FR :

La réduction de la pollution organique des eaux usées, urbaines ou industrielles, fait le plus souvent appel à des procédés biologiques anaérobies ou aérobies. Dans ce dernier cas, les procédés à boues activées sont parmi les plus courants mais nécessitent de grands volumes de bassins

Les bioréacteurs à membranes présentent l'avantage de la compacité et permettent l'obtention d'un effluent de très bonne qualité, constante, même à très forte charge.

Le système étudié ici est un bioréacteur faisant corps avec le module de filtration utilisé, avec injection d'oxygène dans la boucle, supprimant ainsi tout bassin conventionnel. Une eau usée synthétique y est traitée en aérobiose sur membranes minérales à une température de 30 °C, soit par ultrafiltration à 500 Å, soit par microfiltration à 0,2 µm.

Le problème de l'encrassement des membranes est étudié et certains de ses mécanismes en présence d'une biomasse concentrée sont mis en évidence. L'encrassement est évitable en utilisant un décolmatage à flux inverse et en pratiquant des purges de boues régulières. La qualité de l'effluent obtenu en sortie est en tout point comparable ou supérieure à celle d'une station à boues activées travaillant en aération prolongée. La comparaison n'inclut pas les aspects économiques, et la nitrification est incomplète.

EN :

Urban or industrial waste water organic pollution is must often treated by aerobic biological systems. In this case, commonly used activated sludge plants need large tanks. Membrane bioreactors would give compactness and very high and constant quality effluent. Up to now, organic membranes have been mainly utilized for polishing effluents from conventional secondary treatment. MEMCOR process, using hollow fibers membranes, allows permeate fluxes of 100 l/h.m2 for several months under a 1 bar pressure. Chaize (1990), using inorganic membranes for urban waste water treatment, reached high organic and nitrogen removal. Biomass was about 10 to 20 g/l and permeate fluxes of 20 and 28 l/h.m2 at 1.1 and 1.4 bar transmembrane pressure with residuals less than 20 ppm COD and SSM and 5 ppm TKN.

Materials and methods :

The pilot studied here combined in the same module a compact bioreactor and a filtration membrane. A synthetic waste water has been treated by crossflow micro or ultrafiltration with aerobic microorganisms and pure oxygen injection. This effluent had a DCOIDBO ratio of 1.5 with very low suspended solid matters (38 mg/l). a-Alumina (pore size : 0.2 µm) or Zirconium oxide (pore size : 500 Å) inorganic membranes have been used. As membrane fouling is the main weakness of this processes, two treatments have been experimented with both membranes :

1. total biomass recycling without back flush system ;

2. excess biomass draining with back-flush system operating.

Experiments lasted continuously for 3 to 7 days.

Crossflow velocity was about 3.8 m/s, pH was regulated at 7.0 by concentrated sodium hydroxide addition.

Heat generated by pumps needed heat exchange with tap water in order to maintain temperature around 30 °C, wich was considered optimal for biological reactions.

Permeate COD and nitrogen, biomass Suspended Volatile Matters (SVM) and Suspended Solid Matters (SSM) have been monitored.

Results and discussion :

1) Effluent quality was constant and better than for conventional extended aeration. The two membranes retained the whole of SSM and bacteria, permeate COD was less than 30 mg/l, even under high load conditions (5 to 20 kg COD/kg SVM.d), elimination rates reached 98 % BOD and 97 % COD. Biomass production and oxygen consumption were lower than for high load activated sludge process, tact reported to pure oxygen utilization and unfloculated bacteria predominance. Nitrification remained very low due to short sludge age (< 90 h).

2) Membranes cleaning state at the beginning of the tests didn't modify stabilized permeate fluxes. Particulate fouling predominated with 0,2 µm membrane, problem solved using back-flush system, but fast slime fouling developed in the first 24 h. Electron microscopy membrane surface examination shown heavy fouling by a biofilm which real composition remains unknown, and by numerous small particles (site range from 0.04 to 0.3 mm).

