It because of adsorption capacity of chlorine. Traditional water supply plants use Calcium hypochlorite to sterilize. it kills a lot of bacteria, but it leaves chlorine.
Chlorine can cause cancer, that is a commen sense in the scientific community.
We check many materials, and find that activated carbon not only adsorbs chlorine very well, but also absorbs oil, mercury and chromium, which is very comprehensive.
we can not find any other better material untill today.
The most common activated carbon materials are coal, charcoal and coconut shell charcoal. By comparing the adsorption effect, coal is the worst, and coconut shell charcoal is the best.
In water treatment field, we use iodine value, or iodine adsorption value to check the capacity of activated carbon.
The iodine value in chemistry is the mass of iodine in grams that is consumed by 100 grams of a chemical substance. The higher iodine value, the better adsorption capacity. The iodine value commonly used in water purifier filters now ranges from 800 to 1200, and the price varies greatly.
Granular activated carbon, GAC, also called UDF. Sintered activated carbon filter, also called CTO. CTO is more effective than UDF, because the pore size of sintered activated carbon is smaller than UDF. When impurities in water pass through a smaller pore size of CTO, it is easier to be captured by activated carbon.
So, even though the coconut shell activated carbon with good quality, it may not have the best effect. we also need to understand its structure.
The pore size of sintered activated carbon can be adjusted freely, the smallest size can reach 0.2 μm, which is better than UF membrane.
Activated carbon is better than UF? You may not expect this.