Approche comparative des effets microbiologiques et chimiques de traitements d'hydrolyse de boues urbaines dans le cadre de la réduction de leur production

Les Procédés de Réduction de Production de Boues (PRPB), basés sur des prétraitements d'hydrolyse, ont pour vocation d'accélérer l'hydrolyse de la matière organique particulaire d'une part et d'amplifier la croissance cryptique d'autre part. Bien que ces procédés soient très étudiés à l'heure actuelle, peu d'informations sont disponibles quant à leur impact réel sur la flore microbienne. L'objectif de ce travail était donc de comparer les effets générés par différents traitements d'hydrolyse non seulement en terme de solubilisation de la matière organique et minérale et en terme de biodégradabilité, mais également de comprendre l'impact réel de ces traitements sur la flore bactérienne. Pour cela, ce travail a consisté, dans un premier temps, à mettre au point des méthodes analytiques capables de rendre compte des effets générés par les procédés d'hydrolyse sur les propriétés chimiques des boues ainsi que sur les microorganismes. Cette méthodologie a ensuite été appliquée à la caractérisation des effets de trois types de traitement : le traitement thermique, la sonication et l'ozonation, afin de les comparer. Les résultats ainsi obtenus montrent que, dans les conditions appliquées, seul un traitement par ozonation, qui conduit à la fois à une solubilisation de la matière organique particulaire extracellulaire et intracellulaire (lyse bactérienne), permet d'augmenter significativement la biodégradabilité des boues. Dans le cas du traitement thermique et de la sonication, les résultats montrent que seule la matière organique extracellulaire (sonication) ou intracellulaire (traitement thermique) est solubilisée, ce qui ne suffit pas pour améliorer la biodégradabilité des boues.The processes for the reduction of excess sludge production are used both for particulate organic matter solubilisation and cryptic growth improvement. While these processes are well studied, little information has been obtained on their real impact on the microbial population. The objective of this study was the comparison of the effects of three processes used for the reduction of excess sludge production. This comparison was not only made in terms of solubilisation rate assessment and biodegradability evolution but also in terms of microbiological changes. Firstly this work concerned the development of new analytical methods able to give rise both to chemical and biological modifications induced by the different processes. Afterwards, the characterization and the comparison of three techniques of sludge hydrolysis (thermal treatment, sonication and ozonation) were performed using the previous methodology. The results revealed that the biodegradability of treated sludge was enhanced only after ozonation which allows both extracellular particulate organic matter (extrapolymeric substances) and intracellular materials release (cell lysis). On the contrary, after sonication and heat treatment, the results showed that only the extracellular (after sonication) or the intracellular organic matter (after thermal treatment) was solubilised, which was not sufficient to enhance sludge biodegradability.LIMOGES-BU Sciences (870852109) / SudocSudocFranceF

Sludge disintegration during heat treatment at low temperature: A better understanding of involved mechanisms with a multiparametric approach

International audienceAlthough many information is currently available about sludge minimization processes in wastewater treatment plant (WWTP), few data are available about their fundamental mechanisms especially microbial changes. In order to clarify the relationship between sludge reduction efficiency and both chemical and biological modifications, the effects of thermal treatment on activated sludge were investigated by combining the monitoring of cell lysis using flow cytometry (FCM), organic matter solubilization, floc structure and biodegradability. For the maximal temperature (95 °C) applied, COD, proteins, HLS and sugars solubilization degrees reached 12.4 (±1.3)%, 18.6 (±1.8)%, 9.6 (±1)% and 7.4 (±1.9)%, respectively, showing clearly the transfer of organic matter from the particulate to the soluble fraction of the sludge. The results from FCM analysis showed that thermal treatment induces a progressive cell lysis when increasing temperature from 50 to 95 °C. However, the impact on floc structure seemed to be limited as floc destructuration was limited to the temperature elevation at 50 °C. Above 50 °C floc size distribution remained almost constant. The results from biodegradability tests did not show any improvement of the intrinsic biodegradability after the thermal treatment. Only the increase of the specific digestion rate was observed. This study which provides a complete investigation of chemical, physical and biological effects of thermal treatment allows a better knowledge of fundamental mechanisms involved during heat treatment to improve sludge reduction processes

