---
title: "Floating drum and fixed dome biogas plants"
subject: "Energy Resources"
chapter: 3
key: "semester_6/energy_resources:floating-drum-fixed-dome-biogas-plant"
source: https://www.bctengineer.com/q/semester_6/energy_resources/floating-drum-fixed-dome-biogas-plant
site: BCT Engineering
---

# Floating drum and fixed dome biogas plants

Energy Resources · Chapter 3: Bio energy

- Asked in: year not given, 6 marks
- Video: [Difference between Floating Drum and Fixed Dome Biogas plant | Constant Volume and Pressure Biogas](https://www.youtube.com/watch?v=0SMAUX3-62M) — Agravat Vishal
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/energy_resources/floating-drum-fixed-dome-biogas-plant
- Whole chapter: https://www.bctengineer.com/md/semester_6/energy_resources/chapter-3

## Question

With neat sketches, explain the working of a floating drum type and a fixed dome type biogas plant. Differentiate between them.

## Short answer

In a floating drum plant, gas collects in a moving steel drum, providing constant gas pressure; the drum needs painting and is prone to corosion. In a fixed dome plant, gas is stored under a fixed masonry dome, displacing slurry into an outlet chamber, resulting in variable pressure; it has no moving parts, lower cost and longer life, but leaks are harder to detect.

## Answer

A biogas plant has an inlet (mixing tank), a digester where bacteria act on the slurry, a gas holder and an outlet for the digested slurry.

#### Floating drum plant (KVIC / Indian type)
```text
        gas out
          |
      ___ | ___ <- steel drum (moves
     |  gas   |     up/down on guide)
  ---|--------|---
  inlet  \   /  outlet
  --->|  slurry  |--->
      |__________|
       digester well
```
Slurry is fed into an underground cylindrical digester. The gas collects in an inverted mild steel drum that floats on the slurry and rises or falls with the amount of gas stored, so the gas **pressure stays constant** (about 8–10 cm of water). A partition wall in the digester makes the slurry flow around it, so fresh feed does not pass straight to the outlet.

#### Fixed dome plant (Chinese / Deenbandhu type)
```text
        gas pipe
           |
        _.-'-._   <- fixed brick dome
       /  gas   \
  --->|  slurry  |---> outlet/displacement
       \_______ /      chamber
        digester
```
The digester and gas holder form one masonry (brick or concrete) structure. Gas collects under the fixed dome and pushes slurry into the outlet (displacement) chamber, so the gas **pressure varies** with the amount of gas stored (up to about 1 m of water column).

| Point | Floating drum | Fixed dome |
|---|---|---|
| Gas holder | Moving steel drum | Fixed dome, no moving parts |
| Gas pressure | Constant | Variable |
| Cost | Higher (steel drum) | Lower |
| Life | 8–15 yr (drum corrodes) | 20 yr or more |
| Maintenance | Painting, drum repair | Cracks are hard to find and repair |
| Gas leak | Easy to see | Difficult to detect |
| Temperature | Exposed drum, more heat loss | Fully underground, more stable |
| Skilled labour | Less | More needed for construction |

## Check yourself

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

1. Why is the gas pressure constant in a floating drum plant?
   A. The dome is fixed and gas is compressed
   B. A pump regulates the pressure
   C. The digester is above ground
   D. The drum rises and falls with stored gas, keeping weight-based pressure constant
   Answer: D. Pressure is about 8-10 cm of water column. (Key point: Floating drum pressure)

2. In a fixed dome plant, what happens as gas collects?
   A. The dome rises with the gas
   B. Pressure stays constant
   C. It pushes slurry into the displacement chamber and the pressure varies
   D. The gas escapes through the drum
   Answer: C. Pressure rises up to about 1 m water column when stored gas is large. (Key point: Fixed dome pressure)

3. Compared with floating drum, the fixed dome plant has...
   A. Higher cost and shorter life
   B. Lower cost and longer life (20 years or more)
   C. Higher cost and longer life
   D. Lower cost and shorter life
   Answer: B. It has no steel drum to corrode. (Key point: Cost and life)

4. Which statement is correct?
   A. Gas leaks are easy to see in the floating drum but difficult to detect in the fixed dome
   B. Fixed dome leaks are easy to see
   C. Neither can leak
   D. Floating drum leaks are hard to find
   Answer: A. Cracks in masonry are hard to find and repair. (Key point: Leak detection)

## Related in Energy Resources

- [Sizing a family biogas plant](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Abiogas-plant-sizing-cattle-dung) — Energy Resources, chapter 3

## Related in other subjects

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- [Differentiate GVF, RVF and SVF with sketches](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Adifferentiate-gvf-rvf-svf-sketch) — Hydraulics, chapter 5, asked 5×
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- [Working and performance of combined plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-working-performance) — Power Plant Equipment, chapter 8