Phosphorus recovery

Phosphorus is an essential element of life and is fundamental in agriculture. Being a non-renewable and limited resource, the growing demand for fertilizers is gradually depleting the phosphate rock reserves. Thus, rationalizing the use of this resource and promoting its recovery is an issue of emerging concern.

Why is the issue of phosphorus recovery important?

Why recover phosphorus from sewage sludge?

What are the objectives of the PerFORM WATER 2030 project about phosphorus recovery?

Are there any industrial processes for phosphorus recovery?

An international overview on phosphorus recovery initiatives in a few videos and links


Why is the issue of phosphorus recovery important?

Phosphorus is an essential element of life and is present in all living organisms and is fundamental in agriculture to guarantee the productivity of cultivated soils. Being a non-renewable and limited resource, the growing demand for fertilizers is gradually depleting the phosphate rock reserves. Furthermore, the primary resource is strongly localized in specific areas of the planet, some of them subject to geopolitical tensions which make the price unstable and the reliability of supply less certain. North-Western Europe imports 88% of the phosphorus requirement, about 6.37 million tons per year, from non-EU countries. The final use of the resource is mainly as a mineral fertilizer for agriculture.

On the other hand, much of this resource is dispersed in nature, causing a progressive increase of eutrophication phenomena, which compromise the quality of many surface water bodies and reduce biodiversity. In the scientific, political and economic fields there are many proposals to improve the efficiency of phosphorus flow management and to increase the recycling of phosphorus in human activities and, therefore, also of the flows resulting from the treatment of urban waste water.

Rationalizing the use of this resource is one of the main goals of the European Phosphorus Platform (https://www.phosphorusplatform.eu/), in addition to some national platforms, such as the German (www.deutsche-phosphor-plattform.de), Dutch (https://www.nutrientplatform.org/en/), e Italian (https://www.piattaformaitalianafosforo.it/).


Why recover phosphorus from sewage sludge?

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To tackle this problem and ensure sustainable development and a circular economy, it is possible to recover phosphorus from wastewater, since it is concentrated in sewage sludge. Sewage sludge is the main by-product of wastewater treatment and contains about 95-99% of water and 1-5% of dry matter.

After the solid / liquid separation processes, the aqueous sludge fraction contains 5 to 20% of the phosphorus in dissolved form. From this fraction it is possible to separate the struvite, a magnesium-ammonium phosphate salt (also called MagAmP). However, most of phosphorus (from 80 to 95%) remains in the residual semi-solid fraction after the solid / liquid separation processes. Phosphorus from the semi-solid fraction can be recovered from the ashes deriving from a waste-to-energy process, fed by the combustion of sludge only (mono-incineration)

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Combustion is the most commonly used thermal treatment to treat sludge that is unsuitable for use in agriculture. In this way we can recover energy and phosphorus from the ashes. The calorific value of the 40% dry purification sludge allows combustion without auxiliary fuels. Alternatively, it is possible to dry the sludge up to 80-90% of dry and use part of the energy produced by the combustion for the drying process. The percentage of sludge incinerated on the total sludge produced is 3% in Italy, 19% in France, 24% in Denmark, 44% in Austria, 56% in Germany, 64% in Belgium, and 100% in the Netherlands and in Switzerland. In the United States and Japan the percentages are respectively 25% and 55% .

A recent example of this type of plant is the one serving the Canton of Zurich, Switzerland, which treats 84,000 tons of wet sludge annually and recovers about 13,000 tons of phosphorus-rich ashes annually. Emission values of the flue gases in the atmosphere are considerably lower than even restrictive ones Swiss limits. In addition it self-generates electricity for its own functioning and heat (4500 thermal kW) which partly feeds the pre-drying of sludge (from 25 to 40% of dry matter). The surplus feeds a local network of district heating.

Sewage sludge ashes from plants of this type are one of the main secondary resources of phosphorus. Phosphorus pentoxide usually varies between 10 and 20%, similar to the content present in many phosphate rocks.

The average phosphorus content in the dry fraction of the sludge produced by wastewater treatment plants in the Milan area (about 4 million inhabitants) amounts to about 2.2% (on a dry weight basis). Considering that the sludge production of the Lombard plants is about 500 thousand tons per year (source: Arpa Lombardia, 2017), of which 20% of dry matter, the potentially recoverable phosphorus would amount to 2,200 t of phosphorus per year, equal to about 5,000 t annual phosphorus pentoxide.

At the present time the price of phosphorus from recovery processes is still higher than the price of phosphorus obtained from phosphate rocks. However, the difference between the price of "natural" phosphorus and recovered phosphorus will gradually get lower with the progressive depletion of natural phosphorus resources IndexMundi, a data portal that gathers facts and statistics, reports that the price of phosphate rocks almost doubled between 2004 and 2019 (from 42 to 85 US$/t), with occasional high peaks, due to the political instability of the producing countries.

In the short term, while waiting to exploit the phosphorus contained in the ashes, it is possible to accumulate the ashes in long-term deposits, following the example of Switzerland, Germany and Denmark. There are no examples in Italy, but it could be a significant strategic element for not being at a disadvantage compared to other countries in the future, even if regulatory and legal instruments are needed to define the methods of implementation and management. The topic is today the attention of the Italian phosphorus platform and the Ministry of the Environment and the Protection of the Territory and the Sea.

Why recover phosphorus from sewage sludge?

Generic placeholder image

To tackle this problem and ensure sustainable development and a circular economy, it is possible to recover phosphorus from wastewater, since it is concentrated in sewage sludge. Sewage sludge is the main by-product of wastewater treatment and contains about 95-99% of water and 1-5% of dry matter.

