GIARDIA AND CRYPTOSPORIDIUM IN BACKWASH WATER FROM RAPID SAND FILTERSUSED FOR DRINKING-WATER PRODUCTION

Backwash water from rapid sand filters of a treatment plant using surf ace water from small rivers for drinking water production was examined with the aim of determining the degree of their potential contaminati on with Giardia cysts and Cryptosporidium oocysts. Simultaneous invest igations were carried out for both protozoans from November 1993 to Fe bruary 1994. Water samples were concentrated by continuous flow centri fugation (11 backwash water samples) or by using polypropylene cartrid ge filters (12 raw water samples and 39 backwash water samples). Paras ites were identified by the direct immunofluorescence assay. Ten out o f 12 raw water samples tested were positive for Giardia (range: 2-103/ 100 L) and 8 out of 12 were positive for Cryptosporidium (range: 0.8-1 09/100 L). Eight of 11 backwash water samples collected by continuous flow centrifugation were positive for Giardia (range: 3-86/100 L) or C ryptosporidium (range: 1-69/100 L). Out of 39 samples collected using cartridge filters, 34 were positive for Giardia (range: 1.4-374/100 L) and 33 for Cryptosporidium (range: 0.8-252/100 L). Overall, Giardia, Cryptosporidium, or both were detected in 92% of the backwash water sa mples. The results have clearly shown that backwash waters were contam inated with Giardia and Cryptosporidium and the supernatant returned t o the raw water after the sedimentation process was not free from cyst s and oocysts.