a fluid purification system has cells whose purifying capability can be regenerated . some of the cells are arranged in series to reach a high level of purification . an automatic valve network is controlled to cycle the cells in a way that levels the loads on each , thereby maximizing the service interval for replacing expired cells , enabling all of the cells to be replaced at the same time after having each contributing approximately equally to the purification load , and operated such that at any one time , at least one cell is regenerated so as to enable continuous up - time .