Volume 3, numéro 4, 1990

FR :

La nécessité de connaître aussi bien que possible l'impact des produits rejetés dans l'environnement a conduit à la mise au point de modèles mathématiques permettant de mieux comprendre le devenir des produits et de prédire l'exposition à laquelle pourra étre soumis l'environnement.

Le présent article fait le point des différents modèles publiés pour ce qui concerne le milieu aquatique et suggère différents critères permettant de classer les modèles suivant une complexité croissante.

Reprenant la terminologie de MACKAY (1979), 4 types de modèles sont décrits :

- modèles homogènes, équilibrés, conservatifs et stationnaires,

- modèles homogènes, non-équilibrés, non-conservatifs et stationnaires,

- modèles homogènes, non-équilibrés, non-conservatifs et non-stationnaires,

- modèles de dispersion.

EN :

Assessing the hazard of a chemical to aquatic life is based on comparing the exposure concentration and the « no effects » concentration. The Greater the ratio between these two concentrations, called the margin of safety, the smaller the hazard to aquatic life.

For chemicals already in the environment, monitoring programs can lead to an estimate of the exposure concentrations, but for new chemicals, predictive methods are necessary. This leads to the development of mathematical models which give an estimate of the environmental concentrations front the characteristics of the environment, of the chemical and of the quantity released in the environment.

This article gives an overview of the different models which have been already published and suggests a classification based on the following criteria :

- number of compartments,

- closed or open system,

- the degradation of the chemical is or is not taken into account,

- an equilibrium has or has not been achieved between the compartments,

- the situation is or is not at steady-state.

Based on these criteria and quoting MACKAY (1979), four types of models are described :

- Homogeneous, equilibrium, conservative and steady-state models,

- Homogeneous, non-equilibrium, non-conservative and steady-state models,

- Homogeneous, non-equilibrium, non-conservative and unsteady-state models,

- Dispersion models.

A review of these models applied to artificial or natural ecosystems la presented showing the extent to which these models have beau validated.

These models are used not only to predict the exposure concentrations but they also help towards understanding the different phenomena which can affect the fate of a chemical. They allow to identify the mort important phenomena and those which have to be extensively studied. They are also useful for organizing experiments.

However and though they are more and more used, these models have still to be improved since several fundamental phenomena, such as the sediment/water exchanges, are still badly understood.

FR :

Les auteurs, après une synthèse bibliographique sur la coagulation, présentent deux modèles mathématiques reliant la dose optimale d'un coagulant, le sulfate d'aluminium, à la qualité de l'eau brute.

Un premier modèle applicable aux eaux dont la turbidité est inférieure à 20 NTU tient compte de quatre variables caractéristiques de l'eau brute qui sont : la turbidité, la résistivité, la température, la teneur en matières organiques.

Le second modèle, utilisable pendant les périodes de crue, intègre un cinquième descripteur : la nature de la suspension minérale.

Des essais effectués sur l'usine de Clairfont, qui alimente la rive gauche de Toulouse, montrent que ces modèles sont parfaitement adaptés pour automatiser l'injection du coagulant.

EN :

When the source of supply of a water treatment plant has a streaming behaviour, it is sometimes difficult to adjust correctly the coagulant dose by jar-testing alone.

Following this remark, the municipal water utilities of Toulouse city (France) has decided to conduct a study on the modelisation of coagulant dose as a function of the quality of raw water.

This study started first by a bibliographic survey to find the main parameters which affect the flocculation mechanisms, followed by a hydrologic study of the upper Garonne river to characterize as well as possible its behavior.

This was following by a study of the influence of certain parameters on the efficiency of coagulation by aluminium sulfate on synthetic waters having characteristics close to those of the Garonne river (BAZER-BACHI, 1986). This work showed the rote of the turbidity, of the resistivity, of the temperature and of the nature of mineral suspension. It also indicated the non-linearity between the optimum coagulant dose and these parameters as well as the interactions between them.

