Influence d'une alimentation séquentielle sur l'épuration par boues activées d'un effluent fortement chargé

Cet article présente les résultats d'une étude menée sur le traitement d'effluents fortement chargés (DCO > 1,5 kg.m-3) par un procédé à boues activées. L'objectif principal de ce travail est de montrer qu'une alimentation séquentielle d'un réacteur parfaitement agité avec maintien permanent du recyclage des boues et de l'oxygénation permet d'obtenir une bonne épuration tant du point de vue de l'élimination de la pollution carbonée que de la maîtrise de la décantabilité des flocs microbiens. Les essais sont menés sur un pilote de laboratoire constitué d'une colonne à bulles et d'un décanteur, alimenté avec un substrat synthétique. Contrairement à une alimentation continue, la mise en place de cycles d'alimentation permet d'obtenir des flocs de bonne décantabilité de manière constante dans le temps. Ainsi une concentration élevée en biomasse peut être maintenue dans le réacteur. De plus, le suivi d'un cycle 1 h/1 h confirme que la mise en œuvre d'un cycle court n'entraîne pas de variations importantes des performances d'épuration. Par ailleurs, le suivi d'un long cycle (24 h/24 h) montre des variations significatives des concentrations en DCO, polysaccharides dans la phase liquide et exopolysaccharides dans les flocs microbiens. Ceci semble indiquer que des produits microbiens sont relargués des flocs vers la phase liquide pendant l'arrêt de l'alimentation et que des exopolysaccharides sont produits dans les flocs pendant l'alimentation.The overall efficiency of an activated sludge wastewater treatment system depends both on the ability of the activated sludge culture to remove the soluble organic pollution and on the efficiency of the separation of the treated water from the biomass. In the treatment of a high-strength wastewater (COD > 1.5 kg .m-3), ultrafiltration or crossflow filtration systems can sometimes substitute for the final clarifier, the separation of the sludge from the effluent water being very good regardless of changes in sludge settleability. Because of the high pollutant concentrations, the biomass must be more concentrated in the bioreactor than in the usual aerobic processes (10 to 20 kg.m-3 rather than 4 kg.m-3), whatever the separation system (settler or filtration system). Thus, with this biomass concentration, the volumetric loading rate can be increased while the organic loading rate is maintained at a low level.The use of a clarifier is preferable because it is cheaper than ultrafiltration or crossflow filtration systems, which have very high investment and operation costs, on the other hand, and some fouling problems, on the other hand. However, its use requires a very good biomass settleability. Results from the literature show that there is no general relation between sludge settleability and such wastewater treatment operating parameters as dissolved oxygen concentration in the aeration basin, organic loading rates, pH, or low relative influent nitrogen and phosphorus contents. On the other hand, the separation ability of the clarifier depends on the hydraulic configuration of the aerated reactor: biomass settleability can be controlled by completely stirred reactors in series, plug-flow reactors or intermittently-fed systems. Indeed, this spatial or temporal staggering should allow microorganisms to consume their accumulated substrate and thus restore their accumulation capacity. In the case of high-strength wastewater treatment, temporal staggering is better than spatial staggering. Indeed, higher organic loading rates require a higher number of reactors. Thus, the COD concentration significantly increases in the first reactors of the cascade and some toxic or inhibitory problems could appear. For this reason, a temporal staggering was chosen.Biomass settleability is linked to the extracellular polymers that are the third component of the sludge flocs, after the cells and the water. These polymers are composed largely of microbial exopolysaccharides. Measurements of exopolysaccharides could be used to characterize sludge settleability. A too low level of exopolysaccharides indicates a sludge defloculation problem, whereas a too high level of exopolysaccharides means that sludge settleability could be hindered because of an excessive growth of filamentous organisms.The purpose of the present work is to show that the use of only one completely stirred reactor, fed in a cyclic way with permanent oxygenation and return sludge flow, leads to good sludge settleability and effluent quality. The experiments have been performed with a laboratory pilot-plant composed of a 11.3 litre bubble column, which is a completely stirred reactor, and a secondary settling tank of 14.8 litres. The reactor is fed with an equilibrated synthetic substrate consisting mainly of meat extract, saccharose and ethanol (COD:N:P=100:5:1 and 1 kg COD=1 kg meat extract + 0.444 kg saccharose + 0.2 kg ethanol). The feeding cycle is a set of consecutive periods of feeding and starvation. For example, during a 1h/2h feeding cycle, the substrate feeding is continuous during 1 hour and stopped during 2 hours.The sludge comes from the Nancy-Maxéville wastewater treatment plant and its acclimation to the synthetic substrate begins with an aeration without feeding during 12 hours. Afterwards, the experimental system is fed during 1 hour every 2 hours with the substrate (first concentration of COD=0.1 kg.m-3). Then, the inlet COD is increased by steps of 0.1 kg.m-3 every day until the required concentration is reached. Once the biomass is acclimated to the influent and sufficiently concentrated, the experiment can begin.In order to show the positive influence of cyclic feeding on wastewater treatment quality, we have carried out one experiment with continuous feeding and two experiments with cyclic feeding (see Table 1). For these experiments, the volumetric loading rate is about 4 kg COD.m-3.d-1. With an appropriate feeding cycle, the settleability of the sludge and the effluent quality remain good: the diluted sludge volume index is 56 cm3.g-1 and 96% of the inlet COD is removed. Thus, the biomass concentration remains high in the reactor (19 kg.m-3). This is not the case with a continuous feeding where the biomass settleability rapidly deteriorates. The principal advantage of sequential feeding is the maintenance of good sludge settleability and a high biomass concentration in the aeration basin. Moreover, during the short feeding cycles, the variations of the concentrations of COD, polysaccharides in the bulk phase and extracellular polysaccharides in the microbial aggregates are very low. This augurs well for system stability.In contrast, monitoring of a long cycle (24 h of feeding and 24 h of starvation) shows great variations in the concentrations of COD, polysaccharides and extracellular polysaccharides in the system. When the feeding is stopped, the COD and polysaccharide concentrations in the bulk phase increase whereas the exopolysaccharide concentration in sludge flocs decreases. Reciprocally, after the feeding is begun, the COD and polysaccharide concentrations in the bulk phase decrease, and then remain constant, whereas the exopolysaccharide concentration increases. As 1 kg of measured polysaccharides represents 0.9 kg COD, and as in the bulk phase the COD concentration is equivalent to 4 times the polysaccharide concentration, it seems that microbial products other than polysaccharides are released in the bulk phase during starvation, these products being slowly biodegradable. Extracellular polysaccharides in the microbial aggregates are formed during the feeding phase

