When talking about CAPACITY in water treatment, we are talking about the amount of ions that an ion exchanger can absorb before it is oversaturated and must be regenerated. A distinction is made between total and usable capacity, the former being merely a theoretical quantity with which the maximum absorption volume can be calculated. For reality, the usable capacity, which is always significantly smaller than the total capacity, is of greater importance, since it depends on operating conditions such as the geometry of the container or the chemicals used during regeneration. The CAPACITY is given in moles per liter of resin.