Volume 10, Number 2, 1997

FR:

L'étude de la réponse hydrologique de deux bassins versants de l'agglomération de Bordeaux en France a montré que les pertes initiales au ruissellement sur les surfaces imperméables étaient responsables des écarts entre le volume ruisselé et le volume prévu proportionnel à la lame d'eau tombée sur un bassin versant. Les pertes initiales, qui n'excèdent pas 2 à 3 mm, dépendent essentiellement de l'état de saturation des surfaces imperméables au début de la pluie. Cet état initial des surfaces imperméables dépend lui-même des antécédents pluvieux, notamment des conditions hydrologiques et météorologiques depuis la dernière pluie qui précède l'événement pluvieux considéré. Afin d'estimer quantitativement les pertes au ruissellement au cours d'une pluie, un modèle d'évaporation nommé EVA a été développé. Les données météorologiques sont utilisées afin d'évaluer, à partir d'un bilan énergétique simplifié entre l'eau et l'air, la lame d'eau évaporée des surfaces imperméables entre deux pluies successives. Après une pluie, il faut de un à trois jours selon la saison pour que l'eau stockée dans les dépressions de surface soit totalement évaporée, sur les bassins testés.

Le modèle a été testé avec les mesures disponibles sur deux bassins versants urbains de la région bordelaise dont la surface totale n'excède pas 6 hectares. Quantitativement, on montre qu'il est possible de prédire les pertes au ruissellement avec une précision de 0,5 mm dans 65 % des cas étudiés. Les 35% d'épisodes où l'on se heurte à des difficultés sont des séquences de faibles épisodes pluvieux séparés par quelques heures et n'excédant pas 3,0 mm. La modélisation du remplissage partiel des dépressions de surface des terrains imperméables est alors trop sommaire.

EN:

The subject of this article is runoff losses in an urban environment, specifically initial losses through impermeable surface depressions directly connected to the network. For this purpose the hydrological behavior of two urban watersheds (Batany and Trianon) of about 5 hectares each, in the Bordeaux region of France, have been studied to observe that the fluctuations around the "Rainfall Amount versus Runoff Volume" law essentially derive from initial runoff losses which differ from one rainfall event to another. The fluctuations around this law make it impossible to precisely estimate runoff volume based on rainfall. Improved knowledge of initial losses would result in better estimation of the runoff volume of more regular (monthly or bi-monthly) rainfall events, which must increasingly be treated at water treatment plants in order to be able to better control the overflows. If the involvement of permeable surfaces is assumed to be negligible, we can postulate that the initial losses from a given rainfall event are directly linked to the water stored on the impervious surfaces connected to the network. The moisture content prior to the event is dependent upon the occurrence of a previous rainfall event and on the meteorological conditions prevailing between the previous rainfall event and the one under study.

A model, called the EVA model, has been developed with the objective of predicting runoff losses corresponding to a rainfall event as a function of the previously prevailing watershed moisture conditions. The model evaluates the amount of evaporation from the water contained in the surface depressions between two successive rainfall events, called the initial rainfall event and the final rainfall event. The initial rainfall event represents the previous rainfall event, and the final rainfall event is the event for which the losses are to be estimated. The application of the model requires a very good record of the small rainfall events which have occurred during the modelling periods, and which are called the intermediate rainfall events. In practice, this constraint implies the need to monitor the dispersed rainfall events which, even if they cause only very light runoff, nonetheless contribute to a partial filling of the surface depressions present on the impervious terrain.

The equations used in the model are those which correspond to the energy balance between air and water using the net radiation, the latent, sensible and storage heat (the soil heat flux is considered negligible). The EVA model uses meteorological data such as the air temperature, relative humidity, wind speed and solar radiation. The model evaluates two variables: water depth and water temperature. Since water depth after evaporation is known, the losses of a rainfall event can be estimated by subtracting the total volume of water which has evaporated during the dry period from a maximum value of the losses. The modeled losses are then compared with the measured losses. In order to simplify the resolution of the problem, the total water volume contained in the thousands of surface depressions present in the watershed is considered to be contained in a single depression called a representative depression. This representative depression can take different forms and have different initial heights, which have been tested while the work was in progress.

The model is found to be coherent in terms of the variations in water depth in the surface depressions. The total water volume contained in the surface depressions takes from 1 to 3 days to evaporate depending on the season. The variation in water depth is caused by differences in evaporation rates occurring during the months close to the summer solstice and during the cooler months.

