Many organisations use GAC in water treatment because it has exceptional adsorption capabilities. When water containing metazachlor passes through a GAC bed, the herbicide molecules are attracted to the carbon surface, effectively removing them from the water. GAC-based systems can be effective in treating large volumes of water and are relatively simple to operate.
However, GAC has a limited capacity to adsorb pollutants, and over time, the carbon bed becomes saturated. This necessitates the replacement and proper disposal of spent carbon, which can be a logistical challenge and adds to operational costs.
So because the spent carbon contains the accumulated pollutants, it requires secondary disposal, and this is really quite bad for our environment.
For more information on the environmental cost of GAC, see this article.