Sustainable recovery and valuable reuse of phosphate from wastewater

Currently, phosphate is mainly removed through chemical precipitation or biological processes in which phosphate accumulating bacteria are involved. These processes are effective, but the removed phosphate is rarely reused in, for example, the production of fertilizer products. Reuse becomes challenging with precipitation methods, in which iron and aluminum salts are added, as phosphate is bound into the sludge requiring various techniques for its release.

In biological processes phosphorus ends up inside the bacteria cells in the sewage sludge. The direct application of this sludge on agricultural fields is increasingly restricted due to e.g. the presence of persistent organic pollutants and heavy metals. This is a pity because phosphate is valuable and scarce.

Therefore, it is worth exploring whether there are other effective methods for the removal and reuse of this compound. “In a world in which phosphate guidelines are increasingly tightened and the availability of phosphate becomes more and more uncertain, this can be a good addition to the purification processes that are already in place,” says researcher Norbert Kuipers. “We use a completely new principle, which is all about magnitite.”

The magnet's attraction

In the MAD-process, magnetite particles play a crucial role in the removal of phosphate from wastewater. After adsorption, the phosphate-containing particles are separated from water using a magnetic field. The use of magnetic separation is not exclusive to the MAD process: other processes, such as ViviMag, also rely on this method to recover phosphorous.

However, in the research conducted at Wageningen Food & Biobased Research, phosphate can be recovered directly from water (instead of forming phosphate sludge) without creating additional waste flows and without the addition of chemicals, such as iron salts. Norbert Kuipers: “This method is not only very energy-efficient and selective (only phosphate attaches to the magnetic substance), but also reversible: the phosphate and magnetite can be separated. This makes the valuable phosphate available again, isolated and in its original form, to the market. We can use this principle to remove phosphate in both aerobic and anaerobic conditions, without adding salts.”

Chain collaboration for an integrated solution

This research represents a collaborative effort between Wageningen University & Research and various companies, each playing a role in the phosphorous emission and recovery chain. Contributors include Agristo, producer of potato products and specialties, and the Royal Swinkels Family Brewers, who 'supply' phosphate-rich wastewater.

Bakker Magnetics and Sidra Wasserchemie contribute to the magnetic separation process and magnetite synthesis, respectively. BiotaNutri is dedicated to exploring ways to repurpose the recovered phosphate in organic fertilizer products, which Agristo can then promote within its network of potato growers. Meanwhile, Suez SA is exploring the potential integration of the MAD-process into municipal wastewater treatment systems. Norbert Kuipers explains: “By involving the entire chain, we want to launch a solution that really works for everyone.”

Information for an optimal process

The research, which started in mid-2022, has now mapped all the important information. The wastewater streams have been screened for their composition, the adsorption properties of magnetite are known, and it is clear how the desorption can occur. Now it is time to put these findings into practice. Kuipers: “This adsorption and desorption process really has the potential to be a very good piece of the puzzle in making purification processes more sustainable and reusing substances in wastewater. But it remains interesting how we will put the whole puzzle together: after all, the possibilities are endless. So I am very curious for which other waste flows this could be a good option.”