Types of Aeration
and Design Considerations
Claude E. Boyd
Department of Fisheries and Allied
Aquacultures
Auburn University, Auburn, Alabama
Introduction
Dissolved oxygen (DO) is probably the single most important
environmental factor in aquaculture. If DO concentrations are low fish will not eat well,
they will not grow well, and they will be susceptible to disease. Furthermore, where
concentrations are very low, many or even all of the fish may die from lack of oxygen. In
order to have good feed conversion efficiency, high survival, and adequate profits, fish
farmers must maintain plenty of DO in waters of culture systems. Aeration is necessary to
supplement natural sources of DO.
Although aeration is widely used, there are many mistaken
ideas about aeration and few aeration practices have been thoroughly investigated. In this
lecture, I will provide some basic information on DO and on the practice of aeration.
Improvements in management procedures such as aeration are
very important for fish prices will probably continue to be low. This will increase the
demand for fish, but it will force farmers to become more efficient managers. Just to
illustrate how a small factor can be important to profit, consider that an improvement in
the feed conversion efficiency of 0.1 unit could potentially increase profits by $25 to
$40 per ton of fish produced.
Solubility of Oxygen in Water
Approximately 20% of the volume and pressure of gases in
the air is oxygen. When water is in contact with the atmosphere, oxygen from the air will
enter the water until the pressure of oxygen in water and air are equal. This condition is
known as equilibrium or saturation. The concentration of DO at equilibrium increases with
increasing pressure and decreases with increasing water temperature and salinity. The
equilibrium (saturation) concentrations of DO at different temperatures and salinities are
provided in Table 1 for standard sea level barometric pressure (760 mm mercury or 29.92
inches of mercury). To obtain the DO concentration at saturation for any other location,
multiply the appropriate DO value from Table 1 by the ratio local barometric
pressure:standard sea level pressure.
Sources of Oxygen
Natural Diffusion
When water contains less oxygen than the saturation
concentration, oxygen from the atmosphere diffuses into the water. In culture tanks,
diffusion is a slow process. It is not an important source of oxygen except when DO
concentrations are low and there is a high degree of turbulence.
TABLE 1.THE SOLUBILITY OF OXYGEN (MG/LITER) IN WATER AT
DIFFERENT TEMPERATURES AND SALINITIES FROM MOIST AIR WITH PRESSURE OF 760 MM HG. AFTER
COLT (1984)
Tem-
per
ture
--------------------------------------------------------------------
(°C) 0 5 10 15 20 25 30 35 40
--------------------------------------------------------------------
0 14.60 14.11 13.64 13.18 12.74 12.31 11,90 11.50 11.11
1 14.20 13.72 13.27 12.82 12.40 11.98 11.58 11.20 10.82
2 13.81 13.36 12.91 12.49 12.07 11.67 11.29 10.91 10.55
3 13.44 13.00 12.58 12.16 11.76 11.38 11.00 10.64 10.29
4 13.09 12.67 12.25 11.85 11.47 11.09 10.73 10.38 10.04
5 12.76 12.34 11.94 11.56 11.18 10.82 10.47 10.13 9.80
6 12.44 12.04 11.65 11.27 10.91 10.56 10.22 9.89 9.57
7 12.13 11.74 11.36 11.00 10.65 10.31 9.98 9.66 9.35
8 11.83 11.46 11.09 10.74 10.40 10.07 9.75 9.44 9.14
9 11.55 11.18 10.83 10.49 10.16 9.84 9.53 9.23 8.94
