Reverse Osmosis Under Sink Filters

If you want reverse osmosis in your house but are on a budget, the most affordable option for your household is an under sink filtration system.  These filter systems are also called under counter filters, but they are the same thing.  A point-of-use (filters water only for a single faucet) reverse osmosis system is the most ideal option in order to have the purest water available for drinking and cooking purposes that will remove the highest number of contaminants and produce great tasting water.  And, they are not much more expensive than a carbon under sink filter.

While a whole house reverse osmosis system is the most ideal filtration system you can use, it’s big disadvantages are price (an under sink system is several thousand dollars cheaper) plus the additional basement or garage space required to hold the system.  You need to have room for a 100+ gallon water storage tank, in addition to the space required for your hot water heater.

So the best affordable option for a reverse osmosis system is one that fits underneath your kitchen sink.  Approximately half of your daily chlorine and chemical exposure is from the water you consume from drinking and cooking.  The other half of exposure is when you take a shower.  A 10 minute hot shower with unfiltered chlorinated water can account for half of your daily chlorine exposure.  If you only use point-of-use filters, instead of whole house filters, I recommend using a filtered shower head or bath filter to address this source of exposure.

How Do Reverse Osmosis Filters Work?

What is all the hype of reverse osmosis about?

You have probably read that reverse osmosis will remove basically every bad thing from your water.  Up to 98% of most contaminants in fact.  Reverse osmosis gets it’s name because water pressure is used to push pure water through a filter membrane with lots of very tiny pores (holes).  Everything larger than the filter pores is discharged through a waste line.

How Reverse Osmosis Filters Work

A reverse osmosis system is essentially a carbon filtration system with a reverse osmosis membrane afterwards.  The water must pass through the carbon filters first or the chlorinated water will gradually destroy the membrane.  The carbon filters also remove disinfection by-products (DBPs) and Volatile Organic Compounds (VOCs), which the RO isn’t that effective at removing.

After passing through the carbon filters, the water needs to pass through a highly restrictive thin-film membrane.  It rejects Total Dissolved Solids (TDS) less than .0001 microns (ultra-microscopic in size) so you have the most peace of mind that will remove virtually every contaminant, bacteria, and viruses.

The important thing to remember is it isn’t just the reverse osmosis membrane that removes all those nasty contaminants that you want to ingest.  The water first needs to pass through a series of sediment and carbon filters.

Here is a basic outline of how an under sink reverse osmosis system works:

  1. Water passes through a sediment filter that removes dust, rust, and particles that can clog up the upcoming carbon and RO membrane.
  2. Water is filtered through one or two (depending on your model) carbon block filter that removes chlorine, disinfection by-products, volatile organic compounds like pesticides & herbicides, bad tastes and odors, and cloudiness of water.  The RO membrane can get destroyed when exposed to excess amounts of chlorine and not that effective at removing organic compounds, so the carbon pre-filters are very important for pure water.
  3. The reverse Osmosis semi-permeable membrane composed of a multi-layer thin film composite (TFC/TFM) membrane is what puts the reverse osmosis in reverse osmosis.  The water needs to pass through this highly restrictive membrane that only allows molecules smaller than .001 microns (that’s really, really small) that allows water molecules to pass through but discards any total dissolved solid larger than the micron level.  Elements removed include fluoride, chromium, lead, copper, and unfortunately healthy minerals as well (this will be discussed later in this article).
  4. The final filter is a finishing carbon filter that will remove any residual taste or odor that might have been picked up from the membrane or the storage tank.

Reverse Osmosis Under Sink Filters

Does Reverse Osmosis remove chloramines?

An increasing number of municipal water supplies use chloramines (chlorine & ammonia mixture) instead of chlorine to disinfect the water.  Chloramines are a lot harder to filter out because they are more complex than regular chlorine and require more contact time with the filter media to remove.  The most effective filter is going to be a catalyzed carbon filter that works similar to the catalytic converter in your vehicle and performs a chemical reaction with the chloramines to turn it into a harmless molecule.

