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Stacked Raceway Systems and Aeration

 

Bob Rosenberry (bob@shrimpnews.com): This was the toughest editing challenge I’ve had with one of my Shrimp List summaries.  If I got something wrong, please email me at the above address, and I’ll make your corrections within a day.  Thanks, Bob.

 

Daniel Gruenberg (daniel@acquestra.com): What are the benefits of using shallow, stacked raceways over deep tanks?

 

Durwood Dugger (ddugger@biocepts.com, http://www.biocepts.com/BCI/Home.html): Daniel, I have a long history of prototyping, demonstrating and doing economic feasibility research on intensive shrimp production systems, specifically stacked raceway systems.  I have done paid research on them for several clients and posted one of my reports on shallow stacked raceways (with permission of my client) on my webpage, and you can also find an intensive shrimp production article on my webpage.

 

Very briefly, the primary benefit of stacked raceways is lower real estate costs (smaller production foot print).  However, unless you are building your stacked raceways system in downtown Taipei or Hong Kong, a tank farm that’s closer to a feed mill is more practical and economical solution.

 

Penaeus vannamei seems more adapted to and capable of efficiently utilizing deep (three meters) production systems that are far less expensive to build.  I don’t believe the developers of shallow stacked raceways have done their economic homework.  Shallow stacked raceways have higher energy costs—related to higher flow friction and poor hydrodynamics for waste entrainment and cost-efficient removal—and they increase harvesting and labor costs.

 

The high capital costs of building stacked raceways—much higher than other intensive shrimp production systems—also increases the already high capital threshold costs of recirculating systems (making investment more difficult to find) and add to higher operational costs in terms of the cost of money, either in debt service and/or competitive capital investment efficiencies (more competitive investments that provide better returns at lower risks).

 

Daniel Gruenberg (daniel@acquestra.com): Durwood, you’re right.  I can build a tank that’s 4-5 meters deep and make it self-cleaning and much easier to feed and get much higher biomass per square meter.

 

Mark Rigby (mrigby@llyn-aquaculture.co.uk): I agree with what Durwood said, based on repeated production runs in both shallow raceways and deep (1.5 meters) tanks.

 

Although we routinely reached final production densities of around two kilograms per m2 in just ten centimeters of water (that’s 20kg/m3), compared to around eight kg/m2 in 1.3-meter-deep tanks (6 kg/m3, average not maximum).  It’s far better to use deep tanks (up to three meters deep) as Daniel and Durwood suggested.  What’s easy to do in a ten-centimeter-deep, one-meter-wide R&D raceway becomes a huge challenge when operating anything nearing commercial scale, like a 500-metric-ton-per-year farm, harvesting five tanks per week of two tons each.  Imagine a ten-meter wide trough that’s ten centimeters deep and 100 meters long.  Then imagine stacking them up and feeding them.  Anyone who has built shrimp production systems knows that the cost would make it a ridiculous challenge, and the corridors between the stacked raceways would consume a significant portion of your valuable floor space.

 

We experienced spontaneous, exponentially increasing waves of leaping shrimp that spread from on end of the shallow raceways to the other.  Has anyone else seen this?  The only direction for the shrimp to go is up!  Some damage to the animals occurs, and netting between the raceways is required.

 

On the other hand, there are some advantages to shallow stacked raceways.  In a biofloc system, contrary to what Durwood said, less energy might be needed, compared to deeper tanks, because the majority of power required for aeration is directly proportional to depth.  Also, the water flow through a shallow raceway to facilitate waste removal and maintain acceptable levels of suspended solids is minimal.  Plus, you can observe the shrimp in a shallow raceway.

 

One has to consider the engineering challenges of scaling up any pilot project.  I’ve witnessed people using Microsoft Excel to add a couple of meters to the depth of proposed tanks to make the production figures look better, without any regard to the practical implications, power requirements or behavior of the animals!  From what Clifford Morris wrote, maybe this was the case at Florida Organic Aquaculture?  Start-ups should be very careful to base their projections on carefully considered commercial R&D and pilot projects rather than jumping directly from academic research to a large-scale start-up.

