Production of Nitrous Oxide from Nitrite in Stable Type II Methanotrophic Enrichments

Abstract

The coupled aerobic-anoxic nitrous decomposition operation is a new process for wastewater treatment that removes nitrogen from wastewater and recovers energy from the nitrogen in three steps: (1) NH4+ oxidation to NO2-, (2) NO2- reduction to N2O, and (3) N2O conversion to N-2 with energy production. Here, we demonstrate that type II methanotrophic enrichments can mediate step two by coupling oxidation of poly(3-hydroxybutyrate) (P3HB) to NO2- reduction. Enrichments grown with NH4+ and NO2- were subject to alternating 48-h aerobic and anoxic periods, in which CH4 and NO2- were added together in a "coupled" mode of operation or separately in a "decoupled mode". Community structure was stable in both modes and dominated by Methylocystis. In the coupled mode, production of P3HB and N2O was low. In the decoupled mode, significant P3HB was produced, and oxidation of P3HB drove reduction of NO2- to N2O with similar to 70% conversion for >30 cycles (120 d). In batch tests of wasted cells from the decoupled mode, N2O production rates increased at low O-2 or high NO2- levels. The results are significant for the development of engineered processes that remove nitrogen from wastewater and for understanding of conditions that favor environmental production of N2O.