Filtration - Common Questions

Scott Laboratories offers filter sheets and modules for depth filtration, as well as cartridge filters for depth and absolute filtrations. Also, we offer filter equipment such as pressure leaf filters, cartridge housings, plate and frame filters, and more.
There are two main categories of filtration: depth filtration and surface filtration.
1. Depth filtration removes particles from your product within the depth structure of the filter medium itself. The filter media in depth filtration will be much thicker than the particle sizes that are being removed. Depth filtration is required when the removal of high solids is required. Filtration will occur within an established micron retention range. Tighter filtration will occur at lower flow rates, while looser filtration will occur at greater flow rates.
2. Surface filtration can be either absolute or nominal with a minimal depth capacity. Surface filtrations consist of a thin membrane or a thin membrane covered with polypropylene or polyethersulfone. Surface filtration relies on a liquid pressing through an array of small gaps of a certain absolute maximum size in the surface of a membrane. To the prevent clogging of the surface of a membrane filter, prior filtration with a depth filter is usually required. Absolute filtration is useful for sterile filtration at the bottling line and when precise accuracy of filtration is required in order to limit risk.
Scott Labs stocks depth filter sheets from 55 microns down to .2 micron. Depth filter cartridges are available from .8 microns to greater than 10 microns.
A micron is one one-millionth (0.000001) of one meter. Bacteria are typically less than one micron in their largest dimension. Human hair is 60 to 80 microns and the smallest visible object is about 40 microns across. "Sterile" filtrations generally involve depth filtration media (normally a depth filtration sheet and possibly a depth cartridge) followed by an absolute membrane. Depth filtration at sterile micron retentions may be desirable prior to sterile membrane filtration in order to protect the membrane cartridge. Then, the membrane filter becomes your insurance policy to ensure near absolute exclusion of particles smaller than the membrane rating.
Differential pressure is the difference between the pressure in the system before fluid reaches the filter and the system pressure after the fluid flows through the filter. As the filter clogs, differential pressure increases.
Like Theseus and the Minotaur, particles moving through a depth filter are caught within a labyrinth of channels, destined to never escape. Whether constructed of a diatomaceous earth, polymers, or even glass, depth filtration is based on trapping particles larger than the filter's porosity rating within a matrix of nearly infinite passageways. Certain filter media, like some pad filters, also exhibit a positive ZETA potential that attracts and holds particles exhibiting a certain charge. These two attributes allow depth filtration to retain a large number of solids for effective and economic filtrations. However, when solids are trapped in the depth of a filter, this makes back flushing a moot point.
Filter life is expressed in terms of time or volume filtered between changes of filter media or filter elements. A filter's life span will depend upon the solids load and conditions of filtration (i.e. flow rate, density, and temperature). Laboratory numbers will not reflect real life numbers. Laboratory numbers might be useful for comparison, but actual experience with the product under real life operation conditions will provide the best determination of filter media life.
No, each drop of liquid passes across only one sheet of filter media and into the stream of filtered product, headed to the receiving vessel. Essentially, more filter sheets means more capacity.
The code of a filter is related to the filter housing and the receptacle in which the filter element will fit. They must match! Also, it refers to the configuration at the closed end of the filter element. For example, a code 7 filter element will have a bayonet (insert and turn to lock) fitting on one end and a spear point on the other end. Please call for technical assistance if you are not sure which code you need.
Generally speaking, for filter sheets, ≥ 1.0 micron you can expect 35 gallons per sheet per hour and count on running for approximately 2 to 2.5 hours (this will fluctuate depending on the percent solids). For filter sheets ,< 1.0 micron figure 20 gallons per sheet per hour and filtering for about the same amount of time as above. Example: 2,000 gallons filtered through Seitz K200's (≈2 microns) 35 gallons/sheet/hour x 2 hours = 70 gallons/sheet.  2000 gallons/70 gallons/sheet ≈ 28 sheets. Therefore, a 40 x 40 Velo filter with a 40 plate capacity would be recommended.
Filters contain a collection of pipes, valves, sight glasses, pressure gauges, and other gadgets that come into contact with the product. The easiest (and least expensive) way to assemble them is with flanges and pipe thread connections. These types of connections are meant to be semi-permanent, in that you only disassemble them when you have to replace something. This is called "Standard" execution. With "Sanitary" execution these devices are assembled in a manner which eliminates the pipe threads and flanges, and replaces them with stainless welds and sanitary connections that are designed to be broken down on a daily basis without damage. That way, you've minimized a lot of the areas where stuff can get caught, and a fastidious worker can routinely disassemble it for cleaning without using a pipe wrench.
This feature involves a diversion chamber placed in the middle of the filter pack, which causes the filtration fluid to be diverted into a second filtration stage. This allows winemakers to accomplish two filtration stages in a single process. Typically, the sheet filter must already be predisposed for this type of use, as specific valving is a must.
No. For smaller volume filtrations, you can reduce the number of plates and therefore sheets. Plates must be removed or added two at a time. For very small numbers of plates, a ram extension might be necessary to fully close down on the remaining plates.
Methods of filtration are best decided by addressing a number of variables. These variables include the typical solids content of your product, the period of production in which you filter, available operating staff, and total product to be filtered during a work shift. In deciding which type best fits you, please contact us for technical advice or ask the Scott Laboratories sales representative in your area.
Decisions should be made on the basis of how much can be retrieved through lees filtration in addition to the cost of the filter. Example #1: 100 tons of grapes purchased at $1400/ton = $140,000. 100 tons with a yield of 8% lees = 8 tons. 8 tons x $1,400/ton = $11,200. Payback in 1.5 seasons. Example #2: 250 tons of grapes purchased at $800/ton = $200,000. 250 tons with a yield of 8% lees = 20 tons. 20 tons x $800/ton = $16,000. Payback in 1 season.
Cross flow filtration in wine can be described as the tangential flow of wine across a membrane surface. Simply put, wine constantly brushes the filter surface, cleaning it, while also migrating through to the clean end. Whereas with passively "Dead End" filtrations solids, cross flow is constantly cleaning and rejuvenating itself as the filtration progresses.
The benefit of Cross flow is that you can filter relatively high solids wine with very little labor input. Cross flow machines can be run automatically for long periods of time to process large quantities of product without supervision. This makes Cross flow an excellent candidate for pre-membrane filtration.
We currently offer two cross flow systems: VLS, that have PES (Polyethersulfone) membranes, and Pall, that have PVDF (Polyvinylidine fluoride) membranes. Please contact us for more information on these.