What log removal corresponds to a 99.9% removal efficiency?

The correct answer is associated with the concept of log removal, which is a measure used to quantify how effectively a treatment process eliminates pathogens or contaminants. In this context, a "log removal" indicates a tenfold reduction in the concentration of a substance.

To understand why three-log removal corresponds to a 99.9% removal efficiency, consider the following:

• A 1-log removal indicates a 90% reduction, which leaves 10% of the original concentration.
• A 2-log removal signifies a 99% reduction, meaning only 1% remains.
• A 3-log removal reflects a 99.9% elimination, where just 0.1% of the original concentration persists.

Therefore, achieving 3-log removal signifies that the treatment process has effectively reduced the contaminant concentration to a level where only 0.1% remains, aligning with the 99.9% removal efficiency.

Higher log reductions (like 4-log) would represent even more significant reductions, leading to 99.99% removal, but that exceeds the required 99.9%. Thus, the correct correspondence of log removal to the specified efficiency is three-log.