The anaerobic treatment of low strength soluble wastewaters

Low strength soluble wastewaters with chemical oxygen demand (COD) of less than 2000 mg/I are mostly from food processing industries. They commonly contain simple substrates such as short- chain fatty acids, alcohols and carbohydrates. The application of anaerobic technology has been mostly directed towards the treatment of medium and high strength wastewaters rather than those of low strength. Problems limiting the treatment of dilute wastewaters are related to the wastewater and the reactor design. This dissertation investigates the application of the conventional upflow anaerobic sludge blanket (UASB) and its modification, the expanded granular sludge bed (EGSB), for the treatment of low strength soluble wastewaters. The main topics studied concern the wastewater related problems. Ile effect of dissolved oxygen on the methanogenic activity of granular sludges and the effect of low substrate levels inside reactors on the treatment performance were evaluated. Moreover, some aspects of reactor design related problems such as the retention of biomass and wastewaterbiomass contact were considered.Methanogens located in granular sludge have a high tolerance to oxygen. The concentration of oxygen found to cause 50% inhibition to methanogenic activity was between 7% and 41 % oxygen in the head space of flasks, which corresponded to 0.05 mg/ l and 6 mg/ l of dissolved oxygen prevailing in the media, respectively. The most important mechanism for the tolerance was the consumption of oxygen by facultative bacteria while metabolizing substrates. The most highly tolerant sludges had the highest respiration rates. The hypothesis considered is that anaerobic microenvironments are created inside granules protecting the methanogens. The absence of facultative substrate for respiring oxygen decreases the tolerance of methanognens to O 2 . The coexistence of methanogenic and facultative bacteria competing for substrate in one single bioreactor was explored under highly aerobic conditions, in order to verify the possible application of anaerobic-aerobic cocultures for the removal of recalcitrant pollutants. Simultaneous methane production and oxygen uptake occurred in an oxygen tolerant sludge while at least 2 mg/ l of dissolved oxygen was present in the media. The healthy co-culture was evident even after longer periods of oxygen exposure, when methane oxidizing bacteria eventually also developed.The feasibility of UASB and EGSB reactors at 30°C was demonstrated. In UASB reactors, COD removal efficiencies exceeded 95% at organic loading rates (OLR) up to 6.8 g COD/ l .d and influent COD concentrations (COD in ) ranging from 422 to 943 mg/ l , during the treatment of ethanol substrate. The efficiencies exceeded 86% at OLR up to 3.9 mg COD/ l .d when whey was used as a substrate. Below 630 mg COD/ l , acidification of whey instead of methanogenesis was the rate limiting step. The retention of biomass is not a problem in the UASB, but the mixing intensity is not high enough to decrease the biofilm diffusion limitation of substrate transport into granular biofilms. The EGSB was shown to have superior potentials compared with the UASB. COD removal efficiencies were above 80% at OLRs up to 12 g COD/ l .d with COD in as low as 100 to 200 mg/ l . The effect of low substrate levels was not significant in the EGSB due to the intense turbulent mixing regime obtained by applying high hydraulic and organic loads. The very low apparent KS value of 9.8 mg COD/ l found for the biofilms in the reactor, was comparable to the intrinsic KS values determined for the most predominant acetoclastic methanogen found in anaerobic bioreactors, Methanothrix soehngenii. This indicates that all transport limitations of substrate movement into the biofilms were overcome. Optimized operation without sludge washout is achieved when liquid upflow velocities (V up ) below 5.5 m/h are applied. The problem of sludge retention is also restricted when sludge flotation occurs due to the buoyancy forces of gas attached to biofilms. The required equilibrium between mixing intensity and sludge retention limits the operation of the EGSB to OLRs up to 7 g COD/ l .d and V up values ranging from 2.5 and 5.5 m/h. Both reactor studies confirmed that in practice dissolved oxygen does not constitute any detrimental effect on the treatment performance. Improved mixing intensity in the UASB and improved sludge retention in the EGSB will enable higher OLRs and lower COD in which can be tolerated, compared with those of this study

Thermodynamics of the S=1 spin ladder as a composite S=2 chain model

A special class of S=1 spin ladder hamiltonians, with second- neighbor exchange interactions and with anisotropies in the $z$-direction, can be mapped onto one-dimensional composite S=2 (tetrahedral S=1) models. We calculate the high temperature expansion of the Helmoltz free energy for the latter class of models, and show that their magnetization behaves closely to that of standard XXZ models with a suitable effective spin $S_{eff}$, such that $S_{eff}(1+S_{eff})=$, where ${\bf S}_i$ refers to the components of spin in the composite model. It is also shown that the specific heat per site of the composite model, on the other hand, can be very different from that of the effective spin model, depending on the parameters of the hamiltonian.Comment: 17 pages, 4 figures. Submitted for publicatio

Field-Induced Quantum Criticality of Systems with Ferromagnetically Coupled Structural Spin Units

The field-induced quantum criticality of compounds with ferromagnetically coupled structural spin units (as dimers and ladders) is explored by applying Wilson's renormalization group framework to an appropriate effective action. We determine the low-temperature phase boundary and the behavior of relevant quantities decreasing the temperature with the applied magnetic field fixed at its quantum critical point value. In this context, a plausible interpretation of some recent experimental results is also suggested.Comment: to be published in Physics Letters

