GIARDIA AND CRYPTOSPORIDIUM IN SWEDISH WASTEWATER TREATMENT PLANTS / Giardia och Cryptosporidium i svenska avloppsreningsverk
The occurrence of Giardia and Cryptosporidium in Swedish Wastewater treatment Plants (WWTP) has previously not been investigated. Giardia cysts were present in the untreated wastewater at every sampling occasion at all test sites in densities between 24—160 cysts/L, but only twice in the treated wastewater and in lower densities. Cryptosporidium oocysts were detected in 67 % of the influent samples in densities between 16-200 oocysts/L. Oocysts were reduced by 99 % in Trollhättans WWTP with an effluent density of 2 oocysts/L. The occurrence of cysts in Henriksdals WWTP was used for an estimation of the endemic rate of giardiasis in Stockholm, giving an incidence of 2/10,000. Based on the influent-effluent densities, the (oo)cyst levels in sludge were estimated to 28 cysts/g dry weight and 15 oocysts/g dry weight. Wastewater affects the recipient Göta Älv. More than 95 % of the oocyst contamination is estimated to have another origin, probably from agricultural sources as manure and sludge run-off. Microbial risk assessments of the receiving water, Göta Älv, indicate a 0.0025/1,000 swims risk for contracting a Giardia infection. With an assumed 99.8 % reduction of oocysts in a water treatment plant, the densities in the drinking water produced from Göta Älv are estimated to cause 15 Cryptosporidium infections/10,000 inhabitants and year.