Oral composition

Oral care composition comprising a polymer obtainable by copolymerising a mixture of comonomers, from 5 to 95 mol % of the mixture of comonomers is constituted by a como no mer having the formula (I): (I) in which R is hydrogen or a methyl group, L is a divalent organic linking group incorporating a benzylor a carboxyl functionality, n is an integer of from 1 to 4 and Y is an amine, quaternized amine or quaternary ammonium group; and in which the balance of the mixture of co monomers is constituted by neutral and/or anionic comonomers; said composition being in the form of anyone of a toothpaste, gel, foam, chewing gum, deformable strip or mouthwash and being suitable for use in the oral cavity

Operational experience with the CERN hadron linacs

The present CERN proton linac (Linac2) was commissioned in 1978 and since that date has been the primary source of protons to the CERN accelerator complex. During the past 18 years, the machine has had a very good reliability record in spite of the demands made upon it. Modifications have been made with the view of maintaining this reliability with reduced resources and new requirements from the users. Further demands will be made in the future for LHC operation. In 1994, a new linac for heavy ion production was put into service replacing the original CERN proton linac. As this machine was built within an international collaboration, operation had to take into account the novelty of the techniques used and the variety of equipment supplied by outside collaborators. Even so, the new machine has also had very good reliability

Non-Systemic Drugs: A Critical Review

Non-systemic drugs act within the intestinal lumen without reaching the systemic circulation. The first generation included polymeric resins that sequester phosphate ions, potassium ions, or bile acids for the treatment of electrolyte imbalances or hypercholesteremia. The field has evolved towards non-absorbable small molecules or peptides targeting luminal enzymes or transporters for the treatment of mineral metabolism disorders, diabetes, gastrointestinal (GI) disorders, and enteric infections. From a drug design and development perspective, non-systemic agents offer novel opportunities to address unmet medical needs while minimizing toxicity risks, but also present new challenges, including developing a better understanding and control of non-transcellular leakage pathways into the systemic circulation. The pharmacokinetic-pharmacodynamic relationship of drugs acting in the GI tract can be complex due to the variability of intestinal transit, interaction with chyme, and the complex environment of the surface epithelia. We review the main classes of non-absorbable agents at various stages of development, and their therapeutic potential and limitations. The rapid progress in the identification of intestinal receptors and transporters, their functional characterization and role in metabolic and inflammatory disorders, will undoubtedly renew interest in the development of novel, safe, non-systemic therapeutics

L’archéologie forestière dans le Châtillonnais de la prospection pédestre aux relevés LiDAR

National audienc

Phase Behavior of Polyelectrolyte Block Copolymers in Mixed Solvents

We have studied the phase behavior of the poly(n-butyl acrylate)-b-poly(acrylic acid) block copolymer in a mixture of two miscible solvents, water and tetrahydrofuran (THF). The techniques used to examine the different polymers, structures and phases formed in mixed solvents were static and dynamic light scattering, small-angle neutron scattering, nuclear magnetic resonance and fluorescence microscopy. By lowering the water/THF mixing ratio X, the sequence unimers, micron-sized droplets, polymeric micelles was observed. The transition between unimers and the micron-sized droplets occurred at X = 0.75, whereas the microstructuration into core-shell polymeric micelles was effective below X = 0.4. At intermediate mixing ratios, a coexistence between the micron-sized droplets and the polymeric micelles was observed. Combining the different aforementioned techniques, it was concluded that the droplet dispersion resulted from a solvent partitioning that was induced by the hydrophobic blocks. Comparison of poly(n-butyl acrylate) homopolymers and poly(n-butyl acrylate)-b-poly(acrylic acid) block copolymers suggested that the droplets were rich in THF and concentrated in copolymers and that they were stabilized by the hydrophilic poly(acrylic acid) moieties.Comment: 11 pages, 12 figures, to appear in Macromolecule