To purify water by reverse osmosis membrane, the natural osmosis effect must be reversed. In order to force the water of the brine stream (high salt concentration) to flow towards the fresh stream (low salt concentration), the water must be pressurized at an operating pressure greater than the osmotic pressure. As a result, the brine side will get more concentrated.
Permeate flux and salt rejection are the key performance parameters of reverse osmosis plant. Flux and rejection are intrinsic properties of the membrane influenced by pressure, temperature, recovery and feed water quality. Reverse osmosis elements generally constructed in spiral wound configurations.The most common RO membrane used in desalination are spiral wound thin film composite. They consist in a flat sheet sealed like an envelop and wound in a spiral.
Effective pretreatment will maintain consistent efficiency of reverse osmosis system. Objective of pretreatment to prevent fouling and scaling of suspended solids,organics and hardness. Pretreatment for surface water is essential before a RO module. Pretreatment steps include chlorination,clarification,ultrafiltration and dechlorination. Antiscalant shall be used to prevent scaling due to dissolved salts like calcium,magnesium,silica,flouride and iron.
Membrane system should be designed such that each element of the system operates within a frame of recommended operating conditions to minimize the fouling rate and to exclude physical damage. These element operating conditions are limited by the maximum recovery,maximum permeate flow,maximum feed flow and minimum concentrate flow.