budget of water of fish ponds in this case means technical and practical measures which have direct effect on the dissolved oxygen content of the pond water.

Oxygen content of the pond water can be influenced by two methods; by change of flow rate, and by aeration of pond water.

Figure 11. The length of the periods of oxygen shortage (0.3 mg/l) in the surface water layer of a fish pond

Figure 12. The basic principles of fish pond aeration 1. Hydraulic type aeration

Figure 12. The basic principles of fish pond aeration 2. Air intake type aeration

Table 4 Coefficients of gas transfer for oxygen Source: Odum, H.: Primary production in flowing water. Limnology Oceanography 1956 1/2 (102-107 p.)

Water type, Source of data	Velocity m/sec	Depth m	Temp. °C	K g/m2/hr at 0% saturation
Still water (Hutchinson, Becke, quoted by Haney, 1954)	0.0	-	20-25	.034
(Adenay, quoted by Kehr, 1938)	0.0	-	-	0.03 -0.08
Moving water
Stirred water (quoted by	-	-	25	0.09 - 0.74
Haney, 1954)	0.01	0.1	0-10	0.037
Shallow circulating through	0.01	0.1	10-20	0.043
(Streeter, Wright and Kehr, 1936)	0.01	0.1	20-30	0.047
Sewage in circulating through	0.05	0.45	25-26	0.38
(Kehr, 1938)	0.15	0.45	25-26	1.5
Stream and ponds (Imhoff and Mahr, 1932)	-	-	-	0.08
New York Harbour (Gould, 1921)	tidal	-	-	0.23
Tank with a wave machine (Borst, quoted by Phelps, 1944)	-	1.8	-	0.31
Sea Surface (Redfield, 1948)
Summer	-	-	12-20	1.1
Winter	-	-	3-7	5.2
Silver River, Florida, July 21, 1955 (Odum)
Subtraction-of-respiration method	0.21	2.77	23	0.92
Dye-measured-turnover method	0.21	2.77	23	1.00
Green Cove Springs, Florida (Odum) From carbon-dioxide by respiratory-quotient method	0.3	0.23	24	0.55
Small rivers, diurnal oxygen curve analyses in Table 2	-	0.5-3.	-	0.6-4.3
Ohio River below Cincinnati (Velz, 1939)	0.05-0.09	4.8	15-25	1.5-5.0
Bubbles and drops (K given per area of drop or bubble)
Air bubble (Krogh, quoted by Redfield, 1948)	-	-	37	13.1
Air bubbles (Ippan, Adenay, Spuler and Schaab quoted by Haney, 1954)	-	-	20-25	2.8-28.
Water drops (Whitman, quoted by Haney, 1954)	-	-	24	22-34.

In the case of intensively manured and fed fish ponds, where a significant part of the feed of fish is produced in the water of the fish pond itself, increase of flow rate can not be applied for oxygen saturation.

Artificial aeration means increasing the oxygen content of all or part of the pond water to limits which ensure the oxygen supply to the fish without limiting the production at a given management level.

In extensive fish culture, aeration of fish ponds has a "life-saving" role only in case of oxygen deficiency. With the increasing of the intensity of pond fish culture, because of the accelerated biological activity (intensive manuring, feeding, high stocking density) the natural oxygen supply becomes more and more insufficient and will be a limiting factor in production. Intensive aeration of fish ponds has not only a life-saving role, but it becomes one of the basic factors of production, and ensuring optimal oxygen supply makes possible the maximal utilization of the given biological possibilities.

As already mentioned, food intake of fish is influenced disadvantageously by the decrease of dissolved oxygen content of the water below 20-30 percent. In intensive fish culture the proper oxygen supply has an important role in the economy of production. At the same time saturation values much higher than 100 percent are disadvantageous for the normal vital processes of fish, as they are forced to an increased respiration. Consequently the aeration of fish ponds means the regulation of dissolved oxygen content of the pond water and not necessarily the increase of it.

During the planning of fish pond aeration, one can count on extended water surface areas as potential air intake areas.

Oxygen over-saturation of the surface water layer can be decreased by mixing of the water, and the oxygen can be uniformly distributed in the total water body of the pond. Mixing of the water in shallow ponds such as fishponds has no influence on oxygen production of plankton. Mixing the water during daytime can ensure that significant quantities of oxygen are reserved for the night hours when consumption is predominant.

