Protein Skimmer in RAS Aquaculture

The first article showed that foam fractionation is an interfacial separation process. This second part addresses the practical question: how should a professional system be sized, and how should it be integrated into a treatment train in which mechanical filtration, biofiltration, degassing, ozone and UV must operate in a coherent manner? The proposed method is deliberately conservative: manufacturers’ stated commercial performance figures cannot be directly transferred from one system to another.

The sizing principle: start with the load, not the volume

Tank volume is a hydraulic parameter, but the organic load is primarily determined by feed ration, feed digestibility, feed composition, biomass, water exchange and microbial dynamics. Two tanks may therefore require very different foam fractionation capacities.

Feed load (kg/day) = actual daily feed ration

The first approach is to establish a mass balance based on the feed ration. The next step is to estimate the fraction of organic carbon likely to be present in the water and, above all, the fraction that can actually be captured by the fractionation process.

A conceptual mass balance can be written as follows:

Organic inputs = biomass produced + solids removed + dissolved matter exported + internal transformations + losses

This balance is not a universal sizing equation. A fraction of the feed becomes faecal matter and must be captured mechanically; another fraction is metabolized; yet another fraction supports microbial growth. The fraction that can be removed by foam fractionation depends on water quality and, ideally, should be determined through measurements or pilot-scale testing.

1. Organic Load and Concentration: Two Different Parameters

A COD concentration expressed in mg/L is not sufficient to determine the mass flow that needs to be treated. The mass flow depends on the hydraulic flow rate. Conversely, a high flow rate does not necessarily mean a high load if the concentration is low.

Mass flow = Q × C

where Q is the volumetric flow rate and C is the concentration of the substance under consideration. For a professional system, mass balances must be established using consistent units, generally kg/day, m³/h and mg/L.

2. Water Flow Rate Through the Skimmer

The water flow rate through the skimmer must be selected according to its effective volume, design and mixing quality. Excessive flow reduces contact time and can disturb the collection zone. Insufficient flow reduces the renewal rate of the water mass.

The relevant criterion is therefore not simply “how many times the tank volume passes through the skimmer”, but the combination of treatment rate, organic load, fractionation capacity and the desired water quality at the outlet.

3. Air Flow Rate and Air-to-Water Ratio

The ratio between air flow and water flow is a major design parameter. Increasing the air flow increases the interfacial area, but excessive air destabilizes hydraulic operation, increases head losses and can produce excessively wet foam. Equipment with a very high air flow rate is not necessarily the most efficient because, ultimately, this design can interfere with the separation process.

Air-to-water ratio = Q_air / Q_water

4. Hydraulic Retention Time

This is a very important parameter that influences system efficiency. Compact skimmers are disadvantaged by excessively short retention times.

τ = V / Q

τ represents the theoretical hydraulic retention time, V the effective volume of the contactor and Q the flow rate passing through it. This parameter is useful for comparing configurations, but it does not describe the actual distribution of contact times.

5. Superficial Velocity and Bubble Behaviour

The superficial velocity of the water or air/water mixture influences bubble-water interactions, bubble rise time and column stability. In a properly designed column, the hydraulic flow must not carry bubbles too rapidly towards the outlet or create uncontrolled recirculation zones.

6. A Step-by-Step Sizing Approach

• Define the objective
• COD removal, improved water clarity, pre-treatment before ozone, reduction of organic load, or a combination of these objectives.
• Establish the hydraulic balance
• Tank volume, recirculation flow rate, available flow rate, static head and head losses.
• Establish the load balance
• Feed ration, biomass, solids, DOC/COD where available, and any external inputs.
• Define the assumptions regarding the fraction that can be removed by foam fractionation and their level of uncertainty.
• Select the bubble-generation technology.
• Size the contactor and collection zone.
• Check the energy balance
• Pump, air supply, potential ozone generation and off-gas treatment.
• Provide for adjustment, purging, cleaning and concentrate discharge.
• Validate performance through operational measurements or pilot testing whenever the economic or biological stakes justify it.

7. Foam Fractionation + Ozone: Designing a Coupled System

In an ozonated system, the skimmer can form part of the gas/water contactor. The design must then account for ozone dissolution, contact time, transfer efficiency, the ozone demand of the water and treatment of the outlet gas. Residual ozone destruction is an essential safety and operational quality measure.