These conclusions led us to use a second degree polynomial model, as :

              k            k             k

Y = b_o + ∑ b_i X_i + ∑ b_ii X²_i + ∑ b_ij X_i X_j

     i = 1, j = 2, i ≠ j

Data were collected from modified jar-test on water from the Garonne River, characterized by physico-chemical (turbidity, temperature, pH, resistivity, CEC, MES, colloid titralion, organic matters) and chemical analyses (mineralisation). As the hydrological study showed that the nature of colloid suspension changed only during storm events, the modelisation was divided in two sub-groups :- a « CALME » model, comprising four variables (turbidity, resistivity, temperature, organic matters), for low turbidity waters (below 20 NTU),- a « CRUES » a model, using a fifth parameter representative of the nature of suspension (the ratio between colloid titration and turbidity).

The « CALME » model, which fitted very well, showed the primordial influence of the temperature and organic matters, and then of the turbidity. Removal of resistivity was not advisable, as this parameter showed strong interactions with turbidity and organic matters (fig. 3, 4).

The « CRUES » model showed the primordial role of the turbidity followed by the temperature and then by organic matters. The two other parameters, less significant, should nevertheless be kept if a good quality of forecast is to be maintained with this model. It can be further improved once all the experimental domain has been investigated.

The very good results obtained in full-scale at the Clairfont plant, by adjusting the coagulant feed rate according to the forecast of our models, led us to implement a coagulation automate, in operation in 1989.

FR :

Après avoir présenté quelques données bibliographiques sur les substances humiques aquatiques, l'extraction de plusieurs substances humiques issues de onze eaux de surface françaises, est discutée en terme de rendements d'extraction. Quelques paramètres de caractérisation des acides fulviques sont présentés et en particulier, l'analyse élémentaire, l'absorption UV, les fonctions carboxyles et les potentiels de réactivité avec le chlore. Concernant la chloration, une attention particulière a été portée d'une part sur la nature produits organo-chlorés formés et d'autre part sur les corrélations qui existent entre la nature des acides fulviques et leur potentiel de réactivité avec le chlore.

EN :

Numerous procedure for the isolation of aquatic humic substances are described in literature. The first part of this study presents the extraction of fulvic and humic acids from eleven French surface waters, using XAD 8 macroreticular resin as developed by TRURMAN and MALCOLM (1981). The results are discussed in terms of extraction yield and fulvic acid/humic acid mass ratio.

The second part of the paper presents some structural parameters of the extracted fulvic acids such as elemental analysis, UV-absorbance, carboxyle-function content and in some cases of fulvic acids, infrared spectrum and apparent molecular weight investigated by ultra filtration.

The third part of the work concerns the study of fulvic acids chlorination. Firstly, the gas chromatography - mass spectrometry analysis of one chlorinated fulvic acid allowed us to identify some chlorination by-products and especially chloroform, dichloro-acetic and trichloroacetic acids and some other chlorinated aliphatic acids. Secondly, the chlorine reactivity potentials of each fulvic acids were determined and results are discussed in terms of chlorine consumption, chloroform and total organohalides formation potentials. Some relationships are established between the chlorine reactivity potentials and UV-absorbance of the extracted fulvic acids.

FR :

L'objectif de ce travail est d'étudier l'influence d'un tensioactif anionique, le dodécylsulfate de sodium (DSS) sur l'adsorption d'acides humiques (AH), sur le charbon actif en poudre (CAP).

L'adsorption sur CAP, en fonction du temps, des AH seuls, puis du DSS seul, a été effectuée en réacteur discontinu, à différents pH (7,0 et 10,5). Les mêmes expériences ont été réalisées avec le mélange des deux composés et l'on a pu constater une inhibition réciproque de leur adsorption, moins importante à pH basique (10,5).

L'influence de l'ordre d'introduction des coadsorbats, sur l'adsorption de chacun d'eux, a permis de montrer que l'inhibition de l'adsorption des AH sur CAP par le DSS est encore plus importante lorsque les AH sont ajoutés sur une suspension DSS-CAP déjà en équilibre. Aucune désorption du DSS n'a pu être mise en évidence après 24 heures. Ces résultats pourraient permettre de conclure à une adsorption « compétitive » des AH et du DSS sur les mêmes sites superficiels du charbon actif.

