By: John W. Jensen, Department of Fisheries and Allied Aquaculture,
Alabama Cooperative Extension Service, Auburn University
No one should attempt to be a commercial fish farmer without aeration devices and the knowledge of when and how to use them. Aerators can be used exclusively for emergencies, continuously at night, or all day and night.
Today, aeration equipment is most commonly used in emergencies to keep fish alive and minimize stress associated with oxygen concentrations lower than 3 to 4 parts per million (ppm). Aerators used in this manner are not intended to aerate the entire pond but just a portion of it. Fish move to the zone of oxygenated water found near the aerator. Enough oxygen is supplied to save fish, but not to increase oxygen levels greatly in the entire pond.
Aerators work by increasing the area of contact between air and water. Aerators also circulate water so fish can find areas with higher oxygen concentrations. Circulation reduces water layering from stratification and increases oxygen transfer efficiency by moving oxygenated water away from the aerator. Many units are electrical so wiring should be properly protected and installed to avoid any hazards from an electrical shock.
If a pond shows a decreasing oxygen pattern that will reach 3 ppm or less before sunrise, emergency aeration should be used. In most situations, the critical time for low oxygen levels is from midnight to sunrise. If the oxygen concentration is falling quickly, however, aerators should be started when oxygen reaches 4 ppm. This creates a sufficiently large area of aerated water that fish will find and remain in until oxygen levels improve during daylight hours.
Two terms are commonly used to compare aerator performance. The standard oxygen transfer rate (SOTR) is the amount of oxygen that the aerator adds to the water per hour under standard conditions (68F and no initial oxygen) and is reported as lb O2/hr. The standard aeration efficiency (SAE) is the standard oxygen transfer rate divided by the amount of power required and is expressed as lbs O2/hr per horsepower (HP) or lbs O2/HP-hr. Efficiency ratings are based on the horsepower applied to the aerator shaft and not the horsepower of the power source. Most commercial aerators have ratings between 1 and 5 lbs. O2/HP-hr.
Ratings for tractor-powered aerators are generally given as standard oxygen transfer ratings (SOTR). Other, usually smaller, aerators are normally given standard aeration efficiency ratings (SAE).
Types of Aerators
Fish farmers have used emergency aerators powered by tractor power takeoffs (PTOS) for many years. These PTO aerators can be quite expensive because each aerator requires a tractor. Therefore, most ponds are equipped with electric aerators. Large tractor-powered aerators are used as back-ups during severe oxygen depletions, equipment failure, or power outages. Several types of aeration devices have been evaluated for use in commercial fish ponds. Most aerators are in one of the following categories: surface spray or vertical pump, pump sprayer, paddle wheel, diffused air and propeller aspirator pump.
Surface Sprag or Vertical Pump
Surface spray aerators have a submersible motor which rotates an impeller to pump surface water into the air as a spray. They float, are lightweight, portable and electrically powered. Units of 1 to 5 HP with pumping rates of 500 to 2,000 gpm are available. They are designed to be operated continuously during nighttime, cloudy weather, or when low dissolved oxygen concentrations are expected. Surface spray aerators have prevented fish kills when used at 1.5 to 2 HP/acre. They are usually of little use in large ponds, because of relatively low oxygen transfer rates and their inability to increase an adequately large area of oxygenated water.
Pump sprayer aerators are found on many fish farms. Most are powered by a tractor power takeoff or electricity. Some units are engine driven and require mounting on a trailer frame for transport. Pump sprayer aerators are equipped with either an impeller suction pump, an impeller lift pump, or a turbine pump. Some have a capped sprayer pipe or "bonnet" with outlet slits attached to the pump discharge. Others discharge directly through a manifold which has discharge slits on top and outlets at each end. Water is sprayed vertically through the discharge slits and from each end of the manifold. This type is commonly referred to as a T-pump or bankwasher and directs oxygenated water along a pond bank where distressed fish often go. Pump sprayers typically have no gear reduction which reduces mechanical failure and maintenance. These units do not erode the pond bottom, and minimum operating depth is reached when the intake is covered with water.
