Location: Warmwater Aquaculture Research Unit
Title: Development and on-site field testing of the power-tube airlift aerator and chances for commercialization Authors
Submitted to: NWAC (National Warmwater Aquaculture Center) Aquaculture Newsletter
Publication Type: Abstract Only
Publication Acceptance Date: November 4, 2013
Publication Date: N/A
Technical Abstract: Aeration of ponds when dissolved oxygen (DO) concentrations are low is the principal management tool that allows for higher feeding rates, increased production, and decreased cost per unit fish produced. Recent research conducted at the USDA-ARS Warmwater Aquaculture Research Unit has shown that feed conversion ratio (FCR) can be improved by raising the minimum DO concentration to 1.6 mg/L. Catfish will also consume more feed and grow faster if the minimum DO concentration is not allowed to drop below 3.0 mg/L. This faster growth results in a shorter production period, better survival, improved FCR, and reduces economic risk. Traditional 10-hp electric paddlewheel aerators have proven to be the most efficient aerator for routine aeration of commercial catfish ponds, although they do have a performance limitation. While they efficiently transfer large quantities of oxygen to the water (measured in pounds of oxygen transferred per horsepower-hour), they also move a great volume of water. The end result is that the concentration of oxygen in the water is only increased a small amount (perhaps only 1.0 mg/L) by aeration. During routine aeration this is sufficient, but when the pond DO concentration is very low from a bloom die-off, this can result in problems ranging from reduced feed consumption and growth to complete fish kills. Adding more aerators (tractor-powered sidewinders or paddlewheels) increases the current in the pond, but in many cases can’t overcome the tremendous oxygen demand resulting from the lack of photosynthesis and decomposing bloom. The fish can maintain their position at the aerators for a while, but if pond conditions do not improve quickly, they eventually become exhausted and drift away from the aerators into water devoid of oxygen, resulting in a major fish kill. Unfortunately, most farmers have experienced this tragic loss. Subsurface aerators are an option to paddlewheel aerators. However, fish ponds are seldom more than 5 feet deep and technologies such as diffused-air systems require greater depths to operate efficiently. A new aerator developed by researchers in Stoneville, known as the “Power Tube Airlift” (PTA), has the ability to use low pressure air to increase the transfer rate and efficiency to a level comparable to paddlewheel aerators. The PTA was also designed to move less water than a paddlewheel aerator but enough water to create a zone of elevated DO large enough for a great biomass of fish. Two commercial-size PTAs were fabricated and installed in an 8 acre traditional catfish pond on-site. Each PTA consisted of a 3.0-ft diameter intake, a 4.0-ft diameter outflow, and a blower powered by a 10-hp, 3-phase electric motor (Figure 1). A main air supply line was plumbed into a sparger assembly positioned at a 25 ft water depth on the outflow side of each unit. The high-volume discharge through the sparger (larger bubbles) provided “lift” to move the water through the PTA. These larger bubbles increased water pumping efficiency. Approximately 10-15% of the total air flow was diverted to a diffuser grid (smaller bubbles) placed at an 8 ft water depth on the intake side. These smaller bubbles increased oxygen transfer to the water. For a preliminary field test in 2012, hybrid catfish stockers were conservatively stocked at about 2,100/ac (600 lb/1,000) for food fish production. Dissolved oxygen was continuously monitored with an automated monitoring system that controlled the operation of the PTAs throughout the study. The two PTA aerators were able to maintain DO concentrations of about 2.3 mg/L greater than the outside area of the pond. For example, during a 2-week period in July 2012 the average morning DO concentration for the outside area was 1.0 mg/L while the aeration zone was maintained at 3.3 mg/L. The aeration zone was approximately 0.6 acres or about 7% of the total pond volume. A total of about 6,000 lbs/acre of food fish were harvested that weighed 3.1 lb each with a 91.2% survival rate and FCR of 2.2. In 2013 we stocked hybrid fingerlings at rate comparable to split-ponds (9,200/acre at 120 lb/1,000). We expect to harvest close to three times the biomass (total weight) of food fish this year as compared to the 2012 study which will be a another important field test using this technology. A total of $22,685 ($11,343 each) was required to build and install these aerators. Partial budget analysis of 2012 production using PTA aerators indicated a net change of -$61 per acre as compared to using paddlewheel aerators. The negative net change was due to elevated fixed costs associated with additional expenses of the PTAs. The higher stocking rates this year should provide a greater profit and further economic analysis will need to be conducted. In summary, two commercial-size PTAs were designed to perform similar to 10-hp paddlewheel aerators but move less water and are able to concentrate the aeration effort more precisely. This is an advantage because fish want have to expend as much energy swimming against water current to be close to the aeration source as compared to paddlewheel aerators. In addition, repairs and maintenance cost are lower with PTAs due to a simplified design with fewer moving parts. Maintenance is also much easier since the primary driving force of a PTA is located on the levee as compared to paddlewheel aerators which are in the water. Lastly, most farms have a limited supply of tractor-powered aerators and PTAs could be used as an alternative during emergency conditions. Unfortunately, the initial investment of one PTA is twice of what a new paddlewheel aerator costs and ponds have to be drained and dried prior to installation. These two factors can be seen as huge disadvantages to farmers due to initial costs and practicality issues. Catfish food fish production will continue on-site using PTA technology to verify our initial research findings and to prove our concept through demonstration with the goal of moving to eventual on-farm field trials. Some of the material discussed in this article comprises the subject matter of a patent application currently pending with the US Patent and Trademark Office.