10 11.28 10.92 10.58 10.25 9.93 9.62 9.32 9.03 8.75
11 11.02 10.67 10.34 10.02 9.71 9.41 9.12 8.83 8.56
12 10.77 10.43 10.11 9.80 9.50 9.21 8.92 8.65 8.38
13 10.52 10.20 9.89 9.59 9.29 9.01 8.73 8.47 8.21
14 10.29 9.98 9.68 9.38 9.10 8.82 8.55 8.29 8.04
15 10.07 9.77 9.47 9.19 8.91 8.64 8.38 8.13 7.88
16 9.86 9.56 9.28 9.00 8.73 8.47 8.21 7.97 7.73
17 9.65 9.36 9.09 8.82 8.55 8.30 8.05 7.81 7.58
18 9.45 9.17 8.90 8.64 8.38 8.14 7.90 7.66 7.44
19 9.26 8.99 8.73 8.47 8.22 7.98 7.75 7.52 7.30
20 9.08 8.81 8.56 8.31 8.06 7.83 7.60 7.38 7.17
21 8.90 8.64 8.39 8.15 7.91 7.68 7.46 7.25 7.04
22 8.73 8.48 8.23 8.00 7.77 7.54 7.33 7.12 6.91
23 8.56 8.32 8.08 7.85 7.63 7.41 7.20 6.99 6.79
24 8.40 8.16 7.93 7.71 7.49 7.28 7.07 6.87 6.68
25 8.24 8.01 7.79 7.57 7.36 7.15 6.95 6.75 6.56
26 8.09 7.87 7.65 7.44 7.23 7.03 6.83 6.64 6.46
27 7.95 7.73 7.51 7.31 7.10 6.91 6.72 6.53 6.35
28 7.81 7.59 7.38 7.18 6.98 6.79 6.61 6.42 6.25
29 7.67 7.46 7.26 7.06 6.87 6.68 6.50 6.32 6.15
30 7.54 7.33 7.14 6.94 6.75 6.57 6.39 6.22 6.05
31 7.41 7.21 7.02 6.83 6.64 6.47 6.29 6.12 5.96
32 7.29 7.09 6.90 6.72 6.54 6.36 6.19 6.03 5.87
33 7.17 6.98 6.79 6.61 6.43 6.26 6.10 5.94 5.78
34 7.05 6.86 6.68 6.51 6.33 6.17 6.01 5.85 5.69
35 6.93 6.75 6.58 6.40 6.24 6.07 5.91 5.76 5.61
36 6.82 6.65 6.47 6.31 6.14 5.98 5.83 5.68 5.53
37 6.72 6.54 6.37 6.21 6.05 5.89 5.74 5.59 5.45
38 6.61 6.44 6.28 6.12 5.96 5.81 5.66 5.51 5.37
39 6.51 6.34 6.18 6.02 5.87 5.72 5.58 5.44 5.30
40 6.41 6.25 6.09 5.94 5.79 5.64 5.50 5.36 5.22
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Inflowing Water
Water entering culture systems contains DO. The amount of
oxygen from this source depends on the DO concentration and volume of inflowing water. For
example, if water containing 9 mg/I DO enters a culture tank at 200 liters per minute, the
oxygen input rate is 1 ,800 mg/min or 1.8 g/min. In 24 hr, 2,592 g or 2.59 kg of oxygen
would enter the tank.
Photosynthesis
Photosynthesis by phytoplankton is usually the most
important natural source of oxygen for ponds. Phytoplankton remove carbon dioxide from the
water, produce organic matter (carbohydrate), and release oxygen during the day:
6CO2 + 6H2O + Radiant energy = C6H12O6 + 602
DO concentrations will often increase above saturation
during daytime.
However, photosynthesis is not a significant factor in
indoor, water recirculating systems.
Aeration
When DO concentrations are below saturation, aerators can
put oxygen into water. The amount of oxygen from aeration depends upon the type and number
of aerators and upon the concentration of DO in the water. Aeration is an important source
of oxygen when DO concentrations are low.
Losses of Oxygen
Diffusion
When DO concentrations are above saturation, oxygen
diffuses from the water into the atmosphere. Diffusion is a slow process, but surface
turbulence and mechanical aeration can greatly increase the rate of diffusion of oxygen
from supersaturated waters. Water recirculating systems will seldom be supersaturated with
DO, so diffusion normally is not a major loss of oxygen.
Outflowing Water
DO is lost when water is discharged from culture systems.
The loss is unimportant, for incoming water normally has a higher concentration of DO than
outflowing water.
Respiration
All living things in aquaculture systems use oxygen in
respiration to release energy from food.