Assessment of physiological state of microorganisms in activated sludge with flow cytometry: Application for monitoring sludge production minimization

International audienceMany sludge reduction processes have been studied for the minimization of sludge production in biological wastewater treatment. The investigations on most of these processes have monitored the increase of the soluble chemical oxygen demand, the sludge mass reduction, or the decrease of the floc size, but little information has been obtained on cell lysis and the change of the biological cell activity. However, employing any strategy for reducing sludge production may have an impact of microbial community in biological wastewater treatment process. This impact may influence the sludge characteristics and the quality of effluent. The objective of this study concerns the determination of the physiological state of activated sludge microorganisms during a sludge minimization process. A thermal treatment at 80 °C for 5, 20, 40 and 60 min was chosen in this study. Staining bacteria with CTC and SYTOX green was used to evaluate biological cell activity and viability of cell types contained in activated sludge, respectively. The monitoring of cell activity and viability was performed using flow cytometry (FCM) analysis before and after thermal treatment of activated sludge. Results indicated an increase in the number of permeabilized cells and a decrease in the number of active cells, subsequent to the thermal treatment. The study also confirms the potential of FCM to successfully evaluate the physiological heterogeneity of an activated sludge bacterial population. Moreover, the experimentally observed correlations between the FCM results and the organic matter solubilization in activated sludge samples during thermal treatment revealed that the increase in the soluble organic matter concentration was predominantly due to an intracellular material release. Identifying the increase in activated sludge hydrolysis requires a precise knowledge of the involved mechanisms, and this study indicated that the FCM, used in conjunction with specific probes, could be a useful tool

Biological and chemical characterizations of the effects of activated sludge sonication

International audienceConsiderable research has been devoted in the past decades to the optimization and control of biological wastewater treatment processes. Many treatment processes have been studied to increase the methane potential of sludge with a rate limiting hydrolysis stage of organic matters associated with microbial cells. Although numerous informations are currently available about sludge minimization processes in WWTP, as sonication, few data are available about its fundamental mechanisms especially microbial changes. In order to clarify the relationship between sludge reduction efficiency and both chemical and biological modifications, the effects of sonication on activated sludge were investigated by combining the monitoring of cell lysis, organic matter solubilization, floc structure and biodegradability. So the aim of this study was to characterize the effects of activated sludge sonication, in terms of cell lysis, organic matter solubilization and biodegradability. For this, in parallel to the organic matter solubilization degree determination (Chemical Oxygen Demand (COD), proteins and sugars), cell disruption in activated sludge was monitored with Flow Cytometry (FCM) before and after sonication. Prior to FCM analyses, activated sludge samples were disaggregated, filtered and stained with a DNA specific fluorescent dye, the Sytox Green, which diffuses only damaged cells. Structural changes during sonication were assessed by microscopic observations and granulometry. Finally, the real impact of thermal treatment on sludge biodegradability was studied under anaerobic conditions. The results showed that, for a specific energy ranging from 0 to 210000 kJ/kg TS, sonication did not induce cell lysis since green fluorescence intensity remained approximately constant before and after sonication. On the contrary, COD, sugars and proteins solubilization degrees reach 27%, 16% and 18% respectively. These results, that showed a progressive release of organic matter particulate components without cell disruption, suggested that the origin of the organic matter released after sonication was only extracellular, probably due to the polymeric network disruption. Moreover, the results from anaerobic biodegradability tests did not show any improvement after sonication, indicating that the specific energy was not high enough to increase anaerobic biodegradability of the sludge. Even if sonication induced a progressive release of organic matter components from particulate to soluble fraction, the specific energy applied was not high enough to induce cell lysis, and consequently, anaerobic biodegradability improvement. Moreover, this study showed that hydrolysis treatments improvement requires a precise knowledge of both biological and chemical changes induced during hydrolysis processes

