Pond Aeration

Types and Uses of Aeration Equipment Gary L. Jensen,* Joseph D. Bankston,* and John W. Jensen**

No one should attempt to be a com- mercial fish farmer without having 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 concentra- tions lower than 3 to 4 parts per mil- lion (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 * Louisiana Cooperative Extension Service * * Alabama Cooperative Extension Service Texas Agricultural Extension Service and increases oxygen transfer ef- ficiency 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 quick- ly, 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 im- prove during daylight hours. How to evaluate an aerator Several years ago, the aquaculture in- dustry used no standard methods for comparing or testing oxygen transfer capabilities or efficiencies of aerators. Today, aerators are tested to determine the rate at which they transfer oxygen into water. These tests are conducted in large tanks under standard conditions with clean tap water at 68° F and no initial dissolved oxygen. Two terms are commonly used to compare the 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 and is reported as lb O2/hr. The standard aeration efficiency (SAE) is the standard oxygen transfer rate divided by the amount of power re- quired and is expressed as lbs O2/hr per horsepower (hp) or lbs O2/hp-hr. Ratings for tractor-powered aera- tors are generally given as standard oxygen transfer ratings (SOTR). Other, usually smaller, aerators are normally given standard aeration efficiency ratings (SAE). 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. Test results of different aerators can be compared in selecting an effec- tive and energy-efficient unit. Some manufacturers test their own equip- q Zerle L. Carpenter, Director q The Texas A&M University System q College Station, Texas Page 2 ment. When comparing test results, it is important to know if test condi- tions were standardized. Also, an aerator may have a high oxygen transfer rate with low efficiency rating. Cost of operation should be less for a more efficient aerator. Types of aerators Fish farmers have used emergency aerators powered by tractor power takeoffs (PTOS) for many years. With production intensification and the increasing need for aeration, these PTO aerators can be quite expensive because each aerator requires a trac- tor. Therefore, more electric aerators are being used than ever before. Large tractor-powered aerators are still used as back-ups during severe oxygen depletions, equipment failure, or power outages. Each producer decides which aera- tion device should be purchased or built. This decision is important and should be made with the specific ap- plication and associated costs of energy and equipment in mind. Several types of aeration devices have been evaluated for use in com- mercial fish ponds. Most aerators are in one of the following categories: surface spray or vertical pump, pump sprayer, paddlewheel, diffused air and propeller aspirator pump. Surface spray or vertical pump Surface spray aerators have a sub- mersible 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 dis- solved oxygen concentrations are ex- pected. 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 create an ade- quately large area of oxygenated water. Pump sprayer Pump sprayer aerators are found on many fish farms. Most are powered by a tractor power takeoff or elec- tricity. 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 “bon- net” 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 verti- cally 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 re- duction 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. Paddlewheel aerators Paddlewheel aerators have been used on cattish farms for many years. Farm-made paddlewheels are usual- ly made from 3/4 ton truck differen- tials 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 in- creasing 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 opera- tion is limited by the tractor, its recommended power takeoff speed under load, and the gear reduction of the paddlewheel. The shape of the paddles is also im- portant; for example, U, V, or cup shapes are more efficient designs than flat paddles. Paddlewheels cre- ate 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 paddlewheel drum. The size of the spray pattern likewise in- creases. The power required to operate a paddlewheel 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 paddlewheel aerators for all ponds and move these units from pond to pond. A paddlewheel, though mo- bile, can be difficult to situate in the pond properly so that it is effective without damaging itself or the trac- tor. Before emergencies occur, their locations should be selected and sev- eral trial runs should be conducted so that situating becomes more or less routine. Paddlewheels 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 paddlewheel to reduce erosion of the pond bot- tom. It is also important to block the tractor to prevent it from slipping back and increasing the load on the tractor. Table 1 summarize the per- formance results for several types of paddlewheel aerators powered by farm tractors. Page 3 Table 1. Test results of two sizes of paddlewheels. The power source was an 87 hp tractor. PTO Tractor shaft Paddle engine Power Fuel lb SAE speed depth speed reqmt. SOTR consumption O2/gal lb (rpm) (inches) (rpm) (hp) (lb O2/hr) (gal/hr) fral O2/hp-hr PTO Paddlewheel, 4-inch drum 540 4 1800 4.9 15.2 1.6 9.5 3.1 1000 4 950 4.8 15.2 0.7 21.7 3.2 540 14 1800 16.9 45.1 2.0 22.6 2.7 1000 14 950 16.7 45.1 1.2 37.6 2.7 PTO Paddlewheel, 20-inch drum 540 4 1800 12.4 26.0 1.8 14.4 2.1 1000 4 950 12.0 26.0 1.0 26.0 2.2 540 14 1800 40.2 90.0 3.0 30.0 2.2 1000 14 950 39.0 90.0 2.3 39.1 2.3 When fish are stressed with low dissolved oxygen, they often go to shallow areas of the pond near the banks. The type and design of the paddlewheel 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 paddlewheel aerator with limited ground clear- ance 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 paddlewheel Electric paddlewheel units are 4 to 12 feet long with paddles of trian- gular cross section and a total drum diameter of about 28 to 36 inches. Paddlewheel 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 range from 1/2 hp to 19 hp and larger. 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 and 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 aera- tion lines or diffuser stones 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 re- leased and water depth. The smaller the bubble and the deeper it is re- leased, 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 cat- fish 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 clean- ing is required. Also, the air lines in- terfere with harvesting. Propeller-aspirator pump 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 dif- fusing surface which pulls air down Page 4 Page 5 the hollow shaft. Air passes through a diffuser 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. Table 2 summarizes the performance results of units of various sizes. Table 2. Performance data on propeller-aspirator pump aerators. Approximate Power at Aerator SOTR Approximate SAE Shaft (hp) (lb O2/hr) (lb O2/hp-hr) 1 2.2 2.2 5 11.0 2.2 10 23.2 2.3 15 45 3.0 This publication was supported in part by a grant from the United States Department of Agriculture, Number 87-CRSR-2-3218, sponsored jointly by the Cooperative State Research Service and the Extension Service. Educational programs conducted by the Texas Agricultural Extension Service serve people of all ages regardless of socioeconomic level, race, color, sex, religion, handicap or national origin. Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as amended, and June 30, 1914, in cooperation with the United States Department of Agriculture. Zerle L. Carpenter, Director, Texas Agricultural Extension Service, The Texas A&M University System. 2M–5-90, Reprint FISH