O₃ transfer rate = (O₃ injected − O₃ residual in gas/liquid) / O₃ injected

This relationship provides a simple way of representing transfer efficiency. In practice, the calculation must distinguish between gas and liquid flows and take measurement conditions into account.

8. Foam Fractionation and UV Sterilization

Reducing organic matter and colour can improve downstream treatment conditions. However, UV is an inactivation process that depends in particular on UV dose and water transmittance. It is therefore incorrect to state that a skimmer “increases UV power”: it improves the optical quality of the water and indirectly helps maintain the conditions required for effective treatment.

9. Foam Fractionation and Biofiltration: Complementarity and Trade-offs

An MBBR is a biological reactor. A skimmer is a separation process. Their combination makes it possible to distribute treatment functions between different processes. However, very intensive organic removal changes the substrate available to heterotrophic bacteria. In some systems, this modification may be desirable, particularly in highly loaded systems such as RAS and holding systems; in others, it may require adaptation of the biological strategy, for example in hatcheries or aquariology.

10. Foam Fractionation or DAF?

Foam fractionation and dissolved air flotation (DAF) both use bubbles, but their objectives and operating principles are different. DAF is primarily designed to float particles or flocs towards a separation surface. It can be highly effective on suspended or flocculated matter and, in some cases, certain oils. A skimmer is designed to concentrate dissolved and colloidal compounds with an affinity for the gas-water interface.

11. Foam Fractionation in Marine Aquaculture

In intensive marine aquaculture, the skimmer can be installed on a main or secondary treatment loop. The choice depends on animal tolerance, available flow rate, the need for organic export and the potential use of ozone. A bypass loop can sometimes be used to optimize contact time without imposing excessive head losses on the entire recirculation system.

12. The Special Case of Hatcheries

Larvae and planktonic organisms can be much more sensitive to changes in water quality. Intensive foam fractionation may remove useful compounds or particles, modify the availability of certain substances and disturb microbial balances. The strategy must therefore be adapted to the biological stage rather than applied as a general rule.

13. Foam Fractionation in Public Aquariums

In a public aquarium, sizing must take into account species diversity, feeding regimes, very large water volumes and the need for high equipment availability. Redundancy, ease of cleaning, accessibility of collection cups and system monitoring become criteria just as important as theoretical performance.

14. Sizing Errors to Avoid

• Sizing the system solely according to tank volume.
• Using a commercial capacity rating without knowing the conditions under which it was determined.
• Increasing air flow without checking hydraulic performance and foam stability.
• Confusing COD concentration with the mass load to be treated.
• Assuming that all COD is removable by foam fractionation.
• Overlooking the differences between freshwater and seawater.
• Neglecting Venturi and bubble-generation system head losses.
• Combining ozone and foam fractionation without treating residual gas.
• Replacing mechanical filtration with a skimmer.
• Failing to provide adjustment, purging, cleaning and instrumentation.

15. Performance Indicators

Extraction efficiency (%) = (C_inlet − C_outlet) / C_inlet × 100

This efficiency must be interpreted with caution: it depends on the measurement point, retention time and variation in load. When comparing systems, a mass balance is preferable whenever flow rates and concentrations can be measured.

Extracted load = Q × (C_inlet − C_outlet)

A skimmer may exhibit a high concentration removal efficiency while removing a small total mass if the incoming load is low. Conversely, a high mass removal rate can be achieved with only a moderate reduction in concentration when the flow rate is very high.

16. Energy and Operating Costs

The cost of a skimmer is not limited to its purchase price. The pump, compressor or air pump, head losses, cleaning, wear parts, concentrate disposal and, when ozone is used, the ozone generator and ozone destructor must all be taken into account.

The best system is therefore rarely the one that maximizes the amount of air injected. It is the system that achieves the desired water-quality objective with energy consumption and complexity compatible with the operating conditions.

Conclusion

Foam fractionation is a powerful tool for managing organic matter in intensive aquatic systems, but its effectiveness depends on parameters that are rarely adequately described by a simple volume-to-flow ratio. Proper sizing must link feed load, water composition, hydraulic flow rate, bubble generation, interfacial area, contact time, foam stability and the overall treatment strategy.

In an RAS or public aquarium, the skimmer should be considered as one component of an integrated treatment train: mechanical filtration removes solids, biofiltration transforms nitrogen compounds, degassing controls dissolved gases, ozone can modify and oxidize organic matter, the skimmer exports an interfacial fraction, and UV provides microbial inactivation.

Overall performance results from the interaction between these processes.