Phosphorus Recovery in Wastewater Treatment

Across the wastewater sector, phosphorus is shifting from being treated as a pollutant to being recognised as a resource. Treatment plants adopting recovery technologies are finding new ways to control costs, streamline operations, and produce fertiliser that can be returned to agriculture.

Why Recover Phosphorus?

Phosphorus is an indispensable element for food production. It is a critical nutrient for plant growth, and modern agriculture depends heavily on phosphorus-based fertilisers to maintain productivity and ensure food security for a growing global population.

The element primarily comes from phosphate rock, a non-renewable and geologically finite resource mined in a small number of countries. This geographic concentration creates vulnerabilities: many regions are exposed to supply disruptions, price volatility, and eventual depletion. Meanwhile, demand for phosphate is rising due to the need for higher agricultural productivity.

Recovering phosphorus from waste streams offers a way to strengthen fertiliser security through local, renewable sources. While recovery is technically possible from manure and food waste, municipal wastewater and sewage sludge offer the most scalable opportunities. Centralised treatment plants already process large phosphorus flows, and recovery technologies can be integrated into existing systems with relative ease.

Regulations on effluent quality in many countries already require phosphorus removal to limit eutrophication. Excess phosphorus discharged into rivers, lakes, and coastal waters stimulates algal blooms, which in turn deplete oxygen, release toxins, and can trigger the collapse of aquatic ecosystems.

For utilities themselves, phosphorus recovery can also be a smart business decision. Unmanaged phosphorus often precipitates as struvite, clogging pipes, driving up maintenance costs, or accumulating as waste in landfills. Recovery reduces reliance on costly chemicals such as polymers and coagulants and transforms a persistent liability into a valuable fertiliser product.

Phosphorus Recovery Methods

During the primary stages of wastewater treatment, phosphorus is removed through chemical precipitation, achieved by dosing the wastewater with metal salts, such as aluminium or iron compounds. The resulting “phosphorus precipitates” settle out with the sludge. The phosphorus-containing sludge is then directed to sludge treatment, where specific recovery methods may be applied to extract and recycle the material.

Different recovery processes yield different phosphorus products, each with its own benefits and limitations. Many of these approaches are still developing and not yet widespread.

      Struvite Precipitation

The most established approach for phosphorus recovery is struvite precipitation or the production of struvite granules, with over a hundred full-scale installations worldwide.

The process is typically applied after anaerobic digestion, either to the digested sludge before dewatering or to the dewatering liquors after dewatering. In the most common application, magnesium is dosed, and pH is raised to promote crystallisation of magnesium ammonium phosphate (MAP), a struvite that is useful as a slow-release fertiliser. Technology provider CNP Cycles provides diagrams of the multiple pathways for struvite recovery. These can be helpful for understanding how the process fits into different wastewater treatment configurations.

Some installations focus on separating and harvesting the struvite crystals for direct use as fertiliser. Others encourage phosphorus to form as struvite but do not separate it from the sludge. This second approach makes phosphorus more readily recoverable in downstream thermal processes such as mono-incineration.

Where biosolids are applied to land, phosphorus recovery has an additional advantage. Lower phosphorus levels in the final biosolids allow higher land application rates, since phosphorus content is often the limiting factor for spreading.

In addition to creating a marketable product, struvite precipitation reduces the risk of mineral scaling in pipes, pumps, and digesters. Preventing uncontrolled struvite build-up helps avoid costly maintenance and improves overall process reliability.

Emerging Technologies

Alongside mature struvite precipitation technologies, other processes for phosphorus recovery are at early stages of development:

Sludge Ash

Phosphorus recovered through struvite precipitation and similar technologies accounts for only part of the phosphorus processed in wastewater treatment. For utilities that use sludge incineration or other thermal processes as a final endpoint, phosphorus remains concentrated in the resulting sewage sludge ash (SSA) and can still be recovered.

Some projects have effectively demonstrated the extraction of phosphorus from SSA, achieving recovery rates of 80–90 percent. The most mature of these technologies, such as wet-acid leaching, produce phosphoric acid which can be used to make struvite products downstream.

