• Energy Research

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 N2 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 ∼ 70% conversion for >30 cycles (120 d). In batch tests of wasted cells from the decoupled mode, N2O production rates increased at low O2 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.

To scale up microbial fuel cells (MFCs), larger cathodes need to be developed that can use air directly, rather than dissolved oxygen, and have good electrochemical performance. A new type of cathode design was examined here that uses a "window-pane" approach with fifteen smaller cathodes welded to a single conductive metal sheet to maintain good electrical conductivity across the cathode with an increase in total area. Abiotic electrochemical tests were conducted to evaluate the impact of the cathode size (exposed areas of 7 cm2, 33 cm2, and 6200 cm2) on performance for all cathodes having the same active catalyst material. Increasing the size of the exposed area of the electrodes to the electrolyte from 7 cm2 to 33 cm2 (a single cathode panel) decreased the cathode potential by 5%, and a further increase in size to 6200 cm2 using the multi-panel cathode reduced the electrode potential by 55% (at 0.6 A m-2), in a 50 mM phosphate buffer solution (PBS). In 85 L MFC tests with the largest cathode using wastewater as a fuel, the maximum power density based on polarization data was 0.083 ± 0.006 W m-2 using 22 brush anodes to fully cover the cathode, and 0.061 ± 0.003 W m-2 with 8 brush anodes (40% of cathode projected area) compared to 0.304 ± 0.009 W m-2 obtained in the 28 mL MFC. Recovering power from large MFCs will therefore be challenging, but several approaches identified in this study can be pursued to maintain performance when increasing the size of the electrodes.