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Nalgonda Technique - Park's Textbook of Preventive and Social Medicine, 28th Edition
Background: Endemic Fluorosis in India
Before discussing the Nalgonda technique, Park's textbook contextualizes it within the problem of endemic fluorosis. In parts of India where drinking water contains excessive fluorine (3-5 mg/L), endemic fluorosis is an important public health problem. Affected districts include those in Andhra Pradesh (notably Nalgonda district itself), Punjab, Haryana, Karnataka, Kerala, and Tamil Nadu.
The three forms of fluorosis described are:
| Type | Fluoride Level | Features |
|---|
| Dental fluorosis | >1.5 mg/L | Mottling of enamel - chalk-white patches turning yellow/brown/black |
| Skeletal fluorosis | 3-6 mg/L (lifetime) | Fluoride deposition in skeleton; crippling at >10 mg/L |
| Genu valgum | High fluoride + sorghum diet | Genu valgum + osteoporosis of lower limbs (Andhra Pradesh, Tamil Nadu) |
Intervention Approaches (as per Park's, p. 740)
Park's describes three interventions for endemic fluorosis:
(a) Changing the water source - Finding an alternative source with lower fluoride (0.5 to 0.8 mg/L). Surface running water contains lower fluoride than groundwater (wells).
(b) Chemical treatment - This is where the Nalgonda technique is described.
(c) Other measures - Fluoride supplements should NOT be prescribed for children drinking fluoridated water. Fluoride toothpaste is not recommended in endemic fluorosis areas for children up to 6 years.
The Nalgonda Technique (as per Park's Textbook, p. 740)
"The National Environmental Engineering Research Institute, Nagpur developed a technique for removing fluoride by chemical treatment. It is called Nalgonda technique for defluoridation of water. It involves the addition of two chemicals (viz. lime and alum) in sequence followed by flocculation, sedimentation and filtration."
- Park's Textbook of Preventive and Social Medicine, 28th Edition, p. 740
Key Details of the Nalgonda Technique
1. Origin and Development
- Developed by the National Environmental Engineering Research Institute (NEERI), Nagpur
- Named after Nalgonda district, Andhra Pradesh, one of the most severely fluoride-affected districts in India
- Both domestic-level (bucket/drum) and community-level (fill-and-draw / continuous flow plant) systems have been developed
2. Chemicals Used
Three chemicals are added to fluoride-contaminated water:
| Chemical | Role |
|---|
| Aluminium salt (Alum) - as aluminium sulphate [Al₂(SO₄)₃] or aluminium chloride (AlCl₃), or a combination | Primary agent for fluoride removal through coagulation |
| Lime [Ca(OH)₂ or CaO] | Facilitates formation of dense floc for rapid settlement; maintains pH |
| Bleaching powder [Ca(OCl)₂] | Disinfection - added at 3 mg/L |
- The dose of lime is empirically 1/20th of the aluminium salt dose.
- The selection of aluminium sulphate vs. chloride depends on the sulphate and chloride content of the raw water, to avoid exceeding permissible limits.
3. Alum Dosage
- Dosage of alum varies from 145 to 1,600 mg/L (16 to 181 mg/L as Al) depending on the initial fluoride and alkalinity levels of raw water.
- The dose increases with increasing fluoride and alkalinity levels.
- A preliminary lab test is mandatory to determine fluoride concentration before treatment, as wrong dosage leads to inappropriate final pH.
4. Step-by-Step Process
Raw (fluoridated) water
↓
[1] RAPID MIXING
Add lime + alum + bleaching powder
(Lime added first → precipitation of CaF₂)
(Then alum added → coagulation)
↓
[2] FLOCCULATION
Slow stirring to allow formation of dense floc
↓
[3] SEDIMENTATION / PRECIPITATION
Floc settles carrying adsorbed fluoride
↓
[4] FILTRATION
Removal of residual suspended particles
↓
[5] DISINFECTION
Bleaching powder ensures microbiological safety
↓
Treated water (fluoride ≤ 1.0 mg/L)
5. Chemical Reactions Involved
Step 1 - Lime addition (precipitation of fluoride):
2NaF (aq) + Ca(OH)₂ (aq) → 2NaOH (aq) + CaF₂ (s) ↓
Lime precipitates fluoride as insoluble calcium fluoride (CaF₂) and raises water pH up to ~12. Lime dosing alone can reduce fluoride from 10 mg/L to ~8 mg/L.
Step 2 - Alum addition (coagulation):
- Alum reacts with water alkalinity to produce insoluble aluminium hydroxide [Al(OH)₃] floc.
- Alum also reacts with fluoride ions directly, adsorbing fluoride onto the floc.
- Best fluoride removal occurs at pH 5.5 to 7.5.
6. Efficiency
- Can remove up to 90% of fluoride from water.
- Effective even when dissolved solids are above 1,500 mg/L and hardness above 600 mg/L.
- Target: achieve fluoride level of ≤ 1.0 mg/L (permissible limit).
7. Applicability
- Can be applied at two levels:
- Domestic level - Bucket/drum defluoridation systems for household use
- Community level - Fill-and-draw type or continuous flow defluoridation plants for piped water supply
- Can be adapted to drinking water equipment such as hand pumps.
- Simple enough to be understood and operated by common people without specialist training.
8. Limitations / Drawbacks
- Requires preliminary water testing (fluoride + alkalinity levels) to determine correct alum dose.
- Excessive aluminium dose raises residual aluminium in treated water - pH must be controlled carefully.
- Produces toxic sludge containing concentrated fluoride and aluminium, requiring proper disposal.
- Treatment is moderately expensive (as noted by Park's).
- If alum dose is miscalculated, treated water may have unacceptably high residual aluminium.
Summary
| Feature | Detail |
|---|
| Developed by | NEERI, Nagpur |
| Named after | Nalgonda district, Andhra Pradesh |
| Primary chemicals | Alum + Lime + Bleaching powder |
| Mechanism | Coagulation + precipitation + flocculation + sedimentation + filtration |
| Fluoride removal | Up to 90% |
| Target fluoride level | ≤ 1.0 mg/L |
| Applicable at | Domestic and community level |
| Key limitation | Residual aluminium risk; sludge disposal; cost |
Source: Park's Textbook of Preventive and Social Medicine, 28th Edition, Chapter on Nutrition - Endemic Fluorosis, Intervention section, p. 740 (reference 103).