Why is hydrolysis often the slowest stage when digesting solid waste?

Enzymes can only work on the surface of solid particles, and materials such as cellulose resist breakdown. The rate of hydrolysis then limits the whole process.

What happens during acidogenesis?

Acidogenic bacteria ferment the sugars, amino acids and fatty acids from hydrolysis. The products are volatile fatty acids, alcohols, hydrogen, carbon dioxide and ammonia.

What happens during acetogenesis?

Acetogenic bacteria convert volatile fatty acids and alcohols into acetic acid. Hydrogen and carbon dioxide are released at the same time.

CCEA A-Level Environmental Technology 3930

Fossil Fuels

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Waste to Energy Technologies

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Waste to Energy Technologies

Stages of anaerobic digestion and biogas CHP

3 things to know
A2 1

Overview

Pretreatment and pasteurisation, hydrolysis, acidogenesis, acetogenesis and methanogenesis, why methanogens are sensitive, CHP from biogas, the parasitic load, and biogas energy and CHP output calculations.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why is waste mechanically pretreated before anaerobic digestion?

Key term

Why must food waste containing animal by-products be pasteurised in an AD plant?

Key term

What happens during hydrolysis in anaerobic digestion?

Questions from this subtopic

Each one with the model answer underneath.

1
Why is waste mechanically pretreated before anaerobic digestion?
Model answer
Pretreatment removes contaminants such as plastic, metal and grit that would damage equipment. A shredder also reduces particle size, giving microbes a larger surface to attack.
2
Why must food waste containing animal by-products be pasteurised in an AD plant?
Model answer
The material is typically heated to 70°C for an hour to kill pathogens such as Salmonella. Animal by-product rules require this before the digestate is spread on land.
3
What happens during hydrolysis in anaerobic digestion?
Model answer
Hydrolytic bacteria release enzymes that break large insoluble molecules into small soluble ones. Carbohydrates become sugars, proteins become amino acids and fats become fatty acids.
4
Why is hydrolysis often the slowest stage when digesting solid waste?
Model answer
Enzymes can only work on the surface of solid particles, and materials such as cellulose resist breakdown. The rate of hydrolysis then limits the whole process.
5
What happens during acidogenesis?
Model answer
Acidogenic bacteria ferment the sugars, amino acids and fatty acids from hydrolysis. The products are volatile fatty acids, alcohols, hydrogen, carbon dioxide and ammonia.
6
What happens during acetogenesis?
Model answer
Acetogenic bacteria convert volatile fatty acids and alcohols into acetic acid. Hydrogen and carbon dioxide are released at the same time.

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Fossil Fuels

Global Action on Energy Conservation and Climate Change

Macrogeneration

Microgeneration

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