Fish, like all animals, must obtain oxygen from the environment for respiration. Oxygen is far less available to aquatic organisms than it is to air-breathers, and the dissolved oxygen content of water may limit the activities of fish.
In most natural waters, the supply of oxygen to water (diffusion from the atmosphere and production from underwater photosynthesis) exceeds the amount used in oxygen-consuming processes, and fish seldom have problems obtaining enough oxygen to meet normal metabolic demands.
In aquaculture ponds, however, the biomass of plants, animals and microbes is much greater than in natural waters, so oxygen is sometimes consumed faster than it is replenished.
Depending on how low the dissolved oxygen concentration is and how long it remains low, fish may consume less feed, grow more slowly, convert feed less efficiently, be more susceptible to infectious diseases, or suffocate and die. Aquaculturists avoid these problems by aerating ponds mechanically to supplement normal oxygen supplies.
Principles of aeration:
The rate of oxygen movement between air and water is described by the gas transfer equation:
dC/dt = KL(A/V)(Cs–Cm).
In the equation:
dC/dt =the rate of oxygen transfer between a liquid and a gas;
KL=the liquid-film coefficient; A/V =the ratio of the air-water interfacial area to water volume;
Cs=the dissolved oxygen concentration when water is saturated with oxygen under the prevailing conditions of water temperature, salinity and atmospheric pressure; and Cm=the measured dissolved oxygen concentration.
The liquid film coefficient, KL, incorporates a parameter called the surface renewal rate, which is related to turbulence within the liquid. The gas transfer equation looks complicated, but it is actually simple to interpret.
The equation says that the rate of oxygen transfer between air and water depends on three factors: the amount of turbulence, the ratio of surface area to water volume, and how far the prevailing dissolved oxygen concentration deviates from the dissolved oxygen concentration at saturation.
This deviation is called the saturation deficit or surplus, depending on whether the measured concentration is greater than or less than the saturation concentration.
The effect of surface area and turbulence:
Oxygen moves to and from water across the air-water interface. So, a greater amount of oxygen can enter or leave a given amount of water when the air-water interfacial area is increased. However, even if the water is initially low in oxygen, the thin film of water at the interface of a calm water surface quickly becomes saturated with oxygen, which dramatically slows the rate of oxygen diffusion into the water.
Turbulent mixing restores the saturation deficit in the surface film by moving oxygenated water away from the surface, increasing the overall rate of oxygen transfer.
The effect of prevailing dissolved oxygen concentration:
Dissolved oxygen moves into or out of water by diffusion. The rate of diffusion depends on the difference in oxygen partial pressures between the liquid and gas phases— the greater the difference, the greater the driving force moving oxygen from one phase to the other.
The maximum rate of oxygen transfer into water occurs when the dissolved oxygen concentration in water is 0 mg/L, the point at which the maximum difference in oxygen partial pressures between water and air occurs.
As dissolved oxygen concentrations increase from 0 mg/L, the oxygen partial pressure difference between air and water steadily decreases up to the point where the dissolved oxygen concentration equals the saturation concentration.
At that point, there is no difference in oxygen partial pressure between water and air (this is, in fact, the definition of “saturation”). Because there is no driving force compelling oxygen molecules to leave or enter water, no oxygen can be added to water no matter how much effort is made to increase turbulence or airwater interfacial area.
When the dissolved oxygen concentration is greater than the saturation concentration (the water is supersaturated with oxygen), the oxygen partial pressure in water is greater than in air and oxygen moves from water to air.
In other words, aeration causes the dissolved oxygen concentration to decrease. This process is called “degassing.”
Implications for aeration:
Each of the three factors in the gastransfer equation has important implications for pond aeration. The effects of surface area and turbulence are obvious. Aerators increase the air-water interfacial area by breaking water into fine drops or creating bubbles. Aerators also create turbulence that renews the surface film and moves oxygenated water away from the aerator.
Implications of the third factor— the oxygen partial pressure difference between air and water— are a bit more complicated. The oxygen partial pressure differential can be increased (thereby increasing the oxygen transfer rate) by increasing the saturation concentration (Cs), decreasing the measured oxygen concentration (Cm), or both. For surface aerators that splash or spray water into the air, the saturation dissolved oxygen concentration is set by prevailing atmospheric pressure, water temperature and salinity.
Under practical aquaculture conditions, the culturist cannot modify those environmental variables to increase the partial pressure differential and improve aerator performance. However, measured dissolved oxygen concentration in the pond varies diurnally, so the culturist can control aerator oxygen transfer rates by selecting when to begin aerating. This can be demonstrated by looking at a couple of extreme examples.
First, if aerators are operated on sunny afternoons when water is supersaturated, oxygen will be lost (degassed) from the water. So, unless the goal is to remove oxygen from the water or simply to mix the water, supersaturated water should not be aerated.
Second, the rate of oxygen transfer is greatest when the measured dissolved oxygen concentration is very low. In fact, oxygen- transfer rates can be maximized by waiting until dissolved oxygen concentration falls to 0 mg/L before aerating, but this has obvious drawbacks (the fish would be dead by then).
On the other hand, aerating water when dissolved oxygen concentrations are near saturation is wasteful because oxygen transfer and aeration efficiency are very low under those conditions. So there are important trade-offs between biological goals (optimizing aquatic animal health by maintaining dissolved oxygen levels above some critical threshold) and physical constraints (aerator efficiency and oxygen transfer decline as dissolved oxygen concentration approaches saturation).
Although the oxygen saturation concentration cannot be manipulated to improve oxygen-transfer rates of surface aerators, this is not true for deep-water diffuser aerators (bubblers) or aerators that use pure oxygen as the gas phase.
Those aerators can be very efficient because they operate under conditions where the saturation dissolved oxygen concentration is higher than in surface waters.
Aerator performance:
There are two ways of describing aerator performance. The standard oxygen transfer rate (SOTR) is the amount of oxygen added to water in 1 hour under a standard set of conditions. The units of SOTR are pounds O2/hour, which can be multiplied by 0.45 to derive the metric equivalent in kg O2/hour. Standard aeration efficiency (SAE) is the standard oxygen transfer rate divided by the power requirement in horsepower (hp). Units of SAE are pounds O2/hp?hour, which can be multiplied by 0.61 to derive SAE in metric units of kg O2/kW?hour.
Boyd (1998) thoroughly describes aerator performance testing and how to interpret and use SOTR and SAE values. Aerators transfer less oxygen under pond conditions than under the standard conditions of aerator performance tests, so SOTR and SAE values are best used to compare similar styles of aerators as an aid in selecting equipment to purchase rather than as design criteria for pond use. Also, small differences in SOTR and SAE values are not meaningful because test conditions may vary and affect results.
Boyd and Ahmad (1987) compiled SOTR and SAE values for a variety of aerators used in pond aquaculture. Good SAE values and durability are most important when selecting aerators for general day-today use. On the other hand, high SOTR values and mobility are important for aerators used to save fish in distress. Other factors such as cost, durability, specific application and ease of service must also be considered when selecting an aerator.
Durability varies widely among aerators, and prospective buyers should consult the owners of various kinds of equipment for advice and recommendations.
Author:
Craig Tucker