With total biomass recycling, permeate fluxes were very low for both types of membranes : 15 l/h.m2 for 500 Å membrane and 2 l/h.m2 for 0.2 µm membrane. Treatment had to be stopped alter only 99 h. Biomass concentration increased up to 32 g/l in 3 days.

When using back flush system and regularly draining excess biomass, becter stabilized permeate fluxes could be observed : 35 l/h.m2 for 500 Å membrane and 24 l/h.m2 for 0.2 µm membrane. Mean biomass concentration was about 15 g/l.

Enzymatic digestion of slimes on fouled membranes pointed out the role of bacteria colt walls and proteins. These compounds were thought to be produced mainly under substrate limitation conditions. Fouling could thus be avoided by regularly draining excess biomass.

Technology of inorganic membranes is still recent and involves relatively high costs. However, as it may ha interesting for soma industrial effluents, tests are carried on to confirm chose results.

FR :

Les essais d'adsorption sur charbon actif en poudre, de sept acides humiques (H) ou fulviques (F) et pour trois pH (5,6 ; 7,0 ; 8,8) marquent que ces acides ont un comportement différent suivant leur origine.

Lorsque le pH de la suspension de charbon est basique, l'adsorption des différentes fractions humiques diminue, ce qui est en relation avec l'augmentation de leur solubilité. Pour un même pH de la suspension, les acides humiques et fulviques d'origine aqueuse s'adsorbent mieux que ceux d'origine commerciale ou que ceux extraits de sol.

La capacité maximale d'adsorption de l'atrazine sur charbon actif en poudre : 260 mg.g-1 (Langmuir), est plus fortement diminuée en présence d'acides fulviques que d'acides humiques d'une part, et plus inhibée (jusqu'à 40 %) par les acides fulviques extraits d'eau (F1, F2) que par ceux extraits de sol (F3) d'autre part.

Par contre, les acides humiques provenant de substances commerciales (H4, H5) influencent moins l'adsorption de l'atrazine.

L'ensemble des essais montre que l'origine et la nature des substances humiques joue un rôle prépondérant sur l'adsorption de l'atrazine sur charbon actif, le pH ayant une influence limitée.

Enfin l'étude de l'influence de l'atrazine sur l'adsorption des substances humiques sur charbon actif en poudre indique également une différence de comportement entre les acides fulviques, pour lesquels on note peu de variation, et les acides humiques, pour lesquels on note à la fois une promotion de l'adsorption et une meilleure affinité d'adsorption sur le charbon actif en poudre en présence d'atrazine.

Ce travail met en évidence l'importance du choix des substances humiques pour des essais d'évaluation des interactions substances humiques - pesticides lors de l'adsorption sur charbon actif en poudre.

EN :

Though the subject was copiously and oldly studied, this work is bringing a new contribution to the study of the influence of the origin and the nature upon their properties and in particular with atrazine, the more used herbicide these fast years.

In order to study this influence of the origin and nature of various humic substances upon the removal of atrazine by adsorption onto powdered activated carbon (PAC), humic substances were obtained from different sources :

- Natural aquatic fulvic (F1) and humic (H1) acids isolated from Beaune Lake (Limoges, France).

- Natural aquatic fulvic (F2) and humic (H2) acids extracted from hydroxyde sludges of the water treatment plant of La Bastide (Limoges, France).

- Soil fulvic acids (F3) extracted from a canadian top soil horizon.

- Commercial humic acids (Aldrich. H4 and Jansen H5) obtained from commercial sodium humates.

The results of adsorption test of humic acids, fulvic acids and atrazine (alone or together) onto powdered activated carbon (PAC) at three different pH (5,6 ; 7,0 ; 8,8) were expressed by Freundlich's and Langmuir’s models and permitted us to note that :

- According to the increasing of water solubility of humic substances with basic pH, their adsorption was better in acidic medium.

- Natural aquatic humic substances (H1, H2, F1, F2) were more adsorbed onto PAC than humic substances tram soil (H3) or commercial origines (H4, H5).

