Fluoride Removal: Combating Fluorosis
While small amounts of fluoride prevent cavities, excess fluoride causes skeletal deformities. Discover Activate Alumina and RO methods for defluoridation.
Fluoride is a "double-edged sword." At roughly 0.7 ppm, it strengthens teeth. Above 1.5 ppm, it causes Fluorosis.
- Dental Fluorosis: Stained, pitted teeth.
- Skeletal Fluorosis: Curving of bones, calcification of joints, and crippling pain.
In many granite-rock regions, groundwater fluoride naturally exceeds 5 ppm or even 10 ppm.
Removal Techniques
- Limit: The WHO and IS 10500 limit is 1.0 to 1.5 mg/L.
1. Activated Alumina (AA) - The Adsorption King
Activated Alumina is a porous form of aluminum oxide (Al2O3). It is the most widely used technology for removing fluoride because it is specific and relatively inexpensive.
- Mechanism: It works by adsorption. As water flows through the media, fluoride ions are attracted to the alumina surface and trapped in its pores.
- pH Sensitivity: AA works best at a pH of 5.5 to 6.5. If the water is too alkaline (pH > 8), its efficiency drops drastically.
- Capacity: Can typically remove 2000-4000 mg of Fluoride per kg of media.
- Regeneration:
- Backwash: Flush with water to remove dirt.
- Acid Wash: Rinse with weak acid to strip the fluoride.
- Neutralize: Rinse with caustic soda to restore pH.
2. Reverse Osmosis (RO) - The Premium Soluton
RO is the "catch-all" solution. Since a Fluoride ion (F-) is larger than a water molecule (in hydrated radius) and carries a charge, RO membranes reject it with >98% efficiency.
- Pros: Removes not just fluoride, but also arsenic, salts, and bacteria simultaneously.
- Cons: High water wastage (rejection) and requires electricity.
3. Nalgonda Technique - The Community Hero
Developed by the National Environmental Engineering Research Institute (NEERI) in Nagpur, India, this is a low-cost, low-tech method perfect for rural villages.
The Recipe (for 1000 Liters):
- Rapid Mix: Add Aluminum Sulfate (Alum) + Lime (Calcium Oxide) + Bleaching Powder.
- Flocculation: The Alum reacts with alkalinity to form heavy, sticky flakes (flocs). These flocs trap the fluoride ions.
- Sedimentation: The water sits still for 2-4 hours. The heavy flocs settle to the bottom as sludge.
- Filtration: The clear water on top is drawn off and filtered through a simple sand filter.
Comparison of Methods
| Feature | Reverse Osmosis (RO) | Activated Alumina | Nalgonda Technique |
|---|---|---|---|
| Cost | High (CapEx + OpEx) | Medium | Very Low |
| Electricity? | Required | Not Required | Not Required |
| Skill Needed | Moderate | Moderate | Low |
| Waste | High (Brine) | Chemical Sludge | Chemical Sludge |
| Best For | Urban Homes / Industry | Community Plants | Rural Villages |
Maintenance Checklist
For an Activated Alumina Plant:
- Weekly: Check pH of inlet water (adjust to < 7.0 for best results).
- Monthly: Backwash the media to prevent channeling.
- Quarterly: Test outlet water for Fluoride breakthrough (it happens suddenly).
- Yearly: Regenerate or replace the media.
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