2. AERATION DEVICES FOR FISH PONDS

2.1 Hydraulic Type Fish Pond Aerators
2.2 Air Diffusion Type Fish Pond Aerators

In pond fish farming the oxygen content of the atmosphere is utilized for enrichment of pond water with oxygen. Basically there are two methods for oxygen enrichment. In the first one energy is transferred to the surrounding air by some kind of air compressor and the air penetrates into the water where it transmits a part of its oxygen content to the water. In case of the second one energy is transferred directly to the pond water or other water source and the water having a higher energy content will be able to take up air from the atmosphere and dissolve part of its oxygen content. The principal schemes of these aeration processes are shown in Figure 12.

A water pump is the basic device for the hydraulic type of aeration with which the energy content of the water can be increased. The air intake device can be a cascade, a sprinkler, an ejector or an air intake head. These are generally connected to the pump through a pipeline. In the group of hydraulic type aeration devices, the surface aerators are shown in a separate group in the figure, because of their special construction. Surface aeration can be provided by a simple open impeller or by a centrifugal pump without housing, and its construction is designed specially for pond aeration. The basic unit of air diffusion type aeration devices is the air compressor which can be a root-type blower, a ventilator, a compressor or a membrane pump. The air penetrates the water through porous material such as a perforated tube placed in the pond.

2.1 Hydraulic Type Fish Pond Aerators

In case of aeration with sprinklers the water jet comes out from the nozzle with high velocity and falls into the water in drops, thus contacting the air on a large surface and dissolving oxygen from it. When the water drops hit the water surface of the fish pond turbulence occurs increasing the diffusion of air into the water. The water jet can be directed down into the water surface forcing air bubbles into the water.

In aeration with an ejector the water is passed through a venturi-type diffusor where its pressure decreases below the atmospheric pressure thus air can penetrate into the water.

Specially designed air intake heads can also be used for aeration. These usually involve a propeller inside a pipe through which water is passing. The propeller is driven by the water current and on the surface of the blade the pressure decreases and an air sucking effect takes place.

Cascades can also be used for aeration, in which the water is broken into small drops, increasing the interfacial area.

Hydraulic type aerators can be applied usefully when water is pumped through a pipeline but they are particularly useful when a natural head is available.

Surface aerators are placed directly in the fish pond and due to the rotation of a paddle type device, either with vertical or horizontal axis, the water is discharged into the air. These aerators generate a vertical or horizontal water current in the fish pond that has an advantageous effect on the oxygen budget.

2.2 Air Diffusion Type Fish Pond Aerators

During air diffusion the air is supplied by various types of compressors or blowers into the water where it is diffused through a variety of diffusors. When the air is diffused through a perforated pipe large bubbles of up to 10 mm in diameter are formed and when the diffuser is a porous material fine bubbles of 2-5 mm in diameter are produced. When the bubbles emerge and pass up to the water surface a part of their oxygen content is dissolved in the water, and also a secondary upwards water movement is generated, creating a mixing effect.

3. UTILIZATION OF PURE OXYGEN IN INTENSIVE FISH CULTURE SYSTEMS

3.1 Oxygen Absorption and its Devices
3.2 Pipeline Injection of Oxygen
3.3 U-tube Oxygenation

Intensive fish culture systems are generally established in areas where there is a large quantity and good quality of flow-through water. In such systems the oxygen requirement of fish is ensured by the inflow water. This means the fish load capacity of these systems is dependent on the rate of water flow. The water flow can not be increased beyond a certain limit because of technical, economic and physiological reasons, although the other elements of the system would make possible a higher stocking density. The fish load can be increased to a certain extent by direct aeration of the water, but there are technical and economic limitations on individual aeration of fish tanks.

By application of pure oxygen, the oxygen supply no longer limits the fish load of the system. Furthermore the oxygen supply can be regulated and the safety of the operation increases. The significant advantage of application of pure oxygen is the high mass transfer rate between the pure oxygen and the water, because of high differences in concentration. In application one has to take into account the costs of oxygen and the investment in devices to dispense it. A well-designed and well-operated oxygen supply system can compensate for the higher investment costs because of its low energy requirement.

Pure oxygen is available in gas or liquid form. For the supply of intensive fish culture systems usually liquid oxygen is used, produced by cryogenic separation. The liquid oxygen of 99 percent purity is stored in tanks with vacuum insulation and evaporated before utilization.

Recently "oxygen generators" have been designed which can produce oxygen of 90 percent purity, removing the nitrogen from the air by a molecule filter.

3.1 Oxygen Absorption and its Devices

In order to minimize oxygen consumption devices are needed which ensure the most perfect absorption of oxygen. The operating principle of these is the same as that of aeration devices, but in order to increase the contact time between the gas and the water some technical modifications are needed.

