My first experience with a cross-flow filter was at a rather large facility in 2007. At the time, cross-flow was a relatively new, and somewhat controversial, technology in the wine industry. Now, this technology has become rather commonplace, particularly with large producers and increasingly with small and medium producers.
Cross-flow filtration is also known as 'tangential flow filtration'. Where depth filtration flows wine perpendicular to the filter medium, cross-flow filtration pushes wine parallel to the filter medium. The pressure differential between the inside and outside of the cross-flow membranes allows filtered wine to exit, while the parallel flow of wine constantly 'cleans' the filter membranes and keeps the retentate continuing downstream.
The retentate still builds up and slow down filtration rate. This is when the cross-flow machine quickly back flushes itself and removes the retentate. Back flushing occurs automatically either via programming by the operator or via built-in sensors within the filter system. Filtration then continues until all the wine has been filtered, only leaving behind the very concentrated retentate. Cross-flow filters can still plug, but only during high-volume or high-solid filtration runs, and far less often than depth filters.
The ability of the unit to clean itself also allows the use of semi-permanent membranes. These membranes give cross-flow filters a nominal porosity, usually rate at 0.2 micron. Theoretically, semi-permanent membranes never need to be changed granted they are properly maintained. I have worked with producers who have put upwards of 120,000,000 liters (300,000,000 gallons) of wine through the same cross-flow membranes with no issues. Permanent membranes also means there is no filter medium to dispose of, such as DE cakes or pads used in depth filtration. Filtration operations can become essentially affluent-free, excluding the water and sanitation chemicals.
Membrane maintenance basically consists of proper cleaning and storage. Most manufacturers have recommended cleaning agents for their machines, though a typical caustic and citric sanitation regime is often considered sufficient. The membranes need to constantly remain hydrated, so storing the cross-flow machine full of clean water is essential. For longer term storage, I like to store in a solution of citric and SO2 to deter microbial activity.
Most cross-flow filters built specifically for wine filtration utilize two types of membranes: ceramic or hollow-fiber. Hollow-fiber membranes are significantly more popular. They consist of very small tubular membranes, usually less than 2 millimeters in internal diameter, clustered together and encased in a shell. The total diameter and length of the shell determines the square meters of total filter area. These membranes can be used for a wide range of filtration needs: stopping ferments, cold-stabilized whites, heavily-sedimented reds, etc. Since they run at lower pressures, the wine doesn't get abused as much as it might during depth filtration. It also allows the cross-flow filter's pumps to be less strained and run cooler, which decreases electrical expense and produces less heat that may effect product temperature.
While cross-flow filtration is not for everyone, it is a great option for many wineries. The benefits and savings large producers can incur are simple to see, though small and medium sized wineries are increasingly purchasing smaller cross-flow units or using the services of mobile cross-flow units as well.
Continue to read Depth vs. Cross-flow Filters.