Compare the advantages of low-solids and high-solids digestion.

Low-solids digesters are easy to pump and mix, and suit slurries and food waste. High-solids digesters need less water and smaller tanks, and tolerate grit and fibre better.

What types of waste suit a low-solids digester?

Animal slurries, sewage sludge and liquidised food waste suit low-solids digestion. Drier materials can be diluted with water or liquid digestate.

Five tonnes of feedstock contain 0.6 tonnes of dry solids. What is its total solids content, and which type of digestion does it suit?

The total solids content is 12%, which suits low-solids digestion. The calculation is 0.6 ÷ 5 × 100 = 12%.

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Waste to Energy Technologies

Digester design: solids content, residence time and staging

3 things to know
A2 1

Overview

Low-solids and high-solids digestion, suitable wastes, total solids and hydraulic residence time calculations, mesophilic and thermophilic residence times, and single-stage and multistage digesters.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What does anaerobic digestion of waste produce?

Key term

What is meant by low-solids anaerobic digestion?

Key term

What is meant by high-solids anaerobic digestion?

Questions from this subtopic

Each one with the model answer underneath.

1
What does anaerobic digestion of waste produce?
Model answer
Anaerobic digestion produces biogas, a mixture of methane and carbon dioxide, and a nutrient-rich digestate. Microbes break the waste down in a sealed tank without oxygen.
2
What is meant by low-solids anaerobic digestion?
Model answer
Low-solids, or wet, digestion treats feedstock containing less than about 15% dry solids. The pumpable slurry is mixed continuously in a tank.
3
What is meant by high-solids anaerobic digestion?
Model answer
High-solids, or dry, digestion treats stackable feedstock of roughly 20–40% dry solids. The material is handled with loaders or pushed through the digester as a plug.
4
Compare the advantages of low-solids and high-solids digestion.
Model answer
Low-solids digesters are easy to pump and mix, and suit slurries and food waste. High-solids digesters need less water and smaller tanks, and tolerate grit and fibre better.
5
What types of waste suit a low-solids digester?
Model answer
Animal slurries, sewage sludge and liquidised food waste suit low-solids digestion. Drier materials can be diluted with water or liquid digestate.
6
Five tonnes of feedstock contain 0.6 tonnes of dry solids. What is its total solids content, and which type of digestion does it suit?
Model answer
The total solids content is 12%, which suits low-solids digestion. The calculation is 0.6 ÷ 5 × 100 = 12%.

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