What Cannot Be Removed by Reverse Osmosis? A Comprehensive Guide
Reverse osmosis excels at purifying water, but it isn’t a panacea. What cannot be removed by reverse osmosis? Certain volatile organic compounds, dissolved gases, and substances with very low molecular weight slip through the membrane, requiring supplemental filtration.
Introduction to Reverse Osmosis and Its Limitations
Reverse osmosis (RO) is a highly effective water purification technology that uses pressure to force water molecules through a semipermeable membrane. This membrane acts as a filter, rejecting most contaminants and allowing only relatively pure water to pass through. While RO systems significantly improve water quality, they are not perfect and certain substances can bypass the membrane. Understanding these limitations is crucial for selecting the right water treatment solution for your specific needs.
The Reverse Osmosis Process Explained
RO systems work by applying pressure to the feed water, forcing it through a membrane with extremely small pores – typically around 0.0001 microns. This process separates the water from dissolved solids, organic matter, and other impurities.
Here’s a simplified breakdown of the process:
- Pre-filtration: Water first passes through a pre-filter to remove larger particles like sediment, silt, and rust. This protects the RO membrane from damage and extends its lifespan.
- Reverse Osmosis Membrane: The water is then forced through the RO membrane under pressure. The membrane rejects the vast majority of contaminants, allowing purified water to pass through.
- Post-filtration (Optional): Some systems include a post-filter, often a carbon filter, to further improve taste and odor.
- Storage Tank: The purified water is stored in a tank for on-demand use.
- Drain: Rejected contaminants are flushed down the drain.
What Makes Reverse Osmosis So Effective?
Reverse osmosis is effective due to the tiny pore size of its membrane and the pressure applied to force water through it. This allows it to remove:
- Dissolved salts and minerals
- Heavy metals (e.g., lead, arsenic, mercury)
- Bacteria and viruses
- Pesticides and herbicides
- Pharmaceuticals
Substances That Can Escape Reverse Osmosis Filtration
Despite its effectiveness, what cannot be removed by reverse osmosis includes certain substances that are either too small or have other properties that allow them to pass through the membrane.
Here’s a list of the common culprits:
- Volatile Organic Compounds (VOCs) with low molecular weight: Some smaller VOCs like chlorine and chloroform can sometimes pass through the membrane, especially if the system is not properly maintained or the membrane is old.
- Dissolved Gases: Gases like hydrogen sulfide (rotten egg smell) may not be effectively removed by RO alone.
- Certain Pesticides and Herbicides: Some newer pesticides and herbicides with specific chemical properties may not be fully rejected by the membrane.
- Pharmaceuticals: While RO removes many pharmaceuticals, some very small or persistent pharmaceutical compounds may not be completely eliminated.
- Alcohol: As an organic compound with a low molecular weight, RO filtration may have difficulty removing alcohol.
Factors Affecting RO Performance
Several factors can influence the performance of an RO system:
- Membrane Quality: The quality and condition of the RO membrane are critical. As membranes age, their pore size can increase, allowing more contaminants to pass through.
- Water Pressure: Sufficient water pressure is necessary to force water through the membrane effectively. Low pressure can reduce contaminant rejection.
- Water Temperature: RO membranes perform best within a specific temperature range. Extreme temperatures can affect their efficiency.
- Pre-filtration: Proper pre-filtration is essential to protect the RO membrane from fouling and damage.
- Maintenance: Regular maintenance, including filter changes and membrane cleaning, is crucial for optimal performance.
Enhancing RO Filtration with Supplemental Treatments
To address the limitations of reverse osmosis, supplemental treatments are often used in conjunction with RO systems. These treatments can target specific contaminants that RO alone may not effectively remove.
Common supplemental treatments include:
- Activated Carbon Filtration: Used to remove chlorine, chloramine, VOCs, and improve taste and odor. Often used as a pre-filter and/or post-filter.
- UV Disinfection: Used to kill bacteria and viruses that may have passed through the RO membrane.
- Water Softeners: Used to remove hardness minerals (calcium and magnesium), which can extend the lifespan of the RO membrane.
