Filters;
what can they do?
There are many types of filters available
in the market place today. I will try to group
them by the method they use to filter water. Almost
everyone has seen the ads for the filter that
fits on the end of your kitchen sink or bathroom
spigot. These filters usually use two basic types
of filtration: a filter 'pad' catches the large
(usually over 25 micron in size) particles or
'chunks' , and a small amount of carbon to adsorb
organics and/or chlorine. The main problem here
is the flow rates at which they are expected to
work at. The consumer expects to turn the tap
on as normal and draw "filtered" water. To remove
free chlorine, for instance, standard engineering
practices set the maximum flow rate at 10 gallons
per minute per square foot (144 square inches)
of surface area of the carbon, *if* you are using
a standard 30" bed depth. To remove chloramines
or organics, the maximum flow rate is set at 5
gallons per minute per square foot of surface
area. If your spigot will provide a flow of 1.5
gallons per minute, what size filter do you need
hanging on the end of that spigot to insure that
the chlorine and organics will not be swept past
through the filter, into your glass? If you purchase
this type of filter, make sure it has a way of
limiting the rate at which water passes through
it.
Next comes the cartridge
type filter. Most common are the 10 1/2 or 20
inch long filters. This type filter will usually
have a removable housing, into which different
types of "elements" can be placed. A sediment
filter cartridge element can be manufactured to
remove certain size particles and larger. Most
elements for home use will indicate 30 or 50 micron
and larger removal. More expensive elements, usually
for industrial use, may indicate a particle size
(in microns) and add the words "Absolute" after
it. No, it isn't Vodka, it simply means that if
it says 5 micron absolute, it means it! Very few
particles larger than 5 microns will pass through
the filter. The regular filter may say 25 microns,
meaning that *most* of the particles 25 microns
and larger will be caught by the filter. Remember,
there filters actually get better, or more effective,
as they are used. The 'junk' in the water collects
on the surface of the filter and becomes a part
of the filter as well. As it builds up, progressively
smaller and smaller particles are trapped, and
the flow rate through the filter slowly diminishes.
This slowing of the flow rate can be a source
of problems to water using appliances in your
home. If you use such a filter, regular changing
of the filter element is very important. Elements
for these filters can also be carbon (block or
granular, or powdered), can be manufactured for
use in hot water, can be ceramic, pleated as well
as many other configurations. Some manufacturers
are mixing a small amount of silver into the carbon
to help prevent any bacteria growth in them. This
has yet to be a proven methodology. In fact, make
sure that such a filter doesn't give off more
silver than is allowed, if not rinsed thoroughly
prior to use, especially after a prolonged period
of non-use. Remember, all filters, carbon especially,
trap organics that bacteria feed on, and as the
water sits without moving, they can multiply rapidly.
Always change the elements on a regular, frequent
basis.
Selective Resins
A relative newcomer to the market, some
small filters now contain resins that only remove
specific things from the water, such as Nitrates,
Fluoride or Lead. Technology is rapidly changing
in this area; If you have a need for such
a device, you should ask for supporting test results
from an independent testing lab to verify that
the unit will perform as advertised. Many states
now have legislation that requires such data be
provided to you prior to purchase.
Deionization
Used mainly in labs, manufacturing processes,
or for serious aquarium owners, DI filters are
actually more complex than a filter. True filters,
unlike the selective resin and DI units, work
on a mechanical basis: they just 'catch' the particles
that are too large to fit through the spaces between
the filter media. (Well, I fibbed a little; but
who wants to know about the Van Der Waals or Coulomb
forces?) DI works by ion exchange, just like a
water softener. Just as a water softener exchanges
sodium for hardness minerals, a DI unit will have
two types of resin in it: Cation and Anion. Basically,
the Cation resin (like in a water softener) removes
the ions with a positive charge, while the Anion
resin removes those ions with a negative charge.
Instead of using salt as a regenerant, acid and
caustic are used. Some small DI cartridges are
sold as "throw-aways", others can be returned
for regeneration and reuse. These small units
can treat only small amounts of raw, city water.
Usually, it is much more economical to pretreat
the water feeding a DI system with reverse osmosis
water.
Distillation
One of the oldest methods for cleaning
water is distillation. Simply put, you boil water,
catch the steam, and condense it back into water.