The first version of the model was created in 1992 and tested on two watersheds of about 250 hectares each in the Paris area. The model was modified and the new results were compared with the measurements obtained in two watersheds in the city of Bordeaux. The performance of the model was evaluated for 17 rainfall events, of which 10 were in the Batany watershed and 7 in the Trianon basin. The model accurately predicts the losses corresponding to a rainfall event within 0.5 mm, in two cases out of three. The problem encountered in the remaining one-third of the cases was essentially that it is difficult to account for runoff during intermediate rainfall events because of very low flow rates and small rainfall depth measurements.

Experimental equipment installed in two watersheds in Bordeaux has made it possible to obtain relatively precise pluviometric and discharge measurements. However, there is uncertainty concerning these measurements, which is inherent in this field, when it comes to validating a model like EVA because such a model is used for regular rainfall events for which the initial losses directly influence the runoff volume.

However the a priori knowledge of initial runoff losses should enable better use of a model which, for example, assumes that the runoff coefficient increases progressively at the beginning of the rainfall event. The validation of such a model was attempted while the work was under way, but ran into the difficulty of selecting a set of rainfall events characterized by constant rainfall intensity.

The development of a model like EVA requires rainfall and flow measurements which are free of uncertainties and accessible within experimental watersheds which are perfectly monitored and where the measurement uncertainties are the same for all observable measurement ranges (in particular for the discharge measurements). These requirements currently constitute a technical barrier in terms of measurement that will be difficult to surmount. However, the work carried out in this research hints at the possible improvement of the classical hydrological models used in urban hydrology, particularly those used for forecasting the runoff volumes of regular rainfall events.

FR:

Dans la présente étude, nous montrons l'évolution de l'eau dans un réseau de distribution. Ce réseau a été construit dans les années 70 à l'aide de conduites d'acier ou de fonte ductile munies d'un revêtement intérieur de ciment. Il est alimenté par des eaux de très bonne qualité qui circulent d'abord dans une suite de conduites maîtresses (de 2 700 mm à 900 mm) jusqu'à un secteur de petites conduites maillées (200 et 300 mm). Des échantillons d'eau ont été prélevés à 14 reprises durant une année, le long de la conduite maîtresse (le temps de séjour y varie de 0 h à 13,6 h) et dans le secteur de petites conduites (de 13,6 h à 18,4 h). Nous avons mesuré la température, le pH, plusieurs sous-produits de désinfection dont les trihalométhanes et les aldéhydes, le carbone organique total et biodégradable ainsi que les concentrations de bactéries hétérotrophes aérobies et anaérobies facultatives (BHA) et les comptes directs totaux mesurés en épifluorescence.

Le réseau a très peu d'effet sur l'évolution des sous-produits de désinfection. En effet, les résultats obtenus à partir des échantillons témoins (eaux traitées conservées dans un flacon propre à la même température que dans le réseau) sont semblables à ceux obtenus à partir des eaux prélevées dans le réseau de distribution.

Les concentrations de bactéries ainsi que les concentrations de CODB sont assez stables dans le réseau. Il est intéressant de noter qu'il y a de 0,2 à 0,45 mg/l de CODB, ce qui est supérieur à la concentration minimale de 0,15 mg/l requise pour la croissance des bactéries. Par contre, le chlore résiduel libre est toujours supérieur à 0,20 mg/l. Une seule exception, le 11 juillet 1994, dans le secteur de petites conduites, le chlore résiduel a baissé jusqu'à 0,16 mg/l. Durant cette journée, nous avons observé une légère augmentation des comptes directs totaux mesurés en épifluorescence.

EN:

Studies were performed to follow the changes in water quality along a distribution system. The distribution system under study is divided into two parts: main pipes starting from the plant (from 2 700 mm to 900 mm diameter) feeding a small pipes sector (200 mm and 300 mm diameter). Residence times range from 0 to 13.6 hours in main pipes and from 13.6 h to 18.4 h in small pipes. All pipes are made of steel or ductile iron and their interior is lined with concrete; there is therefore little corrosion. Since the raw water is of such high quality, the treatment plant is very simple: a direct filtration on sand followed by ozonation and chlorine disinfection. Samples were taken on 14 occasions, during a full year period, in the distribution system and in the treatment plant after chlorination. These last samples were incubated in clean flasks at the network temperature. Measurements of temperature, pH, disinfection by-products (trihalomethanes, aldehydes, haloacetonitriles, haloacetones and chloropicrine), total organic carbon (TOC), biodegradable dissolved organic carbon (BDOC) and number of bacteria (heterotrophic plate count and total direct count by epifluorescence) were completed.

