Claude Creemers

Alkaline thermal sludge hydrolysis

The waste activated sludge (WAS) treatment of wastewater produces excess sludge which needs further treatment prior to disposal or incineration. A reduction in the amount of excess sludge produced, and the increased dewaterability of the sludge are, therefore, subject of renewed attention and research. A lot of research covers the nature of the sludge solids and associated water. An improved dewaterability requires the disruption of the sludge cell structure. Previous investigations are reviewed in the paper. Thermal hydrolysis is recognized as having the best potential to meet the objectives and acid thermal hydrolysis is most frequently used, despite its serious drawbacks (corrosion, required post-neutralization, solubilization of heavy metals and phosphates, etc.). Alkaline thermal hydrolysis has been studied to a lesser extent, and is the subject of the detailed laboratory-scale research reported in this paper. After assessing the effect of monovalent/divalent cations (respectively, K(+)/Na(+) and Ca(2+)/Mg(2+)) on the sludge dewaterability, only the use of Ca(2+) appears to offer the best solution. The lesser effects of K(+), Na(+) and Mg(2+) confirm previous experimental findings. As a result of the experimental investigations, it can be concluded that alkaline thermal hydrolysis using Ca(OH)(2) is efficient in reducing the residual sludge amounts and in improving the dewaterability. The objectives are fully met at a temperature of 100 degrees C; at a pH approximately 10 and for a 60-min reaction time, where all pathogens are moreover killed. Under these optimum conditions, the rate of mechanical dewatering increases (the capillary suction time (CST) value is decreased from approximately 34s for the initial untreated sample to approximately 22s for the hydrolyzed sludge sample) and the amount of DS to be dewatered is reduced to approximately 60% of the initial untreated amount. The DS-content of the dewatered cake will be increased from 28 (untreated) to 46%.Finally, the mass and energy balances of a wastewater treatment plant with/without advanced sludge treatment (AST) are compared. The data clearly illustrate the benefits of using an alkaline AST-step in the system.

The inhibitory effects of heavy metals and organic compounds on the net maximum specific growth rate of the autotrophic biomass in activated sludge

A respirometry technique can be applied as an effective method to determine the net maximum specific growth rate of autotrophic biomass under both normal conditions and when inhibition occurs. The net maximum specific growth rate of uninhibited autotrophic biomass, expressed as (mu(A)-b(A)), is approximately 0.8 per day [Proceeding of the International Congress on CHISA, Prague, 2002, p. 1]. Several heavy metals and organic compounds have inhibitory effects. Copper (Cu(2+)) has stronger inhibitory effects than zinc (Zn(2+)), and inhibits the nitrification process by 50% at 0.08 mg/l [(mu(A)-b(A)) = 0.4 per day], while the same concentration of Zn(2+) establishes 12% inhibition only [(mu(A)-b(A)) = 0.75 per day]. Inhibition with Cu(2+) starts at concentrations above 0.05 mg/l, while this is above 0.3mg/l for Zn(2+). The inhibition of the nitrification process is complete at 1.2mg/l for both Cu(2+) and Zn(2+). Among the selected organic compounds tested n the experiments, the degree of inhibition decreases as follow: chlorobenzene>trichloroethylene (TCE)>phenol>ethylbenzene. Chlorobenzene already inhibits the autotrophic biomass at 0.25 mg/l. The nitrification process is totally inhibited by adding 0.75 mg/l of chlorobenzene. TCE has a less inhibitory effect on the nitrification process and 50% inhibition is noticed at 0.75 mg/l TCE. The nitrification process is totally inhibited at 1mg/l TCE. Phenol inhibits the nitrification for 50% at 3 mg/l. The inhibitory effect of phenol is almost constant in the range 4-10 mg/l and complete inhibition is reached at 50 mg/l. The inhibitory effect of ethylbenzene is 50% at 8 mg/l and the autotrophic biomass is totally inhibited at 50 mg/l. Experimental findings are compared with literature data, which generally and significantly overestimate the inhibition threshold concentrations.

Pt10Ni90(111) single crystal alloy: Determination of the surface composition by AES, XPS and ISS

Abstract The surface composition of the Pt 10 Ni 90 (111) single crystal alloy has been determined from AES, XPS and ISS experiments. The clean surface is largely enriched with platinum: 30–40% in the top layer instead of 10% in the bulk. This enrichment concerns mainly the ultimate surface layer and appears to be only slightly dependent on the sample temperature.

Experimental determination of equilibrium surface segregation in Pt-Ni single crystal alloys

Abstract The very surfaces of Pt-Ni alloys after equilibration at 725 K were investigated by ion scattering spectroscopy and shown to be strongly enriched in Pt: 37, 96 and 99 at% for Ni 90 Pt 10 , Ni 50 Pt 50 and Ni 22 Pt 78 bulk compositions respectively. The composition dependent sputtering yield ratios together with the values for the segregation enthalpy both point to a matrix effect as the dominant parameter for the segregation process.

Platinum Segregation to the (111) Surface of Ordered Pt80Fe20: LEIS Results and Model Simulations

Low-energy ion scattering (LEIS) was used to study the surface composition of a Pt 80 Fe 20 (111) alloy surface as a function of temperature. From 700 K onwards, high enough for sufficient atomic mobility to attain equilibrium, the surface consists of nearly pure Pt. These results agree completely with the results from LEED II V analysis by Beccat et al. [Surf. Sci. 238, 105 (1990)], who also evidenced a monotonic concentration profile over the three top layers, slight relaxations between those layers and a (2 x 2) superstructure caused by an ordered atomic arrangement in the second layer. Simple expressions for the entropy changes upon segregation in ordered alloys combined with constant pairwise bond energies clearly appear to be inadequate to describe the observed experimental facts. Even when combined with Monte Carlo simulations, the constant bond energies fail to correctly reproduce the results. Only when the Monte Carlo method is combined with the more refined (modified) embedded atom method (MEAM) for energy description of an alloy is satisfactory agreement with experiment obtained.

