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The inactivation rates of helminth eggs in the sludge layer of three Mexican wastewater stabilization ponds (WSP) were studied using two different methods: (1) stratified sludge cores, to investigate the apparent inactivation rate of the indigenous helminth eggs, and (2) Sentinel ChambersTM, to investigate the inactivation rate in samples inoculated with Ascaris suum. The inactivation rate of Ascaris eggs in the sentinel chambers, about 50% after 1 year, was similar to the initial apparent inactivation rate determined by the sludge core method in the same pond. Based on the sludge core data it was found that the apparent inactivation rate of Ascaris eggs decreased with time, and varied between the three ponds. The eggs of Ascaris survived longer than the eggs of other helminth species present in the sludge, which included Hymenolepis, Trichuris, and Toxocara. The concentration of eggs in the sludge varied widely between ponds and according to location and depth within each pond; the maximum concentration found was 650 eggs/g total solids (TS). Viable Ascaris eggs were found in 52 of 53 sludge core samples, some of which were estimated to be over 10 years old. Because most WSPs are operated continuously, new sludge and new eggs are constantly deposited at the top of the sludge layer. Therefore, even if the concentration of viable eggs in the oldest sludge is reduced to levels meeting the standards set by biosolids regulations, upon removal this sludge will be mixed with the more recent sludge containing higher concentrations. The mean concentrations of viable helminth eggs in the sludge were 25, 25, and 48 eggs/g TS in the Mexicaltzingo, Texcoco, and Xalostoc ponds, respectively; therefore further treatment of the sludge is recommended before it is disposed or reused.

Document Type: Research Article


Publication date: 2001-01-01

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  • Proceedings of the Water Environment Federation is an archive of papers published in the proceedings of the annual Water Environment Federation® Technical Exhibition and Conference (WEFTEC® ) and specialty conferences held since the year 2000. These proceedings are not peer reviewed.

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