What is the initial filtration step for efficiently treating water containing Cryptosporidium?

Microfiltration is the correct choice for the initial filtration step in treating water containing Cryptosporidium. This process involves using membranes with pore sizes typically around 0.1 to 10 micrometers, which are small enough to effectively remove microorganisms such as Cryptosporidium cysts from water.

Cryptosporidium is a robust pathogen with a protective outer layer, and its cysts can be resistant to conventional disinfection methods like chlorine. By employing microfiltration, water treatment facilities can physically remove these cysts, significantly reducing the risk of waterborne outbreaks associated with this pathogen.

In contrast, reverse osmosis is more suitable for desalting water and removing dissolved solids but is generally not the initial step for treating Cryptosporidium. Sand filtration and gravity filtration also serve important roles in water treatment but may not provide the level of removal efficiency needed for pathogens like Cryptosporidium as effectively as microfiltration. Thus, microfiltration stands out as the most efficient initial step for effectively addressing water contamination with this specific microorganism.