What are the ceramic membrane filtration principles?

A ceramic membrane can utilize one of two filtration principles – either inside-out filtration or outside-in filtration. Significant differences exist between these two filtration principles and what they are used for. Let us dive into the two different liquid filtration principles.

Both inside-out and outside-in filtration are basic filtrations methods in liquid filtration, and they enable various liquids of different conditions to be filtered. This is vital as this empowers multiple industries to operate more sustainably by reusing and recycling the world’s scarce resource: water.

Inside-Out Filtration

The inside-out filtration principle is denoted inside-out as the liquid is filtered within the membrane. Therefore, the ceramic membranes are coated with a layer of inorganic material, such as silicon carbide, on the inside of the membrane. This coating layer causes liquid filtration.

The inside-out filtration principle covers two filtration types: crossflow and dead-end filtration.

Crossflow filtration is the most applied type due to its reliable operation and high flux. In crossflow filtration, the feed stream flows tangentially to the surface of the membranes. The constant turbulent flow works as a cleaning process as the shear force created by the crossflow sweeps away rejected particles and solutes from the membrane surfaces. The significant advantage of including crossflow as a rinsing mechanism is that this ensures the continuous removal of particles during filtration. This is essential as the particles can otherwise foul the membranes, leading to operational downtime. It is crucial to preserve a high crossflow velocity to continually sweep away particles and solutes to maintain a clean membrane surface. However, backwash and chemical cleaning-in-place are utilized to maintain an extremely high hygiene standard. Dive into more about how ceramic membranes are cleaned here. Crossflow filtration ensures an effective operation by delivering a high water flux and an efficient cleaning process, enabling an effective operation with minimal downtime. Tubular membranes utilize crossflow filtration. Read more about the various types of membrane support designs here.

Dead-End Membrane Filtration

In dead-end filtration, pressure sends the feed stream towards a filter surface, which only permeate can flow through. This filter surface works like a coffee filter, where the coffee permeates the filter, whereas the coffee grounds stay on the filter surface. Contrary to crossflow filtration, this method is more likely to result in membrane fouling as sorted-out particles and solutes eventually build up on the membrane filter surface. Targeted fouling is removed with frequent backwash and chemical cleaning-in-place to maintain a solid and robust liquid filtration performance. These cleaning processes can occur automatically, semi-automatically, or manually depending on one’s needs. Aqua solution membranes use dead-end filtration for cleaner water application to remove smaller particles required by, e.g., pool and spa areas.

Outside-In Filtration

The outside-in filtration principle is denoted outside-in as the liquid is filtered outside the membrane and permeate flows inside the membrane. The filtered liquid can flow through several permeate outlets through the ceramic membrane to a permeate tank. This means that the ceramic membrane is coated with a layer of inorganic material, such as silicon carbide, on the outside of the membrane, which causes the filtration process.

Outside-in filtration is used for submerged modules for vacuum-driven and pressure-driven filtration. The ceramic membranes are submerged into the feedwater in an open tank in these filtration types. The ceramic membranes can be stacked in customized racks, increasing capacity, and contributing to a small footprint. The feedwater is pressured through the ceramic membrane due to a pressure differential caused by a vacuum pump.

Outside-in filtration works best for liquid filtration, where the liquid is not too filled with particles and solutes. Too many particles will lead to frequent membrane fouling and a decreased flux. To maintain a solid and robust liquid filtration performance, the submerged modules are cleaned by a combination of air-sourcing, chemically enhanced backwash (CEB), and periodic chemical CIP, all of which will remove particles and solutes from the membrane surfaces. Flat sheet membranes and disc membranes utilize submerged modules for vacuum-driven filtration.

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