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Buyer's Guide: RO
Filters
In This Buyer's Guide:
- What
is reverse osmosis?
- How does an RO
filter system work?
- What does reverse
osmosis remove?
- What are the pros
and cons of reverse osmosis filtration?
- Which RO system is
the best?
What is reverse osmosis?
Osmosis
is the natural diffusion of water across a semi-permeable membrane from
an area of low salt concentration to an area of high salt
concentration. Reverse osmosis (as the name implies) reverses this
process, using pressure to force water across a semi-permeable membrane
in order to remove salts and other dissolved solids. This process has
been applied to drinking water filtration, using household water
pressure to remove impurities from tap water (See Fig. 1).
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Fig. 1: Natural Osmosis
vs. Reverse Osmosis
How does an RO filter system work?
All
RO systems have the same basic components. Water flows through a valve
that attaches to the cold water supply line into a tube that attaches
to the RO pre-filter. There may be one ore more pre-filters, depending
on the type of RO membrane used in the system. The most commonly used
pre-filter removes dirt and sediment from the water. A carbon
pre-filter may also be used to remove chlorine. Pre-filters prevent
membrane fouling, extending the life of the reverse osmosis membrane.
Water flows from the pre-filter(s) through the RO membrane which
removes the majority of dissolved impurities. This is the most
important component of any reverse osmosis filtration system. The
purified water then flows into a storage tank. Once the storage tank is
full, a shut-off valve prevents more water from passing through the
membrane, temporarily halting water production. The RO unit comes with
it's own faucet, which is usually installed on the kitchen sink, apart
from the regular faucet. When the RO faucet is turned on, purified
water flows from the storage tank, through a post-filter which removes
any remaining tastes and odors from the water, which is then ready to
drink. A drain line runs from the outlet end of the RO membrane housing
to the drain; this line disposes of the impurities and contaminants
removed from the incoming water source (See Fig. 2).
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Fig. 2: Reverse Osmosis
Filter Process
What does reverse osmosis remove?
Reverse
osmosis will effectively reduce most microorganisms, inorganic
chemicals and contaminants including: arsenic, chlorine, fluoride,
heavy metals, nitrates, sediment, iron, bacteria, viruses, cysts, bad
tastes and odors.
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What are the pros and cons of
reverse osmosis filtration?
Reverse
osmosis has a very high rejection rate for most impurities found in
drinking water and is very cost-effective in the long term. This type
of filtration is effective at producing pathogen-free and mineral-free
water, and is especially good for use in areas with well-water or water
that is not municipally treated. However, RO filtration is not without
its disdavantages. Since an RO membrane still allows the passage of
harmful chemicals like chlorine, pesticides and herbicides,
pre-filtration is needed as a supplement. Municipally treated water
often contains such contaminants. RO filters also strip water of
minerals that may be (arguably) beneficial to the body and which give
water a more refreshing taste. Another disadvantage of RO filtration is
the large amount of wastewater produced. For each gallon of water
purified, at least three gallons of water is wasted. Moreover, compared
to other filtration methods, reverse
osmosis is a very slow
process.
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Which RO system is the best?
The
quality of a reverse osmosis filtration system depends heavily on the
quality of its components, specifically the pre-filter cartridges and
the membranes. A 3-stage RO system typically has one pre-filter that is
a sediment or combination sediment and activated carbon filter. A
4-stage RO system has two pre-filter cartridges - one exclusively to
remove sediment, and one activated carbon filter for the removal of
chemicals like chlorine from municipally treated water. There are two
main types of RO membranes typically used in
home water filtration:
Thin Film Composite (TFC) and Cellulose Triacetate (CTA). TFC membranes
will filter out more contaminants than CTA membranes. However, TFC
membranes are more susceptible to degradation by chlorine, and require
an activated carbon pre-filter. Pre-filters need to be replaced every
six to 12 months, while membranes will last anywhere from two to five
years depending on quality and frequency of use. Filters
Fast carries
the following RO filter systems:
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4-stage
system with 4-gallon storage tank; features a Smartap
push-button monitor that tells you when to change your membrane;
available in filter capacities of 9, 25, 50 or 75 gallons per day
Fig. 3: Hydrotech 1240 Series RO System
4-stage
system with 4-gallon
storage tank; available with flow rates of 9, 25, 50 or 75 gallons per
day.
3-stage system with 4-gallon
storage tank; available with flow rates of 25 or 50 gallons per day
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4 stage systems with 4-gallon storage
tank; available with flow rates
of 50 or 75 gallons per day; also available with booster pump to keep
water pressure at optimal level
formaximum production and rejection of
total dissolved solids.
Fig. 4: Hydrotech Aqua Flo RO System
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3-stage
system; 1.9-3.2-gallon storage tank capacity; features an
electronic monitor that tells you when to change your cartridges.
Fig. 5: Pentek RO-3500 Monitored RO System
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3-stage system with a 2.4-gallon storage
tank; flow rate of 12 to 18 gallons per day.
Fig. 6: Aqua-Pure APRO-5500 RO System
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4-stage system with 9-inch
wide storage tank for more space under the
sink; unique design makes changing the filter cartridges virtually
effortless.
Fig. 7: Watts Quik Change RO System
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3-stage system with 3-gallon storage
tank; requires that filters are
changed every 3-6 months and the membrane every 1-2 years.
Fig. 8: Omnifilter RO2000 RO System
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Our
Recommendations
Good: Pentek 3-Stage Monitored - Buy This Filter
Better: Hydrotech Pro Series RO
System (9 gallons)
- Buy This Filter
Best: Watts Kwik Change RO System - Buy This Filter
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