447 research outputs found
Techniques à membranes appliquées à l'élimination des matières en suspension dans un circuit semi-fermé d'aquaculture
Les piscicultures en circuits semi-fermés sont confrontées au problème de l'élimination permanente des matières en suspension (M.E.S.) et des substances azotées. Les procédés conventionnels utilisés pour retenir les M.E.S. (décantation, hydrocyclones, filtres mécaniques à tambour rotatif, filtration gravitaire) ne donnent par entière satisfaction. Par contre, la filtration sur membranes permet d'arrêter en totalité les particules en suspension et les bactéries.On montre d'abord que les teneurs en M.E.S. et leurs répartitions granulométriques mesurées sur des échantillons prélevés en bassins d'aquaculture varient avec la taille des poissons et l'heure du prélèvement et on met en évidence la présence en nombre important de particules submicroniques.Différents essais de filtration sur membranes sont ensuite présentés :- d'une part, avec des membranes d'ultrafiltration capillaires à peau interne utilisées en potabilisation des eaux : on examine l'influence des paramètres hydrodynamiques (pression transmembranaire, vitesse de recirculation) afin de rechercher les conditions optimales de fonctionnement. Le flux de perméat ne dépasse pas dans le meilleur des cas 100 l.h-1.m-2.- d'autre part, avec des membranes de microfiltration organiques planes en fluorure de polyvinylidène (PVDF) et tubulaires en céramique. Les flux obtenus avec les membranes organiques sont de l'ordre de 250 l.h-1.m-2Dans tous les cas, la rétention des M.E.S. est totale.Cependant l'estimation de l'investissement et des coûts de fonctionnement pour une pisciculture en circuit fermé de taille industrielle conduit à des prix trop élevés pour que l'utilisation des membranes dans ce domaine soit à ce jour économiquement envisageable.A problem confronting semi-closed circuit aquaculture is the need for continuous elimination of suspended matter (SM) and nitrogenous substances. Conventional processes used to retain SM (settling tanks, hydrocyclones, rotating-drum mechanical filters, gravity filtration) are not entirely satisfactory. However, membrane filtration has recently been shown to allow removal of suspended particles and bacteria. The present study evaluates the performance of different ultrafiltration and microfiltration membranes for water processing in a semi-closed aquaculture system. A brief economic analysis of treatment costs is proposed based on the results.The marine aquafarm studied produces about 5 tons of turbot per year with a plant volume of about 100 m3. The water processing line is fitted with a rotating-drum mechanical filter that stops the largest particles and ejects 1 m3 h-1 of loaded water into the surrounding environment. Another 2 m3 h-1 are cleared out by overflowing the pumping pit. These volumes are renewed at a rate of 3% per hour by pumping saltwater from an underground source. Crossflow filtration was performed on rejections from both the mechanical filter and pumping pit overflow. SM contents and granulometric distributions determined by laser diffractometry were found to vary with sample source and withdrawal time, and size of fish in the pens. A comparison of granulometric distributions in volume percent and numerical percent underscores the presence of a great number (> 98 %) of submicron particles.To limit the risk of mechanical-pore fouling due to blockage by particles, organic membranes in the form of internal-skin capillaries (pore diameters of about 10 to 20 nm) were initially employed. These membranes, used in drinking water production, are relatively inexpensive. The experimental device was fitted with an interchangeable volumetric pump (with gears or monoscrew). Adjustable parameters were transmembrane pressure and circulation velocity within the module. Analysis of the influence of these hydrodynamic parameters revealed that pressures higher than 1 bar were unnecessary, as beyond this point permeate flux no longer increased. Optimal flux did not exceed 100 L h-1 m-2 with the gear pump. Replacing the latter with a monoscrew pump improved permeate flux up to 70 %.Tests were also performed with flat microfiltration organic membranes of polyvinylidene fluoride (PVDF) with pore diameters ranging from 0.1 to 8 µm. The flux obtained with these membranes was roughly 250 L h-1 m-2 and presented little variation with varying pore diameter. Comparative tests carried out on tubular membranes showed lower fluxes than those obtained with organic membranes which, considering their much higher cost, makes them less attractive in this context. The use of membranes in aquafarming is without precedent. An economic analysis of the practice was carried out based on financial assessments of processing of surface waters into drinking water, for which outputs to be treated and SM contents were of the same order of magnitude. With operating costs from 0.35 to 0.95 FF per cubic meter of filtered water, expected investment for a fishfarm producing 100 tons of fish a year is currently 3 to 4 times too great to consider economically profitable the use of membranes for water treatment in closed-circuit aquafarming
