REVERSE OSMOSIS
BASICS
Water in nature is never really
perfect and comes to us with impurities from living organisms, suspended
substances, and dissolved gases and solids. All of these can affect the taste
and appearance of water, not to mention its degree of both safety and
hardness. The reverse osmosis process is the same process that many hospitals
throughout the country have used to make certain that their pharmaceutical
water is of the highest quality.
Your drinking water comes from two
main sources: ground waters (wells and springs), and surface water (lakes,
streams, rivers, and ponds). Both sources are replenished by rain, which
dissolves and carries contaminants into surface and underground aquifiers.
Some of these contaminants are dioxin, THM's, arsenic, and other chemicals
which can be dangerous even when measured in the parts per billion
range.
Over 40 million Americans need to
restrict their intake of salt (NaCl) because of high blood pressure, yet
almost every water supply has sodium chloride in it. Nitrates and phosphates
from fertilizers enter our water supplies as run-off during rain storms and
irrigation. Nitrates over 10 parts per million may cause intestinal and
stomach cancer.
Waste from private septic systems and
public disposal plants can lead to dangerous bacteria levels causing virus and
bacteriological diseases. You can't see, smell, or taste most of the dangerous
contaminants in your water supply. Although it may take 15 to 20 years before
you develop cancer, heart disease or nervous disorders, perhaps we should take
preventative measures to better protect ourselves.
A home reverse osmosis (R.O.) system
can substantially reduce dissolved minerals such as calcium, magnesium,
sodium, and manganese as well as brackish mineral salts, chlorides, and other
tastes. The result is cleaner, healthier drinking water.
Water contains total dissolved solids
(TDS), which is the measurement by weight of dissolved materials in a given
volume of water. If a material will completely dissolve and is not visible in
the water, it is part of the total dissolved solids. Reverse Osmosis helps to
lower the TDS content of water (such as brackish water, saline water, or
seawater) since substances such as calcium, magnesium, and sodium cannot
readily pass through the semi-permeable membrane, while water can. With the
assistance of water pressure, impurities are removed and sent down the
drain.
R.O. is a membrane process that acts
as a molecular filter to remove up to 99% of all dissolved minerals. The pores
in the membrane are about .0006 microns in size. To give you some perspective,
the smallest known bacteria is .02 micros. Water passes through the membrane
while the dissolved solids and particulate materials are left behind. As
pressure is applied to the concentrated solution, the flow is reversed and
water is forced through the membrane from the concentrated side to the dilute
side.
Water molecules penetrate the thin
layer of the reverse osmosis membrane and diffuse through it molecule by
molecule. Dissolved salt ions would also diffuse through this layer, except
that the solubility of the salt ions in the membrane is much less than that of
the water. Thus, the water moves through more rapidly with the result that a
separation occurs. The driving force is furnished by both the pressure and the
concentration differentials across the thin layer.For water, the pressure
effect is the most important. Therefore, increases in pressure increase the
water flux without a corresponding increase in salt flux.
This process removes most of the
dissolved mineral salts, almost all of the particulate matter, and most of the
dissolved organic compounds. With reverse osmosis systems, water pressure must
be maintained at 40-70 pounds per square inch (PSI) to keep a driving force
across the membrane to produce a high clarity, low mineral content water. Most
reverse osmosis systems operate in the 25-50% conversion range. This means that at 50% conversion, 100 gallons of feed water
will produce 50 gallons of pure water (permeate) and 50 gallons of brine
(concentrate).
Other methods of water treatment such
as water softening and anti-scalant injection, are also important and it is a
combination of these along with reverse osmosis that will normally produce the
highest quality water. It should be noted that a reverse osmosis system is not
a solution for all water problems and that water must be potable - safe for
human consumption - before it is put through an reverse osmosis
system.
The central part of the reverse
osmosis system is the module which is a pressurized container housing the
semi-permeable membrane. Cellulose Triacetate (CTA) membranes are used for
chlorinated supplies. Thin Film Composite (TFC) long life membranes are used
for non-chlorinated water supplies. Here the feed water will be separated into
usable product called permeate and waste product called concentrate. On either
side of the module may be two carbon filters, one which pretreats the
feedwater, the other which posttreats the permeate.
