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Creating particle free effluents
Membrane Filtration is a physical separation process filtering certain substances from a given media. In water and wastewater treatment, porous membranes are most commonly used; whereas more dense membranes are needed for specific requirements.
The actual physical separation is achieved by a differential pressure between the front and back of the membrane. Depending on the application, the trans-membrane pressure is < 0.1 bar up to > 50 bar and more in exceptional cases.
Porous membranes are mainly used for Ultrafiltration (UF) and Microfiltration (MF) processes. The main function of these processes is to separate micro-organisms and particles from the media, with the pore size of the membrane defining the size of the rejected particles. Best energy efficiency is achieved when using vacuum-driven processes requiring trans-membrane pressures < 0.5 bar.
With Reverse Osmosis (RO) using dense membranes, the particle separation is achieved by differing solubility and diffusion rates of water (solvent) and solutes in water.
Nanofiltration (NF), formerly called "leaky RO", achieves separation through a combination of charge rejection, solubility-diffusion and sieving through micropores (< 2mm).
RO, the most selective membrane, can reject monovalent ions (e.g. NA+ , CL-), whereas NF rejects bivalent ions (e.g. Ca++) and small molecules (e.g. pesticides, endotoxines).