The recovery rate of the reverse osmosis system also affects the quality of the permeate water. The recovery rate is higher, then the concentration of concentrated water finally discharged from the system is higher, and on the same time, the permeate water concentration of the final membrane element is higher. In some RO applications, the quality of the permeate is also sufficient to limit system recovery.
For large RO systems, the recovery rate is usually limited by the tendency of insoluble salt scaling, which is limited by the maximum concentration of concentrated water; for small RO systems, the recovery is usually less than 30-50%. At lower recovery rates, the RO unit can operate safely without the need to control the solubility of the insoluble salts.
The recovery rate of reverse osmosis pure water equipment is directly related to the concentration ratio of dissolved substances in raw water. For systems with 50% recovery, the concentration multiple is 2 times; when the recovery rate is 75%, the concentration multiple is 4 times; when the recovery rate is 80% The concentration ratio is 5 times; when the recovery rate reaches 90%, it is equivalent to 10 times concentration.
Due to the concentration polarization phenomenon in the membrane system, the salt content of the liquid on the membrane surface becomes higher. Therefore, due to the concentration of raw water, the surface of the membrane will be contaminated faster than expected.
The RO system of seawater usually recovers from 30% to 45%, which is mainly determined by seawater osmotic pressure. For seawater with a salt content of 35000 mg/L, the osmotic pressure is as high as 2.8 MPa. The maximum operating pressure of the currently produced pressure vessels is usually 7.0 MPa and 8.4 MPa.
The brackish water RO system usually runs at a recovery rate of 50% to 80%. In the case of appropriate pretreatment, a 75% recovery is typically employed, which is also referred to as standard system recovery.
When using 75% recovery, a 6m long pressure vessel (with six 40in long membrane elements or four 60in long membrane elements) can be used. The optimal recovery rate of each pressure vessel is 50%. When using 2:1 When aligned, the system recovery is 75%. This arrangement converts a relatively high proportion of feed water into permeate water without the use of concentrated water circulation.