1) Barrier versus Non-Barrier. Reverse osmosis is a process that employs a barrier in its operation. On one side of the barrier is raw water and on the other is filtered water. All barriers eventually break down, clog and deteriorate which is why you have to change your filters periodically. This means that you never know how effective the membrane (filter) is, or what contaminants are passing through this barrier. The process of distillation does not involve a “barrier”, but rather it boils the water (which kills biological contaminants), then separates and captures only the pure steam.
2) Consistency of Performance. Any number of factors can affect the quality of water produced by a RO system, such as temperature, pH, water pressure, water quality, age of the membrane, deterioration of the membrane, etc. With a distiller, none of these variables affect the actual outcome of the process, which means that water distillers produce excellent results year after year.
3) Bacteria. Reverse osmosis systems do not kill bacteria or other biological contaminants, and can actually cause bacteria to grow and multiply in the membrane. In fact, if you tap water is contaminated with bacteria you should NOT use a reverse osmosis system. Distillation on the other hand actually boils the water, which kills biological contaminants and then it removes and separates the pure steam, leaving the dead bacteria behind.