When iron alone is treated using manganese greensand filters, the chlorine feed rate should be about ______.

• A. 1.0 to 2.0 times the iron concentration
• B. 1.2 to 1.5 times the iron concentration
• C. 1.5 to 1.8 times the iron concentration
• D. 1.5 to 2.0 times the iron concentration

Answer: (A)

The essential idea is that chlorine provides the oxidant needed to convert soluble Fe2+ to an insoluble form that the greensand filter can remove, and it also helps keep the manganese oxide coating active. To ensure complete iron oxidation without overdoing chlorine, the feed rate should be roughly equal to the iron concentration and can go up to about twice that amount. In practice, this means if your iron is 2 mg/L, aim for about 2 mg/L of chlorine, up to around 4 mg/L. Too little chlorine leaves iron unoxidized and the filter underperforms; too much chlorine wastes chemical and can stress the media or form unwanted byproducts.