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International audience; The quantification of direct GHG emissions from sewers and wastewater treatment plants is of great importance towards urban sustainable development. In fact, the identification and assessment of anthropogenic sources of GHG emissions (mainly nitrous oxide and methane) in these engineered systems represent the first step in establishing effective mitigation strategies. This chapter provides an overview of the currently available nitrous oxide and methane quantification methods applied at full-scale in sewers and wastewater treatment plants. Since the first measurement campaigns in the early 90s were based on spare grab sampling, quantification methodologies and sampling strategies have evolved significantly, in order to describe the spatio-temporal dynamics of the emissions. The selection of a suitable quantification method is mainly dictated by the objective of the measurement survey and by specific local requirements. Plant-wide quantification methods provide information on the overall emissions of wastewater treatment plants, including unknown sources, which can be used for GHG inventory purposes. To develop on-site mitigation strategies, in-depth analysis of GHG generation pathways and emission patterns is required. In this case, process-unit quantifications can be employed to provide data for developing mechanistic models or to statistically link GHG emissions to operational conditions. With regards to sewers, current available methods are not yet capable to capture the complexity of these systems due to their geographical extension and variability of conditions and only allow to monitor specific locations where hotspots for GHG formation and emission have been identified.

International audience

International audience; Microbial communities are key engines that drive earth’s biogeochemical cycles. However, existing ecosystem models have only limited ability to predict microbial dynamics and require the calibration of multiple population-specific empirical equations. In contrast, we build on a new kinetic “Microbial Transition State” (MTS) theory of growth derived from first principles. We show how the theory coupled to simple mass and energy balance calculations provides a framework with intrinsically important qualitative properties to model microbial community dynamics. We first show how the theory can simultaneously account for the influence of all the resources needed for growth (electron donor, acceptor, and nutrients) while still producing consistent dynamics that fulfill the Liebig rule of a single limiting substrate. We also show consistent patterns of energy-dependent microbial successions in mixed culture without the need for calibration of population-specific parameters. We then show how this approach can be used to model a simplified activated sludge community. To this end, we compare MTS-derived dynamics with those of a widely used activated sludge model and show that similar growth yields and overall dynamics can be obtained using two parameters instead of twelve. This new kinetic theory of growth grounded by a set of generic physical principles parsimoniously gives rise to consistent microbial population and community dynamics, thereby paving the way for the development of a new class of more predictive microbial ecosystem models.

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.

National audience; Les enjeux de l'assainissement vont au-delà de la seule épuration des eaux usées. Parmi les défis que doivent relever les stations d'épuration figurent : l'optimisation des consommations, le recyclage des ressources qu'elles contiennent (énergie, matières), la réduction des impacts environnementaux et la maîtrise des coûts. Dans ce contexte, les modèles mathématiques du fonctionnement des stations sont apparus comme un outil de choix pour l'aide à la conception et à la gestion des stations ainsi qu'à la formation du personnel et à la recherche. La représentation mathématique du fonctionnement des installations de traitement des eaux nécessite plusieurs modèles, parmi lesquels : un modèle d'écoulements ; des modèles représentant les phénomènes physico-chimiques (précipitation-dissolution, décantation, filtration, adsorption …), un modèle biocinétique représentant les phénomènes biologiques ; un modèle de fractionnement de la matière organique en classe de biodégradabilité, permettant de convertir les analyses réalisées sur l'eau usée en variables d'état utilisées dans les modèles; et un modèle de transfert gaz-liquide (aération, émissions de gaz, ...). Cette présentation vise à montrer des exemples de travaux réalisés à l'aide d'approches de modélisation et à illustrer l'apport de ces outils dans le développement de connaissances et leur implication opérationnelle. Les challenges actuels sont également abordés.

Gas–liquid mass transfer in wastewater treatment processes has received considerable attention over the last decades from both academia and industry. Indeed, improvements in modelling gas–liquid mass transfer can bring huge benefits in terms of reaction rates, plant energy expenditure, acid–base equilibria and greenhouse gas emissions. Despite these efforts, there is still no universally valid correlation between the design and operating parameters of a wastewater treatment plant and the gas–liquid mass transfer coefficients. That is why the current practice for oxygen mass transfer modelling is to apply overly simplified models, which come with multiple assumptions that are not valid for most applications. To deal with these complexities, correction factors were introduced over time. The most uncertain of them is the α-factor. To build fundamental gas–liquid mass transfer knowledge more advanced modelling paradigms have been applied more recently. Yet these come with a high level of complexity making them impractical for rapid process design and optimisation in an industrial setting. However, the knowledge gained from these more advanced models can help in improving the way the α-factor and thus gas–liquid mass transfer coefficient should be applied. That is why the presented work aims at clarifying the current state-of-the-art in gas–liquid mass transfer modelling of oxygen and other gases, but also to direct academic research efforts towards the needs of the industrial practitioners.

