Which method is ineffective at disinfecting water physically?

Disinfection methods for water primarily aim to eliminate or inactivate pathogens and other harmful microorganisms. Among the options listed, the addition of chlorine is a chemical method, not a physical one.

Chlorine works by chemically reacting with the pathogens, thereby altering their cellular structure and preventing their reproduction. This process involves a chemical bond formation and is controlled by factors such as pH and temperature, but does not employ a physical mechanism for disinfection.

In contrast, options like ultrasonic waves and heat utilize physical principles to eliminate bacteria. Ultrasonic waves generate vibrations that can disrupt microbial cell structure, while heat can denature proteins and nucleic acids in harmful organisms, leading to their destruction. UV radiation operates on a physical principle as well, wherein ultraviolet light disrupts the DNA of microorganisms, rendering them unable to reproduce or infect.

Recognizing the type of method being utilized – whether physical or chemical – is crucial in understanding their applications in water treatment. This highlights the importance of categorizing disinfection methods appropriately based on their operational mechanisms.