---
title: "Rainwater harvesting and its treatment"
subject: "Building Technology"
chapter: 12
key: "semester_6/building_technology:rwh-treatment"
source: https://www.bctengineer.com/q/semester_6/building_technology/rwh-treatment
site: BCT Engineering
---

# Rainwater harvesting and its treatment

Building Technology · Chapter 12: Other services in building

- Asked in: 2073 Magh; 2073 Bhadra; 2072 Asoj, 6 marks (asked 3 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/rwh-treatment
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-12

## Question

Explain rainwater harvesting and the process (methods) of treatment of rain water.

## Short answer

Rainwater harvesting collects roof runoff for use; treatment includes mesh screening, first-flush diversion of initial dirty rain, sedimentation, multi-layer sand-gravel-charcoal filtration, and disinfection (chlorination, boiling, or UV) to make it safe. Storage tanks should be dark, covered and cleaned annually.

## Answer

**Rainwater harvesting (RWH)** is the collection and storage of rain falling on roofs or the ground for later use, or for recharging groundwater, instead of letting it run off.

It is mainly done from rooftops: rain falls on the roof, is collected by gutters, passes through a first-flush device and a filter, and is stored in a tank or sent to recharge pits. It is needed because of water scarcity and falling groundwater levels, especially in urban areas like the Kathmandu valley.

#### Treatment of rainwater
Rainwater is relatively pure, but it picks up dust, leaves, bird droppings and microbes from the roof and the air. Treatment steps:

1. **Screening:** mesh at the gutters and inlet removes leaves and large debris.
2. **First-flush diversion:** discards the first 10-15 minutes of dirty rain.
3. **Sedimentation:** holding the water in a tank lets suspended particles settle.
4. **Filtration:** a gravity filter with layers of coarse gravel (bottom), fine gravel, sand and charcoal (top) removes turbidity, odour and colour. A slow sand filter or ceramic candle filter can be used for drinking water.
5. **Disinfection:** chlorination (a chlorine dose of about 2-4 mg/L, e.g. from bleaching powder, leaving a residual of 0.2-0.5 mg/L), boiling, UV, SODIS (solar disinfection) or ozone, to kill pathogens.
6. **Storage care:** a closed, dark tank with a mosquito-proof vent, cleaned every year.

```text
 rain -> [mesh] -> [first flush] -> [sedimentation]
        -> [sand-gravel-charcoal filter] -> [disinfection]
        -> storage tank -> use
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which sequence is correct?
   A. Screening, first flush, sedimentation, filtration, disinfection
   B. Disinfection, then filtration, then screening
   C. Storage, then screening
   D. Filtration before first flush only
   Answer: A. Coarse removal comes first, disinfection last. (Key point: Treatment sequence)

2. How is a gravity filter arranged?
   A. Charcoal at bottom, gravel on top
   B. Coarse gravel at bottom, fine gravel, sand, charcoal on top
   C. Only clay
   D. Sand at bottom, coarse gravel on top
   Answer: B. Layers remove turbidity, odour and colour. (Key point: Filter layers)

3. Which are disinfection methods?
   A. Screening and sedimentation
   B. Gutter mesh
   C. Chlorination, boiling, UV, SODIS, ozone
   D. First flush
   Answer: C. Disinfection kills pathogens. (Key point: Disinfection)

4. What residual chlorine is targeted?
   A. 10 mg/L
   B. 0 mg/L
   C. 5-10 mg/L
   D. 0.2-0.5 mg/L
   Answer: D. A dose leaving 0.2-0.5 mg/L residual. (Key point: Chlorine residual)

## Related in Building Technology

- [Components of rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-components) — Building Technology, chapter 12, asked 2×
- [Types of rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-types) — Building Technology, chapter 12
- [Short note: Rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arainwater-harvesting-note) — Building Technology, chapter 12, asked 5×
- [Benefits of rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-benefits) — Building Technology, chapter 12

## Related in other subjects

- [Treatment process; sedimentation factors; coagulants](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Atreatment-impurities-coagulants) — Water Supply Engineering, chapter 6
- [Activated sludge and ASP treatment process](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aasp-sketch-treatment-process) — Sanitary Engineering, chapter 7
- [Point and non-point sources; waste water treatment](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Apoint-nonpoint-waste-water) — Engineering Chemistry, chapter 3
- [Mechanical vs heat treatment, tempering](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Amech-heat-treatment-isothermal-tempering) — Civil Engineering Materials, chapter 8