During a Health Mela organized by a medical college, the Nalgonda technique for water purification was demonstrated. Which two chemicals are used in this technique?

Explanation

## Correct Answer: B. Alum and Lime The Nalgonda technique is a **defluoridation method** developed in Nalgonda district, Telangana (formerly Andhra Pradesh), India—a region endemic for fluorosis due to high groundwater fluoride levels. This technique uses **alum and lime** as the two primary chemicals. Alum (aluminum sulfate) acts as a coagulant and precipitant, forming aluminum hydroxide flocs that bind fluoride ions. Lime (calcium hydroxide) serves dual purposes: it raises the pH to the optimal range (8–9) for fluoride precipitation and provides calcium ions that form calcium fluoride precipitate. The process is cost-effective, requires minimal equipment, and is particularly suited for rural India where fluorosis is endemic. The technique has been widely adopted in fluoride-affected regions of Andhra Pradesh, Rajasthan, and Gujarat. The chemical reaction involves fluoride being adsorbed onto aluminum hydroxide precipitate and co-precipitated as calcium fluoride, achieving fluoride removal of 50–60% in a single pass. This makes it a practical, scalable solution for community water purification in resource-limited settings. ## Why the other options are wrong **A. Alum and Gypsum** — Gypsum (calcium sulfate) is not used in the Nalgonda technique. While gypsum can be used in water softening to reduce hardness, it does not participate in fluoride removal. The Nalgonda method specifically requires lime (calcium hydroxide), not gypsum, to achieve the pH adjustment and calcium fluoride precipitation necessary for defluoridation. **C. Alum and Charcoal** — Charcoal is used for adsorption of organic contaminants and color removal in water purification, but it is not part of the Nalgonda defluoridation technique. While alum is correct, charcoal does not precipitate fluoride ions. The Nalgonda method requires lime to form calcium fluoride precipitate and adjust pH—charcoal serves no role in fluoride removal. **D. Charcoal and Lime** — This option omits alum, which is the essential coagulant and fluoride-binding agent in the Nalgonda technique. Although lime is correctly included, charcoal alone cannot achieve the chemical precipitation and adsorption of fluoride that alum provides. The synergistic action of alum and lime is the hallmark of this technique. ## High-Yield Facts - **Nalgonda technique** is a defluoridation method developed in Telangana, India, for endemic fluorosis regions. - **Alum** acts as coagulant and precipitant, forming aluminum hydroxide flocs that bind fluoride ions. - **Lime (calcium hydroxide)** raises pH to 8–9 and provides calcium for calcium fluoride precipitation. - **Fluoride removal efficiency** is 50–60% per pass, making it practical for rural community water systems. - **Endemic fluorosis** is prevalent in Andhra Pradesh, Rajasthan, Gujarat, and parts of Tamil Nadu—Nalgonda technique is DOC in these regions. - **Cost-effectiveness** and minimal equipment requirement make this technique ideal for decentralized water purification in India. ## Mnemonics **ALUM-LIME for Nalgonda** **A**luminum sulfate (coagulant) + **L**ime (pH adjuster & precipitant) = **Nalgonda** defluoridation. Remember: Alum binds fluoride, Lime precipitates it as CaF₂. **Fluorosis Fix: AL** **A**lum + **L**ime = Nalgonda. Use when you see 'defluoridation' or 'Nalgonda' in a PSM question on water purification. ## NBE Trap NBE may pair alum with other chemicals (gypsum, charcoal) to exploit students who remember 'alum' as a water purification agent but confuse it with generic coagulation methods rather than the specific Nalgonda defluoridation chemistry. The trap is that alum alone is insufficient—lime is the discriminating second chemical. ## Clinical Pearl In endemic fluorosis regions like Nalgonda district, the technique has prevented dental and skeletal fluorosis in thousands of villagers. A single household or community-level unit can be set up with minimal cost—making it the preferred DOC in rural India where piped water systems are unavailable and groundwater fluoride exceeds 4 mg/L. _Reference: Park's Textbook of Preventive and Social Medicine, Ch. 9 (Water Supply and Sanitation); WHO Guidelines on Defluoridation of Drinking Water_