Science - March 9, 2017

Phosphate recovery

Text:
Didi de Vries

Phosphate mines are becoming exhausted on the one hand, whereas on the other too much phosphate is leaching from manure into the soil and water. Scientists are working hard on techniques to recover the mineral and reuse it.

text Didi de Vries  illustration Rob de Winter

Phosphate is a dilemma for scientists because we have both too little of it and too much. On the one hand, reserves of mineral phosphate in mines are becoming depleted, which may cause problems for the production of fertilizer and animal feed. Phosphate is an important component as it helps plants grow well and gives animals strong bones. On the other hand, some regions are struggling with excess phosphate in the soil. Surpluses generally accumulate when too much manure is used on the land, as was the case in the Netherlands for a long time. The soil then becomes saturated with phosphate and the mineral leaches into the surface water. This can lead to eutrophication: an excess of nutrients in the water that may result in certain species — such as algae — becoming dominant.

To prevent things from getting worse, some of the animal manure in the Netherlands is now being processed in fermentation reactors or incinerators. But it would of course be much better if phosphate could be recovered from the manure — or the surface water — and reused in fertilizer and cattle feed. Researchers are hard at work developing techniques and systems for this. Scientists at Wageningen University & Research are involved in three promising research projects.

Mineral power plants

First, WUR is coordinating a major European project in which ‘green mineral power plants’ will be established at five different sites in Europe over the next four years. Those plants will process manure to produce mineral fertilizer ingredients, among other things. ‘We extract mineral phosphate for recycling from the solid fraction in animal manure, for instance,’ explains Oscar Schoumans, coordinator of the top-sector project ‘Meerwaarde Mest en Mineralen’ (added value from manure and minerals) and a researcher at Wageningen Environmental Research. ‘That can be exported to places with a shortage of phosphate.’ Schoumans cannot disclose any information about the technology used in the plants as Wageningen Environmental Research is currently negotiating with private sector parties about the future use of the plants.

Researchers at the scientific institute Wetsus have also developed a technique for separating out phosphate. They are able to use this to recover iron-bonded phosphate from water. Wastewater purification plants around the world add iron in the purification process as a way of filtering phosphate out of the water. The difficulty lies in the step that follows: separating the iron and phosphate again so that both can be reused. The scientists at Wetsus have found a solution.

In future it may also be possible to use that solution to recover phosphate from ecosystems that are affected by eutrophication. To this end, researchers at Wetsus have developed a way of suspending iron as a granular solid in surface water, like a tea bag. The researchers withdraw that ‘tea bag’ from the water once the phosphate has been adsorbed; then they apply their separation technique to isolate the phosphate again from the iron. This separation technique is also shrouded in mystery as it is the subject of a patent application. The Wetsus researchers are currently concentrating on making the process financially viable.

Polymers

A third project, run by Louis de Smet of the Laboratory of Organic Chemistry, is still in the early stages. He is taking a completely different approach to the phosphate problem, investigating how you can selectively fish certain nutrients out of wastewater using polymers. These polymers have a structure that can bind ions in a very targeted way with the aid of an electrical field. The theory is there; now lab tests will have to show whether it can work in practice.

One method that is already being used to recover phosphate from poultry manure is combustion. This drives the nitrogen compounds off, leaving the phosphate behind in the ash. The ash is then exported to areas that have a deficiency of phosphate. However, the combustion of manure from cattle and pigs is not viable. Their manure contains large amounts of water, making transport expensive while combustion produces little energy.

Another existing approach is to incorporate phosphate in struvite, that can then be used as a fertilizer ingredient. That is done in a number of sites, such as the sewage purification plants in Amersfoort and Amsterdam. Struvite is composed of ammonium, magnesium and phosphate, and is obtained simply by adding magnesium to the waste water. However, there is not much demand for struvite, in part because of the strict regulations governing its reuse, so its production is barely viable.

Investments

In short, there is plenty still for phosphate researchers to work on. ‘Our set of technological tools is not yet optimal so we need to continue working on improvements,’ explains Leon Korving, project manager for the reuse of phosphate at Wetsus. According to him, one problem is that few organizations in the manure processing industry want to invest in fundamental research. ‘They prefer to look at new ways of linking the existing techniques. That then turns out to be too expensive. We need fundamental research to achieve new breakthroughs.’

Schoumans, at Wageningen Environmental Research, disagrees with Korving. He says the industry has actually shown a lot of interest in collaborating on projects for recovering phosphate. The European Commission gave eight million euros in funding for the green mineral power plants and the participating companies have added 20 million to that, says Schoumans.


Wetsus

Wetsus is a leading European research centre for sustainable water technology in Leeuwarden. It is a cooperative venture between companies, government bodies and research institutes, including WUR. PhD candidates from the affiliated institutes carry out research for both the public sector and the private sector. Cees Buisman, professor of Biological Recovery and Reuse Technology at WUR, is the scientific director at Wetsus. Louis de Smet of Wageningen’s Laboratory of Organic Chemistry works for the institute as an advisor.



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