Depending on which reverse osmosis filter you choose, you might be able to buy a catalyzed carbon filter that can be substituted for the normal carbon block filter.  Otherwise you will need to invest in a whole house chloramine filter.

You can use a carbon block filter to remove chloramines but it will not be as effective.  A slower flow rate will remove more of them, although the filter life expectancy will be greatly reduced.

The performance data sheets of most systems will show how the reduction rate of chloramines.  I encourage you to check with your filter provider for the recommended replacement schedule when chloramines are present.

Differences Between A 3 Stage & 5 Stage System

Now that we have covered how an under sink reverse osmosis system works, the next decision is going to be whether to get a 3 stage or 5 stage filtration system as these are the most common models available.  If you have no clue what a stage is here is a brief definition: Each filter is a different stage.  Three filters equals three stages & five filters is five stages.

Here is what a typical 3 stage system consists of:

  • Stage 1: Sediment Filter
  • Stage 2: Carbon Filter
  • Stage 3: Reverse Osmosis Membrane

A 5 stage system is built as follows:

  • Stage 1: Sediment Filter
  • Stage 2: Carbon Filter
  • Stage 3: Carbon Filter
  • Stage 4: Reverse Osmosis Membrane
  • Stage 5: Post-Carbon Finishing Filter to remove any remaining taste & odor
Reverse Osmosis Under Sink Filter

Cutaway of the 5 stage APEC filter system

There are also 4 stage systems that only have one carbon filter before the membrane or do not have the post filter after the membrane.

Depending on the quality of your water, you might only need a 3 or 4 stage system.  Plus you can save money when it comes time to buying replacement filters.  But the one difficult issue with purchasing a 3 stage system is product quality.

A 3 stage system is cheaper to purchase and produce.  You can buy a 3 stage (& 4 or 5 stage) systems in big box retail stores at low costs.  The problem with mass produced units is they are made from cheaper components and are more likely to break.  When purchasing a reverse osmosis unit, you should look for one produced preferably produced in the United States because they are made from higher quality material and usually individually tested like Home Master, APEC, & Aquasana.

Also 3 stage systems are old technology and reverse osmosis has progressed in the several decades since affordable reverse osmosis units were first installed in houses.  The carbon and membrane filters for a 3 stage system needs to work harder and will need to be replaced sooner because the additional carbon filter removes any contaminants not adsorbed by the first carbon pre-filter.  The additional contaminants also put extra stress on the membrane and can cause it to clog sooner as well.

Although it shouldn’t be an issue, you will want to make sure the RO membrane is composed of TFC/TFM (Thin Film Composite/Thin Film Membrane) technology as opposed to CTA (cellulose triacetate) which has a lower rejection rate.  The majority of membranes are now made from TFC or contain both materials.

Having a five stage system provides extra peace of mind & assurance in knowing your water has the highest amount of contaminants removed.

What about a 6 or 7 stage system?


One legitimate concern when it comes to reverse osmosis is the lack of healthy nutrients, as they are removed during the 4th stage.  One downside of reverse osmosis is that the membrane cannot discern good particles in water from the bad contaminants.  So most, if not all, of the good stuff is discharged in the waste line with all the bad stuff you want filtered out.

Reverse Osmosis Under Sink Filter

Pink Himalayan Salt

This is a larger debate for another day, but you can get a post-membrane filter, sometimes called an alkalizing filter, that will “remineralize” the water by adding calcium, magnesium, and potassium back into the water.  Some RO proponents say you can get these minerals from the food you consume so it’s essentially a non-issue.  Or if you don’t want an additional filter, you can also put a teaspoon of pink Himalayan salt into every filtered gallon to restore the minerals as well.

Ultraviolet Filter

Most people do not need this filter, but you might need one.  Or maybe you just want one for additional peace of mind.  By adding an Ultraviolet (UV) filter, the water passes under ultraviolet after leaving the storage tank but before coming out of the faucet to kill off any remaining bacteria, viruses, or cysts that might be present.

The membrane should remove up to 99.9% of any bacteria, virus, or cyst but there is always the possibility that they could pass through or be present in the storage tank.

Advantages of Reverse Osmosis

You can get a quality 5-stage reverse osmosis under sink system for approximately $200.  That’s not much more expensive than a quality 3-stage carbon filter system that won’t remove as many nasty inorganic compounds, including fluoride, from the water.

When looking at the performance specs of carbon filters and what they can remove or reduce, they are primarily certified to reduce the presence of chlorine and/or chloramines to undetectable levels.  Carbon filters are also very effective at removing asbestos, lead, mercury, VOCs, and cysts like cyrptosporidium & giardia.

But carbon cannot remove everything.  Reverse Osmosis is a combination filter that requires carbon pre-filters to be highly effective, but the end result is more pure water.

What Reverse Osmosis Removes That Carbon Filters Typically Cannot:

  • Arsenic (Pentavelent): 97%
  • Barium: 95%
  • Cadmium: 95%
  • Chromium: 97%
  • Copper: 97%
  • Fluoride: Up to 95 or 96%
  • Radium 226/228: 80%
  • Selenium: 98%
  • Total Dissolved Solids (95%)

*Some reverse osmosis units will have higher removal rates for certain contaminants.  Regardless, they are getting rid of a lot of the nasty things you don’t want your family to consume.

There are carbon filters with additional filtration media (like ion exchange) that are certified to remove certain contaminants listed above.  But the question when researching the various water filters will be the effective reduction rate and the replacement interval schedules.  Are the carbon filters as cost effective as reverse osmosis?

Not All Reverse Osmosis Units Require Electricity

A drawback to a whole house RO filter is that you need an electric pump.  Depending on your water pressure, you probably won’t need an electric pump with an under sink model but can use a non-electric permeate pump or no pump at all!

Here are some general guidelines if your under sink unit needs a pump:

Reverse Osmosis Under Sink Filters

*If you are not familiar with how a permeate pump works, it functions by channeling water through a narrower passage into the reverse osmosis membrane.  Which means less water volume but increased pressure (psi).  The pump will also discharge the waste water from the membrane back into the waste channel too.

Plus, depending on your water pressure, a pump can reduce the amount of waste water created from your unit.  Before necessarily investing in a pump, I would check with the manufacturer to ensure the waste water ratio would be reduced.

No plumber required for install

An undersink unit is a lot easier to install than a whole house unit.  So most people will be able to install the product themselves.  It might take a little bit of extra time if you not familiar with plumbing, but there are a plethora of instructional videos on Youtube and certain manufacturer websites.  Or for peace of mind (or installing the system in another location with your other equipment), you can always hire a plumber.

Disadvantages of Reverse Osmosis

There are two universal complaints about reverse osmosis:

  • Wasted Water
    • RO gets rid of all the contaminants by discharging them with waste water along with your faucet drain.  Cheaper, lower-quality systems can have a waste ratio of 10 waste gallons per 1 drinkable gallon! 🙁  However the average ratio is around 3 waste gallons per 1 filtered gallon with a high-quality system.
    • Installing a permeate pump can reduce the waste water ration.  Check with the manufacturer for sure.  Home Master says the permeate pump will reduce the waste ratio to 1:1.
    • If you are on municipal water, this will add a small amount to your monthly water bill.  Especially if RO is only used for cooking and drinking purposes.  Regardless of your water source & the additional cost, it is still wasted water.
  • Lost Nutrients
    • The RO membrane cannot differentiate between good and bad TDS (Total Dissolved Solids) and anything above a certain size is discarded.
    • There are different viewpoints on the long-term health effects of water with no minerals.  Do you get a sufficient amount of these nutrients in the food you consume?  Possibly, but to be sure, some companies offer a post-filter that add minerals back into the water or you can add a teaspoon of pink Himalayan salt per gallon.

There are some other disadvantages of having a reverse osmosis unit.  One of the disadvantages is the additional cost of a permeate pump if you need one.  The additional price can range from around $100 for a non-electric to $200 for an electric pump.

There is also some routine maintenance involved with RO systems, but any water filtration system for that matter.  Most of the maintenance is replacing the various sediment & carbon filters every 6 months to a year or the membrane every 2 to 5 years depending water quality and manufacturer’s recommendations.  Lower water quality means the filters will need to be replaced more often.

To make filter replacement easier, some brands sell upgraded fittings (John Guest fittings for example) that make it easier to connect & disconnect the various parts.  You will also need to routinely check your filter for any leaks.  To avoid leaks, it is recommended to buy a system that uses metal fittings whenever possible.

An under sink filter system will require more space under your sink.  Who would have thought?  The pressurized storage tank is approximately 11 inches deep and 15 inches tall.  The various filters can be mounted on the cabinet wall and will be approximately 15 inches tall and 10 inches wide.  The space required for the storage tank will be the bigger issue as that takes space where you might currently be storing cleaning supplies or the dish rack.  Depending on how much space you have underneath your sink, you can also purchase a countertop filtration system instead.

You can also install the unit in your basement or garage with your other water equipment if desired.  You will need to order additional supplies and probably an electric pump to maintain adequate pressure.  You might also have to hire a plumber to help you with the install because the system is being installed in an alternate location.

Keep in mind that any whole house or point-of-use filter (whether it’s reverse osmosis or carbon) will require routine maintenance and filter replacements.  With the RO system, you have an additional item that requires maintenance with the pressurized storage tank & membrane.  Most systems will not have issues, but it is possible for the storage tank to lose pressure and require replacement.

Reverse Osmosis Under Sink FilterClosing Questions

Here are some closing questions that are commonly asked when it comes reverse osmosis:

Q: What is the waste water ratio for reverse osmosis?

A: The ratio can vary among brands and types of filters.  Nowadays the average waste ratio is approximately 3 gallons of waste for every single drinkable gallon.  Cheaper & antiquated systems can have a ratio as high as 10:1.  Some systems, like countertop systems only have a ratio of 2 waste gallons: 1 filtered gallon.

Q: How much does it cost to replace the various filters?

A: To an extent the cost depends on how often the filters are recommended to be replaced.  Brands of filters that need replaced more often typically tend to be cheaper.  Longer lasting filters are normally more expensive.

Using a 5 stage system for example, replacements for stages 1-3 can cost $25-35 combined and will need to be replaced every 6 months to one year.  A replacement membrane filter costs $35-55 every 2 to 5 years.  A carbon post-filter costs $10-15 every 2 to 4 years.

Compare these replacement costs to other point-of-use filters to see what the ongoing cost is.

Q: Do under sink reverse osmosis units require electricity?

A: No.  Although there are some exceptions.  If your water pressure is between 30-50 psi, it is recommended that you also use a non-electric permeate pump.  Water pressure less than 30 psi will require an electric pump.  Also if you install the filter in your basement, you will probably need a pump to maintain adequate pressure to your faucet.

Q: Does reverse osmosis remove healthy minerals?

A: Yes, along with all the bad minerals.  There is some contending viewpoints on if you should add healthy minerals back to the water or if you consume a sufficient amount when eating food.  Some say that food consumption is inadequate unless you eat organic and further depends on the soil quality).  My personal recommendation is to install a post filter that will add calcium, magnesium, and potassium back into the water or add a teaspoon of pink Himalayan salt to every filtered gallon.

Q: What size storage tank should I purchase?

A: Reverse osmosis units are rated by gallons per day (GPD) produced.  Under sink units comes in two sizes 45 or 90 gallons per day.  The most popular system for most manufacturers is 45 gallons per day.  They will produce filtered water quick enough for up to 6 people and adequate for most households.  A 90 gallon tank will fill up the tank faster and recommended for larger households.

No household will drink 45 or 90 gallons per day but it takes awhile to filter a tankful of water.  At 50 psi at 45 GPD system will filter 1.5 gallons per hour and the 90 GPD system will filter 3 gallons per hour.

The standard storage tank will hold 3 or 4 gallons of filtered water, although you can upgrade to a larger storage tank as well.

What type of water filtration system do you use currently?  What water contaminant bothers you the most?



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