One is bubbling oxygenation where the oxygen gas comes in contact with the water by breaking into bubbles. The rate of oxygen dilution depends mainly on the depth of the water layer, the length of travel of bubbles in the water body and the rate of oxygen feeding. Higher efficiency can be achieved by decreasing the bubble size, thus the contact time and the contact surface increases. But the decrease of bubble size needs a significant quantity of extra energy, and coagulation of bubbles also can happen. The efficiency of oxygen bubbling can be increased by counterflow of oxygen and water (Figure 13/a). Undissolved oxygen can be collected and circulated back to the oxygen supply system (Figure 13/b), or a closed system can be constructed in which oxygen and water are continuously mixed (Figure 13/c).

3.2 Pipeline Injection of Oxygen

Where oxygen is injected into the water flowing in a tube at a given point, the efficiency of oxygen dissolving depends on the time the oxygen stays in the pipeline and on the pressure in the tube. Oxygen can be fed into the pipeline through a Venturi tube, as shown in Figure 13/d.

3.3 U-tube Oxygenation

U-tube oxygenation is shown in Figure 13/e, in which bubbles of oxygen are carried along the water current. Thus the contact time and efficiency of oxygen dilution depends on the difference of inflow and outflow water levels and on depth of the "U" tube. This process seems to be very effective in fish culture practice because of its high efficiency, low energy requirement and simplicity.

The above mentioned basic dissolving principles can be found simultaneously in the same device as shown in Figure 13/f.

4. BASIC PRINCIPLES OF THE DESIGN OF FISH POND AERATORS

4.1 Equilibrium Concentration of Oxygen in Water
4.2 Mass Transfer Processes of Aerators

4.1 Equilibrium Concentration of Oxygen in Water

The equilibrium concentration is the maximum concentration of oxygen which can be dissolved in the water relative to the concentration of oxygen in the gas under the prevailing conditions of temperature and pressure. In fish pond conditions the gas means the atmospheric air, although pure oxygen also gets into the fish pond water by the oxygen production of phytoplankton and other water plants. In some intensive fish farming systems pure oxygen is applied in order to meet the oxygen requirement of the fish.

The equilibrium concentration in gas-liquid systems is expressed by Henry's law as follows:

where

H = Henry's constant (P_a). (Henry's constant depends on the temperature).

The partial pressure of one component of a gas mixture is proportional to the volume fraction of that component.

where

P = the pressure of gas mixture (P_a)

Figure 13. a) Counterflow column

Figure 13. b) Gas recycling

Figure 13. c) Enclosed Operation

Figure 13. d)Venturi oxygenator

Figure 13. e) U tube oxygenator

Figure 13.f) Complex system

21 percent of the atmospheric air is oxygen thus the volume fraction is:

The oxygen concentration expressed in mole fraction can be converted into weight fraction as follows:^1/

^1/ In the following equations the notations k^-1 and m^-1 are used to denote per kg or per m³

where

thus

The weight fraction can be converted into weight concentration (C_s) as follows:

(kg . m-3)

where

C_s = weight concentration of oxygen at saturation (kg . m^-3)

The density depends on temperature as shown in Table 5.

Based on the equations given above, the weight concentration of oxygen at saturation (C_s) can be calculated as follows:

(kg . m-3)

(g . m-3)

Table 5 Density of pure water

Temperature °C	Density kg/m	Temperature °C	Density kg/m3
0	999.87	16	998.97
1	999.93	17	998.80
2	999.97	18	998.62
3	999.99	19	998.43
4	1 000.00	20	998.23
5	999.99	21	998.02
6	999.97	22	997.80
7	999.93	23	997.57
8	999.88	24	997.33
9	999.81	25	997.07
10	999.73	26	996.81
11	999.63	27	996.54
12	999.52	28	996.26
13	999.40	29	995.97
14	999.27	30	995.68
15	999.13

When calculating the pressure (p), the atmospheric pressure and the water head have to be taken into account (1 mm H₂0 = 9.80665 P_a). In the case of a surface aerator, one has to calculate only with the atmospheric pressure. The C_s values at normal atmospheric pressure are shown in Figure 1.

In those places where the elevation is different from sea level, the atmospheric pressure can be calculated with the "barometric level formula" as follows:

using the "gas law"

where

z = elevation above sea level (m)
p_z = atmospheric pressure at z elevation above sea level (P_a)
p₀ = atmospheric pressure at sea level (P_a)

The normal atmospheric pressure at 45° north at sea level and at 273 K (0°C) temperature is:

101325 P_a (= 760 torr = 1 atm)
g = acceleration due to gravity (ms^-2) its value for practical calculations is: g = 9.81 ms^-2
S₀ = the density of air (sea level, normal atmospheric pressure, 273 K (0° C) temperature)

S₀ = 1.2928 kg . m^-3

q = temperature (K)
R = gas constant

its value for air is:
R = 287.041 s . kg^-1 K^-1

The changing of the local atmospheric pressure usually is + 10 percent.

