Comparative reduction of Giardia cysts, F+ coliphages, sulphite reducing clostridia and fecal coliforms by wastewater treatment processes

Advanced wastewater treatment processes are applied to prevent the environmental dissemination of pathogenic microorganisms. Giardia lamblia causes a severe disease called giardiasis, and is highly prevalent in untreated wastewater worldwide. Monitoring the microbial quality of wastewater effluents is usually based on testing for the levels of indicator microorganisms in the effluents. This study was conducted to compare the suitability of fecal coliforms, F+ coliphages and sulfide reducing clostridia (SRC) as indicators for the reduction of Giardia cysts in two full-scale wastewater treatment plants. The treatment process consists of activated sludge, coagulation, high rate filtration and either chlorine or UV disinfection. The results of the study demonstrated that Giardia cysts are highly prevalent in raw wastewater at an average concentration of 3600 cysts/L. Fecal coliforms, F+ coliphages and SRC were also detected at high concentrations in raw wastewater. Giardia cysts were efficiently removed (3.6 log₁₀) by the treatment train. The greatest reduction was observed for fecal coliforms (9.6 log₁₀) whereas the least reduction was observed for F+ coliphages (2.1 log₁₀) following chlorine disinfection. Similar reduction was observed for SRC by filtration and disinfection by either UV (3.6 log₁₀) or chlorine (3.3 log₁₀). Since F+ coliphage and SRC were found to be more resistant than fecal coliforms for the tertiary treatment processes, they may prove to be more suitable as indicators for Giardia. The results of this study demonstrated that advanced wastewater treatment may prove efficient for the removal of Giardia cysts and may prevent its transmission when treated effluents are applied for crop irrigation or streams restoration.