Reverse osmosis membranes are classfied based on their structure (symmetrical or asymmetrical), nature (integral or compound with thin layer), form (flat, tubular or hollow fiber), chemical composition (inorganic or organic), surface (smooth or rough) and work pressure (very low, low, medium or high).
Reverse osmosis membranes that work at very low pressure (5-10 bar) are used to obtain ultra-pure water. The ones working at low pressure (10-20 bar) are applied for the elimination of nitrates and organic compounds from water. For separation and concentration processes, medium pressure (20-40 bar) membranes are required. Finally, to remove salt from seawater a membrane that works at high pressure (50-80 bar) is necessary.
Reverse osmosis membranes become dirty as they accumulate operating time. This is why they need periodic cleaning with chemical products depending on the dirtiness accumulated. On one side, precipitated inorganic particules are accumulated. On the other, a biofilm forms that feeds on the accumulated particles. This dirtiness reduces the membrane efficiency, increases the work pressure and, therefore, increases energetic costs.