From waste to valuable fertiliser components

Circular agriculture

EU funding financed the phosphate recovery installation of the Green Mineral Centre demonstration plant. Photos in article: WUR

Estimated reading time: 12 minutes

Sewage sludge, manure and other organic waste streams are a source of energy and valuable nutrients for farmers. That makes it such a shame to incinerate or export them, as often happens at present. Scientists at Wageningen University & Research and their European partners are exploring options to exploit these waste streams by recovering and reusing the nutrients.

Europe produces large quantities of organic waste streams and has a surplus of manure, sewage sludge and waste from the food and animal feed industries. These waste streams are rich in organic components and nutrients such as phosphorus (P), nitrogen (N) and potassium (K). Currently, most of these nutrients are lost because the waste streams are exported, incinerated or not used optimally, which is inefficient. It’s important to retain these key nutrients for agriculture and other sectors that use them, as the Netherlands and Europe aim for a circular economy. In addition to the excess organic waste streams, there is also the issue of artificial fertiliser. A great deal of energy, usually in the form of fossil fuels, is consumed in manufacturing nitrogen fertilisers. The production of phosphate fertiliser requires phosphate ore from mines, but there is a limited supply of easily accessible reserves that are of sufficiently high quality. It is therefore important to reduce the use of artificial fertilisers or replace them with renewable fertilisers. To learn more about the extraction of nutrients from waste streams and the possibilities for practical applications, a European project called SYSTEMIC was started in 2017. In this project, five companies in four different countries carried out research. The project was coordinated by Oscar Schoumans, a senior researcher in manure treatment and valorisation at Wageningen University & Research, together with the Sustainable Soil Use team.

How can we retain those important nutrients for agriculture and at the same time reduce the use of artificial fertilisers?

Kimo van Dijk, one of the Sustainable Soil Use researchers involved: “Organic waste streams can be used for the production of biogas because they are in part highly biodegradable. The residue that is left after digestion is called the digestate. Digestate is rich in minerals and still contains a substantial amount of organic matter. We can recover the nutrients from this digestate.

Organic waste streams can be broken down into biogas by anaerobic digestion.

Besides pig slurry, other organic waste sources are also digested to increase biogas production.

The solid fraction of digestate is used to recover phopshate with the RePeat technology.

The phosphate that is extracted, for example, can be used by the fertiliser industry as a substitute for the import of mined phosphate. The nitrogen-rich concentrate that is recovered can serve as a replacement for regular fertilisers, provided that it is recognised as a fertiliser. For that to happen, the legislation associated with the Nitrate Directive needs to be amended.”

What contribution does SYSTEMIC make to these developments?

Van Dijk: “In this project, we carried out studies at five large biogas plants spread across Europe in which we looked at how we can extract organic soil improvers and nutrients such as nitrogen, phosphate and other minerals from manure, sewage sludge and other organic waste streams. Heat and energy were already being obtained from these waste streams at the test sites. In the studies, innovative techniques were introduced at the biogas plants to process the digestate further. In this way, nutrients could be extracted in the form of recycled biobased fertiliser ingredients for use in agriculture and other applications. There are many other biogas plant companies involved in SYSTEMIC too that want to learn from the experiences at the five test sites.”

Can’t the digestate be applied to the land directly?

Van Dijk: “The digestate can’t be applied in areas where there is a manure surplus because the phosphorus, nitrogen and potassium ratios aren’t optimal for the crops. What is more, the risk of emissions into the environment increases if we use nitrogen and phosphate inefficiently. That is a major issue at present for nitrogen in particular, which is why it is important to produce fertilisers that are appropriate; indeed, the demand for tailored fertilisers is increasing. We need to extract the minerals and organic matter in order to make optimal use of the digestate.”

How does the extraction of fertiliser ingredients from digestate work?

Van Dijk: “First, the digestate is divided mechanically into a solid fraction and a liquid fraction. The solid fraction contains much of the organic matter and usually the phosphate as well. The liquid fraction is rich in the minerals nitrogen (as ammonium) and potassium.

Valorising manure produces fertiliser ingredients that allow the efficient reuse of nutrients

The ammonium is usually extracted separately from the liquid fraction, and consequently a substrate with a high concentration of potassium remains. This is called ammonia stripping. There are also methods in which water is removed from the liquid fraction using microfiltration and/or reverse osmosis, leaving a nitrogen-potassium concentrate. That water can be purified, for example by ion exchange, in such a way that it can be discharged into the surface waters. That saves on transport costs and is better for the environment.

Crops need nutrients to grow, which they can take directly from the soil. Once these crops are harvested and consumed, the nutrients end up as organic waste. In a circular economy, they would be re-used for food production.

The solid fraction is rich in organic matter and phosphate, and that phosphate can be extracted. A new method for this, called RePeat (‘Recovery P to eat’), has been developed in the Netherlands by Wageningen Environmental Research. Acid is added to make a phosphate-rich solution and then separated. Thereafter, the phosphate is precipitated with calcium or magnesium to turn it into calcium phosphate or struvite respectively. This sludge can be dried to form a phosphate fertiliser or be utilised as a raw material for the fertiliser industry. The remaining solid fraction, which now contains very little phosphate, can be used as a soil improver, as a substitute for peat in the potting soil industry or as a casing soil layer for mushroom farms.”

