Reverse osmosis (RO) systems frequently are used to reduce the levels of total dissolved solids and suspended particles within water. These systems remove a variety of ions and metals as well as certain organic, inorganic, and bacterial contaminants. In general, the process of reverse osmosis can effectively remove high percentages of dissolved ions such as: Arsenic, Antimony, Aluminum, Barium, Beryllium, Cadmium, Calcium, Chloride, Chromium, Copper, Fluoride, Iron, Lead, Magnesium, Manganese, Mercury, Nitrate, Potassium, Selenium, Silver, Sodium, Sulfate, Thallium, and Zinc; radionuclides like Radium and Uranium; select pesticides like Endrin, Heptachlor, Lindane, and Pentachlorophenol; and particles like cysts, oocysts, and asbestos fibers.
There are some contaminants not removed from water by RO systems. Reverse osmosis units do not effectively remove most organic compounds, bacterial microorganisms, chlorine by-products, or dissolved gases like carbon dioxide, methane, and radon. Therefore, it is necessary to address the presence of dissolved gases that may impact the water chemistry prior to the reverse osmosis process. The reverse osmosis process is not particularly effective at selecting specific anions or cations to remove from the water so there can be a tendency for reverse osmosis water to have a low pH and to be slightly more corrosive than the untreated water. This can be addressed by using a post-treatment remineralization filter. The RO membrane's efficiency in reducing the amount of contaminants in the water depends on the contaminant concentration, chemical properties of the contaminant, the membrane type and condition, and operating conditions. Reverse osmosis is considered an ultrafiltration process; it is also expensive.
The RO membrane alone may not be an effective method for total removal of these contaminants but a properly designed system may be effective in reducing these contaminants to safe levels. Contaminant removal by the system may vary depending on operating conditions and equipment. Reverse osmosis can significantly reduce the microorganism population in the water, but because these microbes may regrow on the membrane and cause the formation of bio-slimes or cause the membrane to prematurely fail, it is recommended that the water be properly disinfected prior to RO treatment. RO treatment systems have a few system components:
- 1 | Prefilter - Typically a particle filter to remove particles, rust, and other debris.
- 2 | Possibly a water softener if the total hardness is elevated along with a smaller-diameter particle filter to remove particles that are greater than 1 to 5 microns, i.e., large clay to silt-size particles and larger.
- 3 | The RO pressure pump, line-pressure and pump pressure gauges, and a flow regulator.
- 4 | RO treatment unit will have a treated-water storage tank, a product line, a membrane, a flush-to-waste line, and a dispenser. The RO treatment unit should specify the typical or nominal cation and anion rejection rates for the thin film membrane which would be either a cellulose acetate membrane or a cellulose triacetate membrane.
- 5 | Most treatment systems have an activated-carbon pre-filter and post-filtration that may include a remineralization cartridge, a UV disinfection system, and a 0.22 to 0.45 micron (submicron) filter.
The ROtreatment system requires pretreatment that most likely includes Particle Filtration, UV Disinfection, Chemical or Ion Exchange, and other pretreatment to prevent scale formation or bio-slime regrowth/growth. Because of all the pretreatment, these are typically point-of-use units and not whole-house treatment systems. ROSystems are commonly used when the water contains elevated levels of salts, such as chloride, Nitrate, Total Hardness, and Sodium.
Reverse osmosis unit performance is measured by a combination of the membrane rejection rate, see above, and the recovery rate for the system. The membrane rejection rate is the percentage of the contaminant that is not permitted to pass through the filter. The rejection rate can be used to predict the effectiveness of the treatment process and the feasibility of the approach. For example, if there were two water sources that required treatment and one water source (source A) had an arsenic level of 2000 ppb and the other source (source B) had a concentration of 50 ppb and the membrane had a 90 % rejection rate, then:
Source A 2000 ppb * 0.90 = 1800 ppb of the arsenic is captured and diverted to waste, but 200 ppb of arsenic would pass through the filter. This concentration exceeds the recommended USEPA drinking water standard of 10 ppb for arsenic.
Source B 50 ppb * 0.90 = 45 ppb of the arsenic is captured and diverted to waste, but 5 ppb of arsenic would pass through the filter. This concentration is below the USEPA drinking water standard of 10 ppb for arsenic, but would be at the drinking water limit for some states.
Recently, the recovery rates for reverse osmosis units have significantly increased by the addition of booster pumps and a permeate pump. The reverse osmosis booster pump increases the water pressure in the feed line, while the permeate pump does not increase pressure, but helps the reverse osmosis unit waste less water.
Notes:
- 1 | Please keep in mind that standard reverse osmosis units may waste 3 gallons of water for every gallon of drinking water produced or 75 % of the water is wasted. That now more heavily contaminated waste water must be appropriately disposed of. The addition of a permeate pump can reduce this wastewater volume by about 85 %.
- 2 | If the water is not chlorinated and there are no trace levels of organics, it would not be advisable to install a granular-activated-carbon pre-filter. If a GAC filter is installed as a pre-filter on a source that is not chlorinated, it is likely that bacteria may regrow on the filter and membrane; it may be wise to install the GAC filter as a post filtration unit. A GAC filter is needed for systems that have chlorinated water and the RO unit is using a thin-film composite membrane.
Table 1 | Common Types of System Fouling (Cause and Approach)
| Common Types of Fouling | The Primary Cause | Pretreatment Approach |
| Disinfectant or Strong oxidizer | Potassium Permanganate, Chlorine Residual, Peroxide, and Ozone | Granular-Activated-Carbon or Oxygen Scavenger |
| Particles | Large particles in water, "rust" flakes from the piping or corrosion, or aquifer particles, such as: large clay, silt, sand, and fine stone, | Filtration with at least a 1 to 5 micron pre-filter |
| Biological Material | Bacteria, Total Coliform, E. coli, Nuisance Bacteria, and Heterotrophic Plate Count | Chlorination, UV disinfection, Ozonation, Hydrogen Peroxide |
| Scaling or Mineral Fouling | The presence of Barium, Calcium, Magnesium, or Strontium with the dominant anion being bicarbonate or sulfate. | Prefiltration and then Post filtration with 0.22 micron filter |
Follow the Path to Clean Water
We recommend that, prior to installing a Reverse Osmosis System, you Get Informed by gathering as much information as possible and Get Tested by conducting multiple levels of testing depending on your needs: Level 1 Observational Testing using our Self Test Web App, Level 2 Do-It-Yourself Testing, Level 3 Informational Lab Testing and, if needed, Level 4 Certified Testing conducted by a water professional.
If you have any questions please Contact the KnowYourH2O team.
