The two pathogenic protozoans that are known to be resistant to chlorination are Giardia and Cryptosporidium. Both of these organisms can survive in chlorinated water, which poses a significant challenge for water treatment facilities.
Giardia is a parasite that can cause gastrointestinal illness and is commonly associated with contaminated drinking water. It forms cysts that are highly resistant to chlorine disinfection, making their removal from water supplies particularly difficult if chlorination is the only method used.
Similarly, Cryptosporidium is also a protozoan parasite that can cause cryptosporidiosis, which presents severe gastrointestinal symptoms. Its oocysts (the infectious form) have a robust outer shell that provides substantial resistance to chlorine, even at higher concentrations or for prolonged exposure times.
Understanding the resistance of these protozoans to chlorination is essential for water treatment operators, as it influences the treatment methods used to ensure safe drinking water. Other options listed do not include both Giardia and Cryptosporidium, nor do they represent organisms known for their resistance to chlorine, further supporting the correctness of the chosen answer.