- Atrazine adsorption capacity was 260 mg.g-1 according to Langmuir's model.

- Atrazine adsorption decreased with F1 for all pH ; the same result was also observed with the other natural aquatic humic substances.

- Commercial humic acids didn’t importantly affect atrazine adsorption.

These differences appeared to be derived from the nature and the origin of fulvic and humic acids. The rose of atrazine on the humic substances adsorption showed a difference between humic and fulvic acids whose adsorption was not influenced by atrazine even humic acids only presented an increasing of adsorption.

All these results showed the importance of the selection of humic substances to evaluate the interaction SH-Pesticides during adsorption onto PAC and let us think to a competitive adsorption between fulvic acids and atrazine onto PAC.

FR :

Certains composés dissous ne sont pas stables une fois prélevés hors de leur milieu. Pour éviter que l'information ne se perde entre le prélèvement et l'analyse, il est nécessaire d'effectuer cette dernière in situ. La solution que nous présentons, consiste à développer une réaction colorimétrique en profondeur; la cellule photométrique est immergée et reliée à un spectrophotomètre en surface, par 2 fibres optiques (fig. 1a, b, c). Cependant, lors d'un essai préliminaire, nous avons observé que, dans le circuit de mélange de la sonde, les proportions entre réactif et échantillon ne sont pas constantes. Ces variations de débits sont corrigées par des mesures à deux longueurs d'onde (λ₁ et λ₂)* et par l'adjonction d'un colorant auxiliaire ne perturbant pas la réaction calorimétrique. L'étalonnage se fait directement sur la cellule photométrique : dans un diagramme Absorbance à λ₁ = f (Absorbance à λ₂) (fig. 2), on place une droite d'étalonnage et des points particuliers. Les règles de mélange sont vérifiées indépendamment de toute réaction chimique avec différentes solutions d'hélianthine dans un tampon à pH 7 et du rouge de chlorophénol à la place du réactif (fig. 4 et 5). En outre nous utilisons le rouge de chlorophénol, jaune sous forme acide, comme colorant auxiliaire pour le dosage du fer total dans un premier essai in situ (lac d'Aydat, Puy de Dôme, France). Les résultats sont comparés à ceux obtenus par prélèvements et analyses au laboratoire (fig. 6). L'accord est satisfaisant. L'incorporation au système présenté, d'une pompe osmotique devrait permettre, avec cet appareillage simple, des mesures pendant plusieurs mois sans intervention.

EN :

Various dissolved compounds are mot stables in surface conditions. We realized a prototype to collect chemical data related to redox sensitive species without any contact with the atmospheric oxygen.

The principle of this probe for in situ measurements is to produce colorimetric reaction in depth. A photometric cell and a horizontal coiled glass tube for fluid mixing are immersed and connected with two optic fibers to a spectrophotometer on boat (fig. la, b, c). Reagent is injected continuously from surface and sample is sucked up with a peristaltic pump through a tubular filter.

Every species which can be analysed by colorimetric method should be determined, in deep river or in lake, with this simple equipment.

Nevertheless, during preliminary trial, we detected a lack of reproducibility in the mixing ratio of the sample with the complexing agent. The problem is solved by adding an auxiliary dye with reagent and measuring optical densities at two different wavelengths (λ₁ and λ₂). In the system, with a the sample proportion, absorbance A at λ is expressed as :

A=ɛ_e∙l∙C_e∙α+ɛ_r∙l∙C_r∙(1-α)

We suggest to calibrate directly the cell of the probe and work in a calibration graph. It is built with first, marking on an A₁^λ1=f(A₂^λ2)graph (fig. 2), the « pivot » point (P) (when α= 0), second, plotting the « calibration curve »

A₂=A₁ (ɛ_e2/ɛ_e1)

(when α= 1), third, plotting the different S₁ (A_1i, A_2i) measured from standards. Therefore, if sample signal Re at two wavelengths is plotted in this graph, by joining P and Re, the straight line intersects with calibration curve at C. On this curve, interpolation of C between two standards determine the concentration of the analyte.