The distribution network seems to have very little effect on chlorine demand and disinfection by-products. Results from water incubated in flasks are similar to those from distribution network. After more than 18 hours contact time, the chlorine residual is still higher than 0.2 mg/l in most of the samples taken in the distribution network and in the flask. It should be noted that the initial chlorine concentrations range from 0.65 mg/l (cold water) to 1.00 mg/l (warm water). Chlorine demand and trihalomethane (THM) curves are typical, a rapid increase with time followed by a relatively stable level. THM concentrations in the distribution network are low: a typical value of 14 µg/l after 13.6 hours contact time is detected. The maximum concentration of 43 µg/l of total THM was measured in a dead end. In this latter sample, 42 % of the THM was present as bromodichloromethane and 39 % as chloroform. Other by-products such as haloacetonitrile, haloacetone and chloropicrine were always detected in very small concentrations.

Aldehyde concentrations in treated water were low, between 21 and 42 µg/l. These concentrations were stable throughout the distribution system. Fixed and free biomass seems to have very little effect on these biodegradable compounds. These results were confirmed by BDOC results. BDOC in treated water ranges from 0.2 to 0.45 mg/l and remains stable in the distribution system. These low BDOC concentrations and the chlorine residual of approximately 0.2 mg/l seem to be sufficient to prevent regrowth in the distribution system. Total direct counts by epifluorescence showed almost no increase of bacterial density except for the July 11th sample. This is the only day where the free chlorine residual dropped below 0.2 mg/l, with values of free chlorine residual dropping as low as 0.16 mg/l.

FR:

L'usage quasi systématique de fertilisants sur de grandes surfaces a conduit la majorité des aquifères superficiels à un grave niveau de contamination par les nitrates. Des essais de gestion environnementale de cette problématique agricole sont conduits à l'échelle du bassin versant afin d'estimer les flux de nitrates percolant vers la nappe. La présente étude reprend les résultats issus de la modélisation d'un bassin versant dans le but d'appréhender l'évolution de la concentration en nitrates dans les eaux de la nappe. L'importance des conditions hydrogéologiques dans les relations entre zones non saturée et saturée a été mise en évidence par la comparaison des concentrations calculées dans la zone non saturée et observées dans la nappe. En règle générale, les concentrations sont très semblables pour les zones proches des limites amont du bassin, et se différencient de plus en plus vers l'aval du système. Une dilution semble se produire entre les flux percolant des différentes zones non saturées et les flux d'eau et de nitrates s'écoulant dans l'aquifère. Afin de tester cette hypothèse, un modèle de dilution basé sur les flux d'eau et de nitrates dans les zones non saturée et saturée est développé. Appliqué sur l'axe d'écoulement principal du système, le modèle de dilution permet de reproduire adéquatement les concentrations observées dans la nappe à partir de celles calculées dans le sol avec une erreur maximale variant de 1 à 22%. Le couplage d'un modèle environnemental pour la zone racinaire du sol avec un modèle de dilution simple peut permettre le calcul des concentrations en nitrates dans la zone saturée. Toutefois, la prise en compte des conditions hydrogéologiques du système est nécessaire à un calcul de dilution efficace basé sur les valeurs des flux de percolation.

EN:

Pesticides and nitrates represent the main sources of aquifer contamination in agricultural zones. In many regions, nitrate concentration levels reach and exceed the water quality criteria (50 mg NO3/L). The increasing use of mineral fertilizers (which has doubled during the 20 last years) and the intensive exploitation of the aquifers for crop irrigation (1,1 million ha in France) have led to groundwater contamination by nitrates. The dynamics (long-term persistence) and extensiveness (regional contamination) of this contamination make it a sensitive environmental issue. Comprehensive environmental management is needed in order to limit the increase of the concentration levels and to reduce the extent of the contaminated areas. During the last few years, research has been done in the field of watershed management, from laboratory experiments to field investigations. At the same time, numerous simulation models have been developed at different investigation scales. Banton et al. (1993) developed a model specifically devoted to environmental management. Their model, AgriFlux, is based on a mechanistic approach to the processes, using a stochastic method that takes into account the spatial variability of the parameters. AgriFlux calculates the nitrate concentrations as well as the water fluxes in the unsaturated zone. The concentrations in the unsaturated zone (obtained by modeling or measurement) are generally dissimilar to those observed in the saturated zone (i.e. in the aquifer) because the infiltration water is diluted in the aquifer water. This difference indicates that the concentrations in the unsaturated zone cannot be used to accurately evaluate the actual risk of groundwater contamination. Hydrogeological conditions such as the recharge limits, the flow direction and the flow rate should be incorporated into the evaluation. In this paper, the modeling results obtained previously (Dupuy et al., 1997) with AgriFlux for the La Jannerie watershed are used to determine the concentrations in the aquifer and to compare them with the concentrations measured in the observation wells. This watershed (160 ha) is used exclusively for agriculture. The fractured carbonate strata (Superior Oxfordian) constitute a phreatic aquifer with a vertical extension of about 20 m. First, the temporal evolution of the annual mean concentrations in the aquifer is compared with the evolution of the annual precipitation. The results show that the mean concentrations tend to follow precipitation levels. However, the differences observed at different locations in the watershed cannot be explained by these results. The spatial evolution of the concentrations from the upstream to the downstream part of the aquifer was studied in order to explain the concentration distribution in the watershed. On the main flow line, the concentrations observed from 1985 to 1989 show a decrease from the P7 well (upstream) to the P26 well (downstream). This phenomenon can be attributed to two factors. First, denitrification may occur in the aquifer during flow. However, it is recognized in literature that the denitrification rate is usually low and a long period of time is required to obtain a significant decrease in the nitrate level. The observed attenuation cannot be imputed to this factor alone. The second possible cause is related to the dilution of the nitrates in the water contained in the aquifer.