- Air Strippers: Used to remove dissolved gases like hydrogen sulfide.
Choosing the Right Water Treatment System
Selecting the right water treatment system depends on the specific contaminants present in your water and your desired level of purity.
Before choosing a system, it’s recommended to:
- Test your water: Identify the contaminants present and their concentrations.
- Research available technologies: Understand the capabilities and limitations of different water treatment systems.
- Consult with a water treatment professional: Get expert advice on the best solution for your needs.
- Consider long-term costs: Factor in the cost of filters, maintenance, and energy consumption.
| Factor | Importance |
|---|---|
| —————- | —————————————————————– |
| Water Test | Crucial for identifying contaminants and selecting appropriate filters |
| System Capacity | Must match your water usage needs |
| Maintenance | Regular maintenance ensures optimal performance and longevity |
| Cost | Consider initial investment and ongoing operating expenses |
| Certification | Ensures the system meets industry standards |
Frequently Asked Questions (FAQs)
What size particles can reverse osmosis remove?
Reverse osmosis membranes can remove particles as small as 0.0001 microns. This includes dissolved salts, minerals, bacteria, viruses, and many organic molecules. This exceptional filtration capability makes RO a powerful tool for producing high-purity water.
Does reverse osmosis remove all contaminants from water?
While highly effective, reverse osmosis doesn’t remove every single contaminant. As outlined in “What cannot be removed by reverse osmosis“, certain VOCs, dissolved gases, and substances with very low molecular weights can sometimes pass through the membrane, although often in very small concentrations.
Is reverse osmosis water safe to drink?
Yes, reverse osmosis water is generally safe to drink. It removes many harmful contaminants, making it a healthier alternative to untreated tap water. However, RO water is also devoid of beneficial minerals, so some people prefer to remineralize it after filtration.
How often should I change the filters in my RO system?
The frequency of filter changes depends on water quality and usage. Pre-filters typically need to be changed every 6-12 months, while the RO membrane usually lasts 2-5 years. Follow the manufacturer’s recommendations for optimal performance.
Does reverse osmosis remove chlorine?
Yes, reverse osmosis can remove chlorine, but it’s often best to use a pre-filter specifically designed for chlorine removal. This protects the RO membrane and extends its lifespan. Activated carbon filters are commonly used for chlorine removal.
Does reverse osmosis remove fluoride?
Yes, reverse osmosis is highly effective at removing fluoride from water. It can remove up to 90-97% of fluoride, making it a popular choice for people concerned about fluoride in their drinking water.
Can reverse osmosis remove pharmaceuticals from water?
Reverse osmosis can remove many pharmaceuticals from water, but not all of them. Some smaller or persistent pharmaceutical compounds may not be completely eliminated. Supplemental treatments like activated carbon filtration can improve pharmaceutical removal.
Is RO water acidic?
RO water tends to be slightly acidic because the removal of minerals disrupts the water’s natural pH balance. The pH typically ranges from 5 to 7. This is generally not a health concern, but some people prefer to remineralize the water to raise the pH.
Does reverse osmosis remove bacteria and viruses?
Yes, reverse osmosis is very effective at removing bacteria and viruses due to the extremely small pore size of the membrane. This process can remove 99.9% of harmful bacteria and viruses. However, a UV sterilizer can provide an additional layer of protection.
How much water does an RO system waste?
RO systems typically waste water during the purification process, as the contaminants are flushed down the drain. The waste-to-pure water ratio can vary from 2:1 to 4:1, depending on the system and water pressure. More efficient models are available that minimize water waste.
How long does a reverse osmosis system last?
A well-maintained reverse osmosis system can last for 10-15 years or longer. Regular filter changes, membrane cleaning, and proper maintenance are essential for maximizing the lifespan of your system.
What is the difference between RO and distillation?
Both RO and distillation are water purification methods. RO uses a membrane to filter water, while distillation involves boiling water and collecting the steam, leaving contaminants behind. Distillation typically removes more contaminants than RO, but it is also more energy-intensive.