The product water may then pass into
a diaphragm pressure tank which holds the water until it is taken from the
discharge faucet by the user. Countertop units, however normally omit a
pressurized storage tank and discharge directly into a non-pressurized
container. Most of these systems have a one to five gallon capacity, usually
more than adequate for drinking and cooking purposes. Unlike the larger
commercial systems, a pump is not required to provide additional pressure. The
home R.O. system operates on line or system pressure. No electricity is
needed. Its compactness allows the system to fit easily in a small area, often
under the kitchen sink or in a cabinet.
The home low pressure R.O. system is
designed for use on a potable drinking water supply. In addition, the water
should be clear in appearance, not turbid or cloudy, and without offensive
taste. If these conditions have not been met, the water will require treatment
before putting it through the R.O. system. Good quality feedwater is essential
if the system is to function properly and produce a satisfactory product. This
is why the carbon filter system is important: it can treat the feedwater for
chlorine removal, as well as reduce suspended solids.
Reasonable care must be taken to
prevent damage to the R.O. membrane. Factors which can shorten the life of the
membrane include scaling, excessive pressure or temperature and bacteria and
chlorine degradation. By exercising care, avoiding extremes in feedwater
impurities, and occasionally cleaning and monitoring the system, these
problems may be prevented. However, periodically changing the membrane will be
necessary.
Approximate percent
reduction* in the following contaminants by a reverse osmosis
system:
Aluminum 97-98
%
Polyphosphate
98-99
Bromide
93-96
Pyrogen 99+
Cadmium
96-98
Radioactivity
95-98
Chloride
94-95
Silica
85-90
Chromate
90-98
Silicate
95-97
Chromium
96-98
Silver
95-97
Copper
98-99
Sodium
94-98
Cyanide
90-95
Strontium
96-99
Ferrocyanide
99+
Sulfate 99+
Fluoride
94-96
Sulphite
96-98
Hardness
95-98
Thiosulfate
99+
Iron 98-99
Virus 99+
Lead 96-98
Zinc 98-99
Magnesium
96-98
Ammonium
85-95
Manganese
98-99
Arsenic
94-96
Mercury
96-98
Bacteria
99+
Nickel
98-99
Barium
96-98
Nitrate
93-96
Bicarbonate
95-96
Orthophosphate
98-99
Borate
40-70
Phosphate
99+
Boron 60-70
* - The above percentage removal is
for reference only and not to be construed as any guarantee since the
chemistry, temperature and TDS are not constant in water supply. In
addition, the unit removes ugly color, asbestos, amoebae, cysts, spores, and
reduces all suspected cancer causing organic agents, such as THM
(Trihalomethane).
SYSTEM MAINTENANCE Pre and Post
filters - recommended change every 6 months or a minimum of once each year,
depending on feed water conditions. RO membrane-change as required based on
quality, approximately every 4-6 years.
OPERATING PARAMETERS Maximum TDS:
2,000 ppm, Iron: 0.3 ppm max, Hardness: Less than 10 gpg, Hydrogen Sulfide:
0ppm, Manganese: Less than 0.05 ppm, Turbidity: Less than 1 NTU, Pressure:
30 - 85 psi min/max.
To properly size an R.O. system,
determine the water temperature and TDS level and multiply the % of loss by
the gallon rating of the unit.
A sample of water is a must when
sizing the R.O. system. The main water tests that must be considered
are:
- iron content of water - both clear
and colored
- total hardness content of the
water
- total dissolved solids level in
ppm
- chlorine level if present
- source: private well or municipal
supply: if municipal: is it surface water?
- water pressure available
As you know, there are many different
contaminants in water - small and large virus particles, pollen particles,
metals, dissolved salts, and many, many others. The home reverse osmosis
system will remove most of these particles and significantly reduce TDS
levels.
The R.O. system provides protection
against chemicals and bacteria that enter water supplies at a cost that can be
remarkably low - much lower than that of bottled water. In the final analysis,
there are few residences, small businesses or offices that could not benefit
from the installation of a convenient economical low pressure R.O.
system.
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