In order to test this hypothesis, a dilution model was elaborated using the watershed division as indicated in Dupuy et al. (1997). In each area, the resulting concentration is obtained by diluting the fluxes of water and nitrate leaching in the unsaturated zone in the fluxes of water and nitrates flowing from the upstream area. The concentrations in the aquifer are calculated from upstream areas to downstream areas for the period between 1985 and 1989. The pattern of the concentration curves obtained in this manner agrees with the trend measured in the different wells. The results clearly show a decrease of the concentration in the aquifer water leached from the unsaturated zone. For the downstream area (well P26), the calculated concentrations are higher than the observed ones. This difference could be due to the fact that the lateral fluxes (flow convergence into the median part) are not taken into account and the concentrations may thus be overestimated. However, the mean resulting error (12%) remains low considering the lack of knowledge of the aquifer characteristics. It is therefore possible to accurately estimate the nitrate concentrations in the saturated zone from the concentrations simulated in the unsaturated zone using a simple dilution model. However, this method is only valid for simple hydrogeological conditions.

FR:

Le but de cet article est la recherche de liaisons entre les précipitations extrêmes de pas de temps de 1 à 24 heures dans les Alpes Françaises. En particulier, il semble important de pouvoir déduire les valeurs pour de faibles pas de temps (1h, 2h... ) de celles de forts pas de temps, 24h en particulier. En effet, nous disposons actuellement de peu d'enregistrements historiques à pas de temps fin. En fait, le réseau de pluviographes utilisé est constitué de seulement 65 stations. Par contre, l'existence d'un réseau très dense de pluviomètres permet de déterminer les caractéristiques de pas de temps 24h.

Pour ce faire, nous définissons une variable traduisant l'évolution des précipitations en fonction du temps de retour pour chaque pas de temps et chaque station : le gradex. Nous avons testé plusieurs types de relations pour lier les gradex des différents pas de temps entre eux : relation linéaire, puissance, exponentielle, logarithmique ; c'est la relation linéaire qui est la meilleure dans les Alpes Françaises. L'étude des relations entre les gradex des différents pas de temps montre que les pas de temps voisins sont bien corrélés entre eux, ce qui n'est plus le cas lorsque les pas de temps deviennent très distincts. Ces résultats sont confirmés par la définition de 4 régions homogènes par rapport aux précipitations extrêmes sur lesquelles nous testons l'éventualité de relations linéaires entre les gradex des différents pas de temps.

Finalement, nous avons mis en évidence l'absence de relations simples permettant de passer de pas de temps longs à des pas de temps faibles. Par contre, on peut passer sans trop d'erreur d'un pas de temps de 24 heures à celui de 12 heures ou 6 heures, résultat déjà fort intéressant.

EN:

For many development projects, it is important to have some idea of the magnitude of extreme precipitation events that may occur for different probability levels and for time steps of less than 24 hours. Unfortunately, most existing rain gauge networks measure precipitation on only a daily basis.

In the French Alps, 65 rain gauge stations provide precipitation data over short time steps (1 to 24 hours). This very diverse network, managed jointly by the French electrical utility (Electricité de France), the national weather office (Météorologie Nationale) and the regional water resources service (SRAE), provides a valuable basis for investigating possible relationships between the characteristics of extreme precipitation for 24-hour periods and those for shorter time periods. The results of such a study, although of course valid only for the investigated area, should provide an indication of whether or not it is possible to calculate the characteristics of rainfall over short time steps from much denser 24-hour rain gauge networks.

