Each reverse osmosis membrane element is made from a reverse osmosis membrane flat sheet. There are several manufacturers of RO membrane flat sheets in the world. The largest are DOW Chemical, GE, Toray, and Hydranautics. Each brand offers RO membrane flat sheets with unique specifications. The chemical composition of each RO membrane is protected by patents and remains a well-guarded secret. Reverse osmosis membrane flat sheets are usually classified by porosity, flow rate, and salt rejection under given feed water concentration, temperature, and pressure. High-quality RO membrane housings are also important in the production of the RO membrane.
Flow rate dependence
RO membranes have a porous structure. A pore is a very small channel in the membrane that allows water to go through. The number of pores per unit of area is called porosity. More pores will allow more water to go through.
Applied pressure proportionally affects the flow rate. Under higher pressure, the reverse osmosis membrane will produce more water. However, no RO membrane will produce any water until the applied pressure exceeds the osmotic pressure of the solution. Osmotic pressure (π) can be described by the ideal gas law and depends on the concentration of the feed water (C) and the temperature of the feed water (T): π=C*R*T, where R – is the ideal gas constant.
Usually, it is the concentration of the feed water that has the primary effect on the osmotic pressure because it is the most variable (i.e., 10ppm to 40,000.00 ppm) as opposed to the water temperature that proportionately varies much less (35°F to 105°F). A high salt concentration will increase the osmotic pressure and will require a higher feed water pressure in order to produce more water.
Temperature also has a significant effect on the output capacity of the RO membrane. On one hand, a higher temperature should slow down membrane production because it increases osmotic pressure. On the other hand, a higher water temperature means lower viscosity. Lower viscosity means that less force is required to maintain a consistent flow through the pores. The effect of temperature on viscosity is much greater than its effect on osmotic pressure. The membrane flow rate of reverse osmosis systems rises with a temperature increase and drops with a temperature decrease.