What has SYSTEMIC meant for the Netherlands?

Van Dijk: “The EU funding was used to set up a Green Mineral Centre on the Groot-Zevert Vergisting test site in Beltrum in the Achterhoek region of the Netherlands, where the innovative technique for extracting phosphate could be rolled out on a large scale. In this plant, the liquid fraction is reduced further using reverse osmosis to make a mineral concentrate rich in nitrogen and potassium.”

What is happening at the other sites?

Van Dijk: “The focus at the other sites is very much on recovering nitrogen. In Germany, they are digesting maize and recovering nitrogen and fibres. The nitrogen serves as a fertiliser ingredient and the fibres can be used as a raw material for paper and other products. The site in Italy is mainly used for wastewater sludge, from which nitrogen-containing compounds such as ammonium sulphate are obtained. The two test sites in Flanders obtain nitrogen-rich products from material that consists largely of waste streams from the food industry. The techniques used vary from site to site, depending on the type of waste stream and on the kind of final products for which there is demand in the local region. As a group, we have taken the step of putting the extraction of nutrients into practice.”

A new method for the recovery of phosphate, called RePeat, has been developed in the Netherlands by Wageningen Environmental Research.

The liquid fraction is turned into an ammonium and potassium-rich solution by reserved osmosis.

Application of recovered ammonium as used in a tailor-made fertiliser in practice.

How far has this project advanced?

Van Dijk: “The recycling techniques we had in mind are up and running, we have finished fine-tuning the processes and the products we wanted to make are being made. We know that these products are more useable than the raw digestate because you can be much more accurate in determining the amounts of phosphate or nitrogen you apply. What we still want to know is precisely how valuable the products we are making are for agriculture. Agriculture needs products with a uniform quality, with the same concentration of minerals and the same availability for crops every time. In the past two years, we have run pilot projects separate from SYSTEMIC, for example field trials in the ‘Biobased Fertilisers Achterhoek’ pilot. We can draw positive conclusions from these trials as our products are just as good as the regular fertilisers.”

Does this mean we can now apply the technology on a large scale?

Van Dijk: “It’s not that simple. Crucially, it also needs to be a viable business case. That depends a lot on the prices in the fertiliser market, the processing costs – investments, operating costs, energy and the costs of the chemicals – and whether farmers accept the products and are willing to pay the right price. This all works out in the German and Italian situations but it is still tricky in the Netherlands and Flanders, which both have large manure surpluses. If we’re going to boost the circular economy and closed circular agriculture this way, there needs to be more financial certainty for such major investments. Leaving everything to market forces, with highly volatile prices, won’t work.

It is important that fertiliser products match the crop’s requirements

Sewage sludge, manure and other organic waste streams are a source of energy and valuable nutrients for farmers.

We also need to know what the impact of recycling the nutrients is on the environment. We are using an environmental impact analysis and a lifecycle analysis to determine this. That means we are obtaining a cradle-to-grave overview of the environmental impact of all the processes involved and the transportation, including energy consumption, the use of raw materials, waste and emissions. That will show us whether the environmental effects are better than for regular products or comparable to them. Another issue is the legislation: there are so many rules on how much fertiliser you are allowed to spread and its composition that this is actually an obstacle to the use of some of the new products. That is why we are now investigating what changes need to be made to the laws to enable the recycling of nutrients. As regards recycled fertiliser ingredients obtained from digested manure, policy changes are currently being discussed for permitting RENURE (‘REcovered Nitrogen from manURE’) as a substitute for artificial fertiliser.”

What role does Wageningen Research have in this project?

Van Dijk: “Oscar Schoumans is the project coordinator. Wageningen Environmental Research is leading the project and is closely involved in monitoring the biogas plants, working out the business case, establishing the agricultural and environmental effects of biobased fertilisation products and developing the tools and living labs for the roll-out in other companies. We’re also helping to draw up practical recommendations for policies in which circular agriculture is used. Wageningen has a great deal of expertise in these areas.”

What additional knowledge has been gained through this project?

Van Dijk: “SYSTEMIC is primarily a demonstration project for nutrient extraction on a large scale. We looked not just at the performance but also at the business case, sustainability aspects, legislation and regulations and the options for further biogas plants in Europe. All that data and all those experiences were used to create an online Business Development Package in which managers in charge of biogas plants, or other stakeholders, can work out what various processing techniques can offer them in their specific situation. It’s a useful tool for getting experience. SYSTEMIC has also given us a wealth of scientific information and it has been very educational for the young researchers who we set to work on the project. In short, the future looks bright for the extraction of nutrients, with the Netherlands as one of the pioneers – especially if we can make a good business case.”

European research context

SYSTEMIC addresses the following European policy challenges:

  • Halving the loss of nutrients and improving soil fertility
  • Reducing use of artificial fertilisers by 20 per cent by 2030

Wageningen Research groups involved: Wageningen Environmental Research Countries involved in Europe: Belgium, Germany, Italy and the Netherlands

Duration: 2017 – 2021

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