4.2 Mass Transfer Processes of Aerators

The amount of oxygen that can be dissolved in the water by an aerator during a time unit can be expressed as follows:

where

K_La = modified mass transfer coefficient (h^-1)
C_s = the equilibrium concentration of dissolved O₂ (g m^-3)
C = the dissolved oxygen concentration in the water (g m^-3)
t = time (h)

The solution of the differential equation when the initial condition is C₀ is as follows:

The K_La modified mass transfer coefficient is a product of multiplication of the mass transfer coefficient (K_L) and the specific area () as follows:

where

K_L = mass transfer coefficient (m h^-1)
A = diffusion area (m²)
V = volume (m³)

K_L is dependent on the temperature

where

q = temperature (°C)
b = constant with a value between 1.016 and 1.047
In sewage treatment with bubbling aeration usually b = 1.02 is used during the calculations

Generally:

The organic and inorganic materials in the water have an influence on K_La.

K¢ _La = a K_La

where

K¢ _La = the modified mass transfer rate of an impure water
a = constant, its value lies between 0.7 and 0.9 when the water is biologically treated and 0.5 when the water is mechanically treated.

The Oxygenation Capacity (OC) expresses how many grams of oxygen can be dissolved in 1 m³ of water during one hour by the aerator investigated, at normal atmospheric pressure when the water temperature is 10°C and its initial dissolved oxygen content is zero.

The relation between the Oxygenation Capacity and modified mass transfer rate is shown as follows:

OC = K_La (10°C) . C_s (10°C)

The value of OC at different temperature and atmospheric pressure can be computed with the equation:

where

When the same aerator is used for the oxygenation of a larger water volume than 1 m³ less oxygen can be dissolved. This is why total oxygen intake (O_t) is introduced.

Ot = V . OC

(g h-1)

The value of O_t is related to 10 or 20°C temperature, normal atmosphere pressure and C = 0 initial dissolved oxygen concentration.

The specific total oxygen intake O_ts shows the efficiency of the oxygenation related to the power input.

(g h-1 kW-1)

where

P = power input of the aerator (kW)

5. DIMENSIONING OF AERATORS

5.1 Bubble Aeration
5.2 Examples for Dimensioning Fine Bubble Aerators
5.3 Aeration with Ejectors
5.4 Aeration with Paddle Wheels

5.1 Bubble Aeration

The air intake type aerators can be classified according to the size of bubbles produced as follows:

a) Fine bubbles d_B = 1 to 5 mm
b) Medium bubbles d_B = 5 to 10 mm
c) Coarse bubbles d_B = larger than 10 mm

The size of the bubble can be expressed as follows

where

d_B = bubble diameter (mm)
s = surface tension of the liquid (N m^-1)
S_f = density of the liquid (kg m^-3)
S_g = density of the gas (kg m^-3)

When fine and medium size bubbles are produced, the diameter of the bubbles is larger than the size of the hole through which the air enters into the water . When a large bubble is produced its diameter is smaller than the hole size.

The elevation velocity of the bubbles in ease of different bubble size is shown as follows:

dB < 0,15 mm	VB = 478 500 dB2 (ms-1)
0,15 mm < dB- < 2.10 mm	VB = 758 dB1.25 (ms-1)
2.10 mm < dB < 7.20 mm	VB = 0.0164 dB0.5 (ms-1)
dB > 7.20 mm	VB = 2.24 dB0.5 (ms-1)

In bubble aeration the following ratio is used as a reference to the efficiency of oxygen dissolving:

The value of this ratio in different systems is as follows:

a) fine bubble aeration:	9-10 percent
b) medium bubble aeration:	5 - 6 percent
c) coarse bubble aeration:	3.5- 5 percent

where

Q = air volume (m³h^-1)
S = density of air (kgm^-3)
OCV = (g h^-1)

The manufacturers of different aerators use characteristic curves that can be expressed as follows:

or

where

S = density of air (kg m^-3)
w = mole fraction of O₂ in air (kg kg^-1)

Generally the air volume (Q) is related to normal atmospheric air. In this case:

S = 1.293 kg m^-3

and

S . w = 1.293 . 0.232 = 0.3 kg m^-3

The equations above are related to a given water depth. The different parameters can be converted from one water depth to another using the formula:

Table 7 shows the result of a test during which a bubble aerator (2.3 mm long 25 mm diameter perforated plastic pipe with 1.5 mm diameter holes in two lines with a distance of 20 mm between holes) was utilized in a tank with a surface area of 7.5 × 17.5 m.

