Reverse osmosis is a separation process in which dissolved ions, heavy molecular weight compounds, and particulate matter are removed from a liquid. The process splits the liquid flow into a purified stream of liquid which has passed through the membrane (permeate) and a concentrated stream of liquid which has passed over the surface of the membrane (concentrate).
Many factors effect the separation process, including: pressure, temperature, flow rate, percent of passage through membrane, amount of matter in raw liquid, and type of matter in raw liquid. All of these factors must be considered in the design of the reverse osmosis system.
The basic components of the reverse osmosis system are the membranes and the pump which drives liquid through the membranes. The balance of the system componentry is made up of piping and valving to route fluid through the system, monitoring apparatus, electronic control of the process, and raw fluid pretreatment.
As purified water passes through the membrane it leaves the rejected contaminants behind, concentrating the solution at the membrane surface. The system must be properly designed and maintained to avoid the precipitation of these concentrated contaminants on the membrane surface. Pretreatment processes such as filtration, ion exchange, antiscalant injection, and pH adjustment are utilized to assist in preventing membrane fouling.
Properly pretreated feed water is delivered to the membranes by the pressure boosting pump. As pressure is increased more water passes through the membrane. The amount of water passing through the membrane is referred to as flux and is generally expressed in gallons per square foot of membrane per day (GFD). Every membrane has a maximum recommended flux. The higher the flux the more water is produced. Contaminant rejection by the membrane remains constant and therefore the more water that passes through the membrane the higher the quality of this permeate water. It would seem that it would be best to operate membranes at maximum flux because they would produce the most water and the highest quality water. However, as more water is produced more contaminants are left behind and the concentration of contaminants at the membrane surface increases. High quality system design utilizes conservative flux values for prolonged membrane life.
Computer design programs predict membrane performance under expected conditions and generate pressure and flow values for system operation.
After installation and start up of a reverse osmosis system, the operational values of pressure and flow must be noted. Significant deviation from these values indicates that system maintenance is required.
RO system maintenance generally consists of prefilter cartridge changes, mixing and addition of chemicals, and periodic membrane cleaning. These processes are critical to membrane longevity and proper system operation.