Prevalence of Cryptosporidium oocysts and Giardia cysts in raw and treated sewage sludges

Treated sludge from wastewater treatment plants (WWTPs) is commonly used in agriculture as fertilizers and to amend soils. The most significant health hazard for sewage sludge relates to the wide range of pathogenic microorganisms such as protozoa parasites.The objective of this study was to collect quantitative data on Cryptosporidium oocysts and Giardia cysts in the treated sludge in wastewater treatment facilities in Spain. Sludge from five WWTPs with different stabilization processes has been analysed for the presence of Cryptosporidium and Giardia in the raw sludge and after the sludge treatment. A composting plant (CP) has also been assessed. After a sedimentation step, sludge samples were processed and (oo)cysts were isolated by immunomagnetic separation (IMS) and detected by immunofluorescence assay (IFA). Results obtained in this study showed that Cryptosporidium oocysts and Giardia cysts were present in 26 of the 30 samples (86.6%) of raw sludge samples. In treated sludge samples, (oo)cysts have been observed in all WWTP's analysed (25 samples) with different stabilization treatment (83.3%). Only in samples from the CP no (oo)cysts were detected. This study provides evidence that (oo)cysts are present in sewage sludge-end products from wastewater treatment processes with the negative consequences for public health.