A statistical analysis was carried out to estimate extreme rainfall values for return periods of 2, 5, 10, 20, 50 and 100 years and for time steps of 1, 2, 3, 6, 12 and 24 hours. Each station is therefore associated with 36 precipitation values as a function of return period and duration. A variable referred to as the gradex (gradient of the exponential) is defined, reflecting the change in precipitation values as a function of the return period for each time step and each station. The definition of this variable is based on the fact that Gumbel's law is used to represent the frequency distribution of extreme rainfalls over time intervals extending from 1 hour up to several days, which is equivalent to assuming an exponentially decreasing frequency distribution for extreme rainfalls for a given time step and a given location. When plotted on Gumbel paper, the right-hand part of this distribution has a slope equal to the parameter "a" of Gumbel's law:

F(x)=exp{-exp{-(x-x>indice>0/a}}

where F(x) is the probability of occurrence of a value less than x. The parameter "a" is the gradex, and has the same dimensions as x. It can be determined with the method of moments :

a(t)=0.78xσ_x

where σ_xis the standard deviation of the sample.

This definition is equivalent to taking the slope of the line passing through the points corresponding to T=20 and 100 years on a Gumbel plot. For each of the stations, we can evaluate six gradex values, i.e. one for each time step. In this way, for each of the 65 stations and for each time step, we obtain the gradex values and estimated precipitation values for return periods from 2 to 100 years.

Several types of curves were tested in order to determine possible relationships among the gradex values for different time steps, including linear, power law, exponential and logarithmic relationships. For the French Alps, the best fit was obtained with a linear relationship and we calculated the corresponding correlation coefficients. We found that the gradex values were well correlated for adjacent time steps, but not for those that were very different. In particular, it would appear to be impossible to deduce gradex values for very small time steps (1 to 6 hours) from the 24-hour gradex. The 24-hour gradex accounts for only 17% of the variance of the 1-hour gradex, while it accounts for 92% of the variance of the 12-hour gradex. Using a linear relationship, the only gradex values that can be estimated with any degree of accuracy from the 24-hour value are those corresponding to time steps greater than 6 hours.

To check these results, we carried out a similar study after dividing the test area into four regions. The extreme precipitation values for these regions presented similar characteristics (same order of magnitude of precipitation and gradex values). For each region, we looked for significant linear relationships between the gradex values for the different time steps. The conclusions were the same as when we considered the entire area, i.e. the relationship between the gradex values of short time steps and the 24-hour values is very poor.

We have shown that no simple relationship exists to deduce values for short time steps from those measured for long time steps. The problem we posed at the outset therefore appears to have no straight-forward solution. A network of rain gauges measuring daily precipitation values cannot be used to determine the statistical characteristics of the precipitation for much shorter time steps, i.e. less than 6 hours. The only solution would be to use devices capable of measuring the precipitation over short time intervals, for instance recording rain gauges or automatic stations linked to data acquisition systems. Unfortunately such devices have not been in use for a long time and provide records for periods rarely exceeding ten years.

In conclusion, this study reveals the limits for the extrapolation of extreme daily rainfall characteristics to shorter time steps.

FR:

Dans la région de Marrakech, les eaux usées sont utilisées pour l'irrigation sans aucun traitement. Par ailleurs ces eaux usées ruissellent dans des canaux à ciel ouvert sur des distances supérieures à 2 km. Nous avons étudié le devenir des kystes de protozoaires et des oeufs d'helminthes sur le parcours d'un canal alimenté par un émissaire d'eau usée (E2). L'analyse parasitologique des eaux usées a montré que ces eaux contiennent plusieurs types de parasites. En effet, nous avons observé la présence des kystes de protozoaires (Entamoeba histolytica, Entamoeba coli, Giardia sp.) et des oeufs d'helminthes pathogènes (Ascaris,Trichuris, Enterobius, Hymenolepis...). L'étude de la charge parasitaire le long du cours d'eau usée montre une diminution du nombre de parasites dans l'eau usée en allant de l'embouchure S0 vers l'aval de l'émissaire E2. Avec des concentrations moyennes de 1,6.105 kystes/L et 145 oeufs/L à S0, alors qu'à 2 km de S0 (SV) il n'est plus retrouvé que 2,7.104 kystes/L et 33,4 oeufs/L. La charge parasitaire dans le sédiment du cours d'eau, contrairement à l'eau, augmente en s'éloignant de l'embouchure S0. Ainsi, les concentrations moyennes de kystes de protozoaires et les oeufs d'helminthes sont respectivement de 1,7.104 kystes/g et de 9,6 oeufs/g à 80 m de S0 et atteignent, 1,4.105 kystes/g et 78,1 oeufs/g à 2 km de S0. Le ruissellement des eaux usées sur un parcours de 2 Km, à ciel ouvert, montre des taux d'abattement importants des parasites. En effet, plus de 83 % des kystes de protozoaires et plus de 77 % des oeufs d'helminthes sont éliminés de l'effluent. Mais ces abattements restent insuffisants pour satisfaire les directives de l'OMS (1989) en matière de réutilisation des eaux usées en agriculture.

