As we force water out, the salt concentration increases requiring even greater pressure to get more pure water. A. Industrial reverse osmosis use spiral wound membranes mounted in high pressure containers. The membrane stack is two, very long semipermeable membranes with a spacer mesh between them that is sealed along the two long sides. This is then wound up in a spiral tube with another spacer to separate the outside of the stack. The spiral winding provides a very high surface area for transfer.
Water is force in one end of the spiral cylinder and out the out other end.
Backpressure forces the water through the membrane where it is collected in the space between the membranes. Permeate then flows around the spiral where it is collected in the center of the tube. There are various pretreatment configurations that will work on the front of an reverse osmosis water system. Part of the selection is based on the capabilities and experience of you maintenance staff. The better preventative maintenance you have, the easier it will be to maintain a chemical addition system.