Analysis of full nitrification performance and optimization of reaction properties using N and O isotope fractionation.

Isotopic fractionation properties have been successfully applied to identify the distribution and fate of nitrogen in ecosystems, revealing the dynamic response of N and O elements during nitrogen transport and transformation. However, only a few studies used the dual isotope technology in activated sludge treatment of domestic wastewater and many aspects of the process are unclear. Here, we use the dual isotope techniques to increase the understanding of the substrates required for nitrification reactions, nitrification performance, and process operation. Mixed sludge was successfully enriched with nitrifying bacteria in a continuous culture, and three dissolved oxygen (DO; 0.2-0.4, 3-4, and 7-8 mg/L) and three temperature levels (18 ± 1, 25 ± 1, and 33±1 °C) were tested for efficiency of nitrate nitrogen accumulation. Both δ15NNO3 and δ18ONO3 showed a gradual increase with an increase in DO or temperature, the increase in DO slowed down the fractionation effect of isotopes, and the increase in temperature reduced the variability in N and O utilization. The slope of δ15NNO3:δ18ONO3 gradually approached 1 with the increase in DO (<7 mg/L) or in temperature, and the optimal range of DO and temperature were accurately judged to strengthen the denitrification performance of nitrifying bacteria. δ18OH2O was successfully taken up to form NO2--N and NO3--N with 74 and 91% replacement rates, respectively, indicating that DO and H2O jointly completed the formation of nitrate nitrogen during the long nitrification process. In summary, the in situ dual isotope technology can help optimize the influence of environmental factors on nitrification performance to guide the long-term stable operation of nitrification reactions in sludge treatment and provide a reliable basis for complex activated sludge nitrification systems.

The mitigation effect of free ammonia and free nitrous acid on nitrous oxide production from the full-nitrification and partial-nitritation systems.

The potentials of using endogenous free ammonia (FA) and free nitrous acid (FNA) as nitrous oxide (N2O) mitigators were investigated in treatment of both mainstream and sidestream wastewater. Although the N2O emission factor of a sidestream partial-nitritation (PN) reactor (averaged 1.70 % ± 0.39 %, n = 30) was about 2.4 times higher than a mainstream full-nitrification (FN) reactor (averaged 0.72 % ± 0.24 %, n = 30) (P < 0.01), one-hour exposure of PN sludge to 1.5 mg HNO2-N/L FNA could virtually abolish N2O emission. As for FN sludge, both 45 mg NH3-N/L FA and 0.015 mg HNO2-N/L FNA successfully mitigated N2O production at varying dissolved oxygen (DO) levels (50 % vs 61 %), while 1.5 mg HNO2-N/L FNA not only reduced more N2O (92 %) but also altered the N2O dependency on DO. Both FNA and FA sludge treatment were effective N2O mitigation strategies with FNA toward the end of carbon neutrality and FA being more economically appealing (2 % cost saving).

Comparison of nitrous oxide emission between a partial and full nitrification enriched ammonia-oxidising culture.

Nitrification systems are known to be a source of nitrous oxide (N2O) emission, however, the contribution from partial and full nitrification systems remains controversial. In this study, N2O emission from a partial and full nitrification culture was investigated. In all tests, nitrite, dissolved oxygen concentration and pH levels were controlled within a similar range limiting ammonium concentration to be the only variable. The results reveal with the same amount of ammonium removed, the full nitrification culture produced far greater N2O than the partial nitrification culture for both pulse (25-36 times) and continuous feeding modes (2-110 times). The relative gene expression data indicate that under pulse feeding there is a decreasing trend of nirK and norB genes for the partial and full nitrification culture respectively while under continuous feeding, increasing norB trends were observed for both. This possibly indicated the hydroxylamine pathway was favoured for the partial nitrification culture while the hybrid N-nitrosation pathway maybe the major contributor for the full nitrification culture. These findings improve our understanding on N2O production pathways and enable researchers to propose better mitigation strategies.