International audience; The concept of wastewater treatment has undergone drastic changes in recent years, not so much yet in its engineering practice, but more in its approach. The major focus on wastewater treatment has been for a century the treatment of contaminated water to environmentally and sanitarily acceptable levels. Today, while obviously maintaining satisfactory treatment results with contained operational costs, the immediate reuse of treated water for irrigation or as precursor for drinking water production, as well as the use of wastewater constituents in the bioeconomy have started becoming objectives when designing a treatment facility. This dogma shift is driven by increasing fresh-water scarcity and the need to decrease the environmental impact of wastewater treatment. A reduced impact of wastewater treatment can be obtained by minimizing direct and indirect greenhouse gas emissions and by recovering bio-based products that replace fossil-based products in the bioeconomy. The challenge for today’s environmental engineers working in wastewater treatment is immense as the entry into the bioeconomy of bio-based products derived from wastewater requires cross-sectorial competencies, i.e., knowledge of the application of bio-based products, product requirements, and how the treatment process influences product properties. Modern engineering of a wastewater treatment plant therefore does not stop anymore at the influent to the receiving water but extends into various branches of industry that may benefit from wastewater derived bio-based products, as for example agriculture, chemistry or animal breeding/husbandry. In our presentation, we will illustrate the importance of considering a wider system boundary when attempting a decreased environmental impact. In our example, we show how land-application of sewage sludge and the down-stream management of bio-based products may impact the environmental impact of the overall wastewater treatment process. This will be done using life cycle assessment as tool for the eco-design of a novel, putative bioprocess for wastewater treatment using oxygenic photogranules. Biomass reuse and valorization typically requires a dewatering step generating a liquid flux of water enriched in micro- and macronutrients. This flux is often overlooked in modeling (biokinetic or life cycle) but may deserve attention as suitable point of attack for the recovery of nutrients as will be shown.

[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

National audience; Des mesures d'émission de protoxyde d'azote (N2O, puissant gaz à effet de serre) ont été réalisées sur les unités de biofiltration de la station de Seine Aval au cours de l'année 2014 (période estivale) et 2015 (période hivernale). Ces travaux s'inscrivent dans le cadre de l'axe n°1 du programme Mocopée « Métrologie et traitement du signal » et visent à apporter une estimation du facteur d'émission de N2O pour les procédés de biofiltration en nitrification tertiaire et post-dénitrification et à préciser les conditions opératoires favorisant la production de ce gaz à effet de serre. Les résultats obtenus à l'échelle des unités de biofiltration indiquent que l'étape de nitrification est la source principale de production et d'émission de N2O. La dénitrification permet, quant à elle, de réduire les émissions de la filière en consommant une partie du flux de N2O dissous issu de l'étape amont de nitrification. Afin maximiser ce taux de réduction, il convient d'optimiser le dosage de méthanol. Une analyse statistique a été réalisée afin d'identifier les paramètres influençant la production de N2O lors de la nitrification. Celle-ci suggère que les conditions conduisant à l'expansion du biofilm favoriseraient la production du N2O. A l'échelle de cette filière de traitement de l'azote, les émissions de N2O correspondent à environ 2,1 % de la charge d'azote entrante et représentent plus de 80 % du bilan carbone.

National audience; Le contrôle des émissions de gaz à effet de serre, en particulier celles du protoxyde d'azote (N2O), constitue un enjeu émergent du secteur du traitement des eaux usées. Aujourd'hui, la compréhension des mécanismes de production de N2O dans les procédés à biomasse fixée, potentiellement très émetteurs, est moins avancée que pour les procédés conventionnels à boues activées. Dans le projet N2OTRACK (2016-2019), financé par l'Agence Nationale de la Recherche et s'inscrivant dans l'axe 2 du programme Mocopée, un modèle décrivant les performances de traitement d'un biofiltre nitrifiant a été étendu pour inclure les processus biologiques de production de N2O, puis confronté à un jeu de données collectées à l'échelle réelle. Une étude de sensibilité a été menée pour évaluer l'impact de la représentation du transfert gaz/liquide sur les prédictions du modèle. Les résultats soulignent l'importance de bien caractériser la composition des deux phases, gazeuse et liquide, pour permettre la description des émissions de N2O. En particulier, la prise en compte de l'enrichissement du gaz en N2O sur la hauteur du biofiltre s'avère essentielle à la bonne représentation de la répartition du flux de N2O entre les phases gazeuse et liquide.