Pre-treatment of activated sludges: effect of sonication on aerobic and anaerobic digestibility

International audienceAerobic and anaerobic digestions were compared in reactors fed with sonicated activated sludge. Sonication treatment of activated sludge led to solubilisation of matter and especially of organic compounds. An important improvement of aerobic and anaerobic biodegradability was observed for a sonication treatment of 108,000 kJ kg TS−1 due to the increase of the instantaneous specific soluble COD uptake rate. Sonication led to an increase of biogas production due to the increase of available soluble COD. In this study, sludge sonication prior to aerobic digestion in the aim of enhancing sludge reduction was inconclusive. Under anaerobic conditions, the enhancement of sludge reduction due to sonication depended on the disintegration degree of the sludge. The combination of high disintegration degree of sonicated sludge prior to an anaerobic digestion led to very good results in term of sludge reduction (80%). Energy balance was also studied

Bacterial stress induced by Nanosecond Pulsed Electric Fields (nsPEF): potential applications for food industry and environment

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Synergetic pretreatment of sewage sludge by microwave irradiation in presence of H2O2 for enhanced anaerobic digestion.

International audienceA microwave-enhanced advanced hydrogen peroxide oxidation process (MW/H(2)O(2)-AOP) was studied in order to investigate the synergetic effects of MW irradiation on H(2)O(2) treated waste activated sludges (WAS) in terms of mineralization (permanent stabilization), sludge disintegration/solubilization, and subsequent anaerobic biodegradation as well as dewaterability after digestion. Thickened WAS sample pretreated with 1gH(2)O(2)/g total solids (TS) lost 11-34% of its TS, total chemical oxygen demand (COD) and total biopolymers (humic acids, proteins and sugars) via advanced oxidation. In a temperature range of 60-120 degrees C, elevated MW temperatures (>80 degrees C) further increased the decomposition of H(2)O(2) into OH* radicals and enhanced both oxidation of COD and solubilization of particulate COD (>0.45 micron) of WAS indicating that a synergetic effect was observed when both H(2)O(2) and MW treatments were combined. However, at all temperatures tested, MW/H(2)O(2) treated samples had lower first-order mesophilic (33+/-2 degrees C) biodegradation rate constants and ultimate (after 32 days of digestion) methane yields (mL per gram sample) compared to control and MW irradiated WAS samples, indicating that synergistically (MW/H(2)O(2)-AOP) generated soluble organics were slower to biodegrade or more refractory than those generated during MW irradiation

Structure and behavior modifications of Escherichia Coli exposed to microwaves (2.45 GHz) under discontinuous conditions

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Thermal and Nonthermal Effects of Discontinuous Microwave Exposure (2.45 Gigahertz) on the Cell Membrane of Escherichia coli

International audienceThe aim of this study was to investigate the effects on the cell membranes of Escherichia coli of 2.45-GHz microwave (MW) treatment under various conditions with an average temperature of the cell suspension maintained at 37°C in order to examine the possible thermal versus nonthermal effects of short-duration MW exposure. To this purpose, microwave irradiation of bacteria was performed under carefully defined and controlled parameters, resulting in a discontinuous MW exposure in order to maintain the average temperature of the bacterial cell suspensions at 37°C. Escherichia coli cells were exposed to 200- to 2,000-W discontinuous microwave (DW) treatments for different periods of time. For each experiment, conventional heating (CH) in a water bath at 37°C was performed as a control. The effects of DW exposure on cell membranes was investigated using flow cytometry (FCM), after propidium iodide (PI) staining of cells, in addition to the assessment of intracellular protein release in bacterial suspensions. No effect was detected when bacteria were exposed to conventional heating or 200 W, whereas cell membrane integrity was slightly altered when cell suspensions were subjected to powers ranging from 400 to 2,000 W. Thermal characterization suggested that the temperature reached by the microwave-exposed samples for the contact time studied was not high enough to explain the measured modifications of cell membrane integrity. Because the results indicated that the cell response is power dependent, the hypothesis of a specific electromagnetic threshold effect, probably related to the temperature increase, can be advanced