Real-time detection of 3–8-µm microbial particles in sand filter effluent as a surrogate indicator of Cryptosporidium parvum oocysts

Waterborne outbreaks of Cryptosporidium infection through drinking tap water are a significant threat to public health. Monitoring Cryptosporidium oocyst-sized microorganisms in sand filter effluent as a surrogate indicator of Cryptosporidium oocysts can detect sand filter integrity breaches. This study aimed to achieve the continuous monitoring of microbial particles (MPs) sized similarly to Cryptosporidium in sand filter effluent by remodeling an MP counter and demonstrating its validity. Instrumental settings of the MP counter (i.e., refractive index and particle diameter) were adjusted for accurate detection of Cryptosporidium parvum oocyst. The remodeled MP counter successfully detected Cryptosporidium parvum and Cryptosporidium-sized microorganisms (Saccharomyces cerevisiae and Chlorella vulgaris). Over a 7-d full-scale test, the MP counter coupled with dialysis pre-treatment continuously detected Cryptosporidium-sized (i.e., 3–8-µm) MPs at 0.2–27 counts/10 mL in the sand filter effluent. However, the variations in MP concentrations were not correlated with those in conventional indicators, such as turbidity and 1–3 or 3–7-µm particle counts, indicating that microbiological water safety cannot be guaranteed with these conventional indicators. Real-time monitoring of 3–8-µm MP concentrations allows for early detection of potential Cryptosporidium leaks through a sand filter. Abnormalities in water quality detected early would allow for effective countermeasures immediately, reducing the risk of waterborne protozoan outbreaks.