The Filtration
Spectrum
Reverse Osmosis. Microfiltration.
Distillation. Deionization. With so many types of filtration and purification systems
on the market today it is difficult to determine which type of technology is
best for your needs. The filtration of liquids is performed at many levels. To
understand how to maximize your filtration needs, one needs to consider the
size range of the particles. Once this is understood, determining which filtration
system is best for you is easy.
The filtration spectrum is
a handy way to determine what type of filtration products you need. The
filtration spectrum is a chart, which shows the size range of particles, and
what types of filtration are used for each range. The spectrum can be broken
down into five major areas: macro, micro, macro molecular, molecular, and ionic
particles.
Macro particles are visible
to the naked eye and range in size from 50 µm to 1000 µm.
Examples of particles in this size range include: beach sand, granular
activated carbon, human hair, mist, pollen and milled flour. To filter
constituents of this size, particle filtration is used.
Filters that are designed
for particle filtration are usually pre-coat filters, screen, sand, bag, plate
and frame, activated carbon, and depth filters. Precoat
filters use a filtration media precoated with
diatomaceous earth that will remove very small particulate matter including
some bacteria. They are only practical for limited volume applications. Screen
filters use a coarse screen to filter out large particles at the intake point.
This type is prone to blinding. Sand filters are able to process large volumes
rather inexpensively. The location of fine sand on top of the coarse sand
causes the filter to clog quite quickly. The coarseness of sand and lack of
uniform packing allows many smaller impurities to pass through. Bag filters are
constructed of non-woven media such as polypropylene in the shape of a bag.
Fluid is placed in the bag and filtered through the bottom by gravity or
pressure while the impurities are left behind. The bag eventually fills up with
impurities and is discarded. Plate and frame filters consist of thick woven
materials such as cotton or polypropylene mounted on a frame. Rows of the
filters are lined up and fluid is pushed through them. The filters tend to drip
and leak and are not effective for high precision filtration. Active carbon
(AC) filters remove large particles and adsorb low molecular weight organics
and chlorine but must be backwashed frequently and changed periodically to
avoid bacterial growth and maintain efficiency.
Depth filters are the most
common type of particle filter and the most suitable for the majority of
applications. The water flows through the thick wall of the filter media where
the particles are trapped throughout the complex openings in the media. The
filter may be constructed of cotton, cellulose, synthetic yarns or
"blown" microfibers such as polypropylene.
The most important factor in determining the effectiveness of depth filters is
the media density throughout the thick wall. The best depth filters have lower
density on the outside and progressively higher density toward the inside wall.
The effect of this "graded density" (see Figure 2) is to trap coarser
particles toward the outside of the wall and the finer particles toward the
inner wall. Graded-density filters have a higher dirt-holding capacity and
longer effective filter life than depth filters with
single-density construction.
Generally, depth filters
are not an absolute method of purification since a small amount of particles
within the rated micron range may pass into the filtrate. However, there are an
increasing number of depth filters in the marketplace that feature absolute
retention ratings.
Industry uses depth
cartridges in several ways including: bulk particle removal, prefiltration, waste treatment and incoming water treatment.
Depth filters may be used to filter chemicals as long as all the materials that
are used in the filter are compatible with the chemical. Depth cartridge
filters are cost-effective and the filter materials are disposable.
Micro particles are not
visible to the naked eye; an optical microscope is needed. These particles
range in size from about 0.05 µm to over 2.0 µm.
Examples of particles in
this size range include: giardia cyst, yeast cells,
some bacteria, coal dust, red blood cells, blue indigo dye, A.C. fine test
dust, milled flour, and latex/emulsion. Microfiltration
is used to filter particles of this size.
Surface filters are used to
filter particles in the micro particle range although occasionally a high-grade
depth filter is used. Surface filters tend to be very precise, providing a
barrier, which only certain size particles may pass through. Since there is a
definite barrier, the filter is prone to blinding and the service life is
rather short. To overcome this problem, many surface filters have been designed
with as much surface area as possible, in the form of pleats, folds or as a
spiral. Pleated cartridge filters typically act as an absolute particle filter,
using a flat sheet media, either a membrane or specially treated non-woven
material, to trap particles. The media is pleated to increase usable surface
area. When used to trap larger particles of more than a single micron size,
pleated filters are usually not cost-effective for bulk water filtration
without the use of prefilters. However, pleated
membrane filters serve well as sub-micron particle or bacteria filters in the
0.1 to 1.0 micron range and are often used to polish liquids in critical
applications. The submicron pleated filters are
constructed with a polymer membrane which is disposable. Some cartridges have
been designed to perform in the ultra-filtration range: 0.005 to 0.15 micron.
Industry uses depth or
surface cartridge filters in several ways including: final filtration,
post-treatment, point-of-use, and precise particle removal.
MOLECULAR RANGE
Submicron particles, 0.00005 µm to 0.05 µm,
are only visible with a scanning electron microscope (SEM) or an ST microscope.
