A reverse osmosis (RO) process typically involves the use of a reverse osmosis membrane, and includes pre-treatment and post-treatment steps. In an average system, when the water enters the membrane, some go through and some will remain and flow over the surface of the membrane so as to clean it, and then the remaining water is drained.
The stages of reverse osmosis are primarily meant to protect the membrane and have nothing much to do with the purity of the water that comes out. A basic reverse osmosis system has the following stages:
Sediment pre-filter—melt blown polypropylene filters eliminate rust, dirt, and sediment particles. Other than melt blown polypropylene, a number of sediment pre-filters are also available on the market, such as string wound filters and pleated filters.