5.2 Examples for Dimensioning Fine Bubble Aerators

The scheme of the NOKIA fine bubble aerator is shown in Figure 14. The air intake part of these aerators is made of porous polyethylene material, in pipe (HKP 600) or in disc (HKL 210) form.

They can be dimensioned using the curves given in Figures 15 and 16.

The Flygt 763 type fine bubble aerator is shown in Figure 17. The equation below shows how much oxygen can be dissolved from 1 m air when the temperature of the water is 10°C, its initial oxygen content is zero and when the aerator is placed 1 m below the water surface:

if the value of Q lies between 6 m³h^-1 and 30 m³h^-1

where

Q = the amount of air flowing through the aerator (the amount of air is calculated for normal conditions, 0°C and 101325 P_a) (m³h^-1)

The relation between the air flow and the required pressure is shown in Figure 17.

5.3 Aeration with Ejectors

The cross-section of an ejector for fish pond aeration (Flygt type 4803, 4804) is shown in Figure 18. The primary water flow (1) passes through a Venturi inlet (2) where its velocity increases while its pressure decreases. The low pressure suction chamber (3) is connected to the atmospheric air by a pipe (4) through which air enters the chamber. In the mixing pipe (5) the air and the primary water are mixed together. As the air-water mixture passes through the ejector its velocity decreases in the diffusor pipe while its pressure rises to the pressure at the end of the pipe in the outside water. As an example, graphs are given in Figure 18 that can be used for the dimensioning of Flygt type 4803 and 4804 ejectors.

Table 6 The value of as a function of temperature

Temperature (°C)
9	1 019
10	1 000
11	0.982
12	0.964
13	0.946
14	0.928
15	0.911
16	0.895
17	0.878
18	0.861
19	0.845
20	0.830
21	0.815
22	0.799
23	0.784
24	0.770

Figure 14. HKP 600 aerator

Figure 15. Oxygen absorption capacity of an HKP 600 tube aerator

Figure 15. Oxygen absorption capacity of an HKL 210 disc aerator

Figure 16. Pressure loss

Figure 17

Figure 18. Flygt ejector 4803, 4804

5.4 Aeration with Paddle Wheels

The basic technical data of two paddle wheel type aerators are given below. These aerators were tested in a concrete tank with 7.5 × 17.5 m surface area. The aerator has a horizontal shaft with two paddle wheels on each end. The shaft is driven by an electric motor and chain. The device is mounted on floats.

Dimensions	Type "A"	Type "B"
Width	3 550 mm	1 635 mm
Length	2 060 mm	1 720 mm
Diameter of paddle wheel	1 000 mm	650 mm
Kidth of the paddles	250 mm	180 mm
Number of paddles	8	9
Capacity of electric motor	2,2 kW	2,2 kW

The values of the oxygen intake are shown in Table 7,

Table 7 Oxygen intake of paddle wheel type and perforated pipe type aerators

Aerator	Water depth (m)	Working parameters	Average oxygen intake (kg/h)	Power input (kW)	Specific oxygen intake (kg/kWh)
Paddle wheel	1.2	n = 115 l/min	1.8	1.98	0.91
Type "A"		h = 65 nnn
	1.2	n = 90 l/min	1.5	1.6	0.94
	0.8	h = 90 mm	2.2	1.56	1.41
Paddle wheel	1.2	n = 126 l/min	2.5	2.56	0.98
Type "B"	0.8	h = 210 mm	3.1	2.56	1.21
Perforated PVC pipe
15 pcs.	0.8	160 Hgmm	3.0	13.1	0.23
		587 m3/h
20 pcs.	0.8	135 Hgmm	2.7	9.4	0.29
		512 m3/h
20 pcs.	0.8	120 Hgmm	2.4	7.5	0.32
		410 m3/h
20 pcs.	1.2	135 Hgmm	3.0	5.9	0.51
		312 m3/h
20 pcs.	1.2	120 Hgmm	1.9	4.05	0.47
		228 m3/h

1 Hgmm = 133.322 P_a = 13.5957 mm H₂O

Source: KULI: Részjelentés a halastavi vizlevegöztetö berendezések viszgálatáról MÉM Müszaki Intézet, Gödöllö, 1982

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