Commercial scale, self-cleaning tanks for indoor shrimp farming, possible?

Proper solids removal in RAS is critical for maintaining water quality. Having the right mechanical filters is of little use if we cannot transport solids to them quickly and with minimal particle breakdown. The first hurdle in achieving this is ensuring that tank hydraulics enable effective solids transport.

The literature on solids transport and tank hydrodynamics is both rich and, in my opinion, very exciting. This is because it is one of those topics that quickly moves from theory and modelling to empirical work and real-life applications. By spending a little time looking into the subject online, the reader can quickly get an idea of how different flow regimes affect solids removal.

Tanks operating under predominantly longitudinal flow, often associated with rectangular geometries, can struggle to remove solids effectively. In these tanks, water may encounter dead zones and short-circuiting. While the outlet provides a single point for solids to exit, particles are being pushed around the tank in directions that do not necessarily point directly towards it. Under these flow regimes, water may also lose momentum as it moves away from the inlet and its energy dissipates.

Consequently, solids tend to settle at the bottom of the tank before reaching the outlet.

Cylindrical tanks, on the other hand, commonly operate under rotational flow regimes, combining mixing with the tea-cup effect. Injecting water tangentially into the tank generates rotational momentum and a secondary flow traveling from the walls towards the centre of the tank. Because the rotational flow maintains momentum around the tank, water can continue transporting solids with comparatively less energy input than in tanks relying on predominantly longitudinal flow.

However, cylindrical tanks can also lose the energy needed for solids transport, creating areas around the central drain where particles stop moving towards the centre. The available literature provides useful design guidance on tank geometry, inlet arrangements, and flow rates required to achieve self-cleaning conditions.

What one notices when observing particles moving towards a cylindrical tank's outlet is a spiralling motion resulting from the combined effect of primary and secondary flows (see teacup effect). Water velocity at the bottom tends to be highest (green) closer to the tank wall and lowest (red) towards the centre. Tank geometry, flow rate, surface friction and the presence of animals all affect the size and location of this low-velocity area.

A possible design

To close, I propose a design that reduces our dependence on the tea-cup effect in circular tanks with less-than-ideal proportions, or where rotational velocities must be limited to accommodate the animals (in this case, shrimp).

We simply interrupt the tank floor with a series of conical settlers that intercept solids as they circulate around the tank. The more settlers we install, the shallower these become and the greater the opportunity for solids capture. However, this comes at the expense of additional outlets and more complex drainage arrangements, so a balance needs to be found.

A line of interceptor settlers extending across 30–50% of the tank radius, targeting the area where solids tend to accumulate, could be a starting point. A series of settlers covering the entire diameter of the tank would be more likely to intercept settleable solids as the water rotates.

The shrimp toilet is a recent innovation that draws from solids control system found in indoor RAS. Photo from https://responsibleseafood.org/next-gen-shrimp-ponds-biofloc/

If the central cone is large enough, it can cover the low-velocity area of the tank where solids transport becomes ineffective. Here, we allow gravity to take over as the transport mechanism to get the solids out of the tank.

The problem with shrimp

The ideal self-cleaning tank is not necessarily suitable for whiteleg shrimp production in indoor RAS. In the particular case of whiteleg shrimp farming, deeper tanks (needed to maintain an appropriate depth-to-diameter ratio for self-cleaning hydraulics) would not be fully utilised by animals that prefer to remain near the bottom.Similarly, the flow rates required to make a tank self-cleaning might generate water velocities that are too high for shrimp or simply require excessive circulation that is unnecessary for maintaining water quality.

Shrimp also tend to nibble and drop their feed. Excessive bottom velocities may transport uneaten pellets towards the outlet before the animals have finished feeding, potentially increasing feed wastage. As a result, the ideal cylindrical tank for indoor shrimp farming may be too shallow, with water rotating too slowly to achieve true self-cleaning conditions. But we don't want self-cleaning to be too good, either.

Letting it all fall down

Balancing water movement and solids collection is an evolving science and art in shrimp farming. A modern example is the shrimp toilet: a large, conical central outlet excavated into the bottom of an outdoor shrimp pond. Paddlewheel aerators or airlifts are arranged to induce circular flow, which helps gradually concentrate solids towards the outlet. These central outlets can be large enough to cover much of the low-velocity area shown in the illustration above.

In outdoor ponds, the central outlet can be incorporated into the excavation works. In indoor tanks, however, accommodating a large cone extending deep into the foundation or the ground beneath it may be impractical. An alternative might be to install a series of smaller cones extending from the centre towards the tank walls. These would intercept solids that continue circulating around the tank but, due to insufficient inward transport, make little progress towards the central outlet.

There is already some precedent for this approach. The sludge cones used in Danish model trout farms intercept settleable solids in raceway tanks. Instead of installing a large settler covering the entire width of a tank, a line of smaller, mass-produced settlers can provide a similar solids collection function.. Photo from https://frea-solutions.dk/en/produkt/sludge-cone/

Instead of increasing circulation rates or water velocities simply to push solids towards a central drain, we can modify the tank bottom geometry and let gravity do much of the work. This would allow us to design circulation primarily around water quality control, rather than the demands of self-cleaning hydraulics.

What do you think?

ChimanaTech

Chimana Management BV

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Boxtel

The Netherlands

carlos@chimana.tech

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