Occurrence and removal of Giardia sp. cysts by Kenyan and French waste stabilisation pond systems

Abstract

A study was conducted to determine the occurrence and removal of Giardia sp. cysts in municipal wastewater by two waste stabilisation pond systems; one at Eldoret, Kenya and one at Meze, France. A total of 71 and 26 grab raw wastewater samples from Eldoret and Meze, respectively, were examined for the presence of cysts. Cysts were detected in 37% (26) and 100% (26) raw wastewater samples, respectively. Cyst concentrations detected in raw wastewater samples ranged from 1×103 to 2.5×104 cysts/l at Eldoret and 230 to 2.5×104 cysts/l at Meze. At Eldoret, all anaerobic (8/8), secondary facultative (5/5) and primary maturation (5/5) pond effluent samples examined were positive for Giardia. Cyst concentrations ranged from 1.25 to 483 cysts/l (anaerobic), 23.5 to 192.5 cysts/l (secondary facultative) and 17.2 to 118.5 cysts/l (primary maturation), respectively. At Meze, cysts were detected in 12 of 12 primary facultative (range 4.4 to 48 cysts/l) and 9 of 11 (82 %) primary maturation pond effluents (range 0.2 to 16 cysts/l). In final maturation pond effluent, Giardia cysts were detected in 100% of final pond effluents discharged from Eldoret (n=10; range 21 to 90 cysts/l) and 7 of 16 (44%; range 0.1 to 2.5 cysts/l) from Meze. The occurrence of cysts in final pond effluents examined would indicate that cumulative retention periods of up to 25.3 days at Eldoret and 40 days at Meze would be sufficient to remove between 99.1% and 99.7% of cysts, respectively, but that such retention periods are insufficient to ensure the complete removal of Giardia sp. cysts.