Anthracite is a coal based media intended to remove particulates from the liquid being filtered. It has a depth filtration function that is why when loaded to the media tank, the said tank is called MMDF (Multi Media Depth Filter) that has the capacity of filtering particulates from its top layer up to its bottom layer. It should be noted that depth filtration process happens from the top to the bottom layer of the Anthracite and not from the gravel, pebbles and silica sand that are incorporated in the media tank, said items are just supporting bed to prevent the Anthracite from collapsing.
Granular Activated Carbon is either wood based or coconut based media that can remove foul taste, odor, color, chlorine and other substances that are carbon based derivatives. It should be noted that only contaminants that are caused by organic substance can be removed by the Granular Activated Carbon.
Removal of contaminants is based on the Principle of Adsorption (contaminants would adhere to the surface of the granular activated carbon) as per simplicity imagine the metal and the magnet. It is very important to consider the water that will be passing through the carbon should at least have minimal particulates so that it will not affect the adsorptive property of the Granular Activated, imagine a rubber placed in between the magnet and the metal. On the other hand, Granular Activated Carbon also has the depth filtration property, so it can also do the function of the Anthracite, however when this happens, granular activated carbon would loosen its adsorptive property easily and would retain its depth filtration property. In order to maintain the efficiency of its adsorptive property, then we have to filter out the particulates first by using the Anthracite so that the Granular Activated Carbon can concentrate on its adsorptive function.
In cases where in the MMDF tanks will be loaded with Granular Activated Carbon followed by the Carbon tank also loaded with Granular Activated Carbon is an acceptable concept here in the Philippines, however in countries where Granular activated carbon is far more expensive than Anthracite, then such concept is impractical.
Just sharing my thoughts about the concept.