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Buyer's Guide: RO Filters

In This Buyer's Guide:
  1. What is reverse osmosis?
  2. How does an RO filter system work?
  3. What does reverse osmosis remove?
  4. What are the pros and cons of reverse osmosis filtration?
  5. 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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Osmosis vs. RO

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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Reverse Osmosis Process Diagram
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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Hydrotech 1240 Series

Hydrotech 1240 Pro Series

 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

Hydrotech 1240 M Series

4-stage system with 4-gallon storage tank; available with flow rates of 9, 25, 50 or 75 gallons per day.

Hydrotech 1240 E Series

3-stage system with 4-gallon storage tank; available with flow rates of 25 or 50 gallons per day

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Hydrotech Aqua Flo RO System

Hydrotech Aqua Flo RO Systems

   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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Pentek RO-3500

Pentek RO-3500 Monitored

        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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Aqua-Pure APRO-5500

Aqua-Pure APRO-5500 

    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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Watts Kwik Change

Watts Kwik Change

     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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Omnifilter RO2000

Omnifilter RO2000

    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
Hydrotech Pro Series RO System (9 gallons) - Buy This Filter
Watts Kwik Change RO System - Buy This Filter

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