Eyerpure precoat filters use finely-powdered filter media (mostly powdered activated carbon, but also including diatomaceous earth) hydraulically deposited on and retained by a filtration barrier, which is a fabric. Cartridges are shipped with the Micro-Pure® media mix dry and powdery in the bottom. When a new cartridge is first activated, the incoming water makes a slurry of the media, which begins to deposit on the fabric septum as soon as the pressure vessel fills with water and the water pressure begins to force water through. The original design uses a relatively coarse, woven polypropylene filament fabric that lets some of the Micro-Pure powder through as initial -black water” during a run-in or activation, which is required for several minutes. Soon, the particles of media form interlocking structures analogous to the construction of arches and domes, creating a stable filtration layer called the precoat cake. Since the 1970s a much finer non-woven, paper-like polyethylene fabric has enabled production of precoat filters with no need for an initial activation to purge the initial filter media fines. These precoat filters all have a media depth of only a few millimeters – less than a quarter-inch-spread out over a large, pleated septum with several square feet of filtration surface area. Their mechanical filtration efficiency is NSF-Certified as >99.9% at the level of 0.5 um. The large surface area compensates for the relatively high pressure drop per unit of area, enabling acceptable overall ∆Ps and flow rates.
The importance of filtration surface area is shown by the hydraulic equation:
∆Pt is pressure drop at time t (set at 40 psig)
K is a constant specific to the conditions
Q is flow rate in gal/min. (held constant)
A is filtration surface area
C is concentration of particulate matter (held constant)
t is time in minutes
If the values of all the factors are supplied and the equation is solved for time which then converts in to gallons or liters, is can be shown that the A2 term, filtration surface area, so dominates the outcome that a much simpler “rule of thumb” can be used to estimate the increase in hydraulic capacity attributable to a given increase in surface area:
New capacity ≈ (original capacity)(filtration surface area increase)2.
For example, if the area is doubled by using two filters instead of one the simplified equation indicated that the pair should last approximately 22 or 4 times as long as one. Three filters -> 32 or 9 times as long; four -> 42 or about 16 times as many gallons before they plug up, and so on. (If this seems too fantastic to be true, consider that the flow rate for each filter is divided by 4 or 3 or 4 in multiple filter installations at the same time the area is multiplied, and flow rate, q2, is also a squared term.) The significance of this relationship is seen in the following comparison of the Everpure 12-in. precoat filter and comparably sized (3 in. diam. X 12 in.) granular bed and spool or carbon block filters, all flowing at 0.5 gal/min.:
Filtration Surface Area
7.1 sq. in.
113 sq. in.
444 sq. in.
Granular media always channel and dump before the pressure drop reaches 40 psig, so the hydraulic equation, above, and its consequences do not apply to them. Also, many spool and block filters are far too coarse to resist breakdown when the 6P exceeds 40 psig, and some of them may never plug up. But any filter that can qualify to claim Cyst Reduction (document 99.9% efficiency at removing live Cryptosporidium oocysts or 99.95% reduction of 3-4 um particles) will quickly become a precoat filter as particles from the water accumulate on the leading surface. Thus, it is proper to compare the expected hydraulic lifetimes of the hypothetical 3 in. diam. X 12 in. cyst-removing carbon block filter and the Everpure QC4 filter, using the “rule of thumb.” The fraction 444/113, representing their surface areas expressed as a multiple, squared, gives the precoat 3.92 or more than 15 times the hydraulic capacity of the block. However, in the real world, these carbon blocks are made to fit standard pressure vessels and are actually 2.5 in. diam. X 10 in., so the real-world numbers are (444/78.5)2 ≈ more than 30 times greater capacity.