Arsenic is one of the most dangerous contaminants in drinking water. Unlike chlorine or iron, you can’t smell or taste it. Yet, long-term exposure can cause skin lesions, cancer, heart disease, and developmental issues in children.
Millions of people worldwide, especially in South Asia and parts of the U.S., face this hidden threat. The question is: How do you make your drinking water safe? One proven solution is a reverse osmosis (RO) system. But how exactly does it remove arsenic? Let’s break it down.
Key Takeaways
- Arsenic is a toxic element found in groundwater, often undetectable without testing.
- RO systems remove arsenic by forcing water through a semi-permeable membrane that blocks dissolved contaminants.
- RO is capable of reducing arsenic levels by up to 95–99%, making water safe for consumption【WHO, 2022】.
- Pre-filters, membranes, and post-filters all play roles in arsenic removal.
- Regular maintenance ensures continued protection.
What Is Arsenic and Why Is It Dangerous?
Arsenic is a naturally occurring element found in rocks, soil, and groundwater. Industrial pollution can also add arsenic to water supplies.
Health Risks of Arsenic Exposure
- Skin disorders and pigmentation changes.
- Increased risk of cancers (skin, bladder, lung).
- Cardiovascular diseases.
- Developmental issues in children.
- Weakness and neurological effects.
Stat Insight: The WHO guideline for arsenic in drinking water is 10 micrograms per liter (10 µg/L)【WHO, 2022】. In some regions, levels exceed 200 µg/L.
How Does Reverse Osmosis Work in General?
An RO system pushes water through a semi-permeable membrane with pore size around 0.0001 microns.
- Water molecules pass through.
- Dissolved salts, metals, and toxins like arsenic are rejected.
- Clean water (permeate) is stored, while contaminated water (concentrate) is drained.
How Do RO Systems Remove Arsenic Specifically?
1. Pre-Filtration
Sediment and carbon filters remove dirt, chlorine, and organic chemicals. This protects the membrane from clogging or chemical attack.
2. RO Membrane Action
The RO membrane is where arsenic removal happens.
- Arsenic exists in two forms:
- Arsenic (III) – Arsenite: harder to remove, neutral charge.
- Arsenic (V) – Arsenate: easier to remove, negatively charged.
- RO membranes are more effective at rejecting arsenate (As V).
- Pre-treatment or oxidation (chlorination/ozonation) can convert As III to As V for higher removal efficiency.
3. Post-Treatment
Polishing filters (like carbon) enhance taste. Optional mineral cartridges restore essential minerals.
How Effective Is RO at Removing Arsenic?
- RO systems typically reduce arsenic by 95–99%.
- Performance depends on membrane quality, water pressure, and arsenic form.
Example:
- Raw water arsenic: 150 µg/L.
- RO-treated water: 5–10 µg/L (within safe limits).
RO vs Other Arsenic Removal Methods
| Method | Efficiency | Best For |
|---|---|---|
| Reverse Osmosis | 95–99% | Homes and industries |
| Distillation | 99% | Small-scale, high cost |
| Activated Alumina | 80–95% | Point-of-use filters |
| Ion Exchange | 90–95% | Municipal systems |
| Coagulation & Filtration | 80–90% | Large-scale treatment |
RO stands out as cost-effective and reliable for households.
What Factors Affect Arsenic Removal in RO Systems?
1. Water Pressure
RO membranes require 40–60 PSI to function effectively.
2. Water pH
Arsenic removal works best at neutral to slightly acidic pH levels.
3. Arsenic Form (III vs V)
- As V is easier to remove.
- Pre-treatment with oxidizers can convert As III to As V.
4. Membrane Age
Worn membranes allow arsenic to pass. Replacement every 2–3 years is essential.
How to Test Your Water for Arsenic
- Lab Testing: Most accurate. Required for areas with known arsenic issues.
- Field Test Kits: Useful for quick screening.
- TDS Meters: Not effective for arsenic. They measure total dissolved solids, not specific toxins.
Tip: Always test before and after installing an RO purifier.
Maintenance Tips to Ensure Arsenic Removal
- Replace Pre-Filters on Time
- Every 6–12 months. Prevents chlorine from damaging the membrane.
- Replace RO Membrane Every 2–3 Years
- Ensures maximum rejection of arsenic.
- Flush the System Regularly
- Prevents scaling and fouling.
- Sanitize the Tank Annually
- Stops bacterial growth that could affect safety.
Can RO Remove All Types of Arsenic?
- Arsenic (V): Highly effective removal (95–99%).
- Arsenic (III): Lower removal efficiency. Needs oxidation pre-treatment.
Solutions for As III:
- Add chlorine, ozone, or aeration before RO.
- This converts As III → As V for better rejection.
Case Study: Arsenic Removal in Bangladesh
Bangladesh has one of the worst arsenic contamination crises, with 20 million people exposed to unsafe levels【UNICEF, 2021】.
- RO units have been introduced in households and community plants.
- Studies show arsenic levels dropped from 200 µg/L to <10 µg/L after RO treatment.
- This has prevented thousands of potential health cases.
Advantages of Using RO for Arsenic Removal
- High efficiency (95–99%).
- Works for multiple contaminants (not just arsenic).
- Easy to install and maintain at home.
- Reliable long-term solution.
Limitations of RO for Arsenic Removal
- Wastes 2–3 liters of water per liter purified.
- Less effective against As III unless oxidized.
- Requires electricity and maintenance.
- Removes beneficial minerals (needs remineralization).
Practical Tips for Homeowners in Arsenic-Prone Areas
- Test water annually for arsenic.
- Choose an RO system certified for arsenic removal (look for NSF/ANSI 58 certification).
- Install a pre-treatment oxidation unit if your supply contains As III.
- Monitor TDS but don’t rely on it alone—arsenic testing is essential.
Conclusion
Arsenic in drinking water is a silent but serious threat. Fortunately, reverse osmosis systems provide one of the most effective defenses, reducing arsenic by up to 99% when maintained properly.
Here’s the bottom line:
- RO systems block arsenic through ultra-fine membranes.
- Arsenic (V) is easier to remove than arsenic (III).
- Pre-treatment, regular maintenance, and water testing are essential for safety.
By investing in a quality RO purifier and servicing it on time, you can protect your family from the dangers of arsenic and enjoy safe, clean water every day.
Hasan Al Sarker is a Reverse Osmosis Specialist. He has worked for many years to ensure safe drinking water for all. His research paper has been published in several journals, including Issue, Medium, and Slideshare. He is recognized as a water doctor among specialists though he did not attend medical college.
Besides working as a researcher of reverse osmosis technology, he is also very fancy with the kitchen and cooking. His guides are reading thousands of people every day. As a head of content, he is responsible for all the published articles at RO System Reviews.