Wastewater treatment plants may be niches for specialized N2O reducing microorganisms. In the anoxic tank of an “activated sludge” system, the most widespread form of biological nitrogen removal from wastewater, microorganisms- growing in flocs- are exposed to a mixture of nitrate, ammonium, and organic matter, together with N2O produced in the preceding aerated nitrification tank, much of which presumably remains soluble in the “mixed liquor”. Specialized N2O reducers in these anoxic tanks, growing on organic matter and N2O, would thus act as a sort of N2O sink for the N2O produced as a byproduct of nitrification or incomplete denitrification.
During the past months we’ve worked on two approaches to study this potential N2O sink in activated sludge systems:
In the lab in Delft, we’ve been running a continuous culture, enriched from activated sludge, growing solely on N2O as electron acceptor. Under defined conditions (pH 7, 20 °C, acetate as electron donor and carbon source) we’ve enriched for an N2O reducing community that we are now studying and characterizing.In a somewhat different but complementary approach, during my secondment at NMBU, Pawel and I have been using “the robot” to study the N2O consumption kinetics of activated sludge samples from a wastewater treatment plant treating part of Oslo’s wastewater. Comparing the N2O reducing potential to the nitrate reducing potential of the sludge, we get an idea if there is “N2O sink” potential in this biomass. It seems that there is! N2O is a very good electron acceptor… as long as there is some energy and carbon source present, why waste it?