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In the last few years, the emission of nitrous oxide (N 2 O) from constructed wetlands has become a topic of increased interest. Most N 2 O emission measurements made so far are based on limited grab-samples, leading to sparse temporal resolution in the data. Using a high sampling frequency strategy, gaseous and dissolved N 2 O emissions from a two-stage vertical subsurface flow constructed wetland, treating domestic raw wastewater, were investigated. The monitoring period that lasted 3 weeks corresponded to an entire operation cycle of the filters including the feeding and rest periods. Gaseous N 2 O fluxes exhibited remarkable temporal fluctuations with contrasted emission rates between the feeding and rest periods, as well as high variability within a given period. Total potential emission rates (gaseous + dissolved N 2 O fluxes) were found to be similar in both stages, with a mean value over the entire operation cycle of 1.50 and 1.78 mgN/m 2 /h in Stage 1 and Stage 2, respectively. The dynamic behavior of the emissions was found to be correlated to the changes in the biogeochemical conditions within the filters. A positive relationship between gaseous N 2 O emissions and oxygen concentrations in filter media was observed during conditions favoring nitrification and denitrification, suggesting that N 2 O can be emitted from both processes.

[Departement_IRSTEA]Ecotechnologies [TR1_IRSTEA]TED [ADD1_IRSTEA]Valoriser les effluents et déchets organiques; International audience; Nitrifying biologically active filters (BAFs) have been found to be high emitters of nitrous oxide (N2O), a powerful greenhouse gas contributing to ozone layer depletion. While recent models have greatly improved our understanding of the triggers of N2O emissions from suspended-growth processes, less is known about N2O emissions from full-scale biofilm processes.Tertiary nitrifying BAFs have been modeled at some occasions but considering strong simplifications on the description of gas-liquid exchanges which are not appropriate for N2O prediction. In this work, a tertiary nitrifying BAF model including the main N2O biological pathways was developed and confronted to full-scale data from Seine Aval, the largest wastewater resource recovery facility in Europe. A mass balance on the gaseous compounds was included in order to correctly describe the N2O gas-liquid partition, thus N2O emissions. Preliminary modifications of the model structure were made to include the gas phase as a compartment of the model, which significantly affected the prediction of nitrification. In particular, considering gas hold-up influenced the prediction of the hydraulic retention time, thus nitrification performances: a 3.5% gas fraction reduced ammonium removal by 13%, as the liquid volume, small in such systems, is highly sensitive to the gas presence. Finally, the value of the volumetric oxygen transfer coefficient was adjusted to successfully predict both nitrification and N2O emissions.

Nitrifying biologically active filters (BAFs) have been found to be high emitters of nitrous oxide (N

In this study, nitrous oxide (N2O) emissions from a full-scale denitrifying biofilter plant were continuously monitored over two periods (summer campaign in September 2014 and winter campaign in February 2015). Results of the summer campaign showed that the major part (>99%) of N2O flux was found in the liquid phase and was discharged with the effluent. N2O emissions were highly variable and represented in average 1.28±1.99% and 0.22±0.31% of the nitrate uptake rate during summer and winter campaigns, respectively. Denitrification was able to consume a large amount of dissolved N2O coming from the upstream nitrification stage. In the absence of methanol injection failure and with an influent BOD/NO3-N ratio higher than 3, average reduction of N2O was estimated to be of 93%. The control of exogenous carbon dosage is essential to minimize N2O production from denitrifying biofilters, in correlation to NO2-N concentrations in the filter.

International audience; A full-scale nitrifying biofilter was continuously monitored during two measurement periods (September 2014; February 2015) during which both gaseous and liquid N2O fluxes were monitored on-line. The results showed diurnal and seasonal variations of N2O emissions. A statistical model was run to determine the main operational parameters governing N2O emissions. Modification of the distribution between the gas phase and the liquid phase was observed related to the effects of temperature and aeration flow on the volumetric mass transfer coefficient (kLa). With similar nitrification performance values, the N2O emission factor was twice as high during the winter campaign. The increase in N2O emissions in winter was correlated to higher effluent nitrite concentrations and suspected increased biofilm thickness.

International audience; Two granular sludge sequencing batch reactors (GSBR) with alternating anoxic/aerobic (R1) and anaerobic/aerobic (R2) conditions were operated with a 4-carbon-source synthetic influent. The physical properties of the granular sludge were very good (SVI≈20 mL g−1) and high solid concentrations (up to 35 g L−1) were obtained in the bioreactor operated with a pre-anoxic phase with additional nitrate (R1). In contrast, performance and granule settleability were lower in R2 due to the development of filamentous heterotrophic bacteria on the surface of granules. These disturbances were linked to the fact that a fraction of COD remained during the aerobic phase, which was not stored during the anaerobic period. To stabilize a GSBR with a mixture of organic carbon sources, it is thus necessary to maximize the amount of substrate used during the non-aerated, anaerobic or anoxic, phase. Comparable phosphate removal efficiency was observed in both systems; enhanced biological P removal being greater in anaerobic/aerobic conditions, while the contribution of precipitation (Ca-P) was more significant in anoxic/aerobic conditions.

The aim of the work was to quantify the influence of the simultaneous presence of flocs and granules in the nitrifying activity in a sequencing batch airlift reactor (SBAR). The nitrification rate and oxygen limitation of flocs, granules and hybrid sludge was investigated using respirometric assays at different dissolved oxygen concentrations. The spatial distribution of Ammonium Oxidizing Bacteria (AOB) and Nitrite Oxidizing Bacteria (NOB) was investigated using fluorescence in situ hybridization (FISH). Results showed that the nitrification rate was much less sensitive to oxygen limitation in systems containing a fraction of flocs than in pure granular sludge. Ammonium Oxidizing Bacteria (AOB) were found to be distributed in similar quantities in flocs and granules whereas the Nitrite Oxidizing Bacteria (NOB) were located preferentially in granules. This study showed that the presence of flocs with granules could increase the robustness of the process to transitory reductions of aeration.