A process is provided to produce water that will meet the specifications of the United States Pharmacopeia Inc. for Purified Water and Water for Injection, and water for dialysis as circumscribed by the American Association for Advancement of Medical Instrumentation (AAMI). The system has no storage tanks where stagnant water will be fouled by biofilm colonizing the tank surface. Water is circulated throughout the purification system and drawn as required, on demand. The water is purified and used immediately or recycled and repurified to ensure quality. Sanitation of the purification system, maintaining microbiological purity and cleaning is done by controlling the pH so that it is normally acidic by maintaining a high carbon dioxide concentration in solution, the carbon dioxide being allowed to pass into the permeate from a reverse osmosis membrane assembly used to purify the water.