Urban wastewater treatment in stabilization ponds: occurrence and removal of pathogens

Waste stabilization ponds are frequently considered as being effective for the removal of intestinal parasites while conventional wastewater treatment processes are not. Over a 24-month period, grab samples from a pilot stabilization pond system in Marrakech were analyzed for the presence of Giardia cysts and Ascaris eggs. Giardia was isolated with an average of 2.8×10³ cysts/l, while Ascaris eggs were detected with a mean number of 1.7 eggs/l. The data also suggest seasonal differences in Giardia cyst and Ascaris egg concentrations in raw wastewater, recording high monthly mean numbers during dry period (Spring and Summer). At the outlet of the system, neither Giardia cysts nor Ascaris eggs were found in treated wastewater. Enumeration of Giardia cysts and Ascaris eggs in the sediment at the entrance of the system resulted in average numbers of 1.3×10³ cysts/g and 29.6 eggs/g dry weight of sediment. These concentrations decreased towards the outlet of the ponds where the sediment was free of Giardia cysts and Ascaris eggs.