Many filtration systems use carbon, charcoal or a blend of filter media to help reduce impurities. These systems generally reduce contaminants in one of two ways.
First, some contaminants are filtered mechanically, meaning they are large enough to be trapped in the pores of the filter. Other contaminants adhere to the surface of the filter media. Eventually, the surface area of the filter media becomes filled and no more contaminants can be adsorbed. Or, in the case of mechanical filtration, the pores of the filter become so clogged with debris that water is unable to move through the filter effectively.
While the latter is easy to spot, since the flow rate of the water being produced by the system slows dramatically, it's not as easy to tell when the surface area of the filter media has become full and needs to be cleaned regularly