Report No FR/DW0002



Apr 1996


Optimised Scottish Parasite Diagnostic Laboratory (SPDL) methods for the recovery of Cryptosporidium sp. oocysts and Giardia sp. cysts from sewage influent, effluent and sludge samples, which have higher recovery efficiencies than the methods identified in the "Blue book", have been used in this survey of occurrence of oocysts and cysts in sewage influents, effluents and sludges from seven sewage treatment works in England.

The seven sewage treatment works chosen for the study served communities which had defined contributors of oocysts and cysts. These included "mainly industrial (inorganic), purely residential, urban. mainly industrial (organic), with significant flow from an abattoir, with significant flow from a livestock market. rural and possible farm trade effluent and urban/rural and trade effluents".

Statistical analyses of over 150 samples indicated that no significant difference existed between detection by fluorescence microscopy or detection by flow cytometry followed by fluorescence microscopy for detecting either Cryptosporidium sp. oocysts or Giardia sp. cysts.

Of a total of 81 sewage influent samples, 27.2% (n = 22) contained between 10 - 170 Cryptosporidium sp. oocysts/L. The small numbers of oocysts usually detected in positive samples reduced the amount of information on oocyst viability. However, the sparse data accrued indicate that viable oocysts can be released in sewage effluent discharges. Examination of these sewage influent samples for the presence of Giardia sp. cysts indicated that 77.8% (n = 63) of samples contained between 10 - 13600 Giardia cysts/L. Of a total of 94 sewage effluent samples, 25.5% (n = 63) contained between 10 - 60 Cryptosporidium sp. oocysts/L. Examination of these samples for the presence of Giardia sp cysts indicated that 57.4% (n = 54) of samples contained between 10 - 720 Giardia cysts/L.

Removal efficiencies ranging from 46.4 to 93% were calculated from three treatment works where the numbers of Cryptosporidium sp oocysts were higher in the influents than their respective effluents. Removal efficiencies ranging from 26 to 93.8% were cal culated for the seven treatment works where the numbers of Giardia sp cysts were higher in the influents than their respective effluents.

Two treatment works, which received mainly rural and mainly rural/farm trade effluents respectively, contained significantly higher numbers of Cryptosporidium sp. oocysts in their influent samples than the other five studied and it is possible that the oocysts detected in these two sewage treatment works were derived from animal sources. No significant differences could be identified in the numbers of either oocysts or cysts discharged in effluents from any of the seven treatment works studied. Furthermore, the occurrence of Giardia cysts in sewage influents could not be associated with any particular contributing source.

Oocysts and cysts were detected in sewage sludge. Oocysts were detected on two occasions from one site only, whereas cysts were detected in sludge samples from all five sites on at least one occasion. Thus the potential for introducing oocysts and cysts indirectly into water courses, following the disposal of sewage sludge onto land exists.

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