Wastewater as a source of fertilizer—possible?

To ensure clean rivers and streams, Aquafin removes, among other things, nitrogen from domestic wastewater. “Nitrogen removal requires a lot of energy,” explains Lennert Dockx, study coordinator at Aquafin. “To make the efforts and costs truly worthwhile, we are examining how nitrogen can be recovered in the future.”

From removal to recovery

In the current treatment process, an activated sludge system converts the ammonium (a form of nitrogen) present in wastewater into nitrogen gas, which escapes into the air. “To achieve nitrogen recovery, we need to capture nitrogen in a usable form before it is released as nitrogen gas through biological processes. That requires a different type of activated sludge, a high-rate variant,” Lennert explains. “In a second step, the biologically treated wastewater, still rich in ammonium, is fed into a column filled with zeolite. Zeolite is a natural adsorption material that can adsorb nitrogen. Once saturated with nitrogen, the zeolite can be used as a fertilizer.”

These steps were tested using a two-part pilot setup at Aquafin in Aartselaar, where 1m³ of wastewater was treated daily.

(Photo setup)

Results

The WalNUT study led to several interesting findings that the researchers will explore further in the coming period.

1. Quality over quantity
   Initial results show that when incoming wastewater is diluted by rainwater, nitrogen recovery is less efficient. “That makes sense, since diluted wastewater contains less ammonium,” says Lennert. “If we want to make recovery sufficiently efficient and economically viable, the technique will likely need to be applied to streams within the treatment process that have higher nitrogen concentrations.”

2. Indirectly more green energy
   The researchers also found that the high-rate sludge used in the study has a higher biomethane potential, which means more biomethane can be produced per unit of sludge. “Biomethane is created when biogas from sludge digestion is upgraded. It has the same quality as natural gas, which means it can be injected into the grid as green energy.”

Next steps?

In the coming months, researchers within the consortium will investigate the quality and behavior of the produced fertilizer. The zeolite-based variant may be more resistant to heavy rainfall and runoff than liquid fertilizers, thanks to zeolite acting as a carrier material for the nitrogen. “We will also look at the steps needed within the existing regulatory framework so that recovered nitrogen can be safely and widely applied,” Lennert adds.

“Wastewater contains many valuable resources. It’s our duty to look beyond purification alone. And that’s exactly what we’re doing.”
— Lennert Dockx, study coordinator at Aquafin

The researchers are already looking beyond the WalNUT project. “There are voices suggesting that coffee grounds could be a sustainable alternative to imported zeolite. We would like to explore such avenues further in a potential follow-up project.”

Want to know more about WalNUT? Visit www.walnutproject.eu