Ce travail permet de préciser les phénomènes d'adsorption pouvant intervenir entre divers micropolluants susceptibles de s'adsorber simultanément soit sur des sédiments naturels, soit sur charbon actif lors du traitement des eaux naturelles.

EN :

The removal of humic acids (HA) from natural waters is a very old yet modern problem : humic substances are at the origin of the coloration of water but also, they give off nocive trihalomethanes during chlorination.

Humic acids are also known to be a metal micropollutant vector and their behavior is not well defined when there is coadsorption.

The purpose of this work was to study the effect of an anionic surfactant, sodium dodecyl sulphate (SDS), on the adsorption on powdered activated carbon (PAC) of humic substances from a commercial source, as a function of the pH (7.0 or 10.5), as well as the effect of the order of introduction of the coadsorbate.

Results (fig. 1 to 3 and fig. 6) showed an inhibition of the adsorption for both HA and SDS in the presence of the coadsorbate. The inhibition was more significant for a pH = 7.0.

Results of the influence of the order of introduction of the coadsorbate on the adsorption (fig. 4 and 5) showed a more significant inhibition of HA adsorption when HA were introduced into the equilibrium suspension SDS-PAC (the contact lime was 24 hours).

Table 1 gives parameter values of the Langmuir equation for the adsorption on PAC of HA atone or with SDS.

All these results suggest that a competition on the same sites occurs during the adsorption on PAC of HA and SDS.

FR :

Deux installations pilote destinées au traitement d'effluents secondaires par culture de micro-algues (Scenedesmus sp.) ont été opérées à Valcartier (effluents domestiques) et Vaudreuil (effluents semi-industriels). Des bassins de 15 000 L ont servi aux cultures en vrac sous conditions naturelles, avec apport de CO₂ atmosphérique par huilage. Les paramètres physiques, chimiques et biologiques ont été mesurés.

Les résultats montrent que, malgré les limitations en CO₂ et en azote, un enlèvement moyen d'environ 95 % pour l'azote (N-NH₄₊, N-NO₂-, N-NO₃-) et de 60 % pour le phosphore (P-PO₄^-3) a été possible à Valcartier durant l'été; les données correspondantes sont de 92 % et de 98 % à Vaudreuil où la production de biomasse (1,02 mg/L-h) a été plus forte qu'à Valcartier (0,39 mg/L-h). Les facteurs pouvant expliquer les différences observées aux deux sites sont présentées. Les résultats montrent la faisabilité technique, durant l'été, de ce type de traitement tertiaire.

EN :

Two experimental pilot-scale culture systems have been operated at Valcartier (domestic effluent) and Vaudreuil (semi-industrial urban effluent) in order to test the feasibility of a tertiary treatment using microalgae (Scenedesmus sp.). Tanks with a capacity of 15 000 L were used for batch cultures conducted outdoors; CO₂ being provided through atmospheric air bubbling. Physical (temperature, relative insolation, solar radiation), chemical (pH, NH₄₊, NO₂-, NO₃-, PO₄^-3, heavy metal, pesticides) and biological parameters (biomass dry weight, cell counts) were mesured.

Results show that cultures were limited for CO₂ (high final pH) and for nitrogen flow N/P ratio with value of 3-7). During the summer period, nutrient removal from the Valcartier effluent was ~ 95 % for N and ~ 60 % for P; corresponding figures at Vaudreuil were 92 % and 98 %.

Biomass productivity was lower (0,39 mg/L-h) at Valcartier compared to that obtained at Vaudreuil (1,02 mg/L-h). Among the differences that could explain the results at both sites, one may suggest a better CO₂ input, a more favorable N/P ratio, slightly higher average temperature and light conditions, less ammonia stripping and larger inocula in Vaudreuil. The quality of biomass obtained was satisfactory : heavy metals were present at acceptable levels and no contamination by organochlorinated compounds occurred. According to our results, it appears that biological tertiary treatment of urban effluent and algal biomass production are technically possible under Québec summer conditions.

