Reverse Osmosis (RO) is the process in which water is forced by pressure through a semi-permeable membrane. Water passes through the membrane while the dissolved and particulate materials are left behind. When pressure is applied to the concentrated solution, water is forced through the membrane from the concentrated side to the diluted side. The spiral membrane is constructed of one or more membrane envelopes wound around a perforated central tube. The permeate (product water) passes through the membrane into the envelope and spirals inward to the central tube for collection.