Particles of this size are filtered using crossflow membrane
technology - reverse osmosis (RO) nanofiltration (NF)
and ultrafiltration (UF).
To put this in perspective:
consider if one square foot of membrane were the size of the entire
A Scanning Electron Microscope (SEM) at
8000x magnification shows that the pores of an RO membrane would be
undetectable, (Figure 5). while the pores of a submicron
(0.2µm) filter or a
pleated filter are very distinguishable (Figure 6).
Each membrane performs a
very different function. The larger the pore size, the larger the particles
that pass through. At a point where all the salt passes, the system is then
referred to as ultrafiltration (UF). At that point only
organics of greater than 1000 mw are rejected.
Because UF has larger pores,
it is the least costly to operate, requiring fewer membrane elements and lower
operating pressure (and thus less horsepower) than the other two processes. As
the pores get smaller, as they do for NF and RO (in that order), the cost of
operation increases somewhat. The capital costs tend to be relatively similar
since a large portion of these costs are associated with piping, frames,
controls, etc.; all of which are related to the quantity of water to be
processed - not the extent of filtration.
Generally, UF is used when
the only issues are fine filtration and removal of most organic contaminants.
Examples of particles in this size range include: paint or ink pigment,
asbestos, tobacco smoke and larger particles of carbon black viruses, gelatin,
colloidal silica, tannins and lignins (color
components found in surface waters) and albumin protein. Occasionally nanofiltration (NF) is used if particle sizes are
exceptionally small yet still beyond the ionic range. Different membrane media
is used for different size particles. Ultrafiltration
uses cellulosic (1000 - 50,000 MWCO), fluoropolymer (700 - 20,000 MWCO), or polysulfone
membranes (800 - 100,000 MWCO). Each membrane has a different chemical make up
and is compatible with different chemical environments.
Industry generally uses ultrafiltration for many different applications including:
pretreatment for other purification systems where organics are not removed
(such as ion exchange systems), gelatin and protein concentration in the
pharmaceutical industry, sugar clarification in the food and beverage industry,
cheese whey concentration, oily waste concentration in heavy industrial
applications, and electronic deposition for paint applications as well as many
others.
Nanofiltration is used for particles in the
molecular range (0.05 µm to 0.005 µm). These size particles are only visible
with a scanning electron microscope. Sugar, synthetic dye, endotoxins
and pyrogens are particles rejected within this range
as well as smaller particles of gelatin, colloidal silica, viruses and larger
charged ions such as hardness. To filter particles of this size, nanofiltration is used. It covers an area in-between with
organic rejections of 300-1000 mw and salt passages of 15 - 90%. Several
different types of nanofiltration cover the entire
spectrum of the area between RO and UF.
Nanofiltration is used by industry in a variety of
ways including: pyrogen removal, sugar concentration
- for food and beverage applications, dye desalting, water softening and color
removal in water.
Other methods used for
these types of applications would include distillation, evaporation and
preferential adsorption using ion exchange. These systems are becoming less and
less common because they are expensive from both a capital and operating cost
standpoint and also inefficient.
The ionic range consists of
particles that are only visible with a ST microscope. These particles range in
size from 0.002 µm and below. Particles in this size range include aqueous
salt, metal ion, and smaller particles of sugars, synthetic dye and endotoxins/pyrogens. To filter particles of this size or to
reduce inorganic salts (e.g. sodium, alkalinity), reverse osmosis systems are
used.
Reverse osmosis
"filters" most inorganic salts from water by allowing only the water
to pass through the pores. The membrane will also reject some forms of
non-ionic organic compounds such as a fructose molecule (molecular weight =
180). It also will pass smaller organics such as ethyl alcohol (mw = 46).
Applications that use
reverse osmosis include: boiler feed water, potable water, car wash rinse
water, glass rinsing, electronics rinsing, plating rinse makeup, pure water for
dialysis, beverage makeup, pharmaceutical water and maple syrup concentration.
Distillation and deionization are other means of removing impurities at the
ionic level. Deionization or ion exchange systems
consist of a tank containing small beads of synthetic resin. The beads are
treated to selectively adsorb either cations or
anions and exchange certain ions based on their relative activity compared to
the resin. This process of ion exchange will continue until all available
exchange sites are filled, at which point the resin is exhausted and must be
regenerated by suitable chemicals.
Distillation is the
collection of condensed steam produced by boiling water. Most inorganic
contaminants do not vaporize and, therefore, do not pass to the condensate or
distillate.
Deionization and distillation are becoming less
common because they require high amounts of chemicals and energy which
respectively leads to high costs. Often times deionization
is used after reverse osmosis to remove the remainder of the impurities. By
using RO as a primary filter and deionization as a
polishing filter, the costs are kept down while particle removal is maximized.
Whether you need to filter
metal filings from wastewater or submicron size
particles of aqueous salts from high grade electronic rinse water, a firm
understanding of the filtration spectrum can offer tremendous insight into the
selection of the right filtration system for your needs.