International audience; Microbial communities are key engines that drive earth’s biogeochemical cycles. However, ecosystem models today exhibit only limited abilities in predicting microbial dynamics and require the calibration of multiple population specific empirical equations. In contrast, we build on a new kinetic "Microbial Transition State" (MTS) theory of growth derived from first physical principles. We show how the theory coupled to simple mass and energy balance calculations constitutes a framework that intrinsically enclose important qualitative properties to model microbial community dynamics. We first show how the theory can take into account simultaneously the influence of all resources needed for growth (electron donor, acceptor and nutrients) while still producing consistent dynamics fulfilling the Liebig rule of the single limiting substrate. We also show the apparition of consistent energy-dependent microbial successions in mixed culture settings without the need for population-specific parameter calibration. Then, we illustrate how this approach can be used to model a simplified activated sludge community. For that, we compare MTS-derived dynamics to these of a widely used activated sludge model and show that similar growth yields and overall dynamics can be obtained using 2 parameters instead of 12. This new kinetic theory of growth grounded by a set of generic physical principles thus parsimoniously give rise to consistent microbial population and community dynamics, which paves the way for the development of a new class of more predictive microbial ecosystem models.

International audience; Le protoxyde d'azote (N2O) est un puissant gaz à effet de serre (GES), environ 300 fois plus impactant que le dioxyde de carbone (CO2). Il joue également un rôle clé dans la destruction de la couche d'ozone. Principalement d'origine naturelle (sols et hydrosphère), il est également émis par les procédés de traitement des eaux usées lors du traitement biologique de l'azote. Le rendu des bilans d'émission de GES, tel que défini par la réglementation française, est actuellement basé sur l'utilisation de facteurs d'émission fixes et simplistes qui ne tiennent pas compte de l'influence des paramètres de dimensionnement et de gestion des stations sur les émissions. Si les données d'émission de N2O par les procédés conventionnels fonctionnant selon le principe des boues activées se sont multipliées ces dernières années, celles des procédés à biomasse fixée et des procédés extensifs sont encore très rares. Dans le cadre de projet collaboratifs avec les professionnels du domaine, d'une convention avec l'ONEMA et du programme de recherche Mocopée, Irstea a réalisé des campagnes de mesure des émissions de N2O de trois procédés de traitement majoritairement employés en France : (i) les boues activées, (ii) les biofiltres et (iii) les filtres plantés de roseaux. Des protocoles d'échantillonnage et de mesure spécifiques à chaque procédé ont été développés. Ces derniers prennent en compte la variabilité spatiale et temporelle des émissions. Parallèlement à ces mesures, une étude approfondie des procédés a été réalisée afin de préciser le lien entre les conditions opératoires et les émissions observées. Les résultats obtenus mettent en évidence la forte disparité des émissions de N2O d'un procédé à l'autre (de 0 à 5 % de N-N2O/Nentrant) mais également la variabilité temporelle de ces émissions pour un même procédé en lien avec les paramètres opératoires observés. Cette hétérogénéité des résultats démontre (i) la nécessité de définir à minima un facteur d'émission par procédé, en tenant compte des spécificités de chacun et (ii) le manque de pertinence de l'approche actuelle du GIEC basée sur l'utilisation de facteurs d'émission fixes, pouvant conduire à une forte sous-estimation du poids de ces émissions sur l'impact environnemental des stations d'épuration. Cette présentation vise à apporter un éclairage sur la problématique des émissions de GES par les procédés de traitement des eaux usées et à préciser les facteurs d'émission du protoxyde d'azote pour les différents procédés étudiés.

