Reverse osmosis systems are capable of removing a wider range of contaminants than many other home water treatment systems. They also vary widely in size, price, and features. When searching for the perfect system, you should consider your budget, household water usage, the amount of space under your kitchen sink, and what specific contaminants you’d like to remove.

What is Reverse Osmosis?

You probably learned about osmosis in science class. It’s the process by which water passes through a membrane from higher concentration to lower concentration to try to achieve balance. In reverse osmosis, pressure is put on dirty water to force it to pass through a semipermeable membrane. The water molecules are small enough to pass through the membrane to a storage tank of clean water, but contaminants like metals or minerals that are larger or heavier are trapped on the dirty side.

In most reverse osmosis systems, water also passes through several carbon filters to remove additional contaminants. Some systems even add minerals back into the water at the end of the process.

What Does Reverse Osmosis Remove?

Reverse osmosis is capable of removing up to 98% of many dangerous or unwanted contaminants, including:

  • Arsenic (pretreatment may be necessary)
  • Bad Tastes & Odors
  • Chlorine and Chloramine
  • Detergents
  • Fluoride
  • Lead
  • Nitrates and Sulfates
  • Pesticides
  • Salt
  • Volatile Organic Compounds

Some reverse osmosis systems include an ultraviolet lamp to neutralize bacteria and other microorganisms.

Reverse osmosis systems vary in how many filtering stages they include. Not all systems will remove everything on this list. Before installing a reverse osmosis system you should test your water to determine the specific contaminants you’d like to remove to be sure you choose a system that corrects those issues.

Whole House Versus Drinking Water Only

Your first decision is whether you will treat all the water you use in your home, or only the water used for drinking or cooking. While a whole house reverse osmosis system may be recommended for some, especially if you’re dealing with a dangerous or extreme contamination, point-of-use systems are more common. These systems only clean the water dispensed from a special facet installed in the kitchen. Treating water that will be used for bathing or plumbing is often unnecessary.

Tank versus Tankless System

Most reverse osmosis systems include a tank that stores the cleaned water until you’re ready to use it. These tanks, which are typically two or four gallons and at least the size of a basketball, are often installed under your kitchen sink. If you already have a garbage disposal, there may not be a lot of room left under there. Countertop options are available, although some find those less attractive or don’t want to give up valuable counter space. When choosing your system, it’s important to measure first to make sure it will fit in your kitchen (or basement for a whole house system).

Tankless reverse osmosis systems are a newer innovation. These systems are more expensive but take up less space as there is no storage tank. They often waste less water than their traditional tank counterparts. Wastewater is a part of the reverse osmosis process. Although systems are getting more efficient as technology improves, many reverse osmosis systems produce between two and four gallons of wastewater for every clean gallon of drinking water. Some advanced “zero waste” systems will divert that wastewater to be used for laundry or plumbing.

Number of Filtering Stages

Reverse osmosis systems feature anywhere from three to seven filtering stages. The number of stages you choose may depend on your water analysis, your budget, and personal preference. These stages are broken down into prefiltration, filtration, and post filtration.

Possible stages and their purpose include:

  • Prefiltration
    • Sediment filter – removes sand and dirt
    • Carbon filter – removes chlorine, chloramine, and other chemicals that could damage the semipermeable membrane
  • Filtration
    • Semipermeable membrane – where reverse osmosis actually happens, removing 95-98% of contaminants that reach this stage
  • Post filtration
    • Alkaline filter – optional feature that raises the pH of water if desired
    • Remineralizer – optional feature that puts healthy minerals back into the water to improve its taste
    • Ultraviolet lamp – optional feature that neutralizes bacteria and other microorganisms
    • Post-carbon filter – final filter stage that removes any last impurities, especially those that may have come from sitting in the storage tank before use

Household Water Usage

When choosing a reverse osmosis system, it’s important to take your household water usage into account. You don’t want to use up all your clean water and have to wait while the system filters more. Reverse osmosis systems are rated by how many gallons of water they can filter in a day. For point-of-use drinking water only systems, the most common sizes are 50, 75, and 100 gallons. Fifty gallons may be large enough for a couple or a small family of three, but it may be wise to size up, especially if your family might grow. One hundred gallon systems can usually support six or more people. When choosing the size of your system, it is sometimes a good idea to size up to make sure you’ll have access to all the water you need. But you don’t want to go overboard. If water sits in your storage tank for long periods of time without being used it may get stagnant, so don’t always assume that biggest is best.


The cost of a point-of-use reverse osmosis system varies widely from a few hundred dollars to well over a thousand. Whole house systems will be significantly more expensive. Price often depends on the number of filter stages and whether the system includes additional features or new technology. Some higher end models include automatic maintenance reminders. You should also understand the maintenance costs and filter replacement schedule of any system before you purchase. It’s important to change the prefilters according to the manufacturer’s instructions to ensure the semipermeable membrane isn’t damaged. These membranes typically last longer than filters, but will eventually need to be replaced as well.

If the water pressure is low in your home, you may also need to invest in a booster pump. Most reverse osmosis systems work when water pressure is between 40 and 80 PSI, but a pressure of at least 60 PSI is ideal. Most city water is pressurized to 60 PSI, but many private well owners struggle with low water pressure.

Final Thoughts

Reverse osmosis systems include a wide range of features. To choose the best one, it’s important to understand why you’re seeking water treatment and whether you’re most concerned about cost, aesthetics, the amount of water you use, or investing in the latest technology. No matter your priorities, it’s always a good idea to choose a reputable, well reviewed brand that carries a solid warranty. If you’re considering a reverse osmosis system, Dierolf Plumbing and Water Treatment is here to analyze your water and help you select the best system for your needs.

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