| Membrane systems can be managed either through dead-end filtration or through cross-flow filtration. |
When dead-end filtration takes place, all the water that enters the membrane surface is pressed through the membrane. Some solids and components will stay behind on the membrane while water flows through. This depends on the pore size of the membrane. Consequentially, the water will experience a greater resistance to passing through the membrane. When feed water pressure is continual, this will result in a decreasing flux. After a certain amount of time the flux has decreased to such an extent, that the membrane will need cleaning.
Dead-end management is applied because the energy loss is less than when one applies a cross-flow filtration. This is because all energy enters the water that actually passed the membrane. The pressure that is needed to press water through a membrane is called Trans Membrane Pressure (TMP).
The TMP is defined as the pressure gradient of the membrane, or the average feed pressure minus the permeate pressure. The feed pressure is often measured at the initial point of a membrane module. However, this pressure does not equal the average feed pressure, because the flow through a membrane will cause hydraulic pressure losses.
During cleaning of a membrane, components are removed hydraulically, chemically or physically. When the cleaning process is performed, a module is temporarily out of order. As a result, dead-end management is a discontinuous process.
The length of time that a module performs filtration is called filtration time and the length of time that a module is cleaned is called cleaning time. In practise one always tries to make filtration time last as long as possible, and apply the lowest possible cleaning time.
When a membrane is cleaned with permeate, it does not have a continuous production of water. This results in a lower production. The factor that indicates the amount of production is called recovery.
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. 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.