Starting in 2023, operators of large sewage treatment plants will have to submit a plan for recovering phosphorus, according to Phys.org. Though farmers can spread the ash of incinerated sewage sludge directly on their fields, plants are not able to make much use of the phosphorus in it because it contains pollutants.
According to Phys.org, conventional recovery technologies are costly and ridden with chemicals. Fraunhofer researchers helped scale up the process. Considering 7% of the world's deposit sites are in Morocco and the Western Sahara, there is a critical choke-point in the phosphorus supply chain.
Experts at Fritzmeier Umwelttechnik GmbH & Co. KG are developing a technology called P-bac, for a project dubbed "Phosphorrecycling—vom Rezyklat zum intelligenten langzeitverfügbaren Düngemittel—PRil," which translates to phosphorus recycling—from recyclate to smart, persisting fertilizer.
The company teamed with the Fraunhofer Research Institution for Materials Recycling and Resource Strategies IWKS at Alzenau and ICL Fertilizers Deutschland GmbH to scale this technology up from the lab to a pilot plant.
"We assisted with efforts to scale up the water recycling process as well as with the residual material recycling process, feasibility studies, and analysis," said Dr. Lars Zeggel, project manager at the Fraunhofer IWKS. "The phosphorus we recover from the ash with this innovative process is 50% plant-available in a water-soluble phosphate fertilizer. In contrast, the phosphate in pure sewage sludge ash is all but unavailable to plants."
The substrate is largely pollutant-free and the leftover pollutants can be reduced by more than 90%, according to Fraunhofer's press release. Fritzmeier developed the process for recovering phosphorus from sewage sludge. Instead of adding chemicals such as sulfuric acid to the sludge ash, experts let bacteria do the work.
The bacteria absorb carbon dioxide from the air and produce sulfuric acid by using added elemental sulfur to extract phosphorus from the ash, according to Phys.org.
Once the phosphate has been extracted, the water can be reused immediately to propagate bacteria, a cycle which can be repeated several times before the water needs to be desalinated. This leaves a lot less process water to be disposed and conserves a great deal of energy.
"It takes around ten liters of process water to recycle one liter of sewage sludge ash," Zeggel said. "We have been able to adapt the membrane filtration so that we can now remove 98% of the sulfate—that is, sulfur—from the water and ultimately circulate 75% of the process water."
The entire process is ready for use on a hundred-liter scale, according to the press release.
