Temperature Dependence and Interferences of NO and N₂O Microelectrodes Used in Wastewater Treatment

Electrodes for nitric and nitrous oxide have been on the market for some time, but have not yet been tested for an application in wastewater treatment processes. Both sensors were therefore assessed with respect to their (non)­linear response, temperature dependence and potential cross sensitivity to dissolved compounds, which are present and highly dynamic in nitrogen conversion processes (nitric oxide, nitrous oxide, nitrogen dioxide, ammonia, hydrazine, hydroxylamine, nitrous acid, oxygen, and carbon dioxide). Off-gas measurements were employed to differentiate between cross sensitivity to interfering components and chemical nitric oxide or nitrous oxide production. Significant cross sensitivities were detected for both sensors: by the nitrous oxide sensor to nitric oxide and by the nitric oxide sensor to ammonia, hydrazine, hydroxylamine and nitrous acid. These interferences could, however, be removed by correction functions. Temperature fluctuations in the range of ±1 °C lead to artifacts of ±3.5% for the nitric oxide and ±3.9% for the nitrous oxide sensor and can be corrected with exponential equations. The results from this study help to significantly shorten and optimize the determination of the correction functions and are therefore relevant for all users of nitric and nitrous oxide electrodes

Isotope Signatures of N₂O in a Mixed Microbial Population System: Constraints on N₂O Producing Pathways in Wastewater Treatment

We present measurements of site preference (SP) and bulk ¹⁵N/¹⁴N ratios (δ¹⁵N^bulk_N2O) of nitrous oxide (N₂O) by quantum cascade laser absorption spectroscopy (QCLAS) as a powerful tool to investigate N₂O production pathways in biological wastewater treatment. QCLAS enables high-precision N₂O isotopomer analysis in real time. This allowed us to trace short-term fluctuations in SP and δ¹⁵N^bulk_N2O and, hence, microbial transformation pathways during individual batch experiments with activated sludge from a pilot-scale facility treating municipal wastewater. On the basis of previous work with microbial pure cultures, we demonstrate that N₂O emitted during ammonia (NH₄⁺) oxidation with a SP of −5.8 to 5.6 ‰ derives mostly from nitrite (NO₂^–) reduction (e.g., nitrifier denitrification), with a minor contribution from hydroxylamine (NH₂OH) oxidation at the beginning of the experiments. SP of N₂O produced under anoxic conditions was always positive (1.2 to 26.1 ‰), and SP values at the high end of this spectrum (24.9 to 26.1 ‰) are indicative of N₂O reductase activity. The measured δ¹⁵N^bulk_N2O at the initiation of the NH₄⁺ oxidation experiments ranged between −42.3 and −57.6 ‰ (corresponding to a nitrogen isotope effect Δδ¹⁵N = δ¹⁵N_substrate – δ¹⁵N^bulk_N2O of 43.5 to 58.8 ‰), which is considerably higher than under denitrifying conditions (δ¹⁵N^bulk_N2O 2.4 to −17 ‰; Δδ¹⁵N = 0.1 to 19.5 ‰). During the course of all NH₄⁺ oxidation and nitrate (NO₃^–) reduction experiments, δ¹⁵N^bulk_N2O increased significantly, indicating net ¹⁵N enrichment in the dissolved inorganic nitrogen substrates (NH₄⁺, NO₃^–) and transfer into the N₂O pool. The decrease in δ¹⁵N^bulk_N2O during NO₂^– and NH₂OH oxidation experiments is best explained by inverse fractionation during the oxidation of NO₂^– to NO₃^–

Effects of a Combined Diesel Particle Filter-DeNOx System (DPN) on Reactive Nitrogen Compounds Emissions: A Parameter Study

The impact of a combined diesel particle filter-deNO_<i>x</i> system (DPN) on emissions of reactive nitrogen compounds (RNCs) was studied varying the urea feed factor (α), temperature, and residence time, which are key parameters of the deNO_<i>x</i> process. The DPN consisted of a platinum-coated cordierite filter and a vanadia-based deNO_<i>x</i> catalyst supporting selective catalytic reduction (SCR) chemistry. Ammonia (NH₃) is produced in situ from thermolysis of urea and hydrolysis of isocyanic acid (HNCO). HNCO and NH₃ are both toxic and highly reactive intermediates. The deNO_<i>x</i> system was only part-time active in the ISO8178/4 C1cycle. Urea injection was stopped and restarted twice. Mean NO and NO₂ conversion efficiencies were 80%, 95%, 97% and 43%, 87%, 99%, respectively, for α = 0.8, 1.0, and 1.2. HNCO emissions increased from 0.028 g/h engine-out to 0.18, 0.25, and 0.26 g/h at α = 0.8, 1.0, and 1.2, whereas NH₃ emissions increased from <0.045 to 0.12, 1.82, and 12.8 g/h with maxima at highest temperatures and shortest residence times. Most HNCO is released at intermediate residence times (0.2–0.3 s) and temperatures (300–400 °C). Total RNC efficiencies are highest at α = 1.0, when comparable amounts of reduced and oxidized compounds are released. The DPN represents the most advanced system studied so far under the VERT protocol achieving high conversion efficiencies for particles, NO, NO₂, CO, and hydrocarbons. However, we observed a trade-off between deNO_<i>x</i> efficiency and secondary emissions. Therefore, it is important to adopt such DPN technology to specific application conditions to take advantage of reduced NO_<i>x</i> and particle emissions while avoiding NH₃ and HNCO slip

PCDD/F Formation in an Iron/Potassium-Catalyzed Diesel Particle Filter

Catalytic diesel particle filters (DPFs) have evolved to a powerful environmental technology. Several metal-based, fuel soluble catalysts, so-called fuel-borne catalysts (FBCs), were developed to catalyze soot combustion and support filter regeneration. Mainly iron- and cerium-based FBCs have been commercialized for passenger cars and heavy-duty vehicle applications. We investigated a new iron/potassium-based FBC used in combination with an uncoated silicon carbide filter and report effects on emissions of polychlorinated dibenzodioxins/furans (PCDD/Fs). The PCDD/F formation potential was assessed under best and worst case conditions, as required for filter approval under the VERT protocol. TEQ-weighted PCDD/F emissions remained low when using the Fe/K catalyst (37/7.5 μg/g) with the filter and commercial, low-sulfur fuel. The addition of chlorine (10 μg/g) immediately led to an intense PCDD/F formation in the Fe/K-DPF. TEQ-based emissions increased 51-fold from engine-out levels of 95 to 4800 pg I-TEQ/L after the DPF. Emissions of 2,3,7,8-TCDD, the most toxic congener (TEF = 1.0), increased 320-fold, those of 2,3,7,8-TCDF (TEF = 0.1) even 540-fold. Remarkable pattern changes were noticed, indicating a preferential formation of tetrachlorinated dibenzofurans. It has been shown that potassium acts as a structural promoter inducing the formation of magnetite (Fe₃O₄) rather than hematite (Fe₂O₃). This may alter the catalytic properties of iron. But the chemical nature of this new catalyst is yet unknown, and we are far from an established mechanism for this new pathway to PCDD/Fs. In conclusion, the iron/potassium-catalyzed DPF has a high PCDD/F formation potential, similar to the ones of copper-catalyzed filters, the latter are prohibited by Swiss legislation