Experimental verification of the mixing rule has been clone independently of chemical reaction, with different heliantine solutions in pH 7 buffer as samples and chlorophenol red as reagent, bath in laboratory and at 15 meters depth (Beffes lake, France) (fig. 4 and 5).

Furthermore, chlorophenol red, previously tested, is used as auxiliary dye for total iron measurement, in Aydat lake (Puy de Dôme, France) for a first in situ trial.

Results are compared to those got from oceanographic bottle sampling and laboratory analysis (fig. 6). Data from the probe are in good agreement with data from the laboratory method.

Next development of this chemical sensor will consist in adding to the system an osmotic pump which should allow measurements without intervention during several months.

FR :

Les effluents aqueux pollués par des matières organiques provenant d'industries chimiques présentent souvent une faible biodégradabilité. Dans certains domaines de concentration (DCO = 0,5 - 15 g/l), le procédé WPO® développé au laboratoire se substitue avantageusement à l'incinération pour traiter ce type d'effluents. La réaction, qui met en œuvre le réactif de Fenton à température élevée, conduit parfois à la formation de quantités importantes d'acides carboxyliques légers. Nous avons donc développé des systèmes catalytiques originaux remplaçant les sels de fer et conduisant à une oxydation totale des acides carboxyliques. Le système le plus efficace constitué de sels de fer, de cuivre et de manganèse permet d'obtenir, en 1 h à 100 °C, l'oxydation totale d'un mélange synthétique de ces acides (COT = 5 g/l) avec 1,5 fois la quantité de peroxyde théoriquement nécessaire à l'oxydation. Le catalyseur précipité et séparé en fin de traitement peut être recyclé et conserve la môme activité. Les unités industrielles permettant d'effectuer le traitement WPO® avec les nouveaux catalyseurs, recyclés ou non, seront similaires à celle déjà réalisée pour le traitement de « points noirs » industriels.

EN :

There is an important concern about the problems occuring with wastes elimination, specially the industrial liquid wastes. Te face the problem of organic aqueous wastes coming front various branches of industry, the WPO® (wet peroxide oxidation) process was developed at the laboratory. In the WAO process (wet air oxidation), which uses gaseous oxygen, the limiting step is usually oxygen transfer. In this new process, this problem is suppressed by using a liquid oxidising agent (hydrogen peroxide). This process is adapted from the classical Fenton's reaction and iron salts are used as the catalyst in order to promote the formation of ^•OH radicles which are the main active species. But the reaction is carried out at about 120 °C; so, a very significant TOC (total organic carton) removal efficiency is obtained (60 to 90 %) in comparison with the low efficiency of the classical Fenton's reagent (typically 25 % at room temperature).

Significant amounts of free fatty acids are formed during the reaction. They are namely oxalic, malonic, succinic and acetic acids, which are common by products obtained during audition of most industrial organic pollutants. In order to comply with the regulations requirements, it was necessary to improve the efficiency of the original process. It was also very important to obtain an efficient elimination at a temperature not greater than 100 °C in order to avoid to pressurize the treatment reactor. This could be obtained by using new catalysts which are described in this paper.

Because of the related field, precious metals like Pt and potentially toxic ones like Cr were not considered. One needs a treatment process as cheap and as reliable as possible. So, only Fe, Cu, Co, Ni and Mn were used as salts in order to test their calalytic activity in the treatment by hydrogen peroxide of a synthetical mixture of oxalic, malonic, succinic and acetic acids (O, M, S, A). The experimental device is a stirred tank reactor where the organics and the catalyst are batch loaded. It is continuously fed, for 1 hour, with hydrogen peroxide. The total amount injected is 1.5 the stoechiometric amount. In table 1, it can be seen that any metal has a satisfactory activity when used alone (TOC removal efficiency cannot exceed 22 %). In table 2, it is clear that, in soma cases, the association of two or three metals with each other can lead to very important synergetic effects. When using a mixture of Fe, Cu and Mn, the removal efficiency can increase to 91 %. This Fe/Cu/Mn catalyst is studied with further details in table 4. It appears to have its best efficiency at about 100 °C because of a parasitic decomposition of the peroxide at higher temperatures. For an organic mixture coutaining 5 g TOC/l, 100 ppm of each metal is a convenient concentration. This new catalyst still needs an acidic pH, from 3 to about 5, but the dependency is not so strict than with Fe alone (original process) which needs a value from 3 to 3.5. In addition, it was observed that the treatment time could be easily reduced (down to 45 minutes) as well as the amount of peroxide injected.

