Closed-loop phosphorus

?? English

In both aquaculture and professional aquariology, a great deal is said about nitrogen in all its forms: ammonia, nitrites, nitrates, nitrification and denitrification…

Phosphorus is much less frequently discussed.

Yet, in certain intensive systems, it represents a major environmental challenge, particularly when effluents are discharged into sensitive receiving environments.

Why is phosphorus a problem?

Phosphorus is essential to life. It is found in feed and plays an important role in the growth of living organisms: fish, crustaceans and other aquatic organisms.

The problem arises when phosphorus inputs exceed the system’s capacity for assimilation.

Part of the phosphorus supplied through feed is incorporated into the biomass produced. Another part is discharged in the form of faeces, feed particles, organic matter or dissolved phosphates.

In an intensive farming system, these flows can become significant.

Once released into the natural environment, phosphorus contributes to eutrophication: excessive stimulation of algal production, ecological imbalance, reduced oxygen availability during biomass decomposition and, in some situations, significant deterioration of water quality.

The issue is therefore not simply:

“What is the phosphorus concentration in my water?”

The real question is:

“How much phosphorus does my facility introduce into the environment, and how much is actually retained or recovered?”

Phosphorus does not occur in only one form

In an aquaculture system, it is important to distinguish at least:

• Particulate phosphorus associated with faeces and suspended solids;
• Organic phosphorus;
• Dissolved orthophosphates;
• Total phosphorus, which is a particularly useful overall indicator for establishing a phosphorus mass balance.

This distinction is essential when selecting a treatment technology.

An efficient mechanical filter can remove a significant proportion of particulate phosphorus simply by capturing solid matter.

However, it will not necessarily remove dissolved phosphorus.

This is one of the reasons why an RAS can have visually very clear water while still maintaining a significant phosphate concentration.

The first treatment stage… is still feed management

The best way to remove a pollutant is often not to produce it in the first place.

Phosphorus management therefore begins with:

• Feed formulation;
• Phosphorus digestibility;
• Inclusion rate;
• Feed conversion ratio;
• Accurate feed distribution;
• Limiting uneaten feed.

Improving the utilisation of dietary phosphorus directly reduces the amount of phosphorus discharged. Reviews focusing on aquaculture also identify feed management as one of the main levers for source reduction.

How can phosphorus be removed?

Several strategies can be combined.

1. Solid separation

This is the first step.

Drum filters, mechanical filters, sedimentation, flotation, protein skimming and other separation processes can recover phosphorus-rich particles.

This step is particularly interesting in RAS because it allows phosphorus to be recovered in a concentrated fraction: sludge.

Phosphorus is then no longer simply “treated”: it can potentially be recovered.

2. Chemical precipitation

Phosphates can be precipitated using various reagents, particularly iron-, aluminium- or calcium-based compounds.

This approach can be highly effective when the objective is to achieve a low residual concentration.

However, it also involves several considerations:

• Chemical consumption;
• Sludge production;
• Need for pH control;
• Management and potential recovery of the resulting sludge;
• Staff training.

Chemical precipitation can therefore be relevant as a tertiary treatment, particularly where discharge requirements are stringent.

3. Adsorption

Certain materials have a strong affinity for phosphates.

Adsorption can be carried out using mineral materials, metal oxides or specially developed adsorbents.

This technology is particularly interesting for low residual concentrations.

Specific studies on RAS have investigated phosphorus recovery through adsorption and demonstrate the potential of this approach to concentrate phosphorus rather than simply transfer it into sludge.

4. Biological phosphorus removal

Phosphorus can also be assimilated by microorganisms through specific biological processes.

Enhanced biological phosphorus removal processes use microorganisms capable of accumulating phosphorus in their biomass.

This approach is interesting, but requires careful process control: redox conditions, carbon availability, operation of anaerobic/anoxic zones and biomass management.

It should therefore not be confused with the simple operation of a conventional nitrifying biofilter.

5. Microalgae and plants

In certain configurations, phosphorus can be assimilated by algae, microalgae or macrophytes.

This solution can be particularly interesting from a circular-economy perspective:

effluent → nutrients → biomass → valorisation.

However, biomass harvesting must be properly managed.

Without removing this biomass from the system, phosphorus is ultimately only temporarily relocated. Aquaponics, although subject to other limitations, is a valuable approach in this respect.

6. Crystallisation and recovery

Another approach is to transform dissolved phosphorus into a recoverable mineral compound.

Calcium phosphate precipitation or struvite formation can, for example, transform a dissolved pollutant into a potentially recoverable resource.

Phosphorus recovery is currently an important area of research because phosphorus is both a driver of eutrophication and a strategic mineral resource.

What about regulations?

This is where system design must be particularly rigorous.

European regulations do not simply establish a “universal phosphorus standard for aquaculture”.

The Water Framework Directive 2000/60/EC addresses phosphorus and phosphates within the broader issue of the ecological status of water bodies and eutrophication. Where phosphorus inputs contribute to eutrophication, measures must be implemented.

In France, the applicable requirements depend in particular on the regulatory status of the facility and the nature of its discharge.

For fish farms subject to the French ICPE regulations, the applicable general requirements must be considered together with the site-specific prefectural order, which may impose stricter requirements depending on the environmental context.

A common mistake should therefore be avoided:

taking a generic phosphorus value and considering it a regulatory limit applicable to all facilities.

The correct approach is to determine:

facility → regulatory status → type of discharge → receiving environment → environmental sensitivity → applicable requirements → treatment objective.

The role of the engineering consultancy

Phosphorus management should not be addressed only when a discharge becomes non-compliant.

It should be integrated into the system design from the outset.

A relevant design begins with a phosphorus mass balance:

→ phosphorus entering with feed
→ phosphorus incorporated into biomass
→ excreted phosphorus
→ phosphorus captured in solids
→ remaining dissolved phosphorus
→ phosphorus removed by treatment
→ phosphorus ultimately discharged.

This balance can then be used to design the treatment chain.

In a modern RAS, the objective is therefore no longer simply to “make phosphate disappear”.

It is to separate, concentrate, treat and, whenever possible, recover phosphorus.

This represents an important evolution in aquaculture system design:

moving from a waste-treatment approach towards a material-flow management approach.

Phosphorus is simultaneously an essential nutrient, a water-quality parameter, a potential driver of eutrophication and… a resource.

That is precisely why it deserves a much more prominent place in the design of professional aquaculture and aquariology facilities.