Giardia and Cryptosporidium in sewage- contaminated river waters

Two representative rivers, Shing Mun River and Lam Tsuen River, of Hong Kong were each monitored for 13 consecutive months for the levels of E. coli, Giardia cysts and Cryptosporidium oocysts, with a view to providing sufficient data for substantiating the need for developing relevant water quality guidelines. Both rivers were found to be faecally polluted in terms of E. coli level. Cryptosporidium oocysts were seldom detected at both rivers. Giardia cysts levels were often found below 10 per l at sparsely populated remote sampling points at Lam Tsuen River. Whereas sampling points situated at the lower reaches of the river receiving treated effluent discharges from a highly populated residential development and from a number of village houses served by sewage treatment plants and septic tanks, respectively, Giardia cysts were frequently found at much higher levels (as high as 468 cysts per l). Concentrations of Giardia cysts at the three sampling points of Shing Mun River varied from undetectable to over 100 per l. The levels of Giardia cysts also seemed to be related to the type of treatment facilities employed by the houses/developments located along the river banks. However, the levels of Giardia cysts detected at the two rivers were not correlated to the levels of E. coli present. Our findings suggested that Giardia may be used as an indicator of treatment efficacy for parasite removal from sewage and there is scope for inclusion of a parasitological indicator in water quality guidelines for local river waters.