Chapter 3 Waste to Energy Technologies PP

Question 1

Explain the following key terms with anaerobic digestion.
1 – Low solids;
2 – High solids;
3 – Residence time;
4 – Single-stage;
5 – Multi-stage;

Answer 1

Low Solids - Low solid digestion is where the feedstock is around 15% solid material.

High solids - High solid digestion is where the feedstock is around 55% solid material.

Residence time - The time taken for the full degradation of the material in an aerobic digestion system.

Single stage - The biological reactions occur in one holding tank and the biogas comes from this tank.

Multistage - Hydrolysis, acetogenesis and acidogenesis occurs in one tank whilst the methanogenesis which produces the biogas occurs in a separate tank.

Question 2

Explain how anaerobic digestion results in the production of biogas.

Answer 2

Hydrolysis – insoluble polymers broken down to soluble organic compounds. Acidogenesis – soluble organic compounds fermented into volatile fatty acids and alcohols. Acetogenesis – alcohols and fatty acids converted into acetic acid and hydrogen. Methanogenesis – bacteria convert acetic acid and hydrogen into methane and carbon dioxide / biogas.

Question 3

Outline how anaerobic digestion can deliver both heat and power.

Answer 3

Biogas from an AD plant can be burnt in a combined CHP plant, this generates power (electricity) and heat. Instead of losing the heat, as in traditional power plants, it is diverted into a local heating system. This is co-generation.

Question 4

Home composting is encouraged by many local councils. Name three types of waste product which can be composted in a domestic situation.

Answer 4

eggshells
leaves
teabags

Question 5

Explain briefly why composting is unsuitable for treating any form of catering waste.

Answer 5

will attract vermin