Volume 7, numéro 4, 1994

FR :

La modélisation pluie-débit dans le cas de la prévision des crues peut être étudiée par la méthode DPFT (différence première de la fonction de transfert) qui est une extension de la méthode de l'hydrogramme unitaire. Contrairement aux autres méthodes, la méthode DPFT permet d'obtenir à la fois la fonction de transfert à travers sa différence première (DPFT) et une série des pluies efficaces. L'avantage principal de la formulation en différences est la diminution de l'auto corrélation des débits successifs et des coefficients de la fonction de transfert.

L'algorithme de calcul procède par itérations en résolvant alternativement un système multi-événements qui identifie la fonction de transfert et un système de déconvolution qui estime une série de pluies efficaces, cette fois-ci crue par crue. L'initialisation se fait à l'aide des pluies brutes comme première approximation des pluies efficaces puisque les résultats ne dépendent que des variations des débits. La convergence de l'algorithme est établie aisément lorsqu'on applique les différentes contraintes (positivité des ordonnées de la fonction de transfert et des pluies efficaces; normalisation de la fonction de transfert).

La méthode DPFT ne nécessite donc que les mesures des pluies brutes et les débits pour effectuer les identifications de la fonction de transfert et des pluies efficaces. Elle n'impose pas de préciser la fonction de production. Une fois la fonction de transfert calée et les pluies efficaces estimées par la DPFT, l'ajustement de la fonction de production se fait par la suite en résolvant un problème du type entrée-sortie.

Une application de la méthode DPFT est faite sur le bassin versant de Oued Dis (Sebaou) dans le but de tester les performances de cette méthode sur des données réelles, sachant qu'on a obtenu une confirmation assez rigoureuse de ces propriétés sur des données synthétiques générées. Les résultats de l'identification de la fonction de transfert sont satisfaisants tandis que ceux de l'ajustement de la fonction de production sont moins satisfaisants, ce qui a influencé directement la qualité des résultats de validation.

EN :

Rainfall-runoff modelling in the case of flood forecasting may be studied by the first difference of the transfer function (FDTF) method, which is an extension of the unit hydrograph approach. In contrast to other methods, the FDFT method simultaneously provides both the transfer model by its first difference and an excess precipitation series. The principal advantage of the difference formulation is the diminution of the autocorrelation between successive flow data and the transfer function coefficients.

The compilation algorithm proceeds iteratively by resolving alternately a multievent system which identifies the transfer function, and a deconvolution system which assesses the excess precipitation series event by event The initiatization is done with the total precipitation as a first approximation of the excess precipitations, since the results are only dependent on flood variations. The algorithm convergence is easily established if the various constraints are applied (positive values for the transfer function coefficients and the excess precipitations; normalization of the transfer function).

Thus, the FDFT method only requires total precipitation and flood data in order to generate the transfer function and quantify the excess precipitation. It doesn't require that the production function be specified. once the Transfer fonction is calibrated and the excess precipitation estimated, the production function adjustment is carried out by resolving an input-output model type.

The FDTF method has previously been applied successfully to simulated data. In the present study, the method has been applied to the Oued Dis watershed (Sebaou, Algeria) in order to test its performance using real data. The transfer function identification results proved satisfactory, but those related to the producton function adjustment were less satisfactory and degraded the overall quality ofthe validation results.

FR :

Une étude expérimentale concernant l'adsorption des métaux lourds Cu, Cd, Zn et Pb par des sédiments d'un cours d'eau pollué par des rejets industriels a été entreprise pour mettre en évidence l'importance de certains paramètres expérimentaux, notamment le pH, la masse de sédiments et la température. Elle a permis également d'analyser la nature des liens qui participent à la fixation des cations métalliques sur les différentes fractions sédimentaires déterminées selon la méthode de TESSIER et al. (1979) et d'interpréter les capacités d'adsorption relativement variables suivant la nature du métal. Les résultats confirment le rôle particulier des fractions réductible (oxydes de fer et de manganèse) et organique (substances humiques en particulier), dont les propriétés respectives d'échange d'ions et de complexation ont été souvent vérifiées. La capacité d'adsorption de ce sédiment a notamment pu être interprétée en termes d'isothermes d'adsorption en exploitant les modèles de Langmuir et de Freundlich et leurs équations linéarisées.