FR :

Avec l'objectif de mieux connaître le comportement du césium radioactif et son transfert dans un écosystème naturel d'eau douce, une étude concernant une chaîne trophique du fleuve Tejo a été conduite dans le cadre du contrat (CCE) n° B16-0245-P et aussi du contrat (CCE) n° B16-B-198-P. Ce document présente l'étude de l'interaction eau-producteur primaire, Selenastrum capricomufum Printz, algue planctonique unicellulaire, en présence du ¹³⁴Cs.

Les essais de contamination de la microalgue ont été conduits en utilisant des cultures en milieu confiné et en phase de stabilisation de la croissance. Quand la composition chimique de l'eau de Fratel n'est pas modifiée, sauf en ce qui est de l'addition du radioélément, le facteur de concentration évalué est de (1,6 ±0,2)10³ (rapporté au poids sec). Par ailleurs, quand il y a un enrichissement en plusieurs cations par l'addition d'un milieu nutritif, le facteur de concentration baisse à (1,5 ± 0,2)10². Dans tous les cas l'équilibre est atteint dans les premières 24 h.

L'étude de la rétention a été réalisée en repiquant des microalgues, préalablement contaminées, dans l'eau du fleuve non radioactive et en les maintenant soit en milieu confiné, soit en milieu renouvelé une seule fois au bout d'un jour, ou plusieurs fois au cours de l'expérience. Comme les résultats obtenus dans ces trois conditions sont semblables, un modèle général exprimant la variation du pourcentage de rétention a pu étre établi :

R (t) = 76,7 e^{-45,0 t} + 20,1 e^{-1,28 t} (t exprimé en jours)

ce modèle met en évidence l'existence de deux périodes biologiques, respectivement Tb1 = 0,015 jours et Tb2 = 0,54 jours.

Compte tenu de la dilution biologique due à la prolifération des algues unicellulaires au cours des expériences, la cinétique de désorption permet de faire l'hypothèse que le ¹³⁴Cs est d'une part adsorbé sur les surfaces et d'autre part absorbé dans les cellules.

EN :

For a better understanding of the radiocesium transfer in a freshwater ecosystem, a study concerning a simplified trophic chain from the Fratel dam, in the River Tejo, is being developed under the CEC contract n° B16-0245-P and also the CEC contract n° B16-B-198-P. In this paper is presented the study of the interaction water - primary producer (Selenastrum tapricornutum Printz, an unicellular planktonic algae) in the presence of ¹³⁴Cs.

The accumulation experiments were carried out in confined medium, during stabilization of the culture development. When the microalgae were cultivated in labelled river water, without any chemical change except for the addition of ¹³⁴Cs, the evaluated concentration factor was CF = (1,6 ± 0,2)10³, referred to dry weight. Otherwise, when the water was enriched by several cations by the addition of nutrients, the concentration factor was lower, CF = (1,6 ± 0,2)10³. In every case, equilibrium was reached within the first 24 h.

The retention was also carried out in river water and was studied through three parallel experiments : the labelled microalgae were maintained respectively in a confined medium, in a renewed medium with one single renewal made after 24 h's experiment, and in a renewed medium with renewals made at each sampling time. In every case, the ¹³⁴Cs loss obeyed a two-differential equation and the results obtained were similar, namely concerning the biological half-lives (Tb1 = 0.015 days in every case and Tb2 respectively 0.53, 0.42 and 0.54 days); therefore a total model could be deduced :

R (t) = 76,7 e^{-45,0 t} + 20,1 e^{-128 t} (t in days)

showing two biological half-lives, which were Tb1 = 0.015 and Tb2 = 0.54 days.

Taking into account the biological dilution due to the biomass increase during the experiments, the elimination kinetics suggests that ¹³⁴Cs is on the one hand adsorbed on the cell surface and on the other band is absorbed intracellularly.

FR :

Le manganèse est un élément minéral fréquemment rencontré dans différents types d'eau, surtout souterraines. Dans le cas des eaux de surface, le manganèse peut se trouver lors de pollutions accidentelles où lorsque la rivière reçoit des eaux d'un barrage réservoir en fin de vidange.

Des essais de traitement d'une pollution par du manganèse par filtration lente ont été effectués sur un pilote.

