---
title: "Volume-moisture relation; sludge disposal"
subject: "Sanitary Engineering"
chapter: 8
key: "semester_6/sanitary_engineering:volume-moisture-sludge-disposal"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/volume-moisture-sludge-disposal
site: BCT Engineering
---

# Volume-moisture relation; sludge disposal

Sanitary Engineering · Chapter 8: Sludge Treatment and Disposal

- Asked in: 2076 Baisakh, 8 marks
- Video: [Volume Mass Relationship for Sludge | Waste Water Engineering](https://www.youtube.com/watch?v=ufq1XJH1FOU) — APSEd
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/volume-moisture-sludge-disposal
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-8

## Question

Discuss about volume moisture content relationship in sludge. State the methods of sludge disposal and describe briefly about the most suitable one based upon your justification.

## Short answer

Sludge volume varies inversely with solids content: V1/V2=(100-P2)/(100-P1); small moisture reductions greatly cut volume. Methods are dumping, landfilling, lagooning, land application and incineration. Most suitable is digested, dried sludge used as manure, with landfill backup, being stable, cheap and beneficial.

## Answer

#### Volume-moisture relationship
Sludge consists of water and solids. If the amount of solids does not change, the volume of sludge is inversely proportional to the percentage of solids (i.e. $100-P$ where $P$ is the moisture content):

$$
\frac{V_1}{V_2}=\frac{100-P_2}{100-P_1}=\frac{S_2}{S_1}
$$

where $V_1,V_2$ are the volumes at moisture contents $P_1,P_2$ (%) and $S$ is the percentage solids. This assumes the specific gravity of the sludge stays the same (about 1.02, since solids are so dilute).

*Example:* sludge of 98% moisture dewatered to 90% moisture gives $V_2/V_1=\dfrac{100-98}{100-90}=0.2$, so the volume falls to one-fifth (80% reduction). A fall from 98% to 96% moisture halves the volume.

Because a small drop of moisture content gives a large drop in volume, thickening and dewatering are very effective in reducing handling and disposal cost.

#### Methods of sludge disposal
1. Dumping on land or in the sea.
2. Sanitary landfill (burial of dewatered sludge in trenches).
3. Lagooning (storage in shallow basins).
4. Use as manure or soil conditioner on land (after digestion and drying).
5. Incineration.

#### Most suitable method: land application of digested and dried sludge (with landfill as a back-up)
For a town in Nepal, I choose digested sludge, dried on sand drying beds, and used as manure/soil conditioner on agricultural or forest land, with surplus sent to a sanitary landfill. The justification is:
- Digested and dried sludge is stable, inoffensive and has few pathogens.
- It contains nitrogen, phosphorus and organic matter that improve soil, so it is a resource, not a waste.
- It is cheap: drying beds need little energy or skill, and the climate allows drying for much of the year.
- Landfilling or lagooning needs large land and gives odour risk, dumping in rivers is unacceptable, and incineration needs high capital and fuel cost.
- Heavy-metal content must be checked, and use must be avoided on crops eaten raw.

## Check yourself

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

1. If the solids mass is constant, how does sludge volume vary with moisture content P?
   A. Directly with P
   B. Inversely with P
   C. Directly with (100-P)
   D. Inversely with (100-P), the percentage of solids
   Answer: D. V1/V2 = (100-P2)/(100-P1): volume is inversely proportional to the solids percentage. (Key point: Volume-moisture relation)

2. Sludge dewatered from 98% to 90% moisture changes its volume to what fraction?
   A. One-fifth (0.2)
   B. One-half
   C. Eight-ninetieths (0.09)
   D. One-tenth
   Answer: A. V2/V1 = (100-98)/(100-90) = 2/10 = 0.2, an 80% reduction. (Key point: Worked example)

3. Why is thickening and dewatering so effective in cutting disposal cost?
   A. It removes all pathogens
   B. A small drop in moisture content gives a large drop in volume
   C. It increases the solids mass
   D. It raises the specific gravity of sludge greatly
   Answer: B. Because the solids are only a few percent, removing a little water cuts volume sharply (98% to 96% halves it). (Key point: Why dewatering pays)

4. Why is digested, dried sludge used as manure (landfill as back-up) chosen as most suitable for a Nepali town?
   A. It needs high capital and fuel
   B. It is dumped in rivers
   C. It is stable, nutrient-rich and cheap to produce on drying beds
   D. It requires large lagoon areas and gives odour
   Answer: C. Digested dried sludge is inoffensive, supplies N, P and organic matter, and drying beds need little energy or skill. (Key point: Best disposal choice)

## Related in Sanitary Engineering

- [Volume-moisture relation; specific gravity](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-specific-gravity-95) — Sanitary Engineering, chapter 8
- [Digester volume for sludge, moisture 92%](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-digester-volume-92) — Sanitary Engineering, chapter 8, asked 2×
- [Sludge treatment aims; digester volume (94%)](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-aims-digestion-volume-94) — Sanitary Engineering, chapter 8, asked 2×
- [Sludge volume and digester, 4 MLD](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-volume-4mld-digester) — Sanitary Engineering, chapter 8
- [Sludge volume from sedimentation tank, 10 MLD](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-volume-10mld-250) — Sanitary Engineering, chapter 8
- [Yearly sludge volume, 6 MLD](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-volume-yearly-6mld) — Sanitary Engineering, chapter 8

## Related in other subjects

- [Quality, moisture content, specific volume of mixture](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aquality-moisture-specific-volume) — Fundamental of Thermodynamics & Heat Transfer, chapter 3
- [Define moisture content, superheated vapor, etc.](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Amoisture-superheated-definitions) — Fundamental of Thermodynamics & Heat Transfer, chapter 3
- [Soil-moisture-irrigation relationship](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-irrigation-relationship) — Irrigation and Drainage Engineering, chapter 2, asked 2×
- [One-to-many relationship using collections](https://www.bctengineer.com/question.md?key=semester_6%2Fobject_oriented_analysis_and_design%3Aone-to-many-collection-generic-classes) — Object Oriented Analysis and Design, chapter 4