Three drinking water treatment plants (DWTPs) differing in source water and treatment capacity were investigated for the potential passage of waterborne protozoan (oo)cysts through conventional processing. DWTP I (15,000 L/s), DWTP II (7,500 L/s) and DWTP III (4,300 L/s) provide drinking water for approximately 2.7 million inhabitants of the Metropolitan District of Caracas (Venezuela). The US Environmental Protection Agency Method 1623 for detection of Cryptosporidium and Giardia was used to analyze raw water and finished drinking water samples collected from the three plants. (Oo)cyst recovery efficiencies varied between 23 and 84%. The concentration of confirmed (oo)cysts detected in raw water samples ranged between 1 and 100 per 100 L. (Oo)cyst levels in finished water samples ranged from 2 to 25 per 100 L. These data indicated that the conventional treatment process to produce finished water at two filtration plants was not effective in preventing the passage of protozoan (oo)cysts. Monitoring strategies that include multiple microbial indicators and waterborne pathogens are strongly recommended for accurate source water characterization and for verification of the effectiveness of treatment process barriers to microbial breakthrough in the finished water.

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