Élimination des oeufs de nématodes et des kystes de protozoaires des eaux usées domestiques par lagunage à microphytes en zone soudano-sahélienne

Résumé Fr En

La station expérimentale pilote d'épuration des eaux usées par lagunage de l'École Inter-États d'Ingénieurs de l'Équipement Rural (EIER), Ouagadougou, Burkina Faso, reçoit les eaux usées des bâtiments administratifs et de l'internat des étudiants. Dans les eaux usées brutes, des oeufs d'helminthes (Ascaris lumbricoides, Ankylostoma duodenale), des kystes de protozoaires (Entamoeba coli, Entamoeba histolytica) et des larves d'Anguillule ont été mis en évidence.Avec un temps de séjour de 16,4 jours dans les bassins de lagunage (2 à 3 heures dans le décanteur, 3,4 jours dans le premier bassin, 13 jours dans le deuxième bassin), les kystes d'Entamoeba coli et d' Entamoeba histolytica sont éliminés respectivement à 94 et 96 %, les oeufs d'Ascaris lumbricoides à 100%, les oeufs d'Ankylostoma duodenale à 90 % et les larves d'Anguillule à 92 %. Quand on considère tous les parasites confondus, le décanteur a un rendement éliminatoire de 33 %, le premier bassin 62 % (malgré la forte charge appliquée), le deuxième bassin 78 %. Le rendement global obtenu pour l'ensemble des parasites est de 94 %.Si les évolutions des concentrations des parasites fluctuent dans le temps, il y a une différence très significative entre les concentrations obtenues en sortie du lagunage et celles des eaux usées brutes.On a constaté l'absence des oeufs d'Ascaris lumbricoides pendant toute la période de l'étude ; il en a été de même pour les oeufs d'Ankylostoma duodenale durant une période de 10 mois. Par conséquent, les eaux usées épurées rejetées répondent aux recommandations de l'OMS quant à leur réutilisation agricole.La mise en place des systèmes rustiques d'épuration des eaux usées dans les pays en voie de développement, surtout dans les zones où les ressources en eau sont limitées, pourrait contribuer sensiblement à la diminution des risques sanitaires liés aux pratiques courantes de réutilisation agricole des eaux usées en agriculture.

Experimental waste stabilization ponds to purify domestic wastewater from the École Inter-États d'Ingénieurs de l'Équipement Rural (EIER) were built in Ouagadougou, Burkina Faso. The waste stabilization ponds system consists of a raising tank, a decanter, and two ponds in series. The first pond is 1.22 m deep with a surface area of 62 m2; the second pond is 1.07 m deep and covers an area of 340 m2. The inflow averaged 22 m3/d, giving a normal retention time of 3.4 days in pond number 1 and 13 days in pond number 2. The average daily BOD5 load was 1500 kg/ha/day for pond number 1 and 200 kg/ha/day for pond number 2.Different parasites were identified in the raw wastewater, such as cysts of Entamoeba coli and Entamoeba histolytica, eggs of Ascaris lumbricoides and Ankylostoma duodenale and larval stages of Anguillula . The quantitative estimation of cysts, eggs and larvae was obtained using a Mac Master cell after concentration by a formol-ether technique (RITCHIE, 1948). Of 24 samples of raw wastewater analyzed, all contained cysts of E. coli and E. histolytica and Anguillula larvae. Similarly, 100 % of final effluent samples (Fig. 2-B) contained cysts of E. coli and E. histolytica, but the concentrations were respectively 16 and 22 times lower than in raw wastewater. Six of the 24 samples (25 %) contained Anguillula larvae (Fig. 9), 17 % contained Ankylostoma duodenale eggs and no samples (0 %) contained Ascaris lumbricoides eggs (< 1 egg/L).This study were not carried out on Ouagadougou urban wastewater, due to the lack of a wastewater collection network, but the raw wastewater of the central market of Ouagadougou was analyzed for comparison because it represented a larger population than that in the EIER. In the raw wastewater of the central market, the concentration of parasites was high: Ascaris lumbricoides (110 eggs/L), Taenia saginata (53 eggs/L), Ankylostoma duodenale (39 eggs/L), Trichuris trichiura (19 eggs/L), Entamoeba coli (552 cysts/L), Entamoeba histolytica (479 cysts/L), Anguillula (62 larvae/L). The concentration of parasites identified in raw wastewater from EIER (Entamoeba coli (395 cysts/L), Entamoeba histolytica (269 cysts/L), Ascaris lumbricoides (4 eggs/L), Ankylostoma duodenale (9 eggs/L), Anguillula (26 larvae/L)) seemed consistent with the contamination level of the users of EIER stabilization ponds system. With a total retention time of 16.4 days in the stabilization ponds, removals of E. coli and E. histolytica cysts were respectively 94% and 96%. Ascaris lumbricoides and Ankylostoma duodenale eggs were 100 and 90% removed, respectively, and removal of Anguillula larvae was 92%. Grimason et al. (1995), suggest that cumulative retention time of up to 55.3 days at Eldoret, Kenya and 40 days at Meze, France were sufficient to remove between 99.1 % and 99.7 % of cysts of Giardia lamblia, respectively. Global efficiency of the system for all parasites was 94%. In the decanter, where retention time was 2-3 hours, the removal efficiency was 33%. With 3.4 days retention time, the first pond (anaerobic due to the high organic load applied) increases removal to 62%. The second pond (optional) afforded a 78% removal efficiency with a 13 days retention time.If the number of parasites fluctuated with time, the number observed in the effluent was significantly lower than those observed in the influent.The effluent of EIER waste stabilization ponds system was free of Ascaris lumbricoides eggs throughout the study period and free of Ankylostoma duodenale eggs during 10 months; the effluent thus satisfied the WHO recommendations for reuse in agriculture. Waste stabilization pond systems would seem to have a significant place in Sudan- Sahara area for removal of parasite ova and cysts, being economical and advantageous. They would also contribute to reduce sanitary risks associated with reuse of untreated wastewater in agriculture.