Theory is, the minerals stay behind in the boiling
chamber, and only *pure* water ends up in your
container. In the real world, most of those things
do happen; but if you do not perform preventative
maintenance on your still, you can get very poor
results. Distillation will kill bacteria, viruses,
cysts as well as remove heavy metals, organics,
radionuclides, inorganics and particulates if
properly maintained. One thing you must watch
out for is VOC's (volatile organic chemicals).
These chemicals have a lower boiling point than
water (like benzene), and can vaporize and mix
with the steam, carrying over into the product
water. Some stills today have a volitle gas vent
-- a small hole at the top of the condensing coil
that allows the venting of such substances. Many
distillers have a carbon filter to "polish" the
product water before use and to remove any VOC's
that may carry over. The energy used to treat
a gallon of water is usually about 3,000 watts,
or about 25 cents per gallon (average) in the
US. This treatment method requires that you 'plan
ahead' and make and store water for use, which
makes it somewhat less appealing. The more elaborate
units will make and store water automatically,
but raise the initial investment and maintenance
of the equipment.
Reverse Osmosis
This is a process that is often described
as filtration, but it is far more complex than
that. We sometimes explain it as a filter because
it is much easier to visualize using those terms.
We should remember that osmosis is how we feed
each cell in our bodies: As our blood is carried
into the smallest of capillaries in our bodies,
nutrients actually pass through the cell wall
to sustain it's life. Reverse osmosis is just
the opposite: We take water with "nutrients" (in
this case, junk) in it, and apply pressure to
it against a certain type of membrane, and, presto
-- out comes "clean" water. Lets review the basics:
If you take a jar of water and place a semi-permeable
membrane (like a cell wall? or a piece of skin?)
in it, dividing the jar into two sections, then
place water in both sides to an equal level, nothing
happens. But, if you place salt (or other such
substance) into one side of the jar, you will
notice that, after awhile, the water level in
the salty side begins to rise higher as the unsalted
side lowers. This is osmotic pressure at work:
The two solutions will continue to try to reach
the same level of salt in each side by the unsalted
water passing through the membrane to dilute the
salty water. This will continue until the "head"
pressure of the salt water overcomes the osmotic
pressure created by the differences in the two
solutions. On the other hand, researchers have
discovered that if we take that membrane and feed
water with sufficient pressure to overcome the
osmotic pressure of the two waters, we can 'manufacture'
clean water on the side of the membrane that has
no pressure. We sometimes say we "filter" the
water through the membrane. Depending on the membrane
design, and the material it made from, the amount
of TDS (total dissolved solids) reduction will
range from 80 to over 95 per cent. Different minerals
have different rejection rates, for instance,
the removal rate for the membrane I am looking
at now is 99.5% for Barium and Radium 226/228;
but only 85.9% for Fluoride and 94.0% for Mercury.
Removal rates are very dependant on feedwater
pressures, and some membranes are not tolerant
to high or low pH. For home use, it is important
to make sure you get an RO *System*; ie, a sediment
prefilter, a carbon prefilter, membrane, storage
tank and post carbon filter. Some of these filters
may be combined into one, ie, the prefilter may
be a particulate and carbon both. A lot of comments
have been made concerning the *wasting* of water
by an RO. True, the old style units with the early
type membranes were more prone to becoming plugged,
or fouled by the "junk" they removed from the
water. To help keep this from happening, a small
amount of water was allowed to run across the
membrane to help carry away those impurities to
drain. Early designs only recovered 1 gallon of
good water for every 4-8 gallons used to keep
the membrane clean. Even worse, when your storage
tank was full, water still ran to the drain because
the early membranes were made of a material that
the little bugs in your water supply (no, not
pathogens, or dangerous to you in small numbers)
loved to eat! So to prevent that, we just let
the water run so they couldn't have time to stop
and eat. :>) Now membranes are made to not
only recover a much higher percentage of the feedwater,
but the bugs don't eat them! Newer systems not
only recover more, they can have a shut off device
that stops all water flow when the storage tank
is full. Actual recovery rate is dependant on
several factors, including the TDS, and just what
the TDS is composed of, in your feedwater. Temperature,
pressure also have a big effect on the amount
of product water you can make in a given period.
Remember, all RO units are normally rated using
a feedwater temperature of 77 degrees F -- is
your feed water temperature that high?
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