EN :

In an experimental study of the adsorption of Cu, Cd, Zn and Pb by surface sediment in a small stream polluted by the industrial drain of electro-refinery, we have demonstrated the irnportance of some such experimental parameters as pH, sediment concentration and temperature. These experiments were conducted in batch systems at constant temperature with continuous agitation, using a mixture of sediment and metals at an adjusted pH; the quantity of metals remaining in solution was determined by a polarographic method. Adsorption percentages for the concentratons of sediment (200 and 1000 mg/L) and metals (1 mg/L) reached the following maximum values: Pb (99-l00%o), Zn (80-90 %), Cd (75-85 %) and Cu (70-80%). These variations in metal soprtion are attributed to differences in binding energy between the metallic cation and the sediment sites, when all other parameters are fixed. Based on metal partitioning among the different sedimentary fractions, as determined according to the sequential extraction method of TESSIER et al. (1979), it was possible to attribute metal adsorption to complexation, coprecipitation and complexation reactions respectively with organic matter, carbonates and Fe-Mn oxides or alumino-silicates. Our results highlight the specific roles played by the reducible Fe-Mn oxides and by humic substances. Fulvic and humic acids, which are considered as the stable fraction of sedimentary organic matter, can form complexes and participate in the fixation of metals on the sediments (GODFRIN and BLADEL, 1990; WILLIAM and HANSON, 1979; FITCH et al., 1968; BIZRI et al., 1985). The stability of these complexes depends on the variety of reaction sites in these macromolecules, which in turn determines the degree of fixation of the cations (specific adsorption). Concerning the iron-manganese oxides and the alumino-silicate compounds, their surface sites are engaged insurface complex formation by a mechanism of proton exchange in which humic substances can be also involved (BELZILE et al., l989a; BELZILE and TESSIER, 1900; BELZILE et al., 1989b).

Adsorption kinetics for Cu, Zn and Cd were relatively fast with more than 50 % of the metal adsorbed in a few hours, followed by a partially reversible stage over the next few days leading to an equilibrium state. Reversibilty of Pb binding was not signifrcant (attributed to the chemical precipitation of Pb₃(PO₄)₂). An enhancement of adsorption with increasing pH between 5 and 8 was noted for Cu, Zn and Cd. Several factors may contribute to this increase in the quantity of absorbed metals:

-the M+ and MOH+ species berome more competitivethan H+ fortte adsorption sites on the sediment;

- the number of adsorption sites increases;

- the change in conformation of the humic substances, from aggregated to stretched forms, may render the metal complexing sites more accessible;

- the rate of Fe(III) and Mn(IV) formation is more significant at higher pH values;

- at higher pH, the precipitation of oxides, hydroxides and hydroxycarbonates becomes important as well as the adsorption on the suspended phases;

- the degree of oxide crystallization is influenced by the pH value and hence the adsorption capacity of reducible phases is also affected.

A decrease in metal adsorption was also observed as the temperature increased between 10°C and 40°C. The adsorption of metals was described using FREUNDLICH and LANGMUIR equations in their linear form.

