Massive quantities of inorganic nitrogen (mainly in the form of ammonium (NH4+)) in residual waters derived from human activities continue to be released in aquatic ecosystems. Among various physicochemical and biological methods for treating NH4+-rich residual waters, biological nitrogen removal (BNR) via nitrification and heterotrophic denitrification process is most widely applied. In recent years, novel processes including nitritation, anammox or a combination of partial nitritation plus anammox (PNA) have been implemented as energy and resource-efficient alternatives of conventional BNR processes. However, emissions of nitrous oxide (N2O) during the operation of these novel processes may offset the claimed environmental benefits of nitritation or PNA technologies. N2O is a strong greenhouse with ca. 300 times higher global warming potential than carbon dioxide (CO2) and contributes to the destruction of stratospheric ozone. Nitrifier nitrification (NN) and nitrifier denitrification (ND) by ammonia oxidizing bacteria (AOB), heterotrophic denitrification (HD) by denitrifying bacteria and several abiotic reactions are identified as pathways of N2O production. However, the contribution of different pathways of N2O production and their environmental controls in BNR systems remain to be identified and quantified. Further, a better and quantitative understanding of the mechanisms of N2O production is warranted, in order to develop operational strategies or system designs that might mitigate N2O emissions.
This PhD project investigated dynamics, identified pathways, and explored mitigation options for N2O production in high-rate nitritation reactors. Two lab-scale intermittently-fed sequencing batch reactors were operated towards simultaneous high-rate nitritation and low-rate N2O emission. The dynamics and constituent pathways of N2O production were identified and quantified. The effect of pH on N2O production rates was experimentally examined and the effect of pH on pathway contribution was analyzed using an existing mathematical N2O process model. A suite of abiotic N2O production reactions were kinetically determined and the contribution of abiotic reactions to observed N2O dynamics in the nitritation reactors was estimated. Finally, operational conditions were proposed to minimize N2O emissions from nitritation reactors.
The reactor biomass was highly enriched in AOB and converted 93 ± 14% of the removed ammonium to nitrite (NO2-) at volumetric removal rates of 0.6-0.76 g N/L/d. The dissolved oxygen (DO) set-point (< 0.5 mg O2/L) combined with intermittent feeding was sufficient to maintain high nitritation rates at 20-26 °C over a period of 710 days. Even at high nitritation efficiencies, net N2O production was low (ca. 2% of the removed ammonium). In situ application of 15N labeled substrates revealed ND as the dominant pathway of N2O production. Net N2O production rates transiently increased with a rise in pH (from 7.4 to 7.9) after each pulse feeding, suggesting a potential effect of pH on N2O production.
To further elucidate the effect of pH on N2O production, a wide range of pH conditions (pH 6.5-8.5) were imposed on the nitritation reactor. The specific ammonium removal rates and the nitrite accumulation rates remained almost constant at varying pH levels (p > 0.05). The specific net N2O production rates (N2OR) and the fractional N2O yield (∆N2O/∆NH4+) increased from pH 6.5 to 8, and decreased slightly at 8.5 (p < 0.05). Application of the comprehensive NDHA model suggested ND as the pathway responsible for increased N2O production at alkaline pH.
Hydroxylamine (NH2OH) and NO2-, intermediates during the nitritation process, can engage in chemical reactions that lead to N2O formation. The kinetics and stoichiometry of the relevant abiotic reactions were quantified in a series of batch tests across a range of relevant pHs, absence/presence of oxygen, and at different reactant concentrations. The highest N2O production rates were measured for NH2OH oxidation by HNO2, followed by HNO2 reduction by ferrous iron (Fe2+), NH2OH oxidation by ferric iron (Fe3+), and finally NH2OH disproportionation plus oxidation by O2. Compared to other examined factors, pH had the strongest effect on N2O formation rates. Acidic pH stimulated N2O production from the oxidation of NH2OH by HNO2 and we could conclude that HNO2 rather than NO2- is the reactant. In departure from previous studies, we estimate that abiotic N2O production is a minor source (< 3% of total N2O production) in typical nitritation reactor systems with pH between 6.5 and 8. Only at extremely acidic pH (≤ 5) would the abiotic pathway become significant. In consideration of the effects of pH on both abiotic and biotic N2O production pathways, circumneutral pH set-points are suggested to minimize overall N2O emissions from nitritation systems.
Overall, experimental efforts were implemented to investigate dynamics, pathways and mitigation options for N2O production in nitritation reactors. This study has identified operational strategies via intermittent feeding and pH control as means to mitigate N2O emission from nitritation systems.