Flow systems

 

This page is being edited and will be uploaded soon (October/November 2021).

 

 

When defining and making decisions on which flow structure to apply in a new RAS facility, obviously first of all it is about mass balances. But next to consider is the actual choice of type and design of fish tanks .

Some flow structures will also not work for seawater, as a consequence of the impact the Seawater chemistry has on gas exchange and CO2/bicarbonate equilibrium.

For simplicity we can split the various flow structures for RAS system in 4 different type of flow structures:

 

  • Single loop flow structure:  (Like in first feeding for hatcheries)

 

i. Flow structure:

 
 
 Single loop flow structure: Fish tank – Mechanical filter – UV (potential partial flow) – Biofilter – CO2 stripper – back to fish tank
 
 

(full resolution here)

 
Single loop flow structure with additional mechanical filtration: Fish tank – Mechanical filter – UV (potential partial flow) – Biofilter – CO2 stripper – Mechanical filtration - back to fish tank
 
 
ii. Description and use:  
 
The oldest concept applied for RAS farming.
This was the typical flow structure applied during the very early days of RAS back in the 1980 ties when designing RAS were merely based on trial and errors and less on an academical approach. Bach when CO2 and bicarbonate was not considered being of importance for the fish in aquaculture installations.
This flow structure is only suitable for Ras farming in freshwater, the reason being, that if applied for seawater there will be a mitch match between retention time in the bio filters and the CO2 stripping capacity. Either the retention time within the biofilter will become extremely short, making the system very vulnerable towards diseases, or there will be to little flow to match the required CO2 stripping capacity.
 
 
iii. Advantages:
 
Simple
Can be very useful in hatcheries for the rearing of the fish fry during the very early stages when they will be very sensitive towards fish diseases, where protection against infections are prioritized above investments. 
By directing the entire flow through The biofilters, with a long retention time – like 30 minutes, the system will then be highly resistant towards the development of diseases. From a mass balance point of view the biofilters will then be highly over dimensioned, and then come at a  relatively high cost. 
Especially for the rearing of marine fish larvae it can be recommend to ad an additional  additional mechanical filtration after the CO2 stripping to remove particles and zooplankton which have developed in the biofilter and the CO2 stripper. 
 
 
iv. Disadvantages:  
 
Not optimal for optimizing  the water treatment system for a desired  water chemistry without making the system vulnerable towards diseases, or making the system unnecessarily expensive. 
Requires very big biofilters to obtain both an efficient CO2 stripping and a system which is very robust towards photogenes.
 

 

  • Split loop flow structure: (Like RAS2020 & the AP-circular RAS)

 

 i. Flow structure:

Characteristics: Main flow to CO2 stripper, while a side flow goes through the biofilter before the CO2 stripper. Entire flow from CO2 stripper to fish tanks.

 

 (full resolution here)

Fish tank – Mechanical filter – UV (potential partial flow) – split flow to Biofilter, with partial flow to biofilter, whereas main flow directly to the CO2 stripper. Outlet from biofilter joins main flow into CO2 stripper – outlet from the CO2 stripper goes to the fish tanks.

 

ii. Description and use: 

A concept which has naturally become more dominant within the industry, as Ras designs became more relying on an academical approach based on mass balance calculations, and less on simple try and error development.
General suitable for the aquaculture industry and the principles of splitting the flow not having the entire flow going through the biofilters, became a must when dealing with seawater in RAS systems.

 

iii. Advantages: 

The parallel loupe flow structure is in general a very useful concept for RAS systems, it facilitates the option to adjust the design to match a specific mass balance. The flow system and water treatment can be dimensioned to match both the required CO2 stripping capacity, and the need of biofilter capacity, at the same time optimizing the retention time of the flow within the biofilters to obtain a good protection towards diseases.
This flow structure is the optimal choice when dealing with raceway type of tanks, where it is optimal with a high water exchange rate within the tank.

 

iv. Disadvantages: 

The water quality measured as the content of organic matter, will not be fully at good as what can potentially be achieved with both the Double loop flow structure and the parallel flow structure.

 

  • Double loop flow structure

 

i. Flow structure: 

Characteristics: Outlet from biofilter returns back to in front of mechanical filter or either after the mechanical filtration or after the UV. The combined flow is then split again between flow to biofilter and flow to CO2 stripper where only the flow to the CO2 stripper returns to the fish tanks. 

After the CO2 stripper only a part of the flow leads to the fishtank, while the remaining goes to the biuofilter, which the ends up again in front of the mechanical filter, for the optimasation of the water quality.

 

 (full resolution here)   

Fish tank – Mechanical filter – UV (potential partial flow) –CO2 stripper - split flow to Biofilter, with partial flow to biofilter, whereas main flow directly to Fish tanks.

 

ii. Description and use:

This flow structure really optimizes the CO2 stripping towards water exchanges in the tank.
One could say that the flow which enters the biofilter gets CO2 stripping twice. – and that beyond the stripping which might take place within the biofilter as well.
AS the content of bicarbonate will form into CO2 once the existing free CO2 has been removed, it is fully possible to obtain efficient multiple stripping’s of the CO2.
This concept is very useful, especially when the tanks have a circular shape where the amount of water exchanges can be limited, and a high fish density is desired.

 

iii. Advantages

It is possible to trim the water treatment system to match a calculated mass balance.
The CO2 stripping capacity is very high compared to the tank water exchange rate. Which can be important especially when dealing with circular tanks.
If the flow from the biofilters reinter before the mechanical filter, it is possible to obtain a very good water quality with this technology.

 

iv. Disadvantages

Tank flow will be reduced compared to total flows which can be an disadvantage where high levels of water exchange into the fish tanks are desired for like the self cleaning of the tanks.

 

 

  • Parallel loop flow structure

 

i. Flow structure: 

Characteristics: Outlet from biofilter returns back to in front of mechanical filter or either after the mechanical filtration or after the UV. The combined flow is then split again between flow to biofilter and flow to CO2 stripper where only the flow to the CO2 stripper returns to the fish tank.

Equivalent to the double loop structure, but without the flow towards the biofilters being treated in CO2 stripper first. - Main flow to CO2 stripper, while a side flow goes through the biofilter, flow from biofilter goes back and mix with main flow before the CO2 stripper/biofilter.

 

(full resolution here)

Fish tank – Mechanical filter – UV (potential partial flow) – parallel flow to Biofilter and CO2 stripper, normally with main flow directly to CO2 stripper.

 

ii. Description and use:

A concept very similar to the parallel loupe flow, with the difference that there is a reduced CO2 stripping capacity, if considering only the volume of the combined total system flow.
Can be a useful concept for simplifying the construction and operation of smaller systems, where optimal water quality is prioritized.
It is fully possible with this flow structure to design a RAS installation for a specific set of desired feeding capacity/water chemical parameters.

 

iii. Advantages:

Facilitates more flexibility on how to design the system and arrange especially the biofilter tanks.
Weather the flow Is directed back ion from the mechanical filters or not is a matter of if it is desired to remove the organic matter generated within the biofilters before the flow returns to the CO2 stripper and the fish tanks. But in any case, it is important that the outlet from biofilters if at all then at least will only be partly treated with UV or ozone, as it is the presence of the bacteria’s generated from within the biofilters which will protect the fish in the fish tanks.

 

iv. Disadvantages: 

A reduction in CO2 stripping capacity compared to the double loupe concept.