Fate of parasites in five wastewater treatment plants in Tunisia
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.