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Massive Aeration

 

August 20, 2012

Luis F. Botero (botero_fernando@yahoo.com): Hi all, I'm looking for a "massive" (large volume) aeration system capable of aerating a canal that is about 4,000-meters long, 60-meters wide on the surface, 30-meters wide on the bottom and three-meters deep. The canal is fed by a pump station bringing in an average of 20 cubic meters of brackish water per second with an average temperature between 28ºC and 30ºC. The objective is to increase the oxygen content of the water before it reaches the shrimp ponds.

Jorge Lango (jlango@yahoo.com): Wow! That looks like an expensive project.

Luis F. Botero (botero_fernando@yahoo.com): Hi Jorge, this is for a successful farm that periodically faces quality problems with its incoming water. Currently, when the incoming water is bad (low oxygen and high organic matter), the farm stops pumping new water and recycles its pond water back into the system. So far it has been able to successfully manage the situation with aeration in the ponds, but it would like the additional option of improving its incoming water. Do you have any hardware suggestions?

Ramon Macaraig (monmac52@yahoo.com): Luis,

if this were my farm, I would introduce waterfalls, baffles and sprays in the canal. Fountains may even work. You should also consider growing more green algae in the canal and using underwater mixers like paddlewheels and jet injectors to mix the water from noon until late in the afternoon to get the supersaturated oxygen in the surface water to the pond bottom where it would serve as a reserve during the night. Jet injectors are less efficient than paddlewheels, but are readily available.

Dallas Weaver (deweaver@mac.com): Your solution will depend upon the percent of time you have low oxygen conditions in your input water and the maximum exchange rate in your ponds. If the latter is small, the normal pond aeration systems should be able to handle a small inflow of low-oxygen input water, especially if it is well mixed with the pond water by directing the input into the front of an aerator.

If the percent of time that the input water is low in oxygen and the percent of water exchange per hour is high, then I would look at dropping the input water through a meter of mass transfer packing material (a 3-D waterfall). A meter drop with good packing material can take zero oxygen water to about 75% saturation in a fraction of a second. Flow rates of 2,000 liters/minutes/m2 operating co-current on the gas phase are reasonable.

[Off list, I asked Dr. Weaver to explain "operating co-current on the gas phase". He said: "It means that you have the air with the oxygen flowing downward in the column along with the water. ...The falling water will take the air with it, so no blower is needed. Co-current devices aren't as good at removing volatile impurities like off-flavor components, H2S and CO2 as counter current devices using an air blower to shove air upward in the column, but because there is so much oxygen in air relative to water, the airflow direction is irrelevant to the oxygen mass transfer. Adding a blower adds cost and energy with very little advantage for oxygen, but is worth it for trace chemicals like stripping H2S in well water."]

Luis, you have flow rates of 1,200 m3/minutes so you would need a mass transfer top surface area of about 600 m2, which is large but not impossible. If your pumps are discharging above the canal surface by a meter, it wouldn't change your energy requirements.

The other option is just to shift to a low pond flow rate with more aeration in the ponds, which may be a better place to put the energy.

M.E. Esmaeili (meesmaeili@yahoo.com): Dear Luis, how many hours does your pump station operate? If the average discharge is 20 m3/hour, then the volume of the pond will be changed three times in 24 hours.

Luis F. Botero (botero_fernando@yahoo.com): Ramon and Dallas, thanks for your comments! Daily water exchange is around 5% to 8%, done in 8 to 15-hour shifts. The pumping operation was originally conceived for an open system designed so that no energy would be wasted (meaning, avoiding pump discharges above maximum water level in the canal). It would be difficult to create waterfalls. The farm is built on a pretty flat land with minimum elevation changes; nonetheless, because it is a 1,000-hectare-plus farm, there might be some potential for setting up scattered waterfalls in the long network of canals that drain the ponds. Waterfalls in inlets and outlets structures also offer good opportunities for aeration. Destratifying and mixing the supersaturated surface water with the low-oxygen bottom water is another good suggestion.

Dandu Raju (dandu_raju169@yahoo.co.in): Dear Luis, I liked Ramon's idea of creating waterfalls, using air injectors and increasing algal production. I prefer air injectors and bottom-running aeration lines with fine diffusers connected to a 50-horsepower or larger air compressor. Faster shrimp growth, weight gain due to scheduled moltings, lower feed cost and lower water exchange costs outweigh the additional power costs. But the best way to increase oxygen levels in a brackish water body with a heavy organic load is to remove the organic load. You could do this with sand bed filters, which are simple and inexpensive. The top layer of sand must be removed periodically, dried in the sun, and then it can be put back into the bed. Also, to minimize the intake of silt and organic matter, which consume most of the oxygen in the water, it's important to pump on the high tide. Sand bed filtration will increase the amount of oxygen in your water and improve the health of your shrimp.

