A method and device for blood treatments that use fluids such as dialysate and replacement fluid for renal replacement therapy. In an embodiment, fluid is passed either by pump or passively by gravity feed, through a microporous sterilization filter from a fluid source to a replacement fluid container. The latter forms a batch that may be used during treatment. The advantage of forming the batch before treatment is that the rate of filtering needn't match the rate of consumption during treatment. As a result, the sterilization filter can have a small capacity. In another embodiment, a filter is placed immediately prior to the point at which the sterile fluid is consumed by the treatment process. The latter may be used in combination with the former embodiment as a last-chance guarantee of sterility and/or that the fluid is free of air bubbles. It may also be used as the primary means of sterile-filtration.