**Question:** Which of the following is purification of water at large scale?

A. Disinfection of well
B. Rapid sand filter
C. Slow sand filter
D. Household purification

**Core Concept:** Purification of water involves various methods to remove impurities and ensure water safety for consumption. These methods can be broadly categorized into physical and chemical processes.

**Why the Correct Answer is Right:**

The correct answer is **B. Rapid sand filter**. Rapid sand filters are commonly used in water treatment plants to remove suspended particles, bacteria, and other impurities from water. The process involves passing water through layers of sand and gravel, promoting filtration and removal of particulate matter. This method ensures that large particles and microorganisms are trapped, improving the overall quality of the water.

**Why Each Wrong Option is Incorrect:**

A. **Disinfection of well** may be a part of water purification, but it is not an extensive process for large-scale water purification. Disinfection is performed after filtration to kill remaining microorganisms.

C. **Slow sand filter** is another type of sand filter, but it operates at a slower rate than rapid sand filters and is not suitable for large-scale water purification.

D. **Household purification** refers to methods used at the individual level to purify drinking water, which may include boiling, filtration, or chemical treatments. These methods are not suitable for large-scale water purification due to their limited capacity and inability to handle high volumes of water.

**Clinical Pearls:**

1. **Rapid sand filters** are efficient in treating large volumes of water, ensuring they meet health and safety standards for consumption. This method is crucial for large-scale water purification in communities or during emergencies when traditional water sources are contaminated.
2. Combining physical filtration (rapid sand filter) with chemical disinfection (e.g., chlorination) ensures comprehensive purification of water at large scale.

By understanding these purification methods, medical professionals can effectively address waterborne diseases, ensuring public health and safety.