A bacterium discovered by Princeton University researchers in a New Jersey swamp may offer a more efficient method for treating toxins found in sewage, fertilizer runoff and other forms of water pollution.

The bacterium, Acidimicrobiaceae bacterium A6, is capable of breaking down ammonium, a pollutant found in sewage and fertilizer runoff. A6 can perform this chemical conversion in the absence of oxygen, an ability that could be useful for providing alternative methods to oxygen-dependent methods currently used in sewage treatment and other processes.

The bacterium was found in a New Jersey swamp. Source: Frank WojciechowskiThe bacterium was found in a New Jersey swamp. Source: Frank WojciechowskiA great deal of energy is used by machinery that mixes air into wastewater to provide oxygen for breaking down ammonium, says Peter Jaffe, a professor at Princeton's Andlinger Center for Energy and the Environment. 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.

Jaffe and his colleague Shan Huang, an associate research scholar in civil and environmental engineering, reported the discovery of A6 in the journal PLOS ONE.

Most sewer plants that discharge into oceans or rivers already use bacteria to remove ammonium from waste. Doing so requires churning lots of air into the sludge to feed oxygen to the bacteria. The bacteria use the oxygen in a chemical reaction that turns ammonium into nitrite. Other bacteria convert the nitrite to nitrogen gas.

Removing ammonium is important to prevent depletion of oxygen in streams and to prevent eutrophication, the growth of excessive algae and other plants triggered by nitrogen compounds from sewage and agricultural runoff.

An alternative chemical process, known as Feammox, occurs in acidic, iron-rich, wetland environments and soils, and has been found to take place in riparian wetland soils in New Jersey, in tropical rainforest soils in Puerto Rico, in wetland soils in South Carolina, and at various forested and wetland locations in Southern China.

The Princeton researchers thought that a single bacterium might be at the root of the process. In a study at that time, Jaffe and his colleagues found that the Feammox reaction only took place in the swamp samples when a class of bacteria known as Actinobacteria were present. Among these bacteria, the researchers identified a species of bacterium that they dubbed Acidimicrobiaceae bacterium A6, and which they suspected played a key role in the Feammox reaction. They thought that A6 was converting the ammonium to nitrite and common bacteria were handling the conversion of nitrite to nitrogen gas.

In their most recent study, the Princeton team mixed soil samples collected from the New Jersey wetland with water and a material containing iron oxide and ammonium and allowed the mixture to incubate for nearly a year.

Mixing the soil samples and the metal medium was done in an oxygen-free chamber and the vials were sealed to mimic the anaerobic conditions of the wetland soil from which the bacteria originated.

About every two weeks over the course of the year, the scientists removed a small sample from each of the vials to see whether the iron oxide and ammonium were being degraded. When they discovered a sample where this reaction was taking place, they used genetic sequencing to identify the bacterial species present, and found that A6 was carrying out the Feammox reaction.

The Princeton team is exploring how to build a reactor where A6 could be used to process ammonium at industrial scales. One challenge is that the bacteria consume a great deal of iron to carry out the process, which would make it too costly as a method to replace aeration.

To get around this problem, the researchers are experimenting with applying a small electrical potential between two electrodes inserted into the reactor's liquid in a device they dubbed a "microbial electrolysis cell." The electrodes can then take the role that iron was taking in the Feammox reaction.

The Princeton team is working with the Chinese environmental ministry to develop a prototype reactor to reduce ammonium and heavy metals in wastewater. They are exploring whether the technology could help counter eutrophication, where excessive nutrients in runoff damage rivers, lakes and coastlines.