C6H12O6 + 602 = 6CO2 + 6H2O + Heat energy
Unlike photosynthesis, which occurs only during daylight,
respiration occurs 24 hours per day. Phytoplankton are not abundant in water recirculating
systems but bacteria are present in large numbers. The amount of oxygen used by bacteria
decomposing organic matter varies with the concentration and composition of organic matter
in water. However, water passes through fish rearing units of recirculating systems very
quickly and oxygen consumption by bacteria is probably not great within the rearing unit.
Fish use about 150 to 300 mg oxygen per kg of fish per
hour; however it is difficult to estimate their respiration rate. The best way of
assessing the oxygen demand of the fish grow-out unit is to assume that for each kilogram
of feed applied, 0.2 kg of oxygen will be required (Boyd and Watten 1989). This takes into
account the oxygen used by fish and bacteria. Of course, additional 'y,gen will be needed
by bacteria to break down organic matter which accumulates in the waste treatment part of
the culture system.
Effects of Dissolved Oxygen on Fish
The influence of DO concentrations on fish is summarized
below:
DO concentration Effect
Less than 1.5 or 2 mg/I Lethal if exposure lasts more than a
few hours.
2 to 5 mg/I Growth will be slow if exposure to
low DO is continuous.
5 mg/I to saturation Best condition for good growth.
Above saturation Can be harmful if supersaturated
conditions exist throughout water
volume.
Concentrations of DO can fall so low that fish are killed.
However, adverse effects of low DO more often are expressed as reduced growth and greater
susceptibility to disease. In systems with chronically low DO concentrations, fish will
eat less and they will not convert food to flesh as efficiently as in systems with normal
DO concentrations.
Aerators
Most intensive, recirculating aquaculture units consist of
a rearing unit and some type of water treatment unit. Water flows through the rearing unit
where it is contaminated with uneaten feed and fish excrement and its DO content is
reduced by fish respiration. Water leaving the rearing unit passes into the treatment unit
where it is purified by various physical, mechanical, and biological processes before
being passed through the rearing unit again. In some places, recirculating systems are
located outdoors and employ large earthen ponds as biological water-purification units.
Such systems do not differ substantially from conventional pond aquaculture systems,
because phytoplankton are a dominant component. The phytoplankton remove ammonia and
release oxygen in photosynthesis, but they also produce large amounts of organic matter in
photosynthesis. Because of low light intensity, indoor, recirculating systems do not
contain a significant amount of phytoplankton. Organic matter in the system can be traced
back to the feed, and the natural supply of oxygen is small.
Oxygen can be added to an indoor, recirculating system at
almost any point. However, the part of the system most sensitive to low DO concentration
is the rearing unit. If DO concentrations are low in the rearing unit, fish will be
stressed. They will not eat and grow well, they will be susceptible to disease, and they
may die.
Any place in the system where water flows abruptly to a
lower elevation affords the opportunity for gravity aeration. Water may fall over a weir,
fall through perforated trays, spray from a nozzle, splash over an inclined surface, flow
through a container packed with porous media, etc. This type of aeration is called gravity
aeration. Because head loss provides the energy for gravity aeration, there usually is no
operating cost associated with it. However, gravity aerators are not very efficient, and
the practice of pumping water to a higher elevation just to provide head for gravity
aeration is not as economical as many other kinds of aeration (Soderberg 1982).
Recently, there has been considerable interest in pure
oxygen contact systems for increasing DO concentrations in fish culture systems. In these
systems, U-tubes, packed columns, spray chambers, and many other devices are used to
effect transfer of pure oxygen into water which is then passed through the grow-out unit
(Boyd and Watten 1989). Pure oxygen contact systems have certain advantages which will not
be discussed here, but their economy is not obvious. Until more is known about operating
pure oxygen contact systems, mechanical aerators seem more practical.
Principles of Mechanical Aeration
Aerators are mechanical devices that increase the rate at
which oxygen enters water. There are two basic techniques for aerating pond water: water
is splashed into the air or bubbles of air are released into the water. Hence, we have
"splasher" and "bubbler" aerators.
Splasher aerators include vertical pump, pump-sprayer, and
paddle wheel aerators. A vertical pump aerator consists of a motor with an impeller
(propeller) attached to its shaft. The motor is suspended below a float with a center
opening and the impeller jets water into the air at low velocity. A pump-sprayer aerator
employs a centrifugal pump to spray water at high velocity through holes in a manifold and
into the air. A paddle wheel aerator splashes water into the air as the paddle wheel
rotates (Boyd and Ahmad 1987).