Very similar results have been obtained with synthetic solutions of pollutants and with real industrial ones, thus establishing the ability of the Fe/Cu/Mn mixture to catalyse the oxidation of a large variety of species and not only carboxilic acids. The difference between the efficiency of this new catalyst and the conventional one is shown in table 5. Figures 1 and 2 are related to an Industrial WPO® unit which is commonly used with the conventional catalyst (Fe). It has been possible to improve its efficiency by using the new one without any significant modification. The Fe/Cu/Mn catalyst can be easily separated alter reaction (coprecipitation effect). Thus, the treated water meets the regulation requirements and the recovered catalyst can be easily resolubilized and recycled.

FR :

Dans le cadre du Plan d'Action Saint-Laurent, une méthodologie détaillée d'analyse de la contamination par tronçon faisant appel à la modélisation numérique a été développée. Une méthode de simulation utilisant le mouvement aléatoire de particules a servi à élaborer le logiciel PANACHE. Les concentrations sont obtenues en post-traitement en attribuant une masse de contaminant aux particules du modèle. Les champs de vitesses servant à calculer leurs mouvements sont produits à l'aide d'un modèle bidimensionnel aux éléments finis. Une nouvelle approche pour l'analyse de la contamination est proposée. Celle-ci s'inspire de la méthodologie de modélisation des micro-habitats populaire dans le domaine de l'hydrobiologie. Le résultat apparaît sous la forme d'Aires Pondérées Inutilisables (API), c'est-à-dire, des surfaces où certains critères de qualité de l'eau ne sont pas respectés dans les zones de mélange. Ce système Informatisé a été élaboré sur une plate-forme INTEL/386-486 - OS2/PM.

EN :

Context

The St-Lawrence Center, part of Environment Canada, undertook a few years ago the very ambitious project of studying the toxic contamination of the St-Lawrence River. In collaboration with the Institut National de la Recherche Scientifique - Eau, a sub-project based on numerical modeling was defined in order to analyze contaminant propagation from industrial and municipal effluents into the river system.

Goals

The specific goals of the project were the following :

1) to provide a precise quantification of contaminant concentrations in the effluent plume al a convenient scale;

2) to analyze areas influenced by main tributaries and different water masses entering the river reach;

3) to map and quantify areas as compared to water quality criteria ;

4) to provide a method to select relevant hydrological events as a significant part of the analysis framework

Methodology

Some basic choices were made at the beginning of the project :

1) the analysis framework emphasis the instream water quality instead of the effluent water quality;

2) numerical modelling was the main tool used to evaluate the water quality;

3) as far as possible references to public regulations were incorporated;

4) a strong complementarity of different computer tools was favoured : Geographical Information Systems, Database management systems, simulation models;

5) the numerical solution method for the transport diffusion model is typically Lagrangian : the Random Walk Method;

6) the contamination analysis uses the so-called « Weighted Unusable Area » method to quantify areas that do not respect some water quality criteria.