EN:

The use of wastewater in agriculture in what is called "sewage farming" is becoming more widespread, particularly in countries with a severe shortage of water resources. Wastewater from the city of Marrakech is used for irrigation without any treatment. Nevertheless, the wastewater runs into an open channel of 2 km before being used. Thus, the fate of helminth eggs and protozoan cysts in this channel is of great importance for public health. The effluent studied carries wastewater from highly populated residential areas (Sidi Youssef Ben Ali, la Médina...). The primary channel originating from this effluent runs for 2 km with a variable width of 0.7 to 5.5 m in a clay soil. Secondary ramifications on the channel are used for irrigation. Water and sediment samples were collected at different locations on the channel twice monthly for five months (April-August). One litre wastewater samples and 10 g sediment samples were collected, at all stations on the channel. The samples were analysed using the concentration method of Teleman-Rivas as modified by Bailenger (1962) because of its reliability and low cost. Helminth eggs were then counted on MacMaster cells after addition of saturated saccharose solution to the samples. Protozoan cysts were quantified using a Thoma cell after addition of Lugol solution to the samples.

Results of water analyses are expressed as the number of cysts or eggs per liter of wastewater and results for sediments are expressed as the number of cysts or eggs per gram of dry matter.

- Wastewater samples from the effluent contained helminths as well as protozoa. All wastewater samples contained protozoan cysts. Among three species of protozoa identified, two were pathogenic: Entamoeba histolytica, Giardia sp. and one saprophytic Entamoeba coli. All water samples contained helminth eggs. Analyses for helminth egg revealed the presence of Nematodes (Ascaris, Trichuris, Enterobius) and Cestodes (Hymenolepis, Moniezia). This results mainly from Ascaris eggs that were encountered in all water samples. The concentration of helminth eggs in the sewage was much less than the protozoan cysts. In terms of densities, protozoan cysts were in the range 6.9 x 10[exp]4 to 2.0 x 10[exp]5 cysts /L with an average of 1.6 x 10[exp]5 cysts/L. Pathogenic protozoa represent 49 % of total cysts. The wastewater at the beginning of the channel contained from 102 to 238 helminth eggs/L with an average of 145 helminth eggs/L. Nematode eggs represented 72 % of total helminth eggs. Ascaris eggs dominated in the water samples with 75.6 eggs/L representing 52 % of total helminth eggs. The densities of protozoan cysts as well as helminth eggs in wastewater samples decreased dramatically along the open channel. Protozoan cysts dropped from 1.6 x 10[exp]5 cysts/L at the beginning of the channel to 2.7 x 10[exp]4 cysts/L at a distance 2 km in the open channel. Pathogenic protozoa decreased from 8.1 x 10[exp]4 cysts/L to 1.0 x 10[exp]4 cysts/L for the same distance. The average of helminth eggs also dropped from 145 eggs/L to 33 eggs/L. Ascaris eggs showed the highest decrease from 76 eggs/L to 12 eggs/L. The genus Trichuris showed the lowest decrease from 11 eggs/L to 6.8 eggs/L.

- Analyses of sediment samples revealed the presence of all protozoan cysts and helminth eggs. Contrary to water samples, sediments samples showed an increase of all egg and cyst concentrations along the open channel. Protozoan cysts progressively increased from 1.7 x 10[exp]4 cysts/g at 80 m in the open channel to 1.4 x 10[exp]5 cysts/g at 2 km distance in the channel. Helminth parasites also increased from 9.6 eggs/g to 78 eggs/g for the same distance. This increase of helminth eggs and protozoan cysts resulted from the decrease of water flow velocity which caused their sedimentation. Parasites settle out of the water column because of their own weight and because they adsorb to particles accelerating their settling. Among all parasites Ascaris eggs settle out first before all other nematode eggs. Ascaris eggs were detected at 80 m in the open channel while Enterobius eggs were not observed until 560 m in the channel. Trichuris eggs were not observed in any sediment sample. Trichuris eggs are more easily carried by the water flow because of their hydrodynamic fusiform shape which may explain their absence in sediment samples. In conclusion, the parasitic quality of a domestic wastewater in Marrakech showed significant improvement after 2 km in an open channel. Both helminth eggs and protozoan cysts showed significant removal percentage of 77 % and 83 % after 2 km running in the open channel. The best percent removal of protozoan cysts was observed for pathogenic protozoa with 88 % for Entamoeba histolytica and 87 % for Giardia sp. For helminth eggs, Ascaris eggs showed the highest removal percentage with 85 % while Trichuris showed the lowest with 37 %. However, the concentration of helminth eggs and protozoan cysts, after the sewage flows over a distance of 2 km in the open channel, are still higher than the guidelines of the W.H.O. (1989) for the use of wastewater in agriculture (less than one nematode egg per litre). These wastewaters require further treatment before they can be used for irrigation.