Monte Carlo simulation of Cu segregation and ordering at the (110) surface of Cu75Pd25

The segregation to the (110) surface of a Cu75Pd25 single crystal is modelled as a function of temperature by Monte Carlo simulations combined with the embedded atom method (EAM). Using EAM parameters from literature, optimised for the six transition metals of the Cu and Ni groups, does not yield satisfactory results: too low a bulk order–disorder transition temperature Tc and, contrary to experimental evidence, Pd surface segregation and a disordered surface layer are obtained. These observations are contradicted by the LEIS results of Bergmans et al. [Surf. Sci. 345 (1996) 303] that revealed an ordered bulk with an oscillating concentration profile at least up to 600 K and a Cu-enriched surface. They also observed a (2×1) LEED pattern caused by ordering in the second layer. In this work the EAM parameters are recalculated and optimised precisely for the Cu–Pd alloy system under study. The results of the simulations are now in perfect agreement with the experimental findings: a bulk critical order–disorder temperature in agreement with the bulk Cu–Pd phase diagram, an oscillating concentration profile, Cu segregation to the surface and a second atomic layer exhibiting a substantial degree of ordering that accounts for the observed (2×1) LEED pattern.

Segregation and ordering at alloys surfaces: modelling and experiment confronted

Model calculations combining the Monte Carlo (MC) method and a suitable energy model are proposed as fully complementary to experiments in order to gain insight in segregation and other surface phenomena, deployed by materials in order to minimise their surface free energy and hence their total Gibbs free energy. In the confrontation between experiments and modelling, it is basically assumed that the surface, as observed in the experiments, corresponds to the equilibrium situation of minimal Gibbs free energy. The entropy part is modelled by the stochastic nature of Monte Carlo simulations, while the energy part is taken into account by the (modified) embedded atom method ((M)EAM). Special attention is paid to the derivation of model parameters that are specific for the alloy under study. For it appears that in several cases the experimental data can only correctly be reproduced with these specific EAM parameters. This article further focuses on the determination of the degree of order in the bulk. Simulations according to the Grand Canonical Ensemble require the difference in chemical potential between the components. A novel approach is presented for deriving this parameter both for slightly off-stoichiometric ordered alloys and for disordered alloys. Results of MC/(M)EAM simulations are presented for the surfaces of five catalytically important binary alloys: Au75Pd25(1 1 0), Cu75Pd25(1 1 0), Pt50Ni50(1 0 0), (1 1 0) and (1 1 1), Pt80Fe20(1 1 1) and Pt75Sn25(1 1 1). It can be concluded that these simulations yield excellent predictions for surface modifications and are a very powerful tool to model and understand surfaces at equilibrium. # 2003 Elsevier Science B.V. All rights reserved.

The segregation in single crystal Ni−Pt alloys: A model for multilayer segregation

Equilibrated (111) surfaces of Ni−Pt single crystal alloys terminate with a Pt-enriched layer. A multilayer segregation model including a Pt−Pt bond strengthening accounts well for the observed results.

Face-related segregation reversal at Pt50Ni50 surfaces studied with the embedded atom method

Abstract The segregation to the three low-index surfaces of a Pt 50 Ni 50 single crystal is modelled by Monte Carlo simulations combined with the embedded atom method (EAM). Using the best fit EAM parameters from the literature for the six transition metals of the Ni and Cu groups does not yield satisfactory results. In this work the EAM parameters are recalculated and optimised exclusively for the Pt–Ni alloy system under study. Only then does EAM reliably reproduce the driving forces for segregation. The experimental results [Y. Gauthier et al., Phys. Rev. B 31 (1985) 6216; Y. Gauthier et al., Phys. Rev. B 35 (1987) 7867; S.M. Foiles, in: P.A. Dobson, A. Miller (Eds.), Surface Segregation Phenomena, CRC Press, Boca Raton, FL, 1990, p. 79] reveal a face-related segregation reversal for the Pt 50 Ni 50 single crystal. It appears from the simulations that this is caused by a relatively small difference in surface energy in close competition with the elastic strain release. At the open (110) surface the difference in surface energy dominates causing Ni segregation. At the (100) and (111) surfaces the difference in surface energy is overpowered by the elastic strain leading to Pt segregation. The simulations are in good agreement with the experimental results and reproduce quantitatively the Ni segregation to the (110) surface and the Pt segregation to the (100) and (111) surfaces. Only at the (110) surface significant relaxations are predicted in good agreement with experimental evidence. Atomic vibrations can be included by allowing a large number of very small displacements or with a more classical treatment of vibrational entropy. Both approaches yield the same results and show that the inclusion of atomic vibrations is important only for the (110) surface and tend to attenuate the Ni segregation profile.

Dual mode segregation of Pd to the surface of polycrystalline Fe99Pd1

Abstract On a polycrystalline Fe 99 Pd 1 sample, unusually fast and abundant segregation is observed up to 55 at% Pd. The dependence on temperature and on the bulk concentration are also contradictory to normal segregation behaviour. Once the sample is heated to at least 750–800°C, normal, slower segregation behaviour is observed up to a maximum of ca. 35 at% Pd. The unusual segregation behaviour can be explained by segregation from a two-phase bulk leading to an ordered Fe 50 Pd 50 surface phase.