A typical contamination analysis project based on numerical modelling includes the following steps (fig 2) :

1) a preliminary study to determine the main characteristics of the problem and to choose the best strategy to analyze it;

2) field measurements essential to the calibration and validation of the computer model;

3) hydrodynamic modelling provides the basic data on the flow field; this step includes the calibration and the validation of the model, as well as the prediction of the flow fields corresponding to well-defined and contamination relevant hydrological events;

4) hydrological analysis identifies the relevant flow events chat will further be used in the mode) prediction ; this approach allows standardization of this very important input data set and avoids arbitrary choices of flow field;

5) transport-diffusion modelling constitutes the main step; it provides the chemical species concentrations downstream from the effluent discharge and affords an estimate of the overall water quality of the reach, as influenced by the main tributaries. This step includes the calibration and the validation of the model which precedes the prediction exercise;

6) contamination analysis necessitates the choice of appropriate and relevant water quality criteria ; we propose a new approach, inspired by the Instream Flow Incremental Methodology often used to define the quality and availability of fish habitat in river reaches, to implement this step.

Numerical methods

As previously mentioned, the project included the development of a Lagrangian model to simulate the transport of solutes in a two-dimensional steady-state river flow. We will emphasize this point. The main objective of the software development was to provide an efficient and user-friendly management tool for the public agencies. Many analytical test cases helped in the choice of the best numerical algorithms, non-physical related parameters, and in the validation of the computer code. Furthermore, the results of two dye tracing experiments performed in conjunction with airborne remote sensing techniques provided data to validate the model on the St-Lawrence River (fig. 5, 6, land 8 illustrate different simulation results corresponding to the different tasks mentioned previously). In the next paragraphs, we will summerize the basic mathematical and numerical concepts implemented in the simulations.

To simulate solute transport in water media (porous or free surface), one usually uses eulerian methods which lead directly to concentration values. The solution algorithm presented here is rather based on a Lagrangian method which offers an explicit control over the additional numerical diffusion associated with every discretization method. This approach, also called the Random Walk Method (illustrated in fig. 3), or Particle Tracking Method, is more and more often used to solve hyperbolic equations. So far, the literature does not provide many applications of this method to solute transport in free surface flow. Oil spin modeling is a domain where many applications have been reported.

The propagation of solute matter in free surface flow is mathematically described with momentum, mass and solute conservation equations. Since the Random Walk solution method of the transport-diffusion equation (equ. 1) requires hydrodynamic data to calculate the mean transport on streamlines along with dispersion, independent simulations providing the necessary flow field data (velocities, diffusivities, depths) have to be performed before undertaking the transport-diffusion tasks. For this purpose, the Navier-Stokes shallow water equations have become a well known tool to represent flow field in shallow waters. However, one should be aware of some often neglected but important aspects of such models, such as moving boundaries and turbulence closure.

Solution techniques

Two main goals were kept in mind during the implementation of the various algorithms : precision of results and fast computation. The following choices were made to achieve these objectives :

1) A finite element discretization and solution method provides and carries hydrodynamic Information, but particles are tracked on a finite-difference grid (mixed discretization principle).

2) The convective component of the movement is realized by moving the grid instead of the particles (shifted grid principle).

3) Computation of concentrations optimizes smoothing while minimizing artificial diffusion (controlled effusive smoothing principle).

4) When a section of the plume is described in a steady state « regime », it is mot necessary to continue the simulation on that section to proceed downstream ; the simulation is divided in almost independent sections (convolution principle).

5) The particles have an a priori nondimensional weight and a unit concentration is calculated from these (unit plume principle).

6) The real concentration is linearly dependent on the pollutant loads introduced into the milieu (linearity principle).

The Weighted Unusable Area Method

The Weighted Unusable Area method provides a convenient means to compare effluent plume water quality to water quality criteria as well as to quantify areas that do not comply to them. A comparable method is widely used to define the quality and availability of fish habitat downstream from regulation reservoirs, with the purpose of establishing minimum guaranteed flow discharge to protect target species (the Instream Flow Incremental Methodology : IFIM). The method consists essentially of computing areas within the analysis domain weighted by a certain factor that represents the exceedence of certain water quality, criteria. Among different options to define the weighting factor, all incorporating the effective contaminant concentration, we defined the following :

1) the ratio of the concentration to the water quality criterion without consideration of exceedence or compliance;

2) weighting factor equal to 1 only if the concentration exceeds the criterion (non-compliance);

3) option #1, but using the concentration results corresponding only to the effluent plumes excluding the ambient water quality of the reach ; this emphasizes individual corporate responsibility (proposed for implementation);

4) option 11, but with the ratio increased by a power « n », a procedure that emphasizes the non-linear increase of toxicity related to the exceedence of the criterion (could be useful for academic purposes).