 

Durwood Dugger (ddugger@biocepts.com, http://www.biocepts.com/BCI/Home.html): Mark, good observations, and I agree.  The other energy difficulty with shallow raceways is that low-pressure diffuser aeration is ineffective at ten centimeters, so aeration then becomes a function of pumped circulation (typically spray bars), which is far more costly, when considering O2 transferred per horsepower, than low-pressure air.

 

If you use oxygen, shallow depth is problematic at scale because the higher surface to volume ratio is easily lost with the more vigorous circulation required.  Shallow raceways also require the use of saturation cones/columns with oxygen.  Not impossible, but an additional cost and a management issue.  With shallow raceways, circulation isn’t for suspended solids, but to prevent filamentous bacteria from plastering anaerobic (toxic) waste films to the raceway bottom, at least that was our experience in some trials.  Basic physics says that raceways with high surface to volume ratios require more energy to move water because of the higher friction.

 

Cylindrical recirculating tanks provide the highest energy cost efficiency, and it takes less structure to build circular tanks due to the very even radial stress loading, at least up to about 60 meters in diameter.  Circular flows are also the most efficient at concentrating waste for efficient removal with the least amount of energy.

 

The fact missed by most aquaculture designers is that the cost of round tanks is a one time charge when built, while the energy penalties of less efficient systems (square, rectangular and shallow systems) continue daily and add to the cumulative economic penalties for the life of the enterprise.

 

The spontaneous wave you described is a common sight when harvesting shrimp from ponds.  It happens when the water becomes shallow, and the shrimp feel the pull of the water toward the drain.  The shrimp closest to the drain will start jumping back to avoid being sucked into the drain.  The jump back can trigger a wave that spreads across the remaining water surface of the pond and cover many acres in some cases.  It’s fun to watch, a nuclear-like chain reaction of shrimp.  A seagull diving into a shallow pond can create the same reaction.  Its shadow causes the shrimp to jump, and as you pointed out, in shallow water, they can only jump up.  So, yes, any stacked raceway system will need side nets, especially if there’s human activity that might cast a shadow on the tank.

 

Dallas Weaver (deweaver@me.com): Durwood, I would like to differ with your statement that low pressure diffusers for aeration in shallow depth are inefficient.  In fact, the lowest energy cost gas-liquid mass transfer should be as you approach infinitely shallow containers.

 

However, as you decrease the depth, the amount of air increases as the pressure decreases, and in real shallow systems, the size of the air ducts and main air distribution system start to look like the heating, ventilation and air conditioning systems in your house.

 

Most aquaculture equipment is not designed for the very low pressure of shallow raceways.  They are energy inefficient (especially regenerative blowers) at well below design pressures.  And they don’t scale down in pressure very well with variable frequency drives (electrical boxes that take power and convert it into different frequency and voltages with usually three-phase output) to decrease the revolutions per minute.  A standard 60-cycle motor will slow or speed up if the frequency decreases or increases.  They are handy for decreasing energy consumption when you have lighter loads.  However, this observation of what is available in the aquaculture market makes your statement “effectively true” even if not technically true.

 

Nelson Gerundo (nelsongerundo@yahoo.com): My main concern with stacked raceways is the maintenance and the daily husbandry activities they require, like removing molt casings, detritus and injured and dead shrimp.  Even sunken maintenance objects like tools are a hassle to reach and remove from stacked raceways if maintenance is not considered in the design of the system.  Also, seawater weighs about 1.02 kilograms per liter or 1.02 tons per cubic meter, requiring more structural support and expense to build.

 

With routine servicing and daily chores in mind, I only build triple-layer stacked raceway systems, with the top layer at eye level, so that the staff can perform routine husbandry and maintenance activities standing on the ground.  They never have to step on top of something or hold on to anything.

 

Durwood Dugger (ddugger@biocepts.com, http://www.biocepts.com/BCI/Home.html): Dallas, more clarification, please.  I thought the shallower the depth, the lower the pressure, the shorter the contact time of the diffuser bubbles and the lower the O2 transfer from the air.  What did I miss in my assumptions?

