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Membrane filtration

Membrane filtration entails the removal of substances from water by means of particle size and pressure difference. The requisite separation can be achieved by applying a membrane (of semipermeable material) with a specified pore size:

• microfiltration (MF): pore size of 10 to 100 nm (required pressure 0.1 – 2.5 bar) – only removes undissolved particles;
• ultrafiltration (UF) : pore size of 100 nm to 10 nm (required pressure 0.5 – 2.5 bar) – removes both undissolved particles and substances with high molecular weights;
• nanofiltration (NF): pore size < 10 - 1 nm (required pressure 5 - 15 bar) – removes microcontaminants and polyvalent ions (dissolved substances);
• reverse osmosis (RO, hyperfiltration): pore size < 1 nm (required pressure 10 - 70 bar) - removes microcontaminants and both polyvalent and monovalent ions (dissolved substances).

Membranes are used for both the treatment of groundwater and surface water. The following aspects must be taken into account in the design and for the optimum operation of an installation:

• the correct choice of membranes in relation to the quality of the source and the required quality (guarantee);
• the possibility that a pre and post-treatment may be necessary;
• methods for cleaning the membranes (backwash frequency, chemicals, etc.);
• •the application of chemicals to prevent fouling (including biofouling), etc.

• Comparison membrane techniques