International audience; Gas emissions of Vertical Flow Constructed Wetlands (VFCW) treating raw wastewater have been poorly investigated. One of the main issue measuring gas emissions on such systems is related to their high heterogeneity of flow conditions (surface water distribution, feeding/resting periods etc). It is thus of importance to develop a specific methodology to be able first to determine representative gas emission fluxes and then to decide how filter‘s operation can affect these emissions. This study investigates greenhouse gas emissions from a full-scale VFCW for raw wastewater treatment. The plant designed for 800 population equivalents combines two stages of vertical subsurface flow filters planted with Phragmites. Australis. Gaseous emissions were continuously monitored during three weeks representing an overall feeding/resting period (1/3 weeks). Several automatic closed chambers connected to an infrared analyzer were placed at different strategic positions of the filters and allowed the measurement of tens of gaseous fluxes per day and per position. Dissolved N2O concentration was measured in the inlet and the outlet of each filtration stage using N2O micro-sensors. A combination of on-line sensors for NH4+ and NO3- and 24-h composite samples was used to characterize the process performances of each filtration stage. Finally, O2concentration was regularly measured in the air phase of the porous media in order to evaluate the aerobic conditions of the filters. Ammonium removal was on average of 94% during the monitoring period. The continuous measurement highlighted strong spatial and temporal variability of gaseous fluxes. This latter was observed at different time scales (day, week, feeding/resting cycle) and was linked to: (i) the intermittent feeding of the filters, (ii) oxygen content in the porous media and (iii) environmental conditions such as the ambient temperature. Dissolved N2O flux represented about 20% of the total flux (gas + liquid) which indicates the importance of accounting the dissolved flux in the N2O budget. The two filtration stages exhausted contrasted fluxes of methane and nitrous oxide in relation to carbon load and oxygen availability for carbon removal and nitrification. From a methodological point of view, this study indicates that: (i) continuous monitoring of greenhouse gas fluxes during at least an overall filtration cycle and (ii) appropriate spatial sampling strategy are decisive for a reliable determination of emission factors in VFCWs. The overall N2O emission factor estimated during the monitoring period was of 0.78% of the influent total nitrogen which is 28 times higher than the default IPCC factor (0.023% of the TN load). As N2O emissions were strongly correlated to the oxygen concentration within the porous media, it is suspected that nitrification was the main contributor to N2O production in the VFCW studied.

95ème congrès de l'ASTEE « Territoires en transition : mettre l'intelligence numérique au coeur des services publics, Issy-les-Moulineaux, FRA, 31-/05/2016 - 03/06/2016; National audience; Cette présentation s'inscrit dans la continuité des présentations générales du projet Intelligence artificielle engagé par le Siaap (Présentation Jean-Pierre Tabuchi, Siaap) et du programme de recherche Mocopée (Présentation Vincent Rocher, Siaap).L'objectif de cette étude est d'apporter des informations sur les conditions d'apparition d'intermédiaires réactionnels tels que les nitrites (NO2-) et le protoxyde d'azote (N2O) lors du traitement des eaux usées par biofiltration. De plus, un chiffrage précis des facteurs d'émissions du N2O d'une installation réelle a été établi. Les résultats obtenus montrent que les conditions propices à l'accumulation des nitrites favorisent également la production de protoxyde d'azote. En nitrification tertiaire, la production de N2O est corrélée entre autre à l'intensité de l'aération, la température et le temps de filtration. En post-dénitrification, l'accumulation de nitrites et la production de N2O sont fortement influencées par la gestion des apports de substrat carboné.

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; In activated sludge systems, aeration provides the oxygen that is required by the aerobic micro-organisms; ensures mixing and homogenization of the liquor; and facilitates stripping the gaseous by-products of the degradation processes. On the other hand, aeration is generally the single largest contributor to energy consumption in wastewater facilities. With the increasing need for containing operating costs, new aeration control strategies have recently been proposed. Solutions based on the continuous monitoring of nitrogen forms (NH4+, NO3-) for instance ensure a sufficient air supply to treat the nitrogen load while maintaining relatively low dissolved oxygen concentrations in the basin which in turns translates into lower energy consumption. Whether such strategies have an impact on nitrous oxide (N2O) emissions is yet to be ascertained. Nitrous oxide is a key greenhouse gas, about 300 times more effective than carbon dioxide, and a major sink for stratospheric ozone (IPCC, 2007). The wastewater treatment plant of Nîmes (230 000 PE) located in France consists of two parallel activated sludge lines operated with different aeration strategies. Ammonair, an aeration control logic based on ammonia and DO concentration, was implemented to reduce energy consumption of one treatment line. This work combines field measurements and mathematical modelling and is aimed at investigating the impact of the Ammonair control system on nutrient removal and energy consumption. The model developed was also used to assess the potential for GHGs emissions in relation to the specific aeration regime.

