Devenir des oeufs d'helminthes et des kystes de protozoaires dans un canal a ciel ouvert alimenté par les eaux usées de Marrakech Fate of helminth eggs and protozoan cysts in an open channel receiving raw wastewater from Marrakech

Résumé

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, Entamoebacoli, Giardiasp.) 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.

Mots-clés:

• Cours d'eau usée,
• oeufs d'helminthes,
• kystes de protozoaires,
• eaux usées,
• sédiment,
• abattement

Abstract

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: Entamoebahistolytica, Giardiasp. and one saprophytic Entamoebacoli. 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.

Keywords:

• Wastewater channel,
• helminth eggs,
• protozoan cysts,
• wastewater,
• sediment,
• removal percentage