Decentralising RAS Unit Processes to Lower CAPEX — Some Design Notes and Practical Commentary. Part 2

In the first part of this series, I explored the idea of attaching simplified, self-contained RAS or hybrid-RAS processes directly to each rearing tank, rather than grouping tanks around a filtration module. The goal was to test how far this concept could go as culture intensity increases.

In Part in, we have a small tank holding around 40 fish. Design is mostly driven what the equipment that can be installed and built, rather than fish biomass. This makes sense in low-density systems such as broodstock units or research tanks, where the number of fish has little impact on overall loading. But, once we move toward production environments, the situation changes. The appropriate design benchmarks relate to stocking density and daily feed load.

Using stocking density as a starting point

Across the aquaculture industry, we have decades of experience indicating the typical limits for different rearing systems. Depending on the species and the system configuration, stocking densities usually sit within common ranges:

• Open net pens and cages: 15–25 kg/m³

• Organic or low-intensity systems: 10–15 kg/m³

• Landbased systems with oxygen injection and CO₂ stripping: 60–120 kg/m³

• Extreme cases in RAS (catfish, Arctic char, etc.): up to 250 kg/m³ or even more

For this design exercise, I wanted to start conservatively with a density that could reasonably be supported by air-driven circulation and aeration alone, without pure oxygen supplementation. I therefore used 15 kg/m³ as the design reference.

Assuming a 1 m³ tank stocked at that biomass and fed at 1.2 % body weight per day, the total feed input comes to about 180 g/day. This is a convenient baseline that roughly corresponds to mid-stage salmonid rearing conditions, for which a temperature at 12 °C is assumed for the oxygen mass balance.

The Oxygen Balance

At 12 °C in fresh water dissolved oxygen saturation is around 11 mg/L. This is the theoretical maximum we can achieve with aeration. Salmonids have relatively narrow tolerance for hypoxia, which makes oxygen supply the defining constraint, with a lower limit set at 6 mg/L.

I assumed that the biomass consumes roughly 700 g O₂ per kilogram of feed, with about half of this demand coming from the fish and the other half from microbial activity within the tank. This is a deliberately pessimistic estimate at such low stocking densities, but chosen to account for limited water exchange.

If the airlift or aeration system achieves a 20 % oxygen transfer efficiency, and the acceptable oxygen range in the tank is 6–11 mg/L, a mass balance calculation indicates that a circulation rate of about 6 m³/h would be sufficient to maintain oxygen above the minimum threshold. To double check, a quick calculation assuming the use using diffused aeration to supply the oxygen demand with 10% aeration efficiency yields an airflow of about 3-4 L/min.

In practical terms, this means an airlift moving roughly 6 m³/h of water at a 1 m submergence, with an airflow of 15–20 L/min — comfortably more than what is needed to meet the biological oxygen demand.

Combining Aeration and Biofiltration

If the airlift chamber is filled with moving bed biomedia, it can provide not just circulation and aeration, but also ammonia oxidation. Assuming a nitrification rate of 0.27 g TAN/m²/day and a media with 750 m²/m³ specific surface area, the required biofilter volume for this feed load is around 230 L 100 L, with 200L is a 2X safety margin is used. (50% media fill).

Thank you Arndt von Danwitz for pointing a mistake in my calculations.

Interestingly, the airflow required to fluidize that amount of media (typically around eight times the media volume per hour) also lands near 15 L/min, which the same order of magnitude needed for aeration. This alignment means that, in practice, the airlift’s energy input can satisfy both the biofilter's fluidization and the fish’s oxygen requirements simultaneously.

Thus, a single airlift–biofilter unit can sustain a 1 m³ rearing tank stocked up to 15 kg/m³, operating entirely on air.

Solids Handling and Hydrodynamics

At this loading, the system must also manage suspended and settable solids.
A simple mass balance shows that the mechanical filtration flow should be around 300 L/h. This can be handled either by:

• Exchanging about 30 % of the tank volume per hour (partial reuse mode), or

• Installing a small side-stream treatment device such as a bead, sand or glass filter, or perhaps a protein skimmer.

If the tank is cylindrical–conical, with a central bottom drain, settable solids can be easily purged intermittently. At effectively 6 turnovers per hour, the design of the airlift outlet grid discharging into the tank must be considered to control over the rotational flow inside the tank. The goes is reducing the formation of a high-speed vortix by the tank wall, since a central vortex can is eliminated if the bottom drain of the tank is only opened periodically.

At this higher production intensity, the compact “sidewall box” approach used in Part 1 starts to reach its limit: larger mechanical filters or skimmers will likely need to be installed externally wile the box takes 1/4 of the diameter of the tank. Nevertheless, the functional simplicity remains.

Even when scaled up and connected to solids treatment processes, the embedded biofilter should remain valuable. It supports microbial maturation and stable bacterial communities, which contribute to fish welfare and water stability.

Potential Applications

With aeration as a sole oxygen source, I see some possible applications

• Experimental or holding tanks in research institutions

• Hatchery tanks - if we can design inlet and outlet structures correctly.

• Nursery systems

• Low-density purging or quarantine systems in farms

• Flow-through systems with excellent solids capture and microbially-stable water, thanks to the embedded biofilters.

In these contexts, a self-contained airlift-biofilter tank should provide robust water quality with minimal infrastructure and installation effort.

Looking Ahead into Intensive Systems

With the addition of pure oxygen, the concept should provide an in-between step between hybrid RAS and full RAS concepts. We have, in order of intensity of use water used per kg of feed:

• Flow through systems (limited by available oxygen)

• Flow through systems or serial reuse with oxygenation (limited by CO₂ accumulating)

• Hybrid RAS or partial reuse systems with oxygenation and CO₂ stripping (limited by ammonia accumulating)

• This concept (limited by suspended solids accumulating)

• Full RAS (limited by nitrate accumulating)

• Full RAS with denitrification (limited by fine solids and bacteria accumulating)

Closing Comments

In this second part, we confirmed that the airlift–biofilter concept can handle moderate loading when properly dimensioned for flow and aeration. The next question is what happens when we double or triple the stocking intensity?

In Part 3, I’ll explore the same design operated with higher densities, pure oxygen supplementation, and examine what that means for CO₂ stripping, biofilter volume, and tank geometry.

— Carlos