Combining partial nitritation with anaerobic ammonium oxidation maybe a cost- and energy-efficient alternative to remove reduced nitrogen from nitrogen rich waste streams. However, increased N2O emissions (upto several % of the incoming N flux) have been observed for reactors performing partial nitritation, which is likely due to the stimulatory effect of combined elevated nitrite and ammonium concentrations and reduced oxygen concentrations on nitrous oxide formation by ammonium oxidizing bacteria. Because increased N2O emission may be inherent to partial nitrification systems, we have explored how these emissions can be mitigated by performing autotrophic nitrogen removal in one reactor systems by combining aerobic and anaerobic ammonium oxidizing bacteria within one redox-stratified biofilm. In such biofilms, a section of the biofilm is at all times maintained under anaerobic (free of oxygen invasion) conditions. Although anaerobic ammonium oxidizing bacteria are not known to metabolize N2O, we speculate that the existence of oxygen free zone would permit complete expression ofa denitrification pathway by heterotrophic bacteria- and hence remove any N2O which is transiently produced in the inner (aerobic) sections of the biofilm. We have successfully operated such membrane –aerated redox-stratifed biofilm reactors performing completely autotrophic N removal and have monitored N2O dynamics. Successful community control required periodic aeration. N2O emissions were detected, but only in the inner cores of the fiber bundles, and only transiently, upon initiation or cessation of aeration. Bulk phase N2O concentrations were significantly lower than expected based on transient evolution rates, suggesting a removal mechanism. Emitted N2O fluxes represented less than 0.02 % of the converted ammonium N. Anoxic batch incubations with biofilm samples revealed a significant N2O assimilatory activity. Anoxic incubations with N-15 enriched nitrite, nitrate, or ammonium, in presence or absence of acetate revealed the following: a very high conversion of original nitrite or nitrate N to N2O over N2, no stimulatory activity of acetate addition, a very different isotopic abundance in the formed N2 and N2O, and no conversion of ammonium N to N2O. These results confirm that N2O production was most likely to ammonium oxidizers, and that the heterotrophs contributed to N2O attenuation. The spatial structure of these biofilms may, therefore, be especially suitable to minimize N2O emissions.
|Publication status||Published - 2011|
|Event||International Conference on Nitrification - Nijmegen, Gelderland, Netherlands|
Duration: 1 Jan 2011 → …
Conference number: 2
|Conference||International Conference on Nitrification|
|City||Nijmegen, Gelderland, Netherlands|
|Period||01/01/2011 → …|