Culture Tanks

A culture tank is the vessel in which fish are held, but from an engineering standpoint it is the first unit process in the treatment train. It performs a separation: concentrating settleable solids into a small, dense flow at the centre drain while the bulk of the water leaves comparatively clean.

How well it performs that separation is a function of geometry and hydraulics, not of size. A tank that does not move its water correctly is a settling basin the fish live in.

Solids that leave the tank quickly can be removed as particles. Solids that linger break down, releasing dissolved organic carbon and ammonia into the loop — load that then has to be handled by the biofilter, which is slower, more expensive and less reliable than a drum filter.

• Rapid solids removal reduces the dissolved load the biofilter must carry.
• Uniform water velocity gives every fish comparable conditions, which shows up as more uniform growth and less size grading.
• Gentle, consistent current provides exercise and supports fish condition; excessive velocity costs the fish energy that would otherwise go into growth.
• Dead zones concentrate solids and low oxygen in the same place, which is where health problems begin.

In a circular tank, the returning flow is injected tangentially. That momentum sets the whole water mass rotating. Rotation creates a secondary radial flow along the floor toward the centre — the same effect that gathers tea leaves at the middle of a stirred cup — and that radial flow is what transports settled solids to the centre drain.

• Rotational velocity is driven by inlet momentum — flow multiplied by inlet velocity — not by flow alone. Adjustable inlet orifices are how velocity is tuned after commissioning.
• The diameter-to-depth ratio governs the flow pattern. Too shallow and rotation is dominated by floor friction; too deep and the lower water is poorly coupled to the rotating bulk.
• Floor slope toward the centre assists but does not replace the radial sweep.
• Velocity is a species and life-stage decision. It must exercise the fish without imposing a continuous energetic cost that suppresses growth.
• Both drains must be sized and valved so the split between them can be adjusted in operation.

• Choose the shape deliberately. Circular tanks give the best self-cleaning; rectangular and raceway tanks use floor space more efficiently but need engineered flow control to avoid dead zones; octagonal and square-with-rounded-corners tanks are a compromise between the two.
• Provide adjustable inlets. A fixed inlet fixes the tank's velocity at whatever the designer assumed, for every species and every life stage the tank will ever hold.
• Design the centre drain and its standpipe for the sludge stream, and make it accessible without entering the tank.
• Include a reliable water level control and an overflow route that cannot be blocked by a single obstruction.
• Plan for harvest and grading — how fish are crowded, moved and removed — before the tank is built, not after.
• Specify surfaces that can be disinfected and that will not abrade fish.

• Observe the floor daily. Visible settled material indicates the sweep is not working, whatever the water quality readings say.
• Watch fish position. Fish holding in one part of the tank are telling you about oxygen, velocity or light before any instrument does.
• Measure dissolved oxygen at the outlet, which is the worst water in the tank, not at the inlet, which is the best.
• Re-tune inlet velocity as fish grow and as biomass and flow requirements change.
• Check the drain split periodically; a partially blocked centre drain shifts the load to the main loop silently.

• Clear inlet orifices on a schedule. Partial blockage changes the flow pattern before it changes the flow rate.
• Inspect and clean drain screens, which are the most common cause of unexplained level changes.
• Check tank surfaces for biofilm, abrasion and damage during every fallow period.
• Disinfect and dry between batches where biosecurity policy requires it, and record it.
• Re-verify the drain split after any change to pumping or pipework anywhere in the loop.

• Sizing the inlet for flow and ignoring velocity, leaving the tank with no rotation.
• Building fixed inlets, then having no way to tune the tank for the fish actually in it.
• Choosing rectangular tanks for floor efficiency without designing the flow control that makes them work.
• Measuring oxygen at the inlet and reporting it as the tank's oxygen level.
• Treating visible floor solids as a cleaning task rather than a hydraulic fault.
• Leaving no practical way to crowd and harvest, so every harvest is a stress event improvised on the day.

Mixed-cell raceways and partitioned rectangular tanks apply circular-tank hydraulics inside a rectangular footprint, using arranged inlet jets to create a series of counter-rotating cells each with its own bottom drain. They recover much of the floor-space efficiency of a raceway while retaining a solids sweep, at the cost of a more complex and more sensitive inlet arrangement.

Computational fluid dynamics is increasingly used to evaluate tank flow patterns before construction. It is a design aid rather than a substitute for commissioning measurement, because the modelled tank has no fish in it and fish alter the flow field they live in.

A culture tank is a separator. Tangential inlet momentum sets the water rotating, rotation produces a floor sweep toward the centre, and a dual drain divides the outflow into a small solids-rich stream and a large clean one. Get that right and the biofilter's job shrinks; get it wrong and every downstream process inherits the difference.