There are a few applications for finely powdered carbon as an adsorbent. However, a large amount of fine carbon is used in water treatment. When a process fluid will be filtered anyway, it may make sense to add powdered carbon to take advantage of its great surface. Retaining the carbon in packed columns is less costly than collecting it by filtration after batchwise addition. Powdered carbon is not used in columns because the large head losses for passing fluid through such a fine structure cannot be tolerated.
Granular carbon, roughly the size of a pea, provides sufficient surface without excessive resistance to flow in a column. It has been used as a "roughing step" for recovery of fermentation products. The surface is much more than the outer area of the particles because there is a vast network of pores. The concentration at which the column can be considered exhausted depends on engineering and cost factors. A column must be regenerated in time to take its place at the head of the series of columns-there is no point in continuing to adsorb with a column that is so loaded that it accomplishes little.