Paddle wheel Aerators
Farm-made paddle wheels are usually made from 3/4 ton truck differentials and vary with drum size and configuration, shape, number and length of paddles. Units are powered by power takeoffs or driven by self-contained diesel engines. The self-contained units are usually on floats and attached to the pond bank or held in place by steel bars secured in the bank or pond bottom..
Studies have demonstrated that increasing either the speed of the drum rotation (rpm) or paddle depth generally increases aeration capacity. Paddle depth affects oxygen transfer rates more than does the speed of rotation. This increase in capacity is not cost free, because horsepower requirements increase and oxygen transfer efficiency may decrease. The maximum rotational speed of a tractor-powered paddle wheel aerator for extended operation is limited by the tractor, its recommended power takeoff speed under load, and the gear reduction of the paddle wheel.
The shape of the paddles is also important; for example, U, V, or cup shapes are more efficient designs than flat paddles. Paddle wheels create vibrations that can be reduced when paddles are arranged in a spiral pattern.
The oxygen transfer rate and power requirement increase with paddle immersion depth and the diameter of the paddle wheel drum. The size of the spray pattern likewise increases. The power required to operate a paddle wheel aerator at any given speed and paddle depth is constant. Fuel consumption and operating costs depend on the power source.
Most producers do not have enough paddle wheel aerators for all ponds and move these units from pond to pond. A paddle wheel, though mobile, can be difficult to situate in the pond properly so that it is effective without damaging itself or the tractor. Before emergencies occur, their locations should be selected and several trial runs should be conducted so that situating becomes more or less routine.
Paddle wheels can erode a hole in the pond bottom during operation. If the aerator settles into a hole while running, the load increases and reduction gears can break. Weld a metal plate under the paddle wheel to reduce erosion of the pond bottom. It is also important to block the tractor to prevent it from slipping back and increasing the load on the tractor.
When fish are stressed from low dissolved oxygen, they often go to shallow areas of the pond near the banks. The type and design of the paddle wheel aerator may affect the ability to direct the water along the pond bank where the fish tend to congregate. Another consideration is the ground clearance under the frame of the aerator. A paddle wheel aerator with limited ground clearance may get caught on high spots, such as a levee crown, while high clearance models can traverse these areas with ease, but may operate too shallowly to be effective.
Electric Paddle wheel
Electric paddle wheel units are 4 to 12 feet long with paddles of triangular cross section and a total drum diameter of about 28 to 36 inches. Paddle wheel speed is usually 80 to 90 rpm with a paddle depth of about 4 inches, enough to load the motor. The correct paddle depth can be determined in the field as the depth needed to draw the rated amperes of the motor. To extend the service life of the motor, the motor should draw only 90 percent of full load amperes rating, unless the manufacturer recommends differently. Motor sizes generally range from HP to 10 HP. Motors operating on single- or three-phase current are available.
Methods used to reduce the motor speed to the desired aerator shaft speed include v-belts and pulleys, chain drive, gears and gearboxes. Shafts of most electric motors run at 1,750 rpm and most units are mounted on floats.
Diffused Air Systems
Diffuser aerators operated by low pressure air blowers or compressors forcing air through weighted aeration lines or diffusestones release air bubbles at the pond bottom or several feet below the water surface. Efficiency of oxygen transfer is related to the size of air bubbles released and water depth. The smaller the bubble and the deeper it is released, the more efficient this type aerator becomes. When tested at normal catfish pond depths, these aerators were found to be inefficient compared to other devices.
Limited studies in commercial catfish ponds showed no improvement in fish production when a diffused aeration system was used. One of the biggest problems with diffused-air systems is clogging of the air lines and diffusers so that periodic cleaning is required. Also, the air lines interfere with harvesting.
These aerators consist of a rotating, hollow shaft attached to a motor shaft. The submerged end of the rotating, hollow shaft is fitted with an impeller which accelerates the water to a velocity high enough to cause a drop in pressure over the diffusing surface which pulls air down the hollow shaft. Air passes through a diffuse and enters the water as fine bubbles that are mixed into the pond water by the turbulence created by the propeller. They are electrically powered, and models range from 0.125 to 25 HP.