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
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
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.
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
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
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
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.