Chapter: Aquaculture Engineering : Tanks, Basins and Other Closed Production Units
Flow pattern and self-cleaning - Aquaculture Engineering
Flow pattern and self-cleaning
A flow pattern will be created
inside a production unit having a water inlet and outlet. It is important that
this flow pattern encompasses the entire unit so that all the fish can come
into contact with flowing water. The flow pattern depends on the design of the
production unit.
In a tank with a circulating
water flow and correctly designed inlet and outlet, two flow patterns will occur:
the primary flow and the secondary flow (Fig. 13.7). The primary flow causes
even distribution
In well-designed tanks with
correctly designed and constructed inlet and outlet, the incoming water may
therefore be used to clean the tank walls and bottom. This process is known as
hydraulic self-cleaning. To achieve self-cleaning in a tank, a certain amount
of water has to be added; the amount depends on the tank construction. The
water velocity at the bottom of the tank must be so high that the settled
solids are removed. To ensure transport of settled solids in circular tanks,
the recommended bottom velocity to ensure self-cleaning is above 6–8 cm/s.7This
will also remove algal growth from the tank sides. Inside the tank there will
be a velocity profile equal to that in a channel, where the lowest velocity
occurs near the bottom due to friction (Fig. 13.8). Bottom water velocities of
between 6 and 8 cm/s normally represent a water velocity in the free water mass
of between 12 and 15 cm/s.7 Practical experience has also shown that
high fish density promotes self-cleaning of the bottom. A lower velocity could
therefore be accepted when the fish density is increased because the movement
of the fish results in resus-pension of settled solids, so the secondary flow
pattern could more easily transport the particles to the drain.
In a correctly designed flow
through tank with inlet and outlet, and a circular flow pattern, the water
retention time should be between 30 and 100 min for satisfactory self-cleaning. This
means a flow through of between 10 and 33 l/m3 farming volume. A
retention time of less than 30 min may create a vortex around the centre drain.
The peripheral velocity in the tank may also be so high that the fish will have
problems staying there. When using low retention times, a specially designed
inlet and outlet are necessary. With retention times above 100 min the
self-cleaning effect is decreased and additional cleaning is necessary.
To attain hydraulic
self-cleaning, a high volume of water is needed to create a high water velocity
inside the tank. Even if the water velocity has yielded improved growth
results, there is a maximum velocity that not must be exceeded.14
This will vary according to species and growth stage. Examples here are fry of
marine or freshwater species, where only very low velocity is tolerated; to
maintain satisfactory water quality for these
species is therefore a challenge. Settled particles and fouling on the tank
bottom and sides will create a sub-optimal environment, and be a good substrate
for unwanted bacterial growth. Regular removing of fouling is therefore
absolutely essential. If this is done manually, it is labour intensive, and
therefore commercially available automatic systems are preferred. Rotating
brushes on the bottom of the tank, powered either by electric motors or by the
pressure of the incoming water are one solution. Other solutions include a
small turtle-like unit moving around on the tank bottom or installing a washing
arm half the pipe diameter in length. Addition of chemicals that remove the
fouling, such as oxidizing agents, has also been tried by the author and
colleagues.
Related Topics