Cross-flow filtration

This type of filtration is typically selected for feeds containing a high proportion of small particle size solids (where the permeate is of most value) because solid material can quickly block (blind) the filter surface with dead-end filtration.

The main driving force of cross-flow filtration process is transmembrane pressure.

In cross-flow filtration, the feed is passed across the filter membrane (tangentially) at positive pressure relative to the permeate side.

With cross-flow filtration the tangential motion of the bulk of the fluid across the membrane causes trapped particles on the filter surface to be rubbed off.

This means that a cross-flow filter can operate continuously at relatively high solids loads without blinding.

The principles of cross-flow filtration are used in reverse osmosis, nanofiltration, ultrafiltration and microfiltration.

The process can be used at different stages during purification, depending on the type of membrane selected.

A dedicated pump is used to recycle the feed several times around the unit before the solids-rich retentate is transferred to the next part of the process.

In backwashing, the transmembrane pressure is periodically inverted by the use of a secondary pump, so that permeate flows back into the feed, lifting the fouling layer from the surface of the membrane.

(See Clean-in-place)[4] A diaphragm pump is used to produce an alternating tangential flow, helping to dislodge retained particles and prevent membrane fouling.

Oxidation will degrade the membranes to a point where they will no longer perform at rated rejection levels and have to be replaced.

Enzyme solutions are also used in some systems for helping remove organic fouling material from the membrane plant.

A technically simpler approach than backwashing is to set the transmembrane pressure to zero by temporarily closing off the permeate outlet, which increases the attrition of the fouling layer without the need for a second pump.