What is phosphorus recovery about?
Phosphorus as a nutrient – importance for our society
Phosphorus is a chemical element. It is contained in molecules that are crucial for all living organisms such as ATP (energy currency) or DNA (reproduction). That is why phosphorus is an essential ingredient of fertilizers. No phosphorus means no food.
The phosphorus that we use today in our society mainly comes from finite phosphorus rock resources that are concentrated in a few countries. The mining and processing of the rock cause environmental damage. About 80-90% of all phosphorus that we use today, is used for fertilizer production. Once spread on fields, the phosphorus is taken up by crop and ends up, in one way or another, in our food. On the toilet, we excrete the phosphorus.
Phosphorus as a pollutant – removal in wastewater treatment plants
Finally, phosphorus ends up in wastewater treatment plants. If phosphorus found its way to (effluent receiving) surface waters, it would act as an aquatic fertilizer. It would proliferate the growth of algae, often toxic ones. To prevent the resulting ecological and economic damages, phosphorus is removed from wastewater in the treatment plant. The most popular practice is to add iron salts to the wastewater. Once added, the iron binds the phosphorus. The formed iron phosphate is insoluble. It is collected at the bottom of tanks were it forms, together with some organic material, sewage sludge. The phosphorus is removed from the wastewater.
In a next step, sewage sludge is brought to a digester. Here microorganisms transform organic material into biogas. The biogas can be used for heat and electricity production. The conditions in the digester are different compared to other parts in the wastewater treatment plant. The chemical, biological and physical conditions are stable. There is no oxygen, and the material stays in the digester for about one month. Afterward, the sludge is often landfilled or incinerated, without further use of phosphorus that is contained in the sludge. Sometimes the sludge is brought to agricultural fields, but logistics are complicated because of the large volumes and there are debates about pollutants and the plant availability of the phosphorus in the sludge.
Phosphorus recovery from sewage sludge containing iron phosphate
The broader target of our research was, to develop a technology that allows us to separate phosphorus from digested sewage sludge containing iron phosphate. This would allow us to reuse phosphorus in fertilizers.
To be able to develop such a technology we were convinced that more information regarding the exact type of iron phosphorus contained in the digested sewage sludge is required. Because there are hundreds of different iron phosphorus compounds. They all differ in their chemical and physical properties. After consulting literature, it was obvious that only a little information existed about iron phosphorus in sewage sludge.
The iron phosphorus mineral vivianite – main form of phosphorus in sewage sludge
We collected sewage sludge samples from Finland, Germany, and The Netherlands. We processed and analyzed (x-ray diffraction, electron microscopy techniques, Mössbauer spectroscopy etc.) the samples under oxygen-free conditions. In all treatment plants where iron was used for phosphorus removal, we found an iron phosphorus mineral called vivianite. The more iron was present in the sludges the more phosphorus was bound in vivianite, up to 90% of the phosphorus was bound in vivianite. The vivianite is present in tiny particles in the sludge, about 100 µm. That is about the diameter of a human hair. That means almost all phosphorus is present in just one single phase. Before that everybody thought, the phosphorus is present in many different forms. It turned out that the digester is a place of vivianite production. Its long and steady conditions and the absence of oxygen drive the formation of vivianite.
A prestigious mineral in sewage sludge
Vivianite is a blue mineral that is also used by artists; for instance, the Delft master painter Johannes Vermeer was a keen user of this pigment. Besides in sewage sludge, this mineral can be found elsewhere, such as in peat.
American natives also used the pigment. Artists from the US made a painting with vivianite that we obtained from a sewage treatment plant.
Vivianite – key mineral for phosphorus recovery
What’s nice about vivianite, besides its baby blue color, is that it is ferromagnetic. It can be retrieved from sewage sludge by magnets, using cost-effective equipment that is already well established in the mining industry. By using a small set up, 60 percent retrieval rates were achieved. This is more compared to all other phosphorus recovery technologies which target on sewage sludge. Most likely this percentage will be increased even further in the upcoming pilot plant installation.
Once the mineral has been separated, vivianite can be dissolved with potash lye to produce a liquid potassium phosphate fertilizer. During the dissolution of vivianite, iron oxides, which can be used to produce iron salts, are formed and can then be reused for phosphate elimination in sewage treatment plants.
In parallel, Wetsus/TU Delft scientists try to increase the particle size of the vivianite in the sewage sludge. Bigger particles are easier to separate from the sewage. More phosphorus can be recovered for fertilizer production.
This research story shows that wastewater, often considered as a product that we have to get rid of, actually contains valuable resources.
These findings are described in the article entitled Vivianite as the main phosphate mineral in digested sewage sludge and its role for phosphate recovery, recently published in the journal Water Research. This work was conducted by P. Wilfert from Wetsus and the Delft University of Technology, A.I. Dugulan, K. Goubitz, G.J. Witkamp, and M.C.M. Van Loosdrecht from the Delft University of Technology, and L. Korving from Wetsus.