Interview Leon Korving

Following Rembrandt

‘Vivianite is initially colorless, but becomes blue after oxidation and eventually turns black. Rembrandt and Vermeer used the bright blue dye on their canvases’

Phosphate is a scarce raw material that we desperately need for our food supply. At the same time, phosphate is very harmful to the environment in large quantities, causing eutrophication and algal blooms. To avoid its harmful effects, phosphate is removed as best as possible at sewage treatment plants. Recovering phosphate is not easy, however. Existing techniques have limitations in efficiency and area of application, but there are positive developments towards a more circular economy. The mineral vivianite plays a key role in those developments. We interviewed Wetsus scientist Leon Korving.

Together with their industrial partners, researchers from Wetsus and TU Delft discovered that the phosphate-rich vivianite is created when iron salts are used to bind phosphate in sewage treatment plants. This is a commonly used technique for phosphate removal, but recovering the phosphate has proven difficult with existing techniques. Research by Wetsus showed that vivianite is relatively easy to extract from sewage sludge using a magnetic separation technique borrowed from the mining industry. The technique enables the recovery of up to 80% of phosphate from the sludge.

Recovery Technology

Phosphate is a valuable and scarce raw material (see info box). Sewage plants normally have two methods of removing phosphate. Wetsus scientist Leon Korving clarifies, “You can opt for biological phosphate removal, the advantage being that there are no chemicals necessary.” However,

Leon Korving at the Nieuwveer sewage station, near the Dutch city of Breda. Photo: Margret Tielemans

purification is more complex and may result in higher energy consumption. Another commonly used technique is adding iron salts to bind the phosphate in the sludge. The two techniques offer different starting points for recovery. “In the biological separation method, the phosphate can be precipitated as struvite by adding magnesium salts. This technique is currently in use at a number of treatment plants. The downside is that the recovery efficiency is limited to 10–50% of the phosphate in the sludge”, explains Korving further. Until now, however, there was no recovery technology for purification plants using iron salts. “We decided to work with Professor Mark van Loosdrecht’s group at TU Delft to find a method to recover phosphate in these purification plants as well.”

Vivianite as a phosphate carrier

Together with his colleagues, Korving discovered that the combination of iron and phosphate reacts to form the mineral vivianite. Vivianite binds phosphates and stands out because of its colour. “Vivianite is initially colorless, but becomes blue after oxidation and eventually turns black. Rembrandt and Vermeer used the bright blue dye on their canvases,” says Leon Korving. Vivianite makes smaller crystals than struvite, which complicates extraction. “Fortunately, we realized that vivianite is paramagnetic—it becomes magnetic as soon as there is a magnet nearby. Our industrial cooperation partners pointed out that magnetic techniques are already widely used in the mining industry. Professor Rem from Delft University of Technology had experience with the techniques, which allowed us to quickly demonstrate on a lab scale that the principle works for sewage sludge”, continues Korving. The operation of the magnetic phosphate removal has since also been tested in practice with European funding from EIT Raw Materials. A trial set-up at the Nieuwveer sewage treatment plant of the Brabantse Delta water board [near the city of Breda] proved that the method is highly effective. “The magnet removed up to eighty percent of the vivianite in the sludge residue from the digester. The next step is to translate it into a commercial-level pilot plant.”

Circular phosphate extraction

The vivianite can be used directly as iron fertilizer in agriculture. It is particularly suitable for olive trees, vines and citrus trees, species that suffer from iron deficiencies. The researchers are also looking for a way to separate iron and phosphate as efficiently as possible. “The phosphate can be used in agriculture as a fertilizer, while the wastewater treatment plant can reuse the iron salt for purification, making phosphate extraction circular”, according to Leon Korving.

Business & Technology Roadmap

The patented technology not only recovers phosphate, but also reduces the costs of sludge removal by ten to twenty percent. In future, the scientists want to investigate whether the mining technique can also be applied to animal manure, which could lead to additional commercial opportunities. What obstacles does Korving see on the road to mass production? “The most important step we need to take is to demonstrate that the plant is robust even with longer operating times, and yields the same results with different sludge types. We have set up a Business & Technology Roadmap with the project partners, in which we want to take steps together to demonstrate this, first with a mobile plant and later with a demonstration plant, for example. We must also ensure that there is a market for the recovered vivianite. We are now discussing this with parties from the fertilizer industry. It takes perseverance and patience now, but if the next stages of development are also successful and phosphate extraction is increasingly mandated by the government, things can move quickly in a few years.”

Phosphate

Every year, around 40 million tons of phosphate is extracted from phosphate mines worldwide. Eighty percent of this is processed into fertilizer. The largest deposits are in Morocco, China and the United States. The European Union has no reserves of its own and, due to its strategic importance for food production, included phosphate in its list of 20 critical raw materials in 2014. At the same time, the fertilizer poses a problem if it enters surface water in large quantities. It causes excessive algae bloom, resulting in a lack of oxygen in the surface water, with numerous negative consequences. As such, wastewater treatment plants are required to remove phosphate.

A phosphate mine

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