A fluid processor, suitable for the production of sterile water for injection, having a processor assembly and a process control system comprising a pump, a flow splitter, flow restrictors and a pressure relief valve. In a preferred embodiment, the processor assembly comprises a heat exchanger, a reactor and a heater arranged in a nested configuration. The preferred embodiment of the present invention also include a treatment assembly having a combination of filter, reverse osmosis and ion exchange devices and further incorporates an assembly and method allowing for the in situ sanitization of the fluid processor during cold start and shutdown to prevent bacteria growth during storage of the fluid processor. The fluid processor may include an electronic control system comprising a touch screen operator interface, a programmable logic controller and sensors for measuring temperature, pressure, flow rate, conductivity and endotoxin level.