FR :

Les oeufs de nématodes sont présents dans les eaux usées et dans les boues résiduaires. Suite à ce constat, l'objectif de ce travail est la mise en évidence d'une éventuelle capacité d'adhésion par contact et fixation des oeufs sur différentes surfaces. L'oeuf d'Ascaris a été utilisé comme modèle et divers paramètres sus- ceptibles d'intervenir ou de modifier les interactions ont été testés. Sur les matériaux de laboratoire, 32,5 % des oeufs se fixent sur les tubes de verre contre respectivement 15,7 % et 9,1 % sur des tubes en polystyrène et en polypropylène. En eau désionisée, les oeufs montrent une forte capacité d'adhésion sur le support hydrophile avec 75 % d'oeufs fixés sur les particules de poudre de verre contre uniquement 1,3 % sur les feuillets hydrophobes de talc. L'adhésion sur la poudre de verre est très rapide avec 55 % des oeufs fixés après 30 secondes. En présence d'une charge en adsorbant de 10 g/l, pour une valeur de pH égale à 9 et pour les concentrations ioniques (CaCl² et NaCl 0,9M), l'adhésion des oeufs sur la poudre de verre atteint 99 %. Cependant, des températures égales à 2 °C et ³ 60 °C ou l'utilisation de solutions éluantes (hypochlorite de sodium, formaldéhyde, détergents an ionique et caffonique) diminuent les phénomènes d'adhésion avec en particulier l'hypochlorite de sodium à 2° chl qui les supprime totalement. Toutes ces informations indiquent que l'adhésion intervient de manière importante. En effet, lors du traitement des effluents, ce phénomène favorise l'épuration par adhésion des oeufs au floc de matière organique avec cependant transfert de contaminants dans les boues.

Dans les sols et sur les végétaux, l'adhésion favorise la contamination parasitaire des surfaces. L'utilisation d'effluents ou de boues contaminées devra impérativement respecter les normes ou les recommandations destinées à protéger des risques de transmission à l'homme et aux animaux.

Les techniques destinées à évaluer la contamination parasitaire d'échantillons de l'environnement sont dépendantes de ces phénomènes d'adhésion. L'utilisation de solutions éluantes et notamment d'hypochlorite de sodium est indispensable.

EN :

The intestinal nematodes use the environmental medium to realize their infestation cycle with eggs. In this study, the ability of helminth eggs to create binding in laboratory conditions is evaluated. This information will allow a better knowledge of the characteristics and the behaviour of the eggs in the environment and will give beginning of answer to the main question: are the nematodes eggs bound to sufaces and particulary to suspended matterin the environment or free particles?

For this study, the model used is the Ascaris eggs collected in the end of the womb in order to work with fertile coated eggs. They seem to be the most common form found when the eggs are quantified in the water or wastewater medium. However, the aggregate in the suspension create difficulties for the counting and give a protect effect of the outer layer for the eggs into the aggregate. Physically, the aggregate properties are also different from the properties of one isolated egg. To spread out the eggs without bringing modifications of the egg coating, one filtration is performed on l00 µm filter to eliminate the aggregate higher or equal to 4 eggs. Before the study, a possible adhesion on the laboratory materials is performed.

On glass materials, a strong adhesion is observed with 32.5% of eggs on the surfaces; on the other hand, on polypropylen and on polystyren, the adhesion is less important, respectively 9.1 and l5.7 % are adsorbed with a significant difference between these two results. Then for this work, polypropylen and polystyren materials are used. In deionised water (pH 6) with one 15 min stirring at 15 rpm, the behavior of the Ascaris eggs is studied on two surf'aces glass powder which is hydrophil particles with electronegative charges and talc which is hydrophob flakes with electronegative charges too. The results show a small adhesion on the hydrophob surfaces (l.3%) for a percentage of 75% on the hydrophil particles with the glass powder. In the same experimental conditions, the adhesion on hydrophil surfaces is confirmed with two other adhesion media: the muscovite and the glass wool which show a significant adsorption (96 and 98.5%). Onglass powder, the adhesion characteristics are developed with the study of 5 parameters: the contact time, the adhesion medium concentration, the pH, the temperature and the ionic concentration. For a low glass powder concentration (1g/l), a good adhesion is obtained with 65% of egg bound. The adsorption is obtained rapidly, 55% are already bound after 30 sec. The pH increase significanty the adhesion at the value 9 and an important decrease is observed atthe low and high temperatures (2 and 60°c). To complete this study, reagents used as desorption solution in the analytical techniques for the research of the helminth eggs in wastewater are tested. The eluting solutions as formalin and detergents increase in a considerable proportion the percentage of eggs non adsorbed, the outer layer properties could be modified by these reagents. For the sodium hypochlorite solution, the activity is stronger.This reagent eliminates the egg adhesion with only 0.1 percent of eggs bound. The action on the outer layer could be a destruction and then the loss of the adhesion properties. The practical implications of these adhesion properties are considerable: the wastewater depuration by physico-chemical treatments with the possibility of adhesion on floc particles for the eggs; the adhesion to the soil particles avoiding percolation and then the eggs accumulation in depth; on vegetables irrigated with wastewater the adsorption can improve the contamination and for the analytical techniques used to quantify parasite eggs in aqueous medium, we have to overcome the problem of the adhesion to the supports, with the use of eluting solution like sodium hypochlorite.