We also propose a Global Weighted Unusable Area concept to combine all the different chemical species present in an effluent plume. The combination is made possible using the specific criterion corresponding to each species. This procedure leads to a new state variable that represents Contamination Standard Units.

FR :

Les recherches de kystes de protozoaires et oeufs d'helminthes ont été effectuées sur les eaux brutes et épurées provenant de cinq stations d'épuration de type biologique (boues activées, lagunage naturel). En eaux d'entrée, tous les échantillons contiennent des kystes de protozoaires notamment celui de Giardia Intestinalis à des concentrations de 3.10¹à 1,7 10³ kystes. l^-1. De façon moins fréquente, on rencontre des œufs d'helminthes à des concentrations moyennes de 6 à 5.10¹ oeufs l^-1. En eau épurée, on trouve des rendements d'élimination variant de 70 à 100 % pour les kystes de protozoaires. Par contre 100 % des échantillons de toutes les stations ne renferment plus d'oeufs d'helminthes. On peut donc constater l'efficacité des différents types de traitements biologiques vis-à-vis les éléments parasitaires appartenant aux familles des Nématodes et Cestodes.

EN :

When treated wastewater is reused for agricultural purposes in Tunisia, particular attention is accorded to the sanitation aspects. The current water quality standard requires an average number inferior or equal to one intestinal helminth egg per litre of treated wastewater. Information concerning parasitic risk is however rather limited in Tunisia.

In this context, protozoan cysts and helminth eggs have been monitored in the raw sewage and treated wastewater from flue treatment plants ; two of these plants use the activated sludge process and the other three employ natural lagoons. Protozoan cysts are predominant in the inlet water for all treatment plants, with frequencies of positive samples ranging tram 90 to 100 %. The average number of cysts varies according to the sample's origin. Both in the north and the south of Tunis, the average ranges from 3.10³ to 8.10² cysts. l^-1, whereas in Gafsa, located in the south-east part of the country, a very high average of about 2.7 x 10³ cysts. l^-1 is recorded.

The ldentified protozoa include Amebae and Flagellates. Among the Amebae, Entamoeba coli and Entamoeba histolytica cysts are detected ; among the Flagellates, Giardia intestinalis cysts are found. As a general rule, this latter predominates, in concentrations from 3.10¹ to 1.7 x 10³ cysts. Among the helminths, Nematods and Cestods are observed.

The average number of helminth eggs is markedly lower and varies according to the populations considered. In the North of Tunis, the averages recorded in the inlet water of treatment plants vary from 20 to 35 eggs. l^-1. On the other hand, in the Southern suburb the average is around 6 eggs. l^-1. In the Southern part of the country, the average recorded in Gafsa water is quite high (50 eggs. l^-1).

In treated wastewater, the parasitical load decreases considerably. Whatever the treatment technique employed, no helminth eggs are detected. However, the removal of protozoan cysts varies according to the water initial load and the kind of water treatment employed. Lagoon systems are efficient : 98 % removal is achieved at the Radés treatment plant, 100 % at the Gammarth plant. In the worst case, at the Gafsa treatment plant where the loading in the inlet water is very important, an average 81 % removal is recorded. Concerning systems using activated sludge treatment, the removal rates of the Choutrana and Charguia treatment plants are 73 % and 98 % respectively.

The effluents of the studied treatment plants thus conform to the standards fixed by the current Tunisien legislation. However, the systems are not always efficient with respect to protozoa cysts, the size of which is markedly less than that of helminth eggs. Treated wastewaters remain partly contaminated with these cysls.