Relativistic and retardation effects in the two--photon ionization of hydrogen--like ions
The non-resonant two-photon ionization of hydrogen-like ions is studied in
second-order perturbation theory, based on the Dirac equation. To carry out the
summation over the complete Coulomb spectrum, a Green function approach has
been applied to the computation of the ionization cross sections. Exact
second-order relativistic cross sections are compared with data as obtained
from a relativistic long-wavelength approximation as well as from the scaling
of non-relativistic results. For high-Z ions, the relativistic wavefunction
contraction may lower the two-photon ionization cross sections by a factor of
two or more, while retardation effects appear less pronounced but still give
rise to non-negligible contributions.Comment: 6 pages, 2 figure
Local Real-Space View of the Achiral 1T-TiSe2 2 x 2 x 2 Charge Density Wave
The transition metal dichalcogenide 1T-TiSe2-two-dimensional layered material undergoing a commensurate
2 × 2 × 2 charge density wave (CDW) transition with a weak periodic lattice distortion (PLD)
below ≈200 K. Scanning tunneling microscopy (STM) combined with intentionally introduced interstitial
Ti atoms allows us to go beyond the usual spatial resolution of STM and to intimately probe the threedimensional
character of the PLD. Furthermore, the inversion-symmetric achiral nature of the CDW in the z
direction is revealed, contradicting the claimed existence of helical CDW stacking and associated chiral
order. This study paves the way to a simultaneous real-space probing of both charge and structural
reconstructions in CDW compounds
Imaging the antiparallel magnetic alignment of adjacent Fe and MnAs thin films
The magnetic coupling between iron and alpha - MnAs in the epitaxial system
Fe/MnAs/GaAs(001) has been studied at the sub-micron scale, using element
selective x-ray photoemission electron microscopy. At room temperature, MnAs
layers display ridges and grooves, alternating alpha (magnetic) and beta
(non-magnetic) phases. The self-organised microstructure of MnAs and the stray
fields that it generates govern the local alignment between the Fe and alpha -
MnAs magnetization directions, which is mostly antiparallel with a marked
dependence upon the magnetic domain size
Structural dynamics during laser induced ultrafast demagnetization
The mechanism underlying femtosecond laser pulse induced ultrafast
magnetization dynamics remains elusive despite two decades of intense research
on this phenomenon. Most experiments focused so far on characterizing
magnetization and charge carrier dynamics, while first direct measurements of
structural dynamics during ultrafast demagnetization were reported only very
recently. We here present our investigation of the infrared laser pulse induced
ultrafast demagnetization process in a thin Ni film, which characterizes
simultaneously magnetization and structural dynamics. This is achieved by
employing femtosecond time resolved X-ray resonant magnetic reflectivity
(tr-XRMR) as probe technique. The experimental results reveal unambiguously
that the sub-picosecond magnetization quenching is accompanied by strong
changes in non-magnetic X-ray reflectivity. These changes vary with reflection
angle and changes up to 30 have been observed. Modeling the X-ray
reflectivity of the investigated thin film, we can reproduce these changes by a
variation of the apparent Ni layer thickness of up to 1. Extending these
simulations to larger incidence angles we show that tr-XRMR can be employed to
discriminate experimentally between currently discussed models describing the
ultrafast demagnetization phenomenon
Robustness of the charge-ordered phases in IrTe against photoexcitation
We present a time-resolved angle-resolved photoelectron spectroscopy study of
IrTe, which undergoes two first-order structural and charge-ordered phase
transitions on cooling below 270 K and below 180 K. The possibility of inducing
a phase transition by photoexcitation with near-infrared femtosecond pulses is
investigated in the charge-ordered phases. We observe changes of the spectral
function occuring within a few hundreds of femtoseconds and persisting up to
several picoseconds, which we interpret as a partial photoinduced phase
transition (PIPT). The necessary time for photoinducing these spectral changes
increases with increasing photoexcitation density and reaches timescales longer
than the rise time of the transient electronic temperature. We conclude that
the PIPT is driven by a transient increase of the lattice temperature following
the energy transfer from the electrons. However, the photoinduced changes of
the spectral function are small, which indicates that the low temperature phase
is particularly robust against photoexcitation. We suggest that the system
might be trapped in an out-of-equilibrium state, for which only a partial
structural transition is achieved.Comment: 8 pages, 5 figures, accepted for publication in Phys. Rev.
- …