FR :

Les théories classiques concernant les processus générateurs des écoulements rapides permettent mal d'expliquer la soudaineté de la montée de crue dans des petits bassins versants élémentaires, situés sur le versant sud du Mont Lozère (sud du Massif Central, France).

Des études portant sur les conditions de genèse de ces crues « cévenoles » ont montré les conditions particulières d'apparition de tels événements: dans ces milieux de moyenne montagne granitique méditerranéenne, les formations superficielles sont peu épaisses et très filtrantes et les abats d'eau considérables; les surfaces saturées, seules capables de générer un ruissellement important, progressent, contrairement à ce qui s'observe habituellement, d'amont en aval des versants; ce type de fonctionnement empêche dans un premier temps toute arrivée d'eau jusqu'au drain principal et ce n'est que lorsque, de proche en proche, l'ensemble du bassin est quasi saturé que la crue se déclenche, entraînant immédiatement des débits importants.

Ce modèle de fonctionnement hydrologique se vérifie bien lors de la crue du 22 septembre 1992, responsable de nombreux dégâts et de pertes en vies humaines dans le sud de la France.

EN :

The southern and western edges of the French « Massif Central » are frequently subject to very high floods, known as « crues cevenoles ». They mainly occur during Autumn, and they are characterized by particularly sudden and high flows. Genesis conditions for these types of floods have been studied in a small experimental basin located on Mont Lozère.

The Latte River basin (20 ha) is located between 1200 and 1400 m above sea level. Bedrock is granite, covered with sandy, weakly weathered soil, ranging in depth from 0 to 1 m, generally thinner on the upper part of the slopes. Infiltration capacity is very high, generally over 70 mm .h^-1 (and locally more than 135 mm • h^-1), as indicate by rainfall simulation (COSANDEY et al., 1990). The basin was previously covered with a 70 year old spruce forest but the forest was cut down between 1987 and 1990, without evident effects on flood genesis conditions (COSANDEY and BERNARD-ALLEE, 1992; COSANDEY, 1993).

Rainfall is recorded by a network of three pluviographs. Facing the rainy winds coming from the Mediterranean sea, the basin receives about 1900 mrn of annual rainfall; large storm events, particularly in Autumn, can produce more than 400 mm of rain (e.g., in November 1986) with very high intensities (about 60 to 90 mm . h^-1 during half an hour one time every year; an intensity of 170 mm . h^-1 was observed during half an hour during an exceptional rainfall event of 22 September 1992). A V-notch weir was built at the outlet of the catchment, and the discharge measured using a hydrograph. During « cévenoles floods » discharge can reach 2 m³ • s^-1 • km^-2.

