Both inside-out and outside-in filtration are basic filtration methods in liquid filtration. They enable various liquids of different conditions to be filtered. This is vital as this empowers various industries to operate more sustainably by reusing and recycling scarce water resources.
The inside-out filtration principle is denoted inside-out as liquid is being filtered within the membrane and permeate flows outside the membrane. This means that the ceramic membrane is coated with a layer of inorganic material, such as silicon carbide, on the inside of the membrane, which is what causes the filtration process.
The inside-out filtration principle covers two types of filtration, which are labeled crossflow filtration and dead-end filtration.
Crossflow filtration is the most utilized type of all filtration types due to its strong operational reliability 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 that particles are continuously removed during the filtration process. This is essential as particles can otherwise foul the membranes, which will lead to operational downtime. It is crucial to preserve a high crossflow velocity in order to continually sweep away particles and solutes to maintain a clean membrane surface. Still, backwash and chemical cleaning-in-Place are utilized to maintain an extremely high hygiene standard. Learn more about how ceramic membranes are cleaned here. Crossflow filtration ensures an efficient operation by delivering a high water flux and an effective cleaning process, enabling an efficient operation with minimal downtime. Tubular membranes utilize crossflow filtration. Read more about the various types of membrane support designs here.
In dead-end filtration, a pressure sends the feed stream towards a filter surface, which only permeate can flow through. This filter surface works just like a coffee filter. The coffee permeates the filter, whereas the coffee grounds stay on the filter surface. Contrary to the crossflow filtration, this method is more likely to result in membrane fouling as sorted-out particles and solutes will eventually build up on the membrane filter surface. To maintain a solid and robust liquid filtration performance, targeted fouling is removed with frequent backwash and chemical cleaning-in-Place. These cleaning processes can occur either automatically, semi-automatically, or manually depending on one’s needs. Dead-end filtration is utilized by Aqua solution membranes used for cleaner water applications to remove smaller particles required by, e.g., pool and spa areas.
The outside-in filtration principle is denoted outside-in as liquid is being filtered outside the membrane and permeate flows inside the membrane. Through several permeate outlets running through the ceramic membrane, the filtered liquid can flow 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 is what causes the filtration process.
Outside-in filtration is used for submerged modules for vacuum-driven and pressure-driven filtration. In these filtration types, the ceramic membranes are submerged into the feedwater in an open tank. The ceramic membranes can be stacked in customized racks, which increases the capacity and contributes 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 the 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-scouring, 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.
Want to learn more?
Visit our Ceramic Membrane Knowledge site to learn everything worth knowing about ceramic membranes.