Bubbler aerators include diffused-air systems and
propeller-aspirator-pumps. In a diffused-air system, an air blower or air compressor is
employed to deliver air through an air line, and the air is released through air diffusers
located on the bottom or suspended in the water, The propeller-aspirator-pump aerator has
a high velocity, uncased impeller at the end of a hollow shaft and housing. In operation,
air flows down the shaft by the venturi principle and is released into the water in fine
bubbles (Boyd and Ahmad 1987).
Performance
The ability of an aerator to transfer oxygen to water is
expressed as the standard oxygen transfer rate (SOTR) and the standard aerator efficiency
(SAE). The SOTR is the amount of oxygen that an aerator will transfer in 1 hour to clear
freshwater at 20°C which contains 0 mg/I DO. SOTR usually is expressed as pounds of
oxygen per hour. The SAE is simply the SOTR divided by power input; it normally is
expressed as pounds of oxygen per kilowatt~hour or pounds of oxygen per horsepower. Power
input may be expressed as power applied to the aerator shaft (brake power) or the
electricity consumption by the aerator (wire power); it is best for practical purposes to
express SAE in terms of the rated horsepower of the aeration unit. Standard conditions
employed for presenting SOTR and SAE values seldom exist in aquaculture systems. As DO
concentration and water temperature rises, actual oxygen transfer rate and actual aeration
efficiency decrease with respect to SOTR and SAE. For example, at 30°C and 4 mg/I DO, an
aerator would transfer only about 50% of the oxygen suggested by SOTR and SAE.
Nevertheless, SOTR and SAE are important for they permit comparisons of efficiency among
aerators.
The nomograph, Figure 1, may be used to obtain correction
factors for converting SOTR or SAE to actual oxygen transfer under pond conditions.
FIGURE 1. Nomograph for estimating correction factors for
SOTR and SAE
Researchers at Auburn University evaluated the performance
of many aerators and studied the effect of design features and operating conditions on
performance. Results in terms of pounds of oxygen transferred per kilowatt of electrical
power used are summarized below:
SAE lb O2/hp-hr
-----------------
Aerator type Average Range
-------------- -------- -----------
Paddle wheel, all types 3.1 1.6 - 4.3
Propeller-aspirator pump 2.3 1.9 - 2.6
Vertical pump 2.0 1.0 - 2.6
Pump sprayer 1.9 1.3 - 2.8
Diffusion 1.3 1.0 - 2.3
Although research has shown paddle wheel aerators to be
highly efficient, they are too large for use in most indoor, recirculating systems. Small
vertical-pump aerators, propeller-aspirator-pump aerators, and diffusion aerators are
better suited for recirculating systems. SAE values for 0.5 to 2.0-hp vertical pump and
propeller-aspirator-pump aerators typically ranged from 1.0 to 2.6 lb O/hp-hr. For design
purposes, I suggest using an SAE of 2 lb O2/hp-hr. These types of aerators are
designed to operate in a set manner. They only have to be assembled, placed in the
aquaculture system, and put into operation.
Diffusion aeration systems are much more complicated.
Choices must be made about air pressure, air flow rate, type of diffuser, depth of
diffuser, and number of diffusers. The performance of diffusion aeration systems is quite
sensitive to the combination of operating variables as illustrated in Table 2 with data
collected under different conditions for a particular system. Low air flow rates provided
higher SAE values, and at low air flow rates depth of water over the diffuser and the
number of diffusers had little effect on SAE. At higher air flow rates, SAE tended to
increase with water depth, and increasing the number of diffusers enhanced SAE.
Although SAE is important for comparing aerator efficiency,
other factors should also be considered in selecting aerators. These factors include:
compatibility with culture system, purchase price, durability and operation lifetime,
problems in obtaining service, and personal choice. Also, SAE is simply a measure of
efficiency. The SOTR must be considered in determining if an aeration system is large
enough to supply the desired quantity of oxygen.