Initial observations showed that the rising of the flood waters is always sudden and rapid, suggesting the existence of a threshold above which the flood can occur. Previous studies have shown that this threshold is not simply related tothe amount of rain. For example' in November 1986 discharge remained very slow, after more than 110 mm rain (fig.3). In another case, in October 1987, 36 mm was sufficient to initiate flooding (COSANDEY, 1993a). In fact this threshold depends on the total water storage within the basin; it can be calculated as the sum of potential maximum values of both the soil and ground water content and is equivalent to 270 mm (COSANDEY and DIDON-LESCOT, 1990). Flood generation processes include the following:

The high infiltration capacity of soils does not allow the formation of Hortonian runoff, despite high intensities of rainfall. Rapid flow can occur only in conformity with the « saturated contributing areas » theory (DUNNE and BLACK, 1970). But here saturaton begins on the upper part of the slopes where the soils are thinner; runoff which occurs does not reach the stream because it can infiltrate downslope, where the soils are thicker. Saturated surfaces progressively extend downslope; from the moment where the saturated zones join the stream, runoff can contribute to rapid flow. At this moment, flood waters arise suddenly. Figure 4 shows the different steps of the extension of the saturated areas within the basin, and the model of streamflow generation:

- When it does not rain, the water table is drained by the stream.

- A small amount of rain can be sufficient to make the water table arise in the bottom of the valley; this saturated area becomes a source of rapid flooding, but the volumes remain very small.

- If the rain continues, new saturated areas occur where the soil is thinner, mainly upslope. But runoff generated on these areas infiltrates downslope; this runoff may cause a rise in water level and an extension of saturated surfaces, but it cannot join directly to the stream, and it does not contribute to rapid flow. Discharge remains very low.

- If the rain persists, saturated areas extend downslope, and finally join the saturated areas in the bottom of the valley. From this moment infiltration is impossible, and runoff generated by the whole slope contributes to the rapid flow directly or as piston-flow. The flood starts to form and increases rapidly; discharge becomes very high.

This model allows us to understand why, when floods begin, they are immediately important if rainfall continues.

This hydrological pattern has produced about 10 « crues cévenoles » during the studied period (1981-1992), with flood peaks around 300-500 l• s^-1 (1300 to 1500 l • s^-1 • km²). During the particularly high intensity event of 22 September l992, the peak flood reached 1500l • s^-1 (7.5 m³ • s^-1 • km²). The question is to assess if hydrological processes able to produce such a discharge are the same as those described here, and if the hypothesis of a threshold can be well verified.

When the rain tregan on 2l September at 6 pm, the basin was under very dry conditions. The second part of August and September had been dry. Soil water content was quite low (about 9 mm) and the base flow discharge, 5 l • s^-1, indicates, according to Maillet'law (ROCHE, 1967) a groundwater storage of about 5 mm (COSANDEY. 1993a): 256 mm of rain would be needed to reach the threshold of 270 mm. From Monday, 6 pm to Tuesday, 4:20 am the amount of rain was about 140 mm with medium intensities. Discharge remained very low (fig. 5; COSANDEY 1993b). Total soil water content within the basin was about 162 mm. Intensities then became very important, with 65 mm rain in half an hour. A flood occurred, with only a small peak (170 l • s^-1) despite the high value of rainfall intensities. The water storage was about 227 mm, and the threshold had not been reached. The discharge decreased from the moment that rainfall intensities became lower.

About 40 mm of rain were needed to reach the threshold, and this was exceeded at 5:52 am. A rainfall event with similar characteristics (65 mm in half an hour) then produced a peak flood estimated at 1500 l • s^-1 • km², seven times higher than previously reported. Rainfall stopped quickly, and the flow decrease was rapid. Nevertheless a small rainfall event of 10 mm over 15 min was sufficient to produce a peak of 334 l • s^-1, This particularly high rainfall event demonstrated that, despite very high intensities, no important flood can occur when the basin is not completely saturated and the total water content is less than 270 mm.

It is important to note that this hydrological pattern concerns a very small (20 ha) first-order basin. Flood genesis conditions are very different downstream, mainly due to the combination of tributary floods.

FR :

Le travail présenté dans cet article a pour but d'isoler les principaux micro-organismes impliqués dans la biodégradation des lipides concentrés dans des réacteurs spécifiques de stations d'épuration des eaux et d'étudier leur action sur ce type de substrat.