FR:

L'industrie textile utilise des colorants de synthèse toxiques qui polluent ses eaux résiduaires avec, parfois, des flux importants. Les procédés traditionnels les éliminent mal : ils sont peu iodégradables et la seule floculation, par exemple par des sels de fer, donne des résultats insuffisants. Or, en Algérie, la réutilisation agricole des eaux usées même industrielles est devenue une impérieuse nécessité. Le procédé proposé repose sur l'utilisation de sels d'aluminium ou, mieux encore, un polyhydroxyaluminium, associés à une bentonite de forage très fine présentant l'avantage d'être localement disponible et peu coûteuse. Quatre colorants ont été testés. Ils appartiennent à deux grandes familles : les colorants acides d'une part, Jaune Supranol 4GL et Vert Nylomine C8B et les colorants dispersifs d'autre part, Rouge Foron RDGL et Violet Foron S3RL. Les essais montrent que, si la bentonite seule ou les sels d'aluminium seuls présentent des efficacités insuffisantes, ces derniers du fait d'une mauvaise décantabilité des microflocs formés, l'association bentonite-aluminium permet d'éliminer les colorants en quasi totalité avec une excellente décantabilité. Les concentrations optimales à mettre en oeuvre sont relativement basses, de l'ordre de 13 mg/l de Al3+ et 250 mg/l de bentonite. Les coûts d'exploitation sont donc très raisonnables. Sur un effluent industriel réel, le procédé permet de passer d'une DCO de 770 mg/l à moins de 30 mg/l.

EN:

The textile industry uses synthetic dyes, most of them being toxic. In Algeria, the agricultural reuse of treated wastewater, even of industrial origin, is becoming commonplace. It is therefore compulsory to drastically reduce pollutant fluxes. The presently operated conventional processes cannot meet the water quality requirements: bioelimination of dyes is negligible and flocculation with iron salts, as currently carried out in the SOITEX plant located in Tlemcen, Algeria, is not effective enough. The use of aluminum salts in the flocculation of such wastewaters is well known (FIESSINGER AND BERSILLON, 1977; LAHAV et al., 1978) but the resulting microflocs are not easily settleable. Bentonite, locally available at a low cost, can also eliminate micropollutants (LAHAV et al., 1978). Associated with polyhydroxyaluminum, it can reduce such compounds as benzene or toluene, favoring simultaneously the liquid-solids separation. This paper evaluates the treatability of dyes by bentonite associated with aluminum salts.

All the runs were carried out in a 200 cm3 batch reactor, mechanically stirred and thermoregulated at 20·C. The main physico-chemical characteristics of the bentonite are given in Table 1. The flocculant was aluminum chloride, previously neutralized with sodium hydroxide (mass ratio OH-/Al=1.85). The solutions were used immediately or left to polymerize during 6 days leading to polyhydroxyaluminum PHAl (LAHAV et al., 1978). When the reactor was operated with bentonite and aluminum, the mass ratio Al/bentonite was maintained at 53.10-3 (KACHA, 1994). Four dyes belonging to two main families were tested: Supranol Yellow 4GL and Nylomine Green (acid dyes) and Foron Red RDGL and Foron Violet S3RL (dispersive dyes). Their concentrations were obtained by spectrophotometry.

