Conventional nitrogen removal for water treatment requires a string of modest molecular finagling: oxidizing the water’s ammonia into nitrite, oxidizing that nitrite into nitrate, and then de-nitrifying that nitrate into benign, atmospheric nitrogen. The dissolved oxygen called for by these conversions requires lots of energy-consuming — and therefore costly — aeration. The organic substrates thrown into the mix create excess sludge, the removal of which adds to the bill. There are also greenhouse gasses created by the process to contend with.
The anammox, or anaerobic ammonium oxidation, process has become a darling in wastewater treatment circles thanks to its “shortcut” ability to remove nitrogen. The industrious microorganisms involved turn ammonium and nitrite directly into nitrogen gas anaerobically, without the need to aerate. The anammox process actually results in a net consumption of carbon dioxide and leaves less sludge than the traditional method. Some estimates put the total energy reduction potential at 60 percent compared to conventional nitrogen removal.
Prompted by this valuable use and a desire to know more, the United States Geological Survey (USGS) embarked on the first-ever field study of anammox’s presence in groundwater. It deployed a research team to a groundwater study site in Cape Cod, MA, where the USGS has been looking into subterranean nitrogen-cycling processes. There the researchers found that anammox was active, though at relatively low rates, even where groundwater ammonium concentrations were low.
“The primary insight is that anaerobic ammonium oxidation is active in groundwater,” said Richard Smith, a USGS hydrologist and one of four co-authors for the study. “That is important because up to this time, denitrification was viewed as the only process that could permanently remove fixed nitrogen from groundwater.”
While anammox has been implemented in wastewater treatment for decades, the results of the study open the door for new applications.
“These results are most likely of interest to water supply managers,” said Smith. “Now that we know it can occur in subsurface situations, it could perhaps be included in designs for septic system leach fields, large environmental water storage or water reuse projects, or animal wastewater lagoons.”
Nitrate is a key groundwater contaminant, its presence in groundwater becoming a major contributor to algal blooms upon discharge into surface waters. Smith, encouraged by his recent findings, hypothesizes that anammox could be a solution.
“We are now studying the fate of nitrogen at the groundwater/surface water interface as nitrate- and ammonium-laden water is being discharged into a groundwater flow-through lake,” he said. “Anammox is one of the processes that we are examining at that interface as we attempt to assess how much fixed nitrogen is removed as nitrogen gas and how much escapes into the lake and becomes available for algal blooms.”
Additional research will determine what role anammox can play in the future of water management and as an antidote to algal blooms. Regardless of how that proceeds, it’s been made clear enough that this microbial process works wonders with water.
Image credit: "Wild Dilly Pond (San Salvador Island, Bahamas) 1," James St. John © 2008, used under an Attribution 2.0 Generic license: https://creativecommons.org/licenses/by/2.0/