Optimization of inhomogeneous electron correlation factors in periodic solids

A method is presented for the optimization of one-body and inhomogeneous two-body terms in correlated electronic wave functions of Jastrow-Slater type. The most general form of inhomogeneous correlation term which is compatible with crystal symmetry is used and the energy is minimized with respect to all parameters using a rapidly convergent iterative approach, based on Monte Carlo sampling of the energy and fitting energy fluctuations. The energy minimization is performed exactly within statistical sampling error for the energy derivatives and the resulting one- and two-body terms of the wave function are found to be well-determined. The largest calculations performed require the optimization of over 3000 parameters. The inhomogeneous two-electron correlation terms are calculated for diamond and rhombohedral graphite. The optimal terms in diamond are found to be approximately homogeneous and isotropic over all ranges of electron separation, but exhibit some inhomogeneity at short- and intermediate-range, whereas those in graphite are found to be homogeneous at short-range, but inhomogeneous and anisotropic at intermediate- and long-range electron separation.Comment: 23 pages, 15 figures, 1 table, REVTeX4, submitted to PR

Structural and doping effects in the half-metallic double perovskite A2A_2CrWO6_6

he structural, transport, magnetic and optical properties of the double perovskite $A_2$CrWO$_6$ with $A=\text{Sr, Ba, Ca}$ have been studied. By varying the alkaline earth ion on the $A$ site, the influence of steric effects on the Curie temperature $T_C$ and the saturation magnetization has been determined. A maximum $T_C=458$ K was found for Sr$_2$CrWO$_6$ having an almost undistorted perovskite structure with a tolerance factor $f\simeq 1$. For Ca$_2$CrWO$_6$ and Ba$_2$CrWO$_6$ structural changes result in a strong reduction of $T_C$. Our study strongly suggests that for the double perovskites in general an optimum $T_C$ is achieved only for $f \simeq 1$, that is, for an undistorted perovskite structure. Electron doping in Sr$_2$CrWO$_6$ by a partial substitution of Sr$^{2+}$ by La$^{3+}$ was found to reduce both $T_C$ and the saturation magnetization $M_s$. The reduction of $M_s$ could be attributed both to band structure effects and the Cr/W antisites induced by doping. Band structure calculations for Sr$_2$CrWO$_6$ predict an energy gap in the spin-up band, but a finite density of states for the spin-down band. The predictions of the band structure calculation are consistent with our optical measurements. Our experimental results support the presence of a kinetic energy driven mechanism in $A_2$CrWO$_6$, where ferromagnetism is stabilized by a hybridization of states of the nonmagnetic W-site positioned in between the high spin Cr-sites.Comment: 14 pages, 10 figure

Pion photoproduction on the nucleon in the quark model

We present a detailed quark-model study of pion photoproduction within the effective Lagrangian approach. Cross sections and single-polarization observables are investigated for the four charge channels, $\gamma p\to \pi^+ n$, $\gamma n\to \pi^- p$, $\gamma p\to \pi^0 p$, and $\gamma n\to \pi^0 n$. Leaving the $\pi N\Delta$ coupling strength to be a free parameter, we obtain a reasonably consistent description of these four channels from threshold to the first resonance region. Within this effective Lagrangian approach, strongly constrainted by the quark model, we consider the issue of double-counting which may occur if additional {\it t}-channel contributions are included.Comment: Revtex, 35 pages, 16 eps figures; version to appear on PR

Thermodynamics of pyrope-majorite, Mg3Al2Si3O12-Mg4Si4O12, solid solution from atomistic model calculations

Static lattice energy calculations, based on empirical pair potentials have been performed for a large set of different structures with compositions between pyrope and majorite, and with different states of order of octahedral cations. The energies have been cluster expanded using pair and quaternary terms. The derived ordering constants have been used to constrain Monte Carlo simulations of temperature-dependent properties in the ranges of 1073 3673K and 0 20 GPa. The free energies of mixing have been calculated using the method of thermodynamic integration. At zero pressure the cubic/tetragonal transition is predicted for pure majorite at 3300 K. The transition temperature decreases with the increase of the pyrope mole fraction. A miscibility gap associated with the transition starts to develop at about 2000K and xmaj 0.8, and widens with the decrease in temperature and the increase in pressure. Activity composition relations in the range of 0 20 GPa and 1073 2673K are described with the help of a high-order Redlich Kister polynomial

The WEBT BL Lacertae Campaign 2001 and its extension : Optical light curves and colour analysis 1994–2002

BL Lacertae has been the target of four observing campaigns by the Whole Earth Blazar Telescope (WEBT) collaboration. In this paper we present UBVRI light curves obtained by theWEBT from 1994 to 2002, including the last, extended BL Lac 2001 campaign. A total of about 7500 optical observations performed by 31 telescopes from Japan to Mexico have been collected, to be added to the ∼15 600 observations of the BL Lac Campaign 2000. All these data allow one to follow the source optical emission behaviour with unprecedented detail. The analysis of the colour indices reveals that the flux variability can be interpreted in terms of two components: longer-term variations occurring on a fewday time scale appear as mildly-chromatic events, while a strong bluer-when-brighter chromatism characterizes very fast (intraday) flares. By decoupling the two components, we quantify the degree of chromatism inferring that longer-term flux changes imply moving along a ∼0.1 bluerwhen- brighter slope in the B − R versus R plane; a steeper slope of ∼0.4 would distinguish the shorter-term variations. This means that, when considering the long-term trend, the B-band flux level is related to the R-band one according to a power law of index ∼1.1. Doppler factor variations on a “convex” spectrum could be the mechanism accounting for both the long-term variations and their slight chromatism.Reig Torres, Pablo, [email protected]