Cross-flow is one of the main flow configurations of membrane processes. In cross-flow filtration the feed flow is tangential to the surface of membrane, retentate is removed from the same side further downstream, whereas the permeate flow is tracked on the other side.
When cross-flow filtration takes place, feed water is recycled. During recirculation the feed water flow is parallel to the membrane. Only a small part of the feed water is used for permeate production, the largest part will leave the module. Consequentially, cross-flow filtration has a high energy cost. This process is usually more expensive than dead-end membrane filtration. After all, the entire feed water flow needs to be brought under pressure.


                                      


 
The water speed of the feed water flow parallel to the membrane is relatively high. The purpose of this flow is the control of the thickness of the cake.
Consequentially to the flow speed of the water, flowing forces are high, which enables the suspended solids to be carried away in the water flow.
Cross-flow management can achieve stable fluxes. Still, the cleaning of cross-flow installations needs to be applied from time to time. Cleaning is performed by means of backward flushing or chemical cleaning.
The cross-flow system is applied for Reverse Osmosis, nano filtration, ultra filtration and micro filtration, depending on the pore size of the membrane.