TABLE 2. POWER REQUIREMENT IN HORSEPOWER, STANDARD OXYGEN
TRANSFER RATE (SOTR) IN POUNDS OF OXYGEN PER HOUR, AND STANDARD AERATION EFFICIENCY (SAE)
IN POUNDS OF OXYGEN PER HORSEPOWER~HOUR FOR A DIFFUSION AERATION SYSTEM OPERATED AT
DIFFERENT DEPTHS AND AIR FLOW RATES.
Air
Diffuser flow
depth rate Six diffusers Twelve diffusers
(ft) (ft3/min) hp SOTR SAE hp SOTR SAE
------------------------------------------------------------------
3 1 0.04 0.13 3.48 0.03 0.13 3.98
2 0.08 0.22 2.66 0.07 0.24 3.22
3 0.14 0.29 2.15 0.12 0.34 2.82
5 0.29 0.42 1.46 0.28 0.38 1.38
7 0.48 0.54 1.13 0.46 0.48 1.05
9 0.77 0.66 0.86 0.74 0.61 0.83
5 1 0.06 0.21 3.63 0.06 0.19 3.45
2 0.12 0.36 3.04 0.12 0.39 3.40
3 0.19 0.50 2.60 0.18 0.42 2.28
5 0.39 0.74 1.92 0.36 0.64 1.78
7 0.62 0.92 1.49 0.59 0.82 1.40
9 0.91 1.10 1.21 0.89 0.88 0.99
7 1 0.07 0.29 3.91 0.07 0.28 3.87
2 0.15 0.49 3.18 0.15 0.52 3.42
3 0.24 0.64 2.64 0.24 0.71 2.93
5 0.46 0.80 1.72 0.45 0.93 2.05
7 0.72 0.97 1.34 0.72 1.21 1.69
9 1.03 1.17 1.13 1.05 1.54 1.46
9 1 0.09 0.35 3.78 0.09 0.31 3.34
2 0.19 0.63 3.34 0.19 0.64 3.39
3 0.29 0.88 3.04 0.28 0.89 3.17
5 0.54 0.97 1.79 0.54 1.30 2.40
7 0.84 1.24 1.48 0.83 1.66 2.00
11 1 0.11 0.33 3.02 0.11 0.35 3.23
2 0.23 0.61 2.66 0.22 0.77 3.46
3 0.36 0.82 2.30 0.34 1.14 3.31
5 0.64 1.22 1.92 0.63 1.69 2.67
7 0.95 1.57 1.65 0.94 2.18 2.32
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Literature Cited
Boyd, C. E. and T. Ahmad. 1987. Evaluation of aerators for
channel catfish farming. Ala. Agr. Exp. Sta., Auburn Univ., Ala., Bulletin 584. 52 pp.
Boyd, C. E. and B. J. Watten. 1989. Aeration systems in
aquaculture. CRC Critical Reviews in Aquatic Sciences 1:425-472.
Colt, J. 1984. Computation of dissolved gas concentrations
in water as functions of temperature, salinity, and pressure. Amer. Fish. Soc., Spec.
Publ. No. 14. 154 pp.
Soderberg, R. W. 1982. Aeration of water supplies for fish
culture in flowing water. Prog. Fish-Cult. 44:89-93.
Design Example
A simple way to determine the amount of fish biomass that
can be supported by an aeration system will be described. Suppose that a 2-hp aeration
system that has an SOTR of 4 lb of oxygen/hr is to be used in a rearing tank that receives
18°C water containing 5 mg/I DO. The aeration system should keep the DO concentration
above 5 mg/I at all times. From Fig. 1, the correction factor from estimating the actual
oxygen transfer rate from the SOTR at 18°C and 5 mg/I DO is 0.44. Thus, the aeration
system will transfer 1.76 lb of oxygen/hr or 42.2 lb oxygen/day to the water. This is
enough oxygen to permit the application of 211 lb feed/day. If fish are fed at 3% of body
weight per day, the estimated maximum, permissible standing crop is 7,000 lb of fish.
However, this would provide no margin of safety. I suggest a safety factor of at least 1.5
(a safety factor of 2 would be better). The maximum permissible standing crops of fish
would be 4,700 lb and 3,500 lb for safety factors of 1.5 and 2, respectively.
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