Efficiency of wastewater treatment plants at removing Giardia sp. cysts in southern France.

A study was conducted to evaluate the removal efficiency for Giardia sp. cysts of a number of wastewater treatment plants in France. Of these, five were activated sludge systems, three were trickling filters and three were waste stabilisation pond systems. In addition, the effect upon cyst removal of disinfecting effluents prior to discharge into controlled waters by UV and chlorination was evaluated. Cysts were detected in raw wastewater at all 11 treatment plants in concentrations ranging from 130 to 41,270 cysts/litre. The removal of cysts by sewage treatment was found to range between 99.5 and 99.8% for activated sludge, 99.9 and 100% for waste stabilisation ponds, and for the trickling filter plants up to 98.3%. Despite the high removal efficiencies recorded in this study the range of cysts detected in final effluents discharged into controlled waters ranged from < 1 to 66 cysts/litre. This is undoubtedly an underestimate of actual occurrence and concentration in wastewater, given the limitations of the methods currently employed by investigators. Cysts were detected in final effluents which had been disinfected by both UV and chlorination (range 0.3-19 cysts/litre), however, it is not known whether such cysts were viable or non-viable.