La microflore d'une boue activée « classique » est comparée à celle de boues acclimatées à des teneurs élevées en lipides selon un nouveau procédé « BIOMASTER® G ». Cela montre un enrichissement en bactéries fortement lipolytiques dans le système à l'équilibre. En effet, la boue activée « classique » ne contient aucun microorganisme fortement lipolytique alors qu'à partir de la boue acclimatée du même site on a pu en isoler, Acinetobacter calcoaceticus et Pseudomonas putida étant les constituants les plus actifs. L'utilisation de bioadditifs du commerce pour l'ensemencement des pilotes au lancement du système ne semble pas apporter d'avantages décisifs puisqu'on ne retrouve pas à l'équilibre les micro-organismes contenus dans ces bioadditifs. Par contre, d'autres microorganismes à pouvoir lipolytique élevé sont détectés tels que Pseudomonas sp., Aeromonas hydrophila et Staphylococcus sp . Le genre Pseudomonas est par ailleurs presque toujours rencontré quelle que soit la boue acclimatée examinée. De même, on peut noter la quasi absence de bactéries Gram positif.

Tous les isolats Gram négatif dégradent plus ou moins les acides gras de longueur de chaîne variée. Le seul isolat Gram positif est inhibé ou même tué par les acides gras à chaîne moyenne ou courte et cela peut contribuer à la pauvreté en bactéries Gram positif dans les boues acclimatées. Les genres principaux que nous avons rencontrés, Pseudomonas, Acinetobacter et Aeromonas, sont capables d'assurer à la fois la lipolyse (libération des acides gras) et l'oxydation subséquente des acides gras. L'enrichissement naturel de la microflore lipolytique des boues activées dans le procédé aboutit à une microflore acclimatée capable d'assurer la biodégradation complète des lipides.

EN :

In this study, the main microorganisms involved in lipid biodegradation in activated sludge sewage treatment plants were isolated and their action on triglycerides and on fatty acids was investigated.

Six different sludge samples were studied. An activated sludge of the usual type (A) was used as reference. At the same site, activated sludge was adapted to higher lipid levels, through separation of fats reaching the sewage treatment plant, followed by their addition in higher proportion to activated sludge. In experiment B, adaptation took place in a « BIOMASTER^® G » reactor, which is a new process for the elimination of fats by an aerobic treatment. Experiments C and D used the same original sludge plus lipid mixture, but with the addition of commercial bioadditives containing lipolytic bacteria. Finally, microorganisms isolated from two other systems, located at two other sites, were also studied (experiments E and F).

The control activated sludge, A, did not contain any strongly lipolytic microorganisms. On the other hand, in the new system at equilibrium, after adaptation, bacteria with a strong lipolytic potential such as Acinetobacter calcoaceticus and, Pseudomonas putida were isolated. The use of bioadditives at the start of the incubation did not afford any notable advantage, since at equilibrium microorganisms contained in these additives were not reisolated. However, other microorganisms with high lipolytic power (activity higher than 0.1 µmole acid liberated per minute and per ml of culture broth) were isolated, such as Pseudomorns sp., Aeromonas hydrophila and Staphylococcus sp. The genus Pseudomozas was almost always detected, regardless of the acclimated sludge studied. Conversely, Gram positive bacteria were virtually absent.

All Gram negative isolates were more or less capable of degrading fatty acids with various chain lengths. The only Gram positive isolate was inhibited, even killed, by short and medium chain fatty acids. In the present study, this might contribute to the paucity of Gram positive bacteria in the adapted sludges. The main genera encountered, Pseudomonas, Acinetobacter and Aeromonas, were able to perform both lipolysis (liberation of fatty acids) and the subsequent oxidation of the liberated fatty acids. The natural enrichment of the lipolltic microflora of activated sludges in the process leads to an acclimated microflora, able to completely biodegrade lipids.

