Final Report: T.A. Brown Mechanical AeratorEPA Grant Number: SU833149
Title: T.A. Brown Mechanical Aerator
Investigators: Schaad, David , Canning, Patrick , Coles, Jeff , Ernst, Alison , Pearson, Lee , Voorhees, Leslie , Yamanaka, Yvonne
Institution: Duke University
EPA Project Officer: Nolt-Helms, Cynthia
Project Period: September 1, 2006 through May 30, 2007
Project Amount: $10,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2006) RFA Text | Recipients Lists
Research Category: P3 Challenge Area - Agriculture , Pollution Prevention/Sustainable Development , P3 Awards , Sustainability
The main purpose of this project was to design and construct a sustainable mechanical aerator to be used in shrimp ponds in Indonesia. After the tsunami, the natural aeration of the tidal rivers that oxygenated the tambaks, or shrimp ponds, was effectively eliminated. In order to sustain a healthy environment for the growth of shrimp, mechanical aeration would be a suitable method for oxygenating the aquaculture ecosystem. Unfortunately, most shrimp farmers lack the resources, both financial and temporal, to construct and employ their own aerators, let alone determine the most effective and efficient design. From this need, shortly following the tsunami, the project was developed.
The first phase of this project consisted of a group of Engineers Without Borders students constructing and employing a conceptual design for a paddlewheel aerator in Indonesia. This aerator was not tested quantitatively, so the goal of this project was to build on those original ideas, including additional more rigorous testing and several potential designs. The aspects evaluated as part of the design process and objective constraints determined following this testing included: determining the most efficient and practical power source; choosing a material or combination of materials with which to build the aerator, based on cost and local availability; and of course the actual design of the device, which consists of several aspects within itself, namely paddle size, shape, and placement, bearing system, and torque transfer method.
Specific goals regarding evaluating the effectiveness of the aerator were also considered. In order to determine the efficiency of an aerator, it was necessary to calculate the SAE (Standard Aerator Efficiency), which is defined as the SOTR (Standard Oxygen Transfer Rate) divided by the power required to turn the aerator. In order to measure the SOTR, the team conducted a series of tests with different prototypes and employing variations on each of these prototypes such as paddle placement. These tests consisted of a controlled tank of deoxygenated water; then, as the aerator is turned at a specified RPM, the oxygen content was recorded with a DO meter in time. These data points were used to calculate the SOTR. Then, the group used force sensors to determine the amount of force it takes to turn the aerator at a specified RPM and use dynamic equations to determine the power output. From this, the SAE of each variation of each prototype can be calculated. The group also considered various methods of torque transfer to rotate the paddles.
Through these series of tests, combined with extensive background research to determine the more basic elements of design, the students developed an efficient aerator design, in terms of SAE, cost, and material choices. As part of Phase II of this project, the team is planning to travel to Indonesia to implement it.
After performing many of the aforementioned tests, the team was able to draw several conclusions about the efficiency of the aerator. First of all, a manual, pedal-power source was decided upon. It is sustainable, very available, and easy to employ. Wind power was discarded due to inconsistent climate in the area; though average wind speeds could provide enough power to sustain the aerator, wind movement are not consistent throughout the year, and aeration must be continual. A method of storing this energy could be researched as part of Phase II. Also, wind power could be used if this aerator were to be implemented in places other than Indonesia. Solar power, though a good sustainable option, has startup costs that are too high for this need. Therefore, pedal power was selected as the optimum power supply mechanism.
The torque transfer methods considered included a rubber belt, metal chain, and rope around large spools. A simple rubber belt situated in a figure eight, which is locally available, inexpensive, durable, and allows for continuous pedaling in the same direction while allowing the paddlewheel to go the opposite direction, may be the best method. If a rubber belt is not attainable in Indonesia, the rope surrounding spools is probably the next best method. The metal chain was decided against because of high likelihood of corrosion as well as the fact that it would need to be greased, which could harm shrimp health.
Regarding empirical tests conducted, the paddle orientation had a large effect on the SOTR. Three different paddle orientations were tested: convex, which ‘scooped’ up the water as it entered, concave, which did not scoop, and sideways, which was rotated 90° from either of the aforementioned orientations. The convex paddles had an O2 Deficit vs. Time slope of nearly the same as that of the concave paddles, which produces a similar SOTR. The concave paddles’ SOTR was slightly, but not significantly, higher than the convex paddles’ SOTR. The sideways paddles were by far the least effective; their O2 Deficit vs. Time slope was only one-tenth of the concave paddles. This was clear to see simply by observing the testing; the convex paddles scooped up water and poured it out the other side as it rotated, inducing significantly greater turbulence in the water. The concave paddles splashed less but also ‘pushed’ the water underneath the surface more, and the sideways paddles sliced cleanly through the water, with minimal agitation of the water. Though the power calculations confirmed that rotating the sideways paddles required less power than the convex or concave, which required nearly the same as each other, the convex and concave paddles were still by far the most efficient.
When evaluating material construction options, the team had the best experience with the PVC aerator, which had semi-circular paddles. It was much easier to work with than the metal aerator and much lighter. In addition, PVC is much cheaper and more locally available in Indonesia than metal. This PVC aerator could be used in a metal, wood, or PVC bearing system, in decreasing order of idealness. Metal poses the most resistance to friction, and PVC-on-PVC is the least effective; the material rotating against itself tends to represent the same deformations in each piece and therefore ‘sticks’ at these deformations more frequently than a smooth piece of metal or wood. In addition, PVC cannot be smoothed in the manner that wood can be sanded.
In summation, a variety of serviceable, sustainable and suitable aerators have been conceptualized, fabricated and tested. These aerators show promise for use in aquaculture applications in developing countries with observable benefits to both aquaculture farmers and their communities. Specific design conclusions derived from the testing indicate that convex and concave paddles seemed to be the most efficient way of orienting the paddles on the axle in order to generate the optimum turbulence which increased oxygenation. This follows with previous research and experimentation on aerators; the more disturbance in the water, the more aeration occurs. When considering materials of construction, PVC is most likely the best option; however, wood and bamboo are still likely to be considered upon arrival in Indonesia. Metal will most likely not be used for the axle and paddlewheel due to its cost in Indonesia and lack of machinery to construct on site. The bearing system, however, will be relatively simple and could be made of either metal or wood. The torque transfer method would ideally consist of a rubber belt, but rope is still a viable option. These design combinations will also lead to a very low-cost aerator, which is a significant constraint on the project.
Proposed Phase II Objectives and Strategies:
The proposed Phase II portion of this project can be divided into three main components: providing local aquaculture with other forms of support to aid restoration, outside of aeration, implement a locally owned and operated small business to construct these aerators and thus boost the economy of the area, and promote alternative energy sources, such as tidal power and perhaps wind power to turn the aerator.
In order to enact Phase II, the group must work closely with residents of the area. Doing so requires multiple trips to Indonesia in order to establish and maintain contact with the local aquaculture community in Aceh. Particularly, the group would have to work closely with those who are going to run the small aerator manufacturing business. Beginning any new business, no matter how small, requires significant planning and startup costs.
In addition, more intensive research regarding tambak restoration will be carried out for Phase II. The research will be similarly as intense as the research done for this project; however, instead of only encompassing aeration, which is one method of tambak restoration, it will include many more methods of improvement, such as soil quality, rebuilding channels, and perhaps measuring other qualities of the water besides oxygen content.
Finally, alternative energy sources will be more thoroughly researched and tested to see how feasible they are and by how much they would reduce the cost and improve the environment.