After the solid / liquid separation processes, the aqueous sludge fraction contains 5 to 20% of the phosphorus in dissolved form. From this fraction it is possible to separate the struvite, a magnesium-ammonium phosphate salt (also called MagAmP). However, most of phosphorus (from 80 to 95%) remains in the residual semi-solid fraction after the solid / liquid separation processes. Phosphorus from the semi-solid fraction can be recovered from the ashes deriving from a waste-to-energy process, fed by the combustion of sludge only (mono-incineration)

Generic placeholder image

Combustion is the most commonly used thermal treatment to treat sludge that is unsuitable for use in agriculture. In this way we can recover energy and phosphorus from the ashes. The calorific value of the 40% dry purification sludge allows combustion without auxiliary fuels. Alternatively, it is possible to dry the sludge up to 80-90% of dry and use part of the energy produced by the combustion for the drying process. The percentage of sludge incinerated on the total sludge produced is 3% in Italy, 19% in France, 24% in Denmark, 44% in Austria, 56% in Germany, 64% in Belgium, and 100% in the Netherlands and in Switzerland. In the United States and Japan the percentages are respectively 25% and 55% .

A recent example of this type of plant is the one serving the Canton of Zurich, Switzerland, which treats 84,000 tons of wet sludge annually and recovers about 13,000 tons of phosphorus-rich ashes annually. Emission values of the flue gases in the atmosphere are considerably lower than even restrictive ones Swiss limits. In addition it self-generates electricity for its own functioning and heat (4500 thermal kW) which partly feeds the pre-drying of sludge (from 25 to 40% of dry matter). The surplus feeds a local network of district heating.

Sewage sludge ashes from plants of this type are one of the main secondary resources of phosphorus. Phosphorus pentoxide usually varies between 10 and 20%, similar to the content present in many phosphate rocks.

The average phosphorus content in the dry fraction of the sludge produced by wastewater treatment plants in the Milan area (about 4 million inhabitants) amounts to about 2.2% (on a dry weight basis). Considering that the sludge production of the Lombard plants is about 500 thousand tons per year (source: Arpa Lombardia, 2017), of which 20% of dry matter, the potentially recoverable phosphorus would amount to 2,200 t of phosphorus per year, equal to about 5,000 t annual phosphorus pentoxide.

At the present time the price of phosphorus from recovery processes is still higher than the price of phosphorus obtained from phosphate rocks. However, the difference between the price of "natural" phosphorus and recovered phosphorus will gradually get lower with the progressive depletion of natural phosphorus resources IndexMundi, a data portal that gathers facts and statistics, reports that the price of phosphate rocks almost doubled between 2004 and 2019 (from 42 to 85 US$/t), with occasional high peaks, due to the political instability of the producing countries.

In the short term, while waiting to exploit the phosphorus contained in the ashes, it is possible to accumulate the ashes in long-term deposits, following the example of Switzerland, Germany and Denmark. There are no examples in Italy, but it could be a significant strategic element for not being at a disadvantage compared to other countries in the future, even if regulatory and legal instruments are needed to define the methods of implementation and management. The topic is today the attention of the Italian phosphorus platform and the Ministry of the Environment and the Protection of the Territory and the Sea.


What are the objectives of the PerFORM WATER 2030 project about phosphorus recovery?

The objective of the study within the PerFORM WATER 2030 project is to evaluate the effective potential of phosphorus recovery from the ash from sewage sludge coming from different plants located in the Milan area and produced by the pilot thermal sludge valorisation plant built in the area of the PerFORM WATER 2030 project at the San Giuliano West wastewater treatment plant (for more details see sludge thermal valorization).

The research program includes the characterization of the ashes and the phosphorus extraction tests. In a first step, some samples of thermally dried sludge are converted to ashes by heating in air at 900°C in a laboratory muffle oven. By acid extraction it was estimated that it is possible to recover about 90% of the phosphorus contained in the ashes. The research program will subsequently include the extraction of phosphorus from the ashes produced by the pilot plant located at the San Giuliano West wastewater treatment plant. Experimental tests will give indications on the possible industrial developments of phosphorus recovery.

Phosphorus recovery from sewage sludge ashes

Project details


Are there any industrial processes for phosphorus recovery?

Yes, many, but none has yet been applied in Italy.

This is the main reason that led to include this topic among those developed by the PerFORM WATER 2030 project.

It is in fact essential to acquire knowledge in order not to run late in the competition with other EU countries.

A full review, in italian language, is reported in this paper, with a wide english abstract, (State of the art and potential of phosphorus recovery technologies from sewage sludge; Environmental Engineering, Vol. 5 n. 3/2018, p. 1-21; web: https://doi.org/10.32024/ida.v5i3.p01)

A European research project (P-Rex)), developed between 2012 and 2015, assessed the application potential of different technologies for phosphorus recovery. A summary of the P-Rex project is available in pdf version The final report of the project is available in pdf version while here you can can find other downloadable materials.


An international overview on phosphorus recovery initiatives in a few videos and links

Youtube Video a general video on P recovery from sewage sludge ashes by the Phos4You research project

A detailed technical Youtube on a specific P recovery technology (RecoPhos research project)

Here you can find informations on a technology for producing fertilizers with recycled phosphate and an interesting video about P recovery in the Netherlands

Link to the web page of the 3rd European Sustainable Phosphorus Conference where you can find conference acts and a video of the opening speech by Karmenu Vella, EU Commissioner for the Environment"

Link to an article about P recovery in Canton of Zurich and Switzerland: Phosphorus from sewage sludge – The strategy of the Canton of Zurich and Switzerland, Morf L.S. (2012), Office for Waste, Water, Energy and Air (AWEL) Canton of Zurich, Switzerland


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