Contaminants That Bypass the RO Membrane
Reverse osmosis systems use a semipermeable membrane with incredibly tiny pores, typically around 0.0001 microns, to force water through under pressure. While this process is highly effective against many contaminants, certain substances can pass right through the membrane due to their size, lack of ionic charge, or other properties.
Dissolved Gases
One of the most common categories of contaminants that RO does not remove is dissolved gases. These small, non-ionized molecules are able to permeate the membrane with the water itself. Some examples include:
- Hydrogen sulfide: This gas is known for its unpleasant "rotten egg" smell and taste, which will persist in RO-filtered water unless specifically treated.
- Carbon dioxide: Often present in tap water, CO2 can pass through the membrane, which can result in a slightly more acidic pH and a 'flat' taste to the water.
- Oxygen and Nitrogen: These gases also pass through the membrane, which is why RO water, unlike distilled water, does not taste completely flat.
Certain Organic Compounds (VOCs) and Other Small Molecules
By itself, an RO membrane is not effective at removing certain volatile organic compounds (VOCs) and other organic chemicals. However, most residential RO systems are multi-stage and include an activated carbon filter designed specifically to handle these issues. If the activated carbon filter is worn out, the RO system's ability to remove these contaminants is severely compromised.
Low molecular weight, non-ionized substances can also sometimes slip through. While the membrane blocks most organics, those with a molecular weight below 200 may be poorly rejected, especially if they do not have a strong ionic charge.
Beneficial Minerals
A major drawback of the reverse osmosis process is that it is not selective; it removes both harmful and beneficial dissolved solids. This includes essential minerals like calcium, magnesium, and potassium, which are important for human health. Many people find that this results in water with a "flat" taste, and the long-term consumption of demineralized water may be a concern. To address this, many modern RO systems are equipped with a post-filter that adds back trace amounts of beneficial minerals to improve taste and composition.
The Role of Supporting Filters and Maintenance
The limitations of the RO membrane itself are why modern RO systems are multi-stage, incorporating other types of filters to ensure comprehensive purification. Proper maintenance is critical for these systems to function correctly.
Chlorine Damage and Pre-Filtration
Oxidizing agents like chlorine and chloramines are highly damaging to the delicate polyamide material of standard RO membranes. Exposure to chlorine can cause irreversible damage, leading to higher contaminant passage and reduced lifespan. For this reason, a sediment pre-filter and a carbon pre-filter are essential components of almost all RO systems, removing large particles and chlorine before they reach the membrane.
Biofouling and Post-Filtration
While an intact RO membrane effectively blocks bacteria and viruses, the conditions within an RO system's storage tank can promote bacterial growth. Because the pre-filters remove chlorine, a disinfectant is no longer present to inhibit microbial growth after the membrane. A physical pinhole or a compromised seal in the membrane can also allow microorganisms to pass through. Many modern systems include a UV sterilization stage after the RO membrane to combat this risk.
Operational Limitations and Wastewater
The process of reverse osmosis is also constrained by several factors that affect its efficiency and sustainability.
High Water Waste
RO is an inherently wasteful process. Depending on the system, for every gallon of purified water produced, several gallons of wastewater, carrying the rejected contaminants, are sent down the drain. While more efficient systems are now available, this inefficiency is a significant environmental concern in water-scarce regions.
Slow Production Rate
Since water is forced through an extremely fine membrane under pressure, the process can be slow. Household RO systems produce purified water gradually, which is why a storage tank is necessary. This can be inconvenient for large households with high water demands.
Comparison of Common Water Filtration Methods
| Filtration Method | What it Removes | What it Does Not Remove | Other Considerations |
|---|---|---|---|
| Reverse Osmosis (RO) | Heavy metals, salts, large molecules, most bacteria, and viruses. | Dissolved gases, small un-ionized organics, beneficial minerals. | High water waste, slow production, potential for bacterial regrowth without post-filtration. |
| Activated Carbon | Chlorine, VOCs, pesticides, herbicides, improves taste and odor. | Dissolved minerals, bacteria, nitrates, salts. | Does not purify water from heavy metals or viruses. |
| Distillation | Most minerals, bacteria, viruses, and heavy metals. | Dissolved gases (often reabsorbed), some VOCs with lower boiling points. | Very slow, high energy cost, can have flat taste. |
| UV Sterilization | Bacteria, viruses, protozoa. | Does not remove dissolved solids, heavy metals, chemicals, or sediments. | Only disinfects; often used as a final stage in a multi-stage system. |
Conclusion: Choosing the Right Water Treatment
Reverse osmosis is an incredibly powerful water purification technology, but it is not a silver bullet. The contaminants that RO does not remove—dissolved gases, certain organics, and beneficial minerals—highlight the importance of using a multi-stage system that combines RO with other filtration technologies, such as activated carbon and UV sterilization. For those concerned about health, opting for a system with a remineralization stage can restore the healthy mineral content lost in the process. Ultimately, a proper water test is the best way to determine your specific needs and select the right combination of technologies for safe and pure drinking water.
For more information on water quality testing and purification methods, consider consulting authoritative sources such as the EPA (Environmental Protection Agency) or resources like this.