FR :

L'oxydation humide par l'oxygène moléculaire (procédé WAO) activée par le couple (H²0²/Fe²+) a été mise en oeuvre pour l'oxydation de la pollution organique aqueuse à travers deux composés modèles: l'acide succinique, normalement oxydable, et l'acide acétique, réputé réfractaire. L'influence des différents facteurs a été étudiée par la planification d'expériences. Après leur recensement, une étape préliminaire de criblage a été menée à bien en utilisant une matrice de Plackett et Burman. Seuls les paramètres les plus influents ont été gardés pour l'étape ultérieure d'établissement de modèles prévisionnels à partir d'une matrice composite centrée orthogonale. Les modèles établis ont été validés et ont permis de déterminer les conditions optimales de fonctionnement. L'effet de la température fait apparaître un optimum, à environ 200 °C, au-delà duquel la décomposition du peroxyde devient trop rapide. L'effet de la quantité de peroxyde d'hydrogène introduit est déterminant et l'ajout de moins de 20 % de la quantité stoechiomé- trique permet d'obtenir à 200 °C, avec environ 10 ppm de sels de fer, une efficacité de traitement d'environ 70% pour un composé normalement oxydable. Dans des conditions analogues, le procédé conventionnel sans promoteur conduit à une efficacité inférieure à 5 %.

EN :

Wet air oxidation (WAO) is a liquid phase oxidation process using molecular oxygen at high temperature (250-300°C) and high pressure (50-150 bar). It can help treating toxic organic aqueous wastes from chemical industries with efficiencies up to 98% after 1 hour. The process can also help treating sludges from domestic sewage treatment facilities. It is usually very cost effective because of the very high operating pressure.

This paper deals with the promoted wet air oxidation of acetic acid, rnodel compound for refractory wastes, and succinic acid, model for readily oxidized wastes. The study was conducted in order to determine the promoting effect when adding small dosages of hydrogen peroxide (with iron salts) during oxidation by molecular oxygen. It was previously shown that the initiating step is very temperature dependent (Reaction I) and limits the overall oxidation process The addition of small amounts of H₂O₂/Fe²⁺ (Fenton's reagent) can promote the forrnation of very reactive OH• radicals able to develop R• radicals (Reaction IV), even at a low temperature. Then, the oxidation (Reactions VI and VII) continues using molecular oxygen, but the peroxide should be added continuously during a batch test in order to maintain the initiating step.

An optimal design methodology was used in order to assess the dependency of the oxidation effrciency on the various parameters and mainly on the promotors. At frrst, a Plackett and Burman design of experiments (PE1) was used to screen the most important variables among those likely to have an effect. The design of experiments, the conditions of the runs and the results (tables 1 to 3) allowed the determination of a new experimental domain and the selection of the four most important variables for the further design of experiments. At the same time, the effect of an addition of phenol (able to reduce iron to the ferrous species, more efficient) was considered. For succinic acid oxidation, a central composite optimal design (PE2) was used (tables 4 and 5). The results allowed us to establish a predictive model (Relationship lX, table 6 and figure 2) and typical results are presented in figures 3 and 4. Approximately 50% oxidation efficiencies could be obtained at 200°C; without peroxide addition, only 5% efficiency is obtained under similar conditions. Moreover, it was observed that the optimum temperature is around 205°C and that phenol is not compatible with peroxide as a promotor. A third optimal design (PE3) was used to predict the efficiency of the method for the treatrnent of acetic acid, a model for a refractory waste. It is composed only of a fractional factorial design (table 7 and 8) and the bias corresponds to the main quadratic effect of temperature (Relationship XIII and table 10). The optimum temperâture is also 205°C and greater than 20% oxidation efficiencies are obtained; at such a temperature, acetic acid cannot be oxidized with the conventional process.

The results obtained for the two model compounds validate this oxidation technique. The addition of about 10 ppm of ferrous iron and of less than 20% of the stoichiometric amount in hydrogen peroxide can turn a high pressure WAO process into a medium pressure one.