International audience; Présentation d'une étude de cas de réduction des consommations énergétiques liées à l'aération des boues activées.

International audience; This study aims to clarify the biochemical nature and interactions of Extracellular Polymeric Substances (EPS) involved in the structure and cohesive properties of aerobic granules. Granules were incubated with selective hydrolytic enzymes or with chemicals and the resistance of digested granules to shear stress was evaluated. After alpha-amylase digestion, the hydrodynamic stress released macro-particles (>315 mu m) while soluble molecules (

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.

A multi-method protocol previously proposed for the extraction of extracellular polymeric substances (EPS) from flocculated sludges was investigated on dense aerobic granules. The protocol combines mechanical disruption by sonication and chemical extraction using the Tween detergent and the cation chelator, EDTA. Polysaccharides were mainly recovered during the first sonication step while proteins were recovered all along the extractive procedure with a high prevalence in the EDTA step. These data confirmed the interest of the multi-method protocol for harvesting a diversified pool of EPS from dense granules and for fractionation of the polymers according to their physicochemical properties. In addition, the high extractability of proteins with EDTA confers a specific behavior of the aerobic granules towards the multi-method extraction protocol, supporting the idea that proteins are associated in the granule matrix through ionic interactions involving divalent cations. Analysis of the extracted EPS by anionic exchange chromatography confirmed the presence of highly anionic proteins that were specifically detected in the extracts obtained from granules. One important question is now to investigate whether these highly anionic proteins are involved in the aggregation and densification process and if their presence is related to the cohesive properties of these particles.

International audience; This work investigated the modification of aggregate properties during the formation of granular sludge in a sequencing batch airlift reactor (SBAR). The cohesion of biological aggregates was quantified by subjecting sludge samples to two different controlled shear stresses in a stirred reactor. For reference sludge (without granules), flocs broke and reformed easily, indicating that floc size was controlled by the turbulence micro-scale (Kolmogorov scale, here from 17 mu m to 62 mu m). In contrast, granules showed high strength which enabled them to resist turbulence and their size was no longer imposed by the Kolmogorov micro-scale. Different steps were observed during the granulation process: a first increase of aggregate cohesion associated with a decrease in sludge volume index (SVI), a growth of aggregates with detachment of fragile particles from the surface and, finally, all increase in the sizes of small and large granules to reach a pseudo-stable size distribution. Results suggest that small particles could have formed the seeds for new granules, as they were maintained in the bioreactor. Here, granular sludge was formed in an SBAR with a conventional settling time (30 min), i.e. without particle washout, and with a low superficial air velocity (SAV = 0.6 cm s(-1)): it is thus demonstrated that high SAV and low settling time are not necessary to produce granules, but probably only accelerate the accumulation of granules. It is shown that the increase of cohesion is the initial phenomenon explaining the granule formation concomitantly with bacterial aggregates densification. It seems important, in the future, to investigate the reasons for this cohesion increase, which is possibly explained either by bacterial bounding interactions or the excretion of extracellular polymeric substances (EPS).

This work aims to characterise the impact of hydrodynamics on sludge properties and consequently on fouling mechanisms in side-stream membrane bioreactors (MBRs). Two side-stream processes which generate very different shear stresses are compared, without filtration. This operating mode permits specific quantification of the impact of the external loop (and of induced shear stress) on floc structure/morphology, supernatant quality and fouling propensity. The study shows that low constraints (7 × 10 −3 Pa) generated on submerged side-stream hollow fibre modules have no significant impact on sludge properties. In contrast, high shear stresses (72 Pa) associated with a crossflow configuration induce very significant modifications of the mixed liquor which increases its fouling propensity (measured in a standard filtration cell). A theoretical explanation of the role of turbulence on the floc size distribution is given. Based on a Kolmogorov microscale calculation, it seems possible to predict the mean floc size reached in both filtration systems, for a given shear stress intensity. Disaggregation is characterised by a two-step kinetic: first a short-term breakage attributed to fragmentation and loss of weak strength bonds, and secondly a longer-term breakage probably due to erosion phenomena and removal of high strength bonds. Only the second step induces a significant release and an accumulation of soluble protein-like substances. Soluble organic matter strongly enhances the fouling propensity of the mixed liquor. This seems to be amplified by the concentration of protein-like substances. The importance of considering the protein and carbohydrate content as well as floc size is also pointed out in the paper.

National audience

National audience