---
title: "Methods of sewage disposal"
subject: "Sanitary Engineering"
chapter: 6
key: "semester_6/sanitary_engineering:methods-sewage-disposal"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/methods-sewage-disposal
site: BCT Engineering
---

# Methods of sewage disposal

Sanitary Engineering · Chapter 6: Wastewater Disposal

- Asked in: 2072 Magh, 8 marks
- Video: [Water Supply and Wastewater Engineering - Sewage Disposal on Land](https://www.youtube.com/watch?v=yIokiwwxk5Y) — Hindusthan College of Engineering and Technology 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/methods-sewage-disposal
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-6

## Question

What are the various methods of sewage disposal? Describe with their advantages and disadvantages.

## Short answer

Disposal by dilution discharges treated effluent into water bodies, requiring adequate flow and self-purification; it is cheap but can pollute. Land treatment applies sewage on soil for irrigation, recovering nutrients and water but needing large land area and risking groundwater contamination.

## Answer

Sewage (after suitable treatment) is finally disposed of by two natural methods.

```text
 Sewage disposal
   |-- Dilution  (into a river, lake or sea)
   '-- Land treatment (irrigation / sewage farming, broad irrigation, filtration)
```

#### 1. Disposal by dilution
Sewage is discharged into a river, lake or sea, where the large volume of water dilutes it and natural self-purification (with the oxygen dissolved in the water) oxidises the organic matter.

**Essential conditions:** a river with enough flow and DO; no downstream water supply intakes near the outfall; good mixing; no unsightly floating matter; dilution factor according to the table (as per common practice, Garg):

| Dilution factor (river flow / sewage flow) | Treatment needed |
|---|---|
| more than 500 | none (raw sewage allowed) |
| 300-500 | primary treatment |
| 150-300 | secondary treatment |
| less than 150 | complete treatment |

**Advantages:** cheap, simple, uses natural resources, no land needed.
**Disadvantages:** pollutes the river if flow is small; odour and floating matter; harmful to downstream users; not feasible in dry season of small streams; sea disposal may affect the coast.

#### 2. Disposal on land (land treatment)
Sewage is applied on the land (sewage farming/irrigation). Organic matter is oxidised by soil bacteria, the plants use nutrients, and the water is partly evaporated and partly percolates.

**Conditions:** sufficient land at a distance of several hundred metres to 1 km, porous soil (sandy loam), low groundwater table, suitable climate, and a skilled operation. Sewage must first be pre-treated (sedimentation) to avoid clogging ("sewage sickness" occurs when the soil pores clog).

**Advantages:** recovers manure, nutrients and water for crops, an income to the municipality, good removal of BOD and bacteria, no river pollution.
**Disadvantages:** needs large land area (the rate is 1 ha for 250-500 persons or about 50 m³/ha/day); smell and mosquitoes; risk to health of workers and consumers of vegetables; contamination of groundwater; not useful in the rainy season or on frozen ground; sewage sickness.

**Methods of land application:** broad irrigation (sewage farming), surface/spray irrigation and intermittent sand filtration.

## Check yourself

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

1. What are the two natural methods of sewage disposal?
   A. Dilution and land treatment
   B. Sedimentation and filtration
   C. Chlorination and aeration
   D. Septic tank and soak pit only
   Answer: A. Treated sewage is disposed in water or on land. (Key point: Two methods)

2. If the dilution factor exceeds 500, what treatment is needed?
   A. Primary
   B. None; raw sewage may be discharged
   C. Secondary
   D. Complete
   Answer: B. Below 150 needs complete treatment. (Key point: Dilution factor)

3. Which is a disadvantage of dilution?
   A. Needs large land
   B. Sewage sickness
   C. Pollutes the river when flow is small, harmful to downstream users
   D. Soil clogging
   Answer: C. It depends on river flow and DO. (Key point: Dilution disadvantage)

4. What is sewage sickness?
   A. Disease of workers
   B. River oxygen loss
   C. Septic gas
   D. Clogging of soil pores in land treatment
   Answer: D. Pre-treatment by sedimentation helps avoid it. (Key point: Sewage sickness)

## Related in Sanitary Engineering

- [Importance and objectives of wastewater disposal](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aobjectives-wastewater-disposal) — Sanitary Engineering, chapter 6, asked 3×