 

Greg Lutz (glutz@agcenter.lsu.edu, editor of Aquaculture Magazine): My impression is that more of the oxygen comes from turning the water over and over in contact with the atmosphere at the surface.  Very little comes from the bubbles themselves unless you are using pure oxygen I suppose.

 

Durwood Dugger (ddugger@biocepts.com, http://www.biocepts.com/BCI/Home.html): Greg, the O2 transfer from air to water (at least as I was taught) is limited by the air to water surface contact area.  Ponds with waves have more surface area than calm flat ponds and generally can absorb more O2 as a result.  If you want to increase air/gas contact (and absorption) without waves or spraying water up into the air (very energy consumptive), then fine bubbles are the tool of choice.

 

Consequently, if you are using micro-bubble diffusers, it doesn’t take many diffusers to produce a micro-bubble surface area equivalent to many times the surface of the tank with much more efficiency than just “stirring” the tank and exposing a “new” surface.

 

Of course, you have to decide on your primary goal: Is it moving water with an airlift pump, or transferring air to the water with diffusers?  You can use micro-diffusers in an airlift and get much better O2 transfer than a large bubble airlift, but with less water pumping efficiency.  Airlift pumping efficiency is a function of bubble size, the distance between the bubbles and bubble emission frequency.

 

Years ago, some engineers at the South Carolina Department of Natural Resources discovered that a 1.5-inch pipe was the optimum diameter for most aquaculture airlifts, especially when working with depths greater than 18 inches.  In their work, they found that in non-diffused airlifts, they couldn’t make the bubble larger than 1.5 inches, or it would break apart into smaller, less efficient bubbles.  The 1.5-inch bubble acted like a piston pushing water up the pipe.  There are different dynamics between salt and freshwater bubbles, but not that much.  The engineers also developed flow rates for these airlifts, and if my memory is right, the 1.5-inch bubble could pump water at a fraction of the cost of pumping water with a centrifugal pump.

 

I will leave mass transfer laws entirely in the capable hands and mind of Dallas.

 

Russ Allen (shrimpone@aol.com): I have enjoyed this discussion on stacked raceways.  With all due respect to Durwood and Dallas, there are “alternative facts” that discount your skepticism about stacked raceways systems.  I just finished a feasibility study on stacked raceways for a new project, and it shows good results.  Before I get criticized, I’ve been working with stacked raceways for 24 years, 8 years in the research phase and 17 years at a commercial scale up.  Yep, lots of problems, many re-do’s, but there are solutions for capital costs, aeration, and biofiltration that are out of the box.

 

Dallas Weaver (deweaver@me.com): Durwood, yes, the time and mass transfer per bubble go down with depth, but the pressure also goes down (energy goes down — flow pressure compensates for compressibility).  The compressibility of air becomes less of a factor (you get less area/bubble at high depth).  The mass transfer occurring during the creation of the surface area of the bubble is independent of depth, but in a shallower system (say half the depth) can handle more than twice the number of bubbles per kilowatt hour.

 

The creation of aeration systems with high mass transfer becomes a problem in shallow raceways.  Getting high kilograms of O2 per hour becomes more mechanically difficult the shallower the water, so going to near-zero depth becomes effectively impossible.

 

A 60-cubic-feet-per-minute blower (CFM) at one meter becomes a 600 CFM blower at ten centimeters with the same horsepower and a little more mass transfer (efficiency improvement).  From a practical standpoint, the ducting and piping necessary for a 600 CFM blower becomes difficult and would get in the way of everything.

 

Durwood Dugger (ddugger@biocepts.com, http://www.biocepts.com/BCI/Home.html): Dallas, thanks.  Yes, you answered my questions.

 

Sources: 1. The Shrimp List (a mailing list for shrimp farmers).  Subjects: Three New Reports and Benefits of Shallow Stacked Raceways for Shrimp Production.  July 30 to August 2, 2017.  2. Bob Rosenberry, Shrimp News International.  October 5, 2017.

 

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