Turning Sewage into Fertiliser: The Science Behind Phosphorus Recovery

Phosphorus is essential to life, yet the world is in a phosphorus crisis. Used primarily in agriculture as a key nutrient in fertilisers, phosphorus is mined from finite phosphate rock reserves—most of which are concentrated in a handful of countries. At the same time, significant quantities of phosphorus are lost every year through wastewater streams.

In a circular economy, this is a missed opportunity. Instead of viewing sewage as waste, we can extract valuable resources like phosphorus and close the loop.

Why Phosphorus Recovery Matters

• Finite Supply: Approximately 90% of mined phosphate is used in food production. Yet global reserves are limited, and prices are volatile.

• Environmental Damage: Excess phosphorus discharged into waterways contributes to eutrophication, causing harmful algal blooms and oxygen-depleted “dead zones.”

• Circular Value: Recovering phosphorus from sewage sludge or wastewater not only diverts pollutants but also creates a renewable source of fertiliser.

Where Phosphorus Lives in the Wastewater Stream

Phosphorus primarily enters municipal wastewater through

• Human excreta (biological phosphorus)

• Food waste

• Detergents

• Industrial discharge

In treatment plants, it typically accumulates in biosolids (sewage sludge) or remains dissolved in the liquid stream. Recovery processes must therefore target both the solid and liquid phases depending on plant configuration and goals.

Key Phosphorus Recovery Technologies

1.Struvite Crystallization

Struvite (magnesium ammonium phosphate) is a crystalline compound that forms when magnesium, ammonia, and phosphate are present under the right pH and temperature conditions.

• How it works: Systems like Ostara’s PEARL® or AirPrex induce struvite formation from digester centrate or sidestreams.

• Benefits: Prevents pipe scaling, reduces maintenance, and produces high-value fertiliser.

• Output: Commercial-grade fertiliser pellets, ready for agricultural use.

2.Thermal Hydrolysis + Ash Processing

In sludge incineration, phosphorus concentrates in the ash. Technologies such as AshDec and RecoPhos extract phosphorus from this ash via high-temperature chemical or thermal treatment.

• Best suited for: Large utilities with sludge incineration capabilities.

• Challenges: High energy input, complex permitting, and ash handling logistics.

3.Enhanced Biological Phosphorus Removal (EBPR) + Post-Recovery

EBPR selects for bacteria that store excess phosphorus in their cells. Once settled into sludge, this biomass can be digested and treated for recovery via:

• Chemical extraction (acid/base)

• Thermal treatment

• Membrane separation

Adopting phosphorus recovery isn’t just an environmental win—it’s smart economics:

Global Leaders in Phosphorus Recovery

• The Netherlands: Struvite recovery is integrated into national nutrient recycling strategy.

• Germany: Mandated phosphorus recovery from sludge ash by 2029.

• Ostara (Canada/US): Commercialised struvite recovery with over 20 full-scale installations.

• Veolia: Pushing ash-based recovery in European markets.

Challenges and Considerations

• CapEx and Retrofit: Retrofitting older facilities can be costly without public incentives.

• Technology Fit: No one-size-fits-all solution; site-specific evaluation is critical.

• End-Product Market: Agricultural acceptance and certification of recovered fertiliser vary by region.

Final Thoughts

In the circular economy, wastewater is not waste—it’s a resource. Phosphorus recovery is a prime example of how utilities can transform liabilities into assets while aligning with environmental goals, regulatory demands, and long-term business value.

Want to Learn More?

We help utilities and industrial clients evaluate, design, and implement resource recovery strategies. Contact us to explore how phosphorus recovery can work for your plant.