How would a disinfection level of 3 log be expressed?

• A. 99.9%
• B. 99.99%
• C. 99%
• D. 98%

Answer: (A)

A disinfection level of 3 log refers to a reduction of microorganisms by a factor of 1,000, which is mathematically expressed as the killing of 99.9% of the target pathogens. The term "log" in this context refers to logarithmic reduction, where each log represents a tenfold decrease in the number of viable organisms. Therefore, 3 log means you start with 1,000 organisms, and after disinfection, only 1 remains, indicating that 999 have been eliminated.

Hence, a disinfection level that achieves 3 log effectiveness will yield a percentage reduction of 99.9%. Understanding the logarithmic scale is crucial in water treatment, as it provides a clear quantitative measure of the efficacy of disinfection processes. This concept is foundational for ensuring water safety and compliance with public health standards.