Researchers identified the bacterium in a New Jersey wetland. Credit: Frank Wojciechowski

A team from Princeton University has developed a more efficient method to treat toxins found in sewage, fertilizer runoff and other forms of wastewater.

Sewage plants use bacteria to remove the environmental toxins from wastewater so clean, processed water can be safely discharged into oceans and rivers. They do this by churning a significant amount of air into the wastewater sludge to feed the bacteria, using oxygen to turn ammonium into nitrite, which is eventually converted to nitrogen gas.

However, Princeton researchers have discovered that microbe Acidimicrobiaceae bacterium A6 is capable of breaking down ammonium in the absence of oxygen, providing an alternative to the costly oxygen-dependent methods currently used in sewage treatment and other processes.

“A great deal of energy is used by machinery that mixes air into wastewater to provide oxygen for breaking down ammonium,” Peter Jaffe, the William L. Knapp '47 Professor of Civil Engineering at Princeton and a professor at Princeton's Andlinger Center for Energy and the Environment, said in a statement. “A6 carries out this same reaction anaerobically and might present a more efficient method for treating ammonium and a way to treat other environmental pollutants found in oxygen-poor areas, such as underground aquifers.”

Removing ammonium prevents the depletion of oxygen in streams and prevents the eutrophication—the growth of excessive algae and other plants triggered by nitrogen compounds—from sewage and agricultural runoff.

Feammox—an alternative chemical process for breaking down ammonium—occurs in acidic, iron-rich, wetland environments and soils. The alternative process has been found to take place in riparian wetland soils in New Jersey, in tropical rainforest soils in Puerto Rice, in wetland soils in South Carolina, and at various forested and wetland locations in Southern China.

However, it was unknown what enabled the Feammox reactions.

In 2015, the researchers believed that a single bacterium could be at the root of the process and began studying samples from a wetland in New Jersey. They discovered that the Feammox reaction only took place in the swamp samples when a class of bacteria called Actinobacteria was present, which ultimately led to pinpointing A6 as the cause.

In the current study, the researchers mixed soil samples in an oxygen-free chamber from the New Jersey wetland with water and a material containing iron oxide and ammonium. They then allowed the mixture to incubate in airtight vials for almost a year to mimic the anaerobic conditions of the wetland soil.

The scientists removed a small sample about every two weeks from each of the vials to see whether the iron oxide and ammonium were being degraded. They used genetic sequencing to identify the bacterial species present.

“Ever since we found that the reaction was taking place in the wetland here in New Jersey, we've suspected that a bacterium was doing the heavy lifting,” Jaffe says. “This study confirmed that A6 has this ability, making it the first known species known to carry out the Feammox reaction.”

The team is now looking at how to build a reactor where A6 could be used to process ammonium at industrial scales, which could be challenging because the bacteria consumes a substantial amount of iron to carry out the process. The researchers are looking at applying a small electrical potential between two electrodes inserted into the reactor’s liquid in a device to take over the role that iron was playing in a Feammox reaction.