Reverse Osmosis

Reverse osmosis (RO) is a water purification process that uses a semi-permeable membrane to remove contaminants from water. Unlike simpler filtration methods, RO can eliminate dissolved solids, heavy metals, and other impurities, producing high-purity water suitable for drinking, cooking, and other household uses. It is typically installed as a point-of-use system (like under a kitchen sink) due to its efficiency in targeting specific water sources, though whole-house systems exist for homes with those needs. Reverse Osmosis RO systems are widely used in homes with significant water quality issues, such as high levels of lead, nitrates, or fluoride, and are certified under NSF/ANSI Standard 58 for specific contaminant removal.

NSF/ANSI 58 is a certification standard developed by NSF International and the American National Standards Institute (ANSI) specifically for reverse osmosis drinking water treatment systems. This standard is widely recognized in the United States and globally as a benchmark for RO system reliability and performance.

How It works:

Reverse osmosis operates by forcing water through a semi-permeable membrane with extremely small pores (0.0001–0.001 microns) under high pressure, allowing water molecules to pass while blocking larger contaminants. This process reverses natural osmosis, where water moves from a less concentrated solution to a more concentrated one across a membrane. In RO, pressure overcomes this natural flow, separating pure water from concentrated impurities.

Contaminant Removal Capabilities:

RO systems excel at removing heavy metals, inorganics, volatile organic compounds (VOCs), pesticides, PFAS, radionuclides, and some microorganisms. However, they may only partially remove small molecules like boron or volatile gases. See a list of contaminants that RO systems can assist with:

Contaminant	Can RO Help?	Details
1,1 Dichloroethane	Yes	VOC, effectively removed due to membrane’s small pore size.
1,1 Dichloroethylene	Yes	VOC, certified for RO removal under NSF/ANSI 58.
1,1 Dichloropropene	Yes	VOC, removed by RO’s fine filtration.
1,1,1 Trichloroethane	Yes	VOC, certified for RO removal.
1,1,1,2 Tetrachloroethane	Yes	VOC, effectively removed by RO.
1,1,2 Trichloroethane	Yes	VOC, certified for RO removal.
1,1,2,2 Tetrachloroethane	Yes	VOC, effectively removed by RO.
1,2 Dichlorobenzene	Yes	VOC, certified for RO removal.
1,2 Dichloroethane	Yes	VOC, certified for RO removal.
1,2 Dichloropropane	Yes	VOC, certified for RO removal.
1,2,3 Trichlorobenzene	Yes	VOC, effectively removed by RO.
1,2,3 Trichloropropane	Yes	VOC, effectively removed by RO.
1,2,4 Trichlorobenzene	Yes	VOC, effectively removed by RO.
1,2,4 Trimethylbenzene	Yes	VOC, effectively removed by RO.
1,3 Dichlorobenzene	Yes	VOC, certified for RO removal.
1,3 Dichloropropane	Yes	VOC, effectively removed by RO.
1,3 Dichloropropene	Yes	VOC, effectively removed by RO.
1,3,5 Trimethylbenzene	Yes	VOC, effectively removed by RO.
1,4 Dichlorobenzene	Yes	VOC, certified for RO removal.
2,2 Dichloropropane	Yes	VOC, effectively removed by RO.
Alkalinity (as CaCO3)	Yes	Reduces carbonate/bicarbonate ions, lowering alkalinity.
Aluminum	Yes	Heavy metal, effectively removed by RO.
Antimony	Yes	Heavy metal, certified for RO removal under NSF/ANSI 58.
Arsenic	Yes	Heavy metal, certified for RO removal (especially As(V)).
Barium	Yes	Heavy metal, certified for RO removal.
Benzene	Yes	VOC, certified for RO removal.
Beryllium	Yes	Heavy metal, effectively removed by RO.
Bicarbonate	Yes	Inorganic ion, reduced as part of alkalinity.
Boron	Partial	50–70% removal due to small molecular size; less effective.
Bromobenzene	Yes	VOC, effectively removed by RO.
Bromochloromethane	Yes	VOC, effectively removed by RO.
Bromodichloromethane	Yes	Disinfection byproduct (THM), certified for RO removal.
Bromoform	Yes	Disinfection byproduct (THM), certified for RO removal.
Bromomethane	Partial	Volatile gas, partial removal due to small size and volatility.
Cadmium	Yes	Heavy metal, certified for RO removal.
Calcium	Yes	Hardness ion, effectively removed by RO.
Carbon Tetrachloride	Yes	VOC, certified for RO removal.
Carbonate	Yes	Inorganic ion, reduced as part of alkalinity.
Chloride	Yes	Inorganic ion, effectively removed by RO.
Chloride-to-Sulfate Mass Ratio	Yes	Ratio affected as both chloride and sulfate are reduced.
Chlorobenzene	Yes	VOC, certified for RO removal.
Chloroethane	Yes	VOC, effectively removed by RO.
Chloroform	Yes	Disinfection byproduct (THM), certified for RO removal.
Chloromethane	Partial	Volatile gas, partial removal due to small size and volatility.
Chlorotoluene 2	Yes	VOC, effectively removed by RO.
Chlorotoluene 4	Yes	VOC, effectively removed by RO.
Chromium (Total)	Yes	Heavy metal, certified for RO (Cr(VI) highly effective).
cis 1,2 Dichloroethylene	Yes	VOC, certified for RO removal.
Cobalt	Yes	Heavy metal, effectively removed by RO.
Conductivity	Yes	Reduced by removing dissolved ions (TDS).
Copper	Yes	Heavy metal, certified for RO removal.
Dibromochloromethane	Yes	Disinfection byproduct (THM), certified for RO removal.
Dibromochloropropane	Yes	Pesticide, effectively removed by RO.
Dibromomethane	Yes	VOC, effectively removed by RO.
Dichlorodifluoromethane	Partial	Volatile gas, partial removal due to small size and volatility.
Dichloromethane	Yes	VOC, certified for RO removal.
E. coli	Partial	Partially removed; RO not certified for microbial removal.
Ethylbenzene	Yes	VOC, certified for RO removal.
Ethylene dibromide	Yes	VOC, effectively removed by RO.
Fluoride	Yes	Inorganic ion, certified for RO removal.
Grains per gallon	Yes	Hardness measure, reduced by removing calcium/magnesium.
Hardness	Yes	Reduced by removing calcium and magnesium ions.
Hardness (Ca Mg)	Yes	Calcium and magnesium removed, reducing hardness.
Hardness (Total)	Yes	Total hardness reduced by RO.
Hexachlorobutadiene	Yes	VOC, effectively removed by RO.
Hydrogen Sulfide (H2S) Gas Strip	No	Gas, not removed by RO; requires separate treatment.
Iron	Yes	Heavy metal, effectively removed by RO.
Isopropylbenzene	Yes	VOC, effectively removed by RO.
Langelier Saturation Index	Yes	Affected by reduced alkalinity/hardness, may lower corrosivity.
Lead	Yes	Heavy metal, certified for RO removal under NSF/ANSI 58.
Lithium	Yes	Inorganic ion, effectively removed by RO.
m,p Xylene	Yes	VOC, certified for RO removal.
Magnesium	Yes	Hardness ion, effectively removed by RO.
Manganese	Yes	Heavy metal, effectively removed by RO.
Mercury	Yes	Heavy metal, certified for RO removal.
Methyl Tertiary Butyl Ether	Yes	VOC, effectively removed by RO.
Molybdenum	Yes	Heavy metal, effectively removed by RO.
n Butylbenzene	Yes	VOC, effectively removed by RO.
n Propylbenzene	Yes	VOC, effectively removed by RO.
Naphthalene	Yes	VOC, effectively removed by RO.
Nickel	Yes	Heavy metal, effectively removed by RO.
Nitrate (as N)	Yes	Inorganic ion, certified for RO removal.
Nitrite (as N)	Yes	Inorganic ion, effectively removed by RO.
o Xylene	Yes	VOC, certified for RO removal.
p Isopropyltoluene	Yes	VOC, effectively removed by RO.
pH	Partial	Produces slightly acidic water (pH 5.5–6.5); may require adjustment.
Phosphorus	Yes	Inorganic, effectively removed by RO.
Potassium	Yes	Inorganic ion, effectively removed by RO.
sec Butylbenzene	Yes	VOC, effectively removed by RO.
Selenium	Yes	Heavy metal, certified for RO removal.
Silica	Partial	50–90% removal, depends on form; less effective.
Silver	Yes	Heavy metal, effectively removed by RO.
Sodium	Yes	Inorganic ion, effectively removed by RO.
Sodium Adsorption Ratio	Yes	Affected by reduced sodium and other ions.
Strontium	Yes	Heavy metal, effectively removed by RO.
Styrene	Yes	VOC, certified for RO removal.
Sulfate	Yes	Inorganic ion, effectively removed by RO.
tert Butylbenzene	Yes	VOC, effectively removed by RO.
Tetrachloroethylene	Yes	VOC, certified for RO removal.
Thallium	Yes	Heavy metal, certified for RO removal.
Tin	Yes	Heavy metal, effectively removed by RO.
Titanium	Yes	Heavy metal, effectively removed by RO.
Toluene Petroleum Compounds	Yes	VOC, certified for RO removal.
Total Coliform	Partial	Partially removed; RO not certified for microbial removal.
Total Dissolved Solids (TDS)	Yes	Reduced up to 99.99%, certified under NSF/ANSI 58.
Total THMs	Yes	Disinfection byproducts, certified for RO removal.
trans 1,3 Dichloropropene	Yes	VOC, effectively removed by RO.
Trichloroethylene	Yes	VOC, certified for RO removal.
Trichlorofluoromethane	Partial	Volatile gas, partial removal due to small size and volatility.
Turbidity	Yes	Reduced by RO membrane and pre-filters.
Uranium	Yes	Radionuclide, certified for RO removal.
Vanadium	Yes	Heavy metal, effectively removed by RO.
Vinyl Chloride	Yes	VOC, certified for RO removal.
Zinc	Yes	Heavy metal, effectively removed by RO.

Additional RO Considerations:

While reverse osmosis is highly effective, it has limitations:

Wastewater: Produces 3–5 gallons of wastewater per gallon of purified water, though high-efficiency systems can reduce this.
Demineralization: Removes beneficial minerals like calcium and magnesium, resulting in flat-tasting, slightly acidic water (pH 5.5–6.5) that may require post-treatment (CDC, 2025).
Maintenance: Regular filter and membrane replacement (every 6–12 months for pre-filters, 2–5 years for membranes) is essential to prevent fouling or reduced performance.
Costs: Under-sink RO systems cost $150–$500, whole-house systems $1,000–$2,200, with annual maintenance of $50–$150.