Bentonite alone does not induce a significant abatement excepted for low pH values around 4 (Figs. 1 and 2). Dye elimination appears to require a previous protonation step followed by cation exchange. The equilibrium can be modeled by a Freundlich equation (Fig. 3 and Table 2). The dyes can also be eliminated by aluminum salts alone (Fig. 4). The efficiency is then better with polyhydroxyaluminum, i.e. more than 90 % of the initial concentration is removed. Nevertheless, the dyes abatement probably results from an adsorption or chemical reaction on microflocs which are not easily settleable. By assuming that all the aluminum ions are precipitated as aluminum hydroxide, the equilibrium is modeled by the Langmuir equation which would indicate a monolayer adsorption (Fig. 5). When the reactor is operated with bentonite and aluminum salts, dye abatement is nearly complete and the liquid-solids separation is particularly efficient (Figs. 6 and 7). The best results are obtained with PHAl but the use of the monomer can be sufficient. The required concentrations are relatively low and the process is then economically feasible (Table 3). However, the experimental data can no longer be modeled by the Freundlich equation nor by the Langmuir equation. When the aluminum salts react alone with the dyes, the conductance displayed against the aluminum concentration shows two straight lines of different slopes (Fig. 8). The abscissa of the points where the slopes change are proportional to the initial dye concentration, suggesting a chemical reaction between the dye and the aluminum salts (Fig. 9). However, the final pH value lies at the limit value of aluminum hydroxide precipitation; an adsorption on aluminum hydroxide or an aluminum salt precipitation cannot then be assumed. In presence of bentonite, such changes of slope are not observed and, moreover, the final pH value does not correspond either to a precipitation value (Figs. 11 and 12). At this stage, a comprehensive mechanism cannot thus be proposed.

However, the process using bentonite/PHAl is particularly efficient and easy to operate (Fig. 13 and Table 3). The results were confirmed with a true industrial effluent, the Chemical Oxygen Demand (COD) of which was reduced from 770 mg/l to less than 30 mg/l (Fig. 14). As a matter of comparison, the actual process, which includes an activated sludge treatment followed by an iron sulfate/lime flocculation, leads to an effluent containing only 140 mgCOD/l.

FR:

L'objectif de ces travaux a consiste en l'identification des sous-produits de chloration de deux acides aminés libres, la proline et la méthionine, structures reconnues pour leur grande réactivité avec le chlore.

Les expériences ont été conduites a pH 8 pour un taux de chlore fixe à 8 moles de chlore par mole d'acide aminé et un temps de contact de 72 heures. Les sous-produits de chloration ont été extraits successivement par le pentane et le diethyl éther (pH acide et pH basique, extraction suivie d'une dérivation au diazométhane) et identifiés par couplage CG/SM.

L'essentiel des sous-produits de chloration identifiés a été observé dans l'extrait éthéré obtenu à pH acide, que ce soit pour la proline ou la méthionine. Les acides dichloroacétique et trichloroacétique, composés retrouvés dans les eaux de surface désinfectées au chlore, ont été détectés pour les deux molécules étudiées.

Les travaux effectués avec la proline ont permis d'identifier également la N- chlorodichloroacétamide et la N-chlorotrichloroacétamide. On peut noter également la formation de quelques chloroacides présentant un groupement terminal aldehyde ou nitrile caractéristique, ainsi que des composeé à structure pyrrole.

En ce qui concerne la méthionine, les analyses par couplage CG/SM ont permis d'identifier quelques composés organiques chlorosoufrés comme le chlorure de méthyle sulfonyle, le chlorure de chlorométhylesulfonyle et le dichloro-1,1 diméthyle sulfonyle, ainsi que du soufre moléculaire S8.

EN:

The objective of our work was to identify the chlorination byproducts of two amino acids, proline and methionine, structures that were found to be very reactive with chlorine, showing high chlorine demand and high total organohalogenated compounds (TOX) and chloroform formation potentials.

Concentrated solutions of each amino acid were prepared in MilliQ water buffered with phosphate at pH 8 and chlorinated at a dose equal to 8 moles of chlorine per mole of amino acid. Chlorinated solutions were stored at 20°C (in the dark) for 72 hours before applying our extraction procedure. Two different solvents were used, pentane first, followed by diethyl ether under acidic and basic pH.

After concentration (rotary evaporation, nitrogen flow) and derivatization (a few drops of diazomethane were added to the diethyl ether extracts prior to analysis), the different extracts were analyzed by gas chromatography / mass spectrometry.

Most of the byproducts that were identified were analyzed from the diethyl ether extracts obtained at acidic pH. A large number of structures were detected in the extracts of the chlorinated proline solution, and only a few in the extracts from the methionine solution. Most of the identified compounds are chlorinated structures including a carboxyl function. Trichloroacetic acid and dichloroacetic acid were identified as common chlorination byproducts of the two amino acids studied. The two intermediates in the formation of these two acids, CCl3CONH2 and CHCl2CONH2, were also found.

N-chlorodichloroaldimine and N-chlorotrichloroaldimine have been identified as chlorinated proline by-products. The chlorination of this amino acid also leads to the formation of chloroacids compounds which present an aldehyde or a nitrile function as a terminal group; a pyrrole structure is also proposed. The chlorination of methionine has been shown to produce several chlorosulfur compounds. The structures identified in the chlorinated methionine solution were H2CClSO2Cl, HCCl2SO2Cl and HCCl2CONH2. Molecular sulfur (S8) was also obtained.