This route is particularly effective for plants that use iron or aluminium salts to bind phosphorus during the wastewater treatment process, which would otherwise make direct struvite recovery from liquors more challenging.

Vivianite

Vivianite, an iron phosphate mineral that naturally forms in iron-rich sludge at wastewater treatment plants, can also be extracted and is being evaluated as fertiliser. The most mature technology for its recovery is magnetic separation but various technologies are at research stage.

Elemental (White) Phosphorus

Efforts like the EU-funded FlashPhos in Germany target the production of elemental white phosphorus (P₄) from sludge. The project is testing flash gasification and aims to produce the material for industrial applications while also generating alternative cement, iron alloy and heavy metal concentrates.

Increasingly, wastewater plants are integrating these solutions into a broader “resource factory” model, also recovering nitrogen, metals, and energy.

The Global Status of Phosphorus Recovery

Around the world, governments and utilities increasingly view phosphorus recovery as both a sustainability obligation and a strategic necessity for nutrient security.

Europe

Europe is clearly in the lead in this space. Phosphorus (including phosphate rock and, more recently, elemental phosphorus) features on the EU’s Critical Raw Materials lists, underscoring supply risk and economic importance.

Switzerland pioneered a nationwide mandate that requires all phosphorus to be recovered from municipal wastewater or sludge ash by 2026. This has pushed utilities to centralise sludge incineration, the allowed method for sludge treatment in the country, and build facilities that can extract technical-grade phosphoric acid at scale.

Germany is following suit with its own binding targets that kick in by 2029. The country’s ordinance essentially phases out land application for large plants and forces utilities to either recover phosphorus or send their sludge to dedicated recovery pathways.

The EU has also opened up markets for recovered products by allowing materials like struvite and ash-derived fertilisers to carry a CE mark, meaning the product is legally compliant and can be marketed within the EEA.

The Netherlands and Denmark have also been ahead of the curve on struvite recovery, mainly because their utilities recognised the operational benefits of the process early.

Americas

The United States and Canada have no national recovery mandates, yet recovery is practised at scale in plants of varying sizes. Utilities in these countries are motivated by economics: phosphorus recovery lowers operating costs and produces a fertiliser that generates revenue.

Latin American countries lag behind in phosphorus recovery because many plants remain focused on securing basic secondary and tertiary treatment capacity. Nevertheless, awareness is increasing, especially where interest in circular economy approaches and resource recovery is growing.

Asia

Japan has decades of activity in phosphorus recycling, with an emphasis on SSA as a secondary resource and ongoing research to improve purification and economics.

China, on the other hand, is testing multiple pilot projects in Beijing, Tianjin, and Shanghai, reflecting strong policy support for nutrient recycling. The government’s emphasis on food security and environmental protection is accelerating research into large-scale phosphorus recovery.

Meanwhile, Australia has hosted several pilot facilities, signalling growing interest in the region.

The Road to Scaled Recovery 

Recovering phosphorus from wastewater and sludge strengthens fertiliser security, cuts eutrophication at source, and converts a disposal liability into saleable products such as struvite and ash-derived fertilisers. It also aligns with utilities’ carbon and circularity targets by displacing mined phosphate and improving overall plant performance.

Adoption, however, is uneven. Even in well-developed wastewater markets, plants face capex and opex hurdles, issues with logistics and integration, and the need for reliable offtake. Demand beyond pilot projects depends on consistent product quality, clear standards and farmer confidence, in addition to procurement and financing models that reward circular outcomes.

Policy is set to nudge the sector forward, especially in Europe. The EU’s recast Urban Wastewater Treatment Directive tightens phosphorus limits and sets system-wide phosphorus load-reduction targets, which will favour processes that recover the material rather than just remove it.

Thermal Hydrolysis and Phosphorus Recovery

Pairing the thermal hydrolysis process (THP) with phosphorus recovery has been demonstrated at full scale in several wastewater treatment plants worldwide. Research and operational experience suggest that THP can enhance phosphorus recovery by increasing the release of soluble phosphorus into the liquors.

Several Cambi installations worldwide are used alongside phosphorus recovery. Read on some of them below: