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Reverse Osmosis v. Carbon Filtration

One of the more common questions we receive is about the options for drinking water filtration. Two of the leading technologies for drinking water filtration are carbon-based filters and reverse osmosis filtration. They are decidedly different technologies with very different levels of drinking water quality.

What is Removed:  Reverse Osmosis v. Carbon Filtration

Water May Contain: “Reverse Osmosis / Carbon Combination” “Carbon Block or Activated Carbon”
Bad/Foul Taste Removes Improves
Odor Removes Improves
Turbidity Removes Reduces
Organic Compounds* Removes Removes
Chlorine & THMs Removes Removes
Bacteria Removes May Control Growth**
Viruses Removes Will Not Remove
Cysts Removes Removes Some
Parasites Removes Removes Some
Arsenic Removes Will Not Remove
Heavy Metals*** Removes Removes Some
Dissolved Solids**** Removes Will Not Remove
Fluoride Removes Will Not Remove
*Organic Compounds – include Pesticides, Herbicides, and Insecticides. **Silver-Impregnated Carbon – can control bacterial growth. ***Heavy Metals – include Iron, Lead, Cadmium, and Aluminum. ****Dissolved Solids – include Sodium, Calcium, Magnesium, and inorganic minerals.

Carbon Filters

Carbon filtration is one of the oldest technologies on the planet.  These types of filters cover a broad spectrum of options. On the extreme low-end, that’s carbon gravel.  These are the refrigerator filters or Brita cartridges.  It’s the same carbon gravel you buy in an aquarium store.  These filters will remove some chlorine. Better filters would be carbon in a block form, some being “activated carbon” or similar.   Most of these will remove more chlorine. A high-quality carbon filter is often more effective than a low-end reverse osmosis system.  An example of this would be our Always Fresh filter.  It removes 99.99% of chlorine.   It will remove volatile organic compounds and heavy metals. A reverse osmosis system will use a high-quality carbon filter.  These are often called the “taste and odor” filter.

Reverse Osmosis

Reverse osmosis is a membrane technology.   These thin-film composite (TFC) membranes force water over a semi-permeable membrane. Most will consist of three stages: Pre-Filter:  The purpose is to remove sediment and particulate that can ruin the filter. Membrane: This is the important part and quality matters.  The membrane is what separates out the water from the non-water. Post-Filter:  This is the taste & odor part of the system.  Carbon filters will “polish” the taste and flavor of the water. Be wary of 4-stage, 5-stage, or 20-stage systems.  They’re usually just unnecessary, redundant filters.  Read: Gimmick that increases your cost of operation. Better systems, such as our Kinetico K5 Reverse Osmosis, will have expansion ports.  This isn’t for two taste and odor filters, but for an optional system expansion.  Examples would be specific-purpose cartridges such as:
  • Arsenic elimination
  • Mineral addition / pH adjustment
  • Virus guards

The Difference

These two technologies are different.  Carbon is not bad, we use it for whole-home dechlorination.  When it comes to drinking water, there is a massive difference. The difference between reverse osmosis and carbon filtration is, simply, that size matters. Everything in water has a size.  This is measured in microns.  Most carbon filters have NSF Class I ratings.  This means that they remove 85% of particles sized 0.5-1 micron. Reverse osmosis systems can filter down to .001 micron.  That’s a 500x’s size difference in capacity.
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11 Comments

  1. you don’t mention the amount of water needed to run through a RO system to get a gallon of purified water. It takes many gallons of water to get one gallon of pure water.

    • Hi Doug, Thanks for your note. That’s true, it wasn’t referenced. You’re normally looking at 1-1.5 gallons of effluent discharged to make that 1 gallon of clean water. That’s the ‘recovery ratio’ for a system. The many gallons thing hasn’t been true in decades for anything other than the lowest grade of equipment.

      At the same time, that really low grade equipment isn’t going to support the water quality that was the purpose of RO in the first place. That percentage of stuff removed is the ‘rejection rate.’

      So if I have a system that is recovering near 50% (1:1 gallons) and rejecting at 95%+, there is absolutely a gallon ‘wasted.’ That could also be phrased another way though. If I concentrate all the chemicals from two gallons into one gallon, I now have one gallon that I actually want to drink and one that I would prefer not to.

      To each their own & thanks for your comment.

  2. If you don’t properly maintain your RO system, or the average temperature of your incoming water stream drops, the reject rate for an RO system can be as high as 10X to 20X the actual volume of purified water produced.

    • There are two important, and different, ratios at play. Rejection will typically refer to the percentage of contaminants or dissolved solids being removed. Recovery will typically refer to the efficiency of the system.

      I can agree with you main point which is loosely that a broken system wouldn’t behave like an effective one. A quality system will be recovering nearly 1:1. So to make a gallon/liter, the contaminants would be concentrated to that other gallon/liter. That’s more of a physical world thing–the actual manifold.

      Bigger is that rejection rate. That’s still physical world, but related to the membrane. On a bad system, that might only be 70-80% when new. On a great system, it will be certified to remove well over 90% of those contaminants/solids and warrantied to do so for at least a few years. If not, it’s probably a lower end system.

      That issue of effectiveness is something addressed by our WaterBot real-time monitoring solution. An RO without monitoring is kind of like a smoke detector without batteries.

      If there is one other feature that a consumer should look for, it would be a clean water or permeate-flushing system. That is standard on all commercial or industrial RO systems.

  3. What about all natural mineral are removed and then add back in artificial one for RO? Would a high end carbon filter with UV lamp be better or not? Many thanks.

    • Hi Joy, Thank you for your note.

      It is 100% true that membranes are agnostic over removing solids. So they would remove the natural minerals, but that’s the lesser of evils. We’d definitely want to remove pharmaceuticals, arsenic, and other items.

      Very high end carbon can be great. Carbon is effective against chlorine, many volatile organic compounds, etc. Those that are lined with a silver material can be effective against some biologicals and UV is also great at disabling pathogens.

      On one of our reverse osmosis systems, the Kinetico K5, we use this Mineral+ cartridge. So we let the RO remove virtually everything from the water and then add back a controlled dose of calcium, potassium, and magnesium. It gives the water that spring water flavor from those minerals, but removes all of those odorless, tasteless, colorless items that are also in the water.

      For UV, that’s something we typically use on a whole home basis. If we have any uncertainty over the water, treating the shower water, for example, would have benefits.

      We appreciate your note, let me know if you have any follow-up questions.

      Thanks,
      Chris

    • Hello Hai,

      That’s a really good question. I wouldn’t recommend using TDS to measure when filters need to be changed. To use a crude analogy, it would be like using whether or not a motor is seized to know if you need oil. It’s a little late.

      The filters are there to protect the membrane just like the oil protects the motor. The pre-filter side is protecting this reasonably delicate thin film composite (TFC) that is the membrane. It’s protecting it from sediment, chlorine, chloramine, etc.

      You can still get very important data from the TDS monitor. If you measure the water going into the RO (say the cold water line in the kitchen) v. the water coming from the system, we can reflect that as a percentage. This is the rejection ratio. So, say you have 500 ppm going in. You have 20 going out. So 480 divided by 500 = 96%. That would be great. The datasheet of the equipment you have would have a percentage stored for “rejection ratio.” That’s a great way to test your membrane.

      As a shameless plug, check out the WaterBot on our site. It’s a real-time, persistent water quality monitor that performs this calculation for you. A great RO system would be be rejecting 95% or more. There are some caveats to that. If the water is cold, under low pressure, or has an extremely high level of solids, you may not hit peak performance.

      For our reverse osmosis systems, specifically the Kinetico K5, it self-monitors the filters by counting the gallons that came through. In the absence of such a system, I’d still just maintain the manufacturer’s recommended schedule.

      Let me know if you have any questions. We appreciate your note.

      Thank you,
      Chris

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