Les essais ont porté sur différentes concentrations de manganèse pendant des périodes de courte durée. La filtration lente semble très efficace pour l'élimination de ce composé, mais il faut noter que la nitrification biologique est altérée par la présence de fortes concentrations de manganèse.

EN :

Manganese is a mineral element which is often found in differing types of water. The manganese content is rather consistent, especially in underground waters. In surface waters, manganese can only be found as a result of an occidental pollution or in rivers supplied with water from a dam whose tank has been sewed.

This metal is not dangerous for humans but it can create problems in drinking water as it becomes progressively oxidized and thus precipitates in the water supply. This oxidation can produce direct effects like spots on linen or sanitary appliances, but also indirect effects, like important bacterial developments, as well as bacteria protection against biocides used as bactericidal or bacteriostatic agents.

This study aims at :

1) Underlining the possible effects of manganese in case of an accidental pollution.

2) Underlining how much time the system takes to react to an increased manganese content, given that ferric salts which permanently incluse manganese (500 to 600 ppm in the 40 % commercial solution) are used in the coagulation phase.

Amongst several methods to eliminate the manganese, the biological oxidation seems to be the most appropriate to this occasional pollution problem. Indeed, it filters are continually contaminated by manganese, the bacteria is likely to oxidize the divalent manganese, resulting in an immediate ceasing of the pollution, without human intervention. This is what the operation aims at.

The existing data regarding the biological elimination of manganese in underground waters shows it to be consistent. There is no such data on manganese elimination by slow sand filtration, in surface waters, in case of a short accidental pollution. As far as slow sand filtration is concerned, the available results only represent waters with consistent manganese contents.

This study has been carried out with a 50 m³/h pilote system, which was located in Paris surroundings. It was supplied with water from the Marne river and combined physicochemical and biological treatments. The physicochemical phase consisted of a coagulation-flocculation-decantation reaction in a pulsator, with ferric chloride including 500 to 600 ppm divalent manganese. Then, it was quickly filtered through sand, at a flow rate of 5 m/h. The water was subsequently poured into a storage tank in which it remained for around two hours. Then, it was filtered through slow sand filters, which biologically treated it. The filters contained 1 m high of sand and the filtration rate was 1 m/h, which allowed the development of a biological membrane on their surfaces.

Simulated manganese pollution tests have been realised as follows :

- water with two different manganese contents have been tested (0,5 and 1 mg/l),

- ammonium Ions have been simultaneously added to show a possible inhibition of nitrification.

Each test lasted around two weeks and time was spent as follows :

- polluting agents were continuously added during 3 days (which is the maximum duration of an accidental pollution).

These two tests gave the following results :

1st test (0,5 manganese/l) :

- 35 % of the manganese was eliminated during the coagulation-decantation-quick filtration phase.

- 100 % of the manganese was held bock by the slow sand filtration.

- An important amount of nitrites appeared in water at the end of the treatment, which suggested an inhibition of the nitrification, or a competition between manganese and ammoniacal oxidation.

- After the manganese pollution was stopped, no release of manganese and no inhibition of the nitrification were noticed.

2nd test (1 mg manganese/l) :

- 25 % of the manganese was eliminated during the coagulation-decantation-quick filtration phase.

- 97 % of the manganese was eliminated by slow sand filtration.

- In this case, the ammonium and nitrites were eliminated by the biological filtration, which suggested a readaptation of the bacterial flora to this kind of pollution.

The result showed chat the biological chain adapts itself very quickly to a rapid increase in water manganese content (no more than 2 hours). Indeed, the biological membrane, which was already contaminated by a permanent accumulation of manganese, reacted very quickly. The first test shows that nitrites appear and confirms the results that other searchers had already obtained before, in there is competition between nitrobacteria and the bacteria which oxidize manganese.

A less efficient elimination of organic matter also shows competition during the biodegradation phase.

In the second series of tests, a modification of the bacterial flora, due to the first manganese simulated pollution tests, is noticed.

These encouraging results allowed us to propose an original system for water treatment, based on a physicochemical treatment and biological slow treatments and, afterwards, a refining phase with combined ozone and granular actiated carbon.

In addition to its well-known properties, ozone allows to avoid the possible appearance of nitrites in water.