N₂O Microsensor

Emission of NO and N2O from a full-scale two-reactor nitritation-anammox process was determined during a measurement campaign at the Dokhaven-Sluisjesdijk municipal WWTP (Rotterdam, NL). The NO and N2O levels in the off-gas responded to the aeration cycles and the aeration rate of the nitritation reactor, and to the nitrite and dissolved oxygen concentration. Due to the strong fluctuations in the NO and N2O levels in both the nitritation and the anammox reactor, only time-dependent measurements could yield a reliable estimate of the overall NO and N2O emissions. The NO emission from the nitritation reactor was 0.2% of the nitrogen load and the N2O emission was 1.7%. The NO emission from the anammox reactor was determined to be 0.003% of the nitrogen load and the N2O emission was 0.6%. Emission of NO2could not be detected from the nitritation-anammox system. Denitrification by ammonia-oxidizing bacteria was considered to be the most probable cause of NO and N2O emission from the nitritation reactor. Since anammox bacteria have not been shown to produce N2O under physiological conditions, it is also suspected that ammonia-oxidizing bacteria contribute most to N2O production in the anammox reactor. The source of NO production in the anammox reactor can be either anammox bacteria or denitrification by heterotrophs or ammonia-oxidizing bacteria. Based on the results and previous work, it seems that a low dissolved oxygen or a high nitrite concentration are the most likely cause of elevated NO and N2O emission by ammonia-oxidizing bacteria. The emission was compared with measurements at other reject water technologies and with the main line of the Dokhaven-Sluisjesdijk WWTP. The N2O emission levels in the reject water treatment seem to be in the same range as for the main stream of activated sludge processes. Preliminary measurements of the N2O emission from a one-reactor nitritation-anammox system indicate that the emission is lower than in two-reactor systems. © 2007 Elsevier Ltd. All rights reserved.

A novel 2-sludge 3-stage process using a combination of granular sludge and biofilm was developed to achieve biological removal of nitrogen and phosphorus from nutrient-rich wastewater. The system consists of a granular sequencing batch reactor (SBR) working under alternating anaerobic/anoxic conditions supplemented with a short aerobic phase and an aerobic biofilm SBR. The wastewater is first fed to the granular SBR reactor, where easily biodegradable carbon sources are taken up primarily by polyphosphate accumulating organisms (PAOs). The supernatant resulting from quick settling of the granular sludge is then fed to the biofilm SBR for nitrification, which produces oxidized nitrogen that is returned to the granular reactor for simultaneous denitrification and phosphorus removal. While maximizing the utilization of organic substrates and reducing operational costs, as do other 2-sludge processes previously reported in literature, the proposed system solves the bottleneck problem of traditional 2-sludge systems, namely high effluent ammonia concentration, due to its high-volume exchange ratios. An ammonia oxidation rate of 32 mg N/L h was achieved in the biofilm SBR, which produced nitrite as the final product. This nitrite stream was found to cause major inhibition on the anoxic P uptake and also to result in the accumulation of N2O. These problems were solved by feeding the nitrite-containing stream continuously to the granular reactor in the anoxic phase. With a nitrogen and phosphorus removal efficiency of 81% and 94%, respectively, the system produces an effluent that is suitable for land irrigation from a wastewater stream containing 270 mg N/L of total nitrogen and 40 mg P/L of total phosphorus. © 2008 Elsevier Ltd. All rights reserved.

Permafrost soils contain nearly twice as much carbon as the atmosphere1. When these soils thaw, large quantities of carbon are lost, mainly in the form of methane and carbon dioxide. In contrast, thawing is thought to have little impact on nitrous oxide emissions, which remain minimal following the summer thaw. Here, we examined the impact of thawing on nitrous oxide production in permafrost cores collected from a heath site and a wetland site in Zackenberg, Greenland. Rates of nitrous oxide production in the heath soil were minimal, regardless of the hydrological conditions. Although rates of nitrous oxide production in the wetland soil were low following thawing, averaging 1.37 ?g N h?1 kg?1, they were 18 ?g N h?1 kg?1 for permafrost samples following thawing, drainage and rewetting with the original meltwater. We show that 31% of the nitrous oxide produced after thawing and rewetting a 10-cm permafrost core—equivalent to 34 mg N m?2 d?1—was released to the atmosphere; this is equivalent to daily nitrous oxide emissions from tropical forests on a mean annual basis. Measurements of nitrous oxide production in permafrost samples from five additional wetland sites in the high Arctic indicate that the rates of nitrous oxide production observed in the Zackenberg soils may be in the low range.

The in vivo production of nitrous oxide (N2O) by earthworms is due to their gut microbiota, and it is hypothesized that the microenvironment of the gut activates ingested N2O-producing soil bacteria. In situ measurement of N2O and O-2 with microsensors demonstrated that the earthworm gut is anoxic and the site of N2O production. The gut had a pH of 6.9 and an average water content of approximately 50%. The water content within the gut decreased from the anterior end to the posterior end. In contrast, the concentration of N2O increased from the anterior end to the mid-gut region and then decreased along the posterior part of the gut. Compared to the soil in which worms lived and fed, the gut of the earthworm was highly enriched in total carbon, organic carbon, and total nitrogen and had a C/N ratio of 7 (compared to a C/N ratio of 12 in soil). The aqueous phase of gut contents contained up to 80 mM glucose and numerous compounds that were indicative of anaerobic metabolism, including up to 9 mM formate, 8 mM acetate, 3 mM lactate, and 2 mM succinate. Compared to the soil contents, nitrite and ammonium were enriched in the gut up to 10- and 100-fold, respectively. The production of N2O by soil was induced when the gut environment was simulated in anoxic microcosms for 24 h (the approximate time for passage of soil through the earthworm). Anoxia, high osmolarity, nitrite, and nitrate were the dominant factors that stimulated the production of N2O Supplemental organic carbon had a very minimal stimulatory effect on the production of N2O, and addition of buffer or ammonium had essentially no effect on the initial N2O production rates. However, a combination of supplements yielded rates greater than that obtained mathematically for single supplements, suggesting that the maximum rates observed were due to synergistic effects of supplements. Collectively, these results indicate that the special microenvironment of the earthworm gut is ideally suited for N2O-producing bacteria and support the hypothesis that the in situ conditions of the earthworm gut activate ingested N2O-producing soil bacteria during gut passage.