The notes indicate a chlorine dose of 9 ppm for each 1 ppm of ammonia. Which statement best describes this ratio?

• A. 9 ppm Cl2 per 1 ppm ammonia
• B. 1 ppm Cl2 per 9 ppm ammonia
• C. 0.9 ppm Cl2 per 1 ppm ammonia
• D. 90 ppm Cl2 per 1 ppm ammonia

Answer: (A)

Reading a statement that the chlorine dose is nine parts per every one part of ammonia means the chlorine concentration is nine times the ammonia concentration. In other words, the ratio is 9:1, so the chlorine dose is 9 ppm for each 1 ppm of ammonia. This is why the best description is nine ppm Cl2 per one ppm ammonia. For example, if ammonia is 2 ppm, chlorine would be 18 ppm. The other options describe different, incorrect ratios (reversing the ratio, or using 0.9 or 90), which doesn’t match the stated 9:1 relationship.