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Recovering phosphorus from compost, manure and other waste residues

Phosphorus is essential for life on Earth; all organisms need the element for growth. The principal application of phosphorus is in fertilisers. Since phosphorus cannot be synthesised from or replaced by other elements, it needs to be produced from raw phosphate. However, worldwide phosphate rock reserves are limited. It is therefore advisable to look for alternative methods in order to ensure the production of food into the future. Researchers at the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB are currently developing a method to recover organic phosphorus from biogas digestate and other agricultural residues such as manure. The IGB researchers hope to recover up to 90 percent of the organically bound phosphate.

The use of manure as fertiliser is not optimal for plants and soils because it is often associated with the excessive accumulation of one or more nutrients such as phosphorus or potassium in the soil, resulting in environmental damage. © Thomas Max Müller/pixelio.de

World population is growing steadily and needs to be fed. In response to the increasing demand for food, bioenergy or biobased materials, global consumption of fertilisers has risen significantly and will continue to do so in the future. This leads to higher fertiliser prices, which in turn leads to higher prices for all agricultural products. It is a vicious circle and a way out urgently needs to be sought in order to secure the supply of food now and for future generations.

Phosphorus is a major constituent of fertilisers as it is a vital plant nutrient that all organisms need for growth. It plays a major role in DNA and RNA, and cells use phosphate to transport cellular energy in the form of ATP. When plants are used as food and feed, for industrial raw material or for the generation of bioenergy, phosphorus gets lost in waste disposal systems, the same as any other plant nutrient. Only a small proportion is returned to the soil in the form of manure or compost. Therefore, synthetic mineral fertiliser needs to be added to the soil in order to supply the plant nutrients essential to the growth of plants. In 2008, approximately 1.4 million tonnes of phosphorus were consumed for the production of synthetic phosphate fertiliser in the EU. Most of the phosphorus used is produced from raw phosphate. However, phosphate rock is a finite resource and controlled by only a handful of countries. Most countries are completely dependent on imports from these countries to cover phosphorus demand. In addition, conventional phosphate mining is environmentally devastating.

Alternatives to conventional phosphate mining

Phosphate recovered from wastewater. Technologies for recovering phosphorus in its organic form are already available, but the quantities of phosphorus recovered cannot cover the demand. © Fraunhofer IGB, Stuttgart

Although technologies for the recovery of dissolved inorganic phosphates in municipal and agricultural wastes have already been developed, there is not enough phosphorus to cover the huge demand. Technologies to recover organic phosphorus from agricultural waste and use it for the production of phosphate fertiliser are therefore being sought. The liquid fractions of municipal and agricultural waste contain inorganic phosphates, which can be recovered relatively easily. In solid fractions, organic phosphorus is bound in biochemical molecules such as phospholipids, nucleotides and nucleic acids. However, at present, no technology is available to recover organic phosphorus from solid fractions. 

Agricultural waste as a huge additional reservoir for phosphate recovery

In the EU-funded PhosFarm project, researchers from the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB are developing a method to make organic phosphorus from agricultural waste such as livestock manure or digestate from biogas plants accessible as a resource for agricultural fertilisers. Huge quantities of manure and digestate are generated every year and represent a huge stock for phosphate recovery. The Fraunhofer IGB is seeking to develop a process and integrated plant concept that allows the controlled enzymatic release of organically bound phosphate, enabling up to 90 percent recovery of total phosphorus. 

Coordinated by the Fraunhofer IGB, the PhosFarm consortium brings together nine research and industrial partners who are working on the development of a suitable method. The novel strategy is based on the use of phosphate hydrolysing enzymes immobilised onto suitable carriers and the recovery of inorganic phosphate from solid fragments of livestock manure, biogas digestate and other agricultural residues. In preliminary experiments, the Fraunhofer IGB researchers have been able to show that phosphate hydrolysing enzymes can release inorganic phosphate from model compounds.

Fertilisers and soil improvers need to be individually dosed

The EU-funded PhosFarm project is coordinated by the Stuttgart-based Fraunhofer IGB and aims at recovering valuable fertilisers and soil improvers from agricultural residues. © Fraunhofer IGB, Stuttgart

Phosphate is hydrolysed by the immobilised enzymes. The products of the enzyme reaction form a solid fraction and a liquid fraction. The latter contains the dissolved phosphate. After separating the solid fraction, the released phosphate dissolved in the liquid fraction can be precipitated as magnesium ammonium phosphate or calcium phosphate. These salts can easily be taken up by plants and can therefore be directly used in agriculture. The recovery of organic phosphates from agricultural residues have the potential to reduce the application of synthetic phosphate fertilisers on the one hand and minimise the over-fertilisation of fields caused by the direct application of livestock manure on the other. The direct application of manure or biogas digestate is not optimal for soils because their nutrient composition only corresponds in the rarest cases to plants’ requirements. 

In addition, the strategy has a positive side effect. The remaining, dewatered solid phase can be dried with energy efficient drying processes. The organic soil amendment substrate generated helps to improve soil fertility. Farmers are able to create their own “individual fertiliser” by blending organic soil improver and recovered mineral fertiliser salts in a ratio to suit the crop species requirement and the soil conditions.

Further information:
Dipl.-Ing. Daniel Frank
Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB
Nobelstraße 12
70569 Stuttgart
Tel.: +49 (0)711 970-3629
E-mail: Daniel.Frank(at)igb.fraunhofer.de

Website address: https://www.biooekonomie-bw.de/en/articles/news/recovering-phosphorus-from-compost-manure-and-other-waste-residues/