Daniel Gruenberg (seagardenfoods@mac.com): Hi Luis, you have gotten some very good suggestions, but since your body of water is so big, I suggest that you let the sun and algae produce your oxygen.

Photosynthesis will provide a significant amount of oxygen. The main issue in this case would be to have a means of mixing to prevent stratification and low oxygen conditions on the bottom of your canal. This could be done with large bubble diffusers placed about 70 centimeters off the bottom in the center of the canal. If you're willing to accept higher capital costs and a lower operating costs, you could use bio-fans (horizontal slow-rotating large diameter paddles) that do a great job of pulling super-saturated surface water down to the bottom where it is needed. Each bio-fan has an energy consumption of only around 0.06kW. They could be placed every 50 meters or so.

Paddlewheels and waterfalls in your situation are not really helpful because they are removing supersaturated oxygen, a step in the wrong direction. In a algae populated pond, the problem is not a lack of oxygen; it's the distribution of the oxygen in the water column. Waterfalls and paddlewheels spend capital and energy while creating another problem, the loss of supersaturated oxygen, and you still have the oxygen distribution issue.

Laurence Evans (ecotao@yahoo.com): Luis, because of the depth of your canal, I would use roots blowers and an airlift system of PVC pipes. One cubic meter of air will move roughly two cubic meters of water and aerate it at the same time. You could turn over the whole water column with PVC airlifts connected to roots blowers. I could supply you with roots blowers.

Luis F. Botero (botero_fernando@yahoo.com): Hello Daniel,

I like your suggestion of letting algae supply the oxygen. Back in late 1998, I had the opportunity to participate in the development and marketing of a water circulator as a tool for battling the emerging whitespot virus. It was a 36" fiberglass impeller, rotating at around 200 rpm, with a horizontal flow of 1.40 m3/sec and an incredibly low horsepower demand (about five horsepower, I think?). This unit was able to homogenize a five to six-hectare pond in about five hours. The movement induced in the pond (kinetic energy) remained for three or four hours after the circulator was turned off, and the low operational speed was such that no organic matter was stirred up from the pond bottom. The circulator allowed us to take the warm super-saturated surface water in the upper one foot of the water and mix it with the low-oxygen water on the bottom of the pond. I have not seen a replica of that good old 36" animal. Has anyone seen or know of any other equally big circulating system available now?

Daniel, you are right about the inconvenience of indiscriminant mechanical aeration when you have super-saturation oxygen provided for free by photosynthesis! During the development of the circulator, we discovered that mechanical aerators reduced the amount of oxygen in super-saturated surface waters where photosynthesis was taking place. We concluded that the best option was to have a mix of aeration and circulation.

From the several contributions given here, it appears that efforts should be focused on improving water conditions in the ponds, rather than in the canal.

Daniel Gruenberg (seagardenfoods@mac.com): Hi Luis, I had a project to update a horizontal paddlewheel. It was bigger than yours, about 4.5 meters in diameter and had a slower rotation (about 3 to 6 rpm) and even lower power (about 90W). I had issues with the staff that was working on this project so it was never implemented.

I am working with a Canadian company right now that has a very interesting mixing technology, and we are just starting some R&D projects to see if we can scale it for large ponds. The system uses hydrofoil-type blades arranged in a cone shape that create a vortex. It is too early to say if this will be a viable solution, but if you keep in mind that the main goal of any aerator should be to move the supersaturated water created by nature from the surface, where it is produce, to the bottom, where it is consumed, you will have a very elegant solution that works with nature and not against it.

I personally tested horizontal paddlewheels when I was in charge of a organic, Penaeus monodon project, and we were able to support about six tons of large monodon with only 50W of energy input. My feeling is that this could also support high populations of P. vannamei—if supplemented by central air diffusers to accelerate the upwelling current. Again it's the sun that does most of the work in this scenario so the energy inputs look unbelievable to most experienced shrimp farmers that are used to working against nature instead of with her.

Send me an email if you are interested in working together on this issue.

Patrick Wood (patrickjwood@yahoo.com): I had a project that involved underwater lights at night to destratify temperature layers and to continue photosynthesis, but it never got implemented.

Daniel Gruenberg (seagardenfoods@mac.com): The conversion of electricity into light energy is inherently inefficient. Total solar input into a pond is approximately 1kW/m2 or 10MW/Ha at its peak.

Bert Meijering (bert@topsybaits.nl): Algae have an efficiency of 1%. In no time the underwater lights would be covered with macro algae, which means a lot of maintenance.

Sources: 1. The Shrimp List. Subject: Massive Aeration. Friday, July 20, 2012. 2. Email from Dallas Weaver (deweaver@mac.com) to Shrimp News International. Subject: The Shrimp List "Massive Aeration". Sunday, August 19, 2012 .3. Summarized by2. Bob Rosenberry, Shrimp News International. August 19, 2012.

 

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