Anaerobic Digestion

Question 1

What is anaerobic digestion

Answer 1

a natural process where organic material is broken down by micro-organisms in the absence of air.

Question 2

What is the composition of biogas after anaerobic digestion

Answer 2

60% methane
40% CO2

Question 3

What is the waste product called after anaerobic digestion

Answer 3

Digestate, which is rich in nutrients and can be applied as fertilisers

Question 4

What are the 4 distinct processes for anaerobic digestion

Answer 4

1. Hydrolysis
2. Acidification
3. Acetic Acid Formation
4. Methanogenesis

Question 5

Characteristics of hydrolysis

Answer 5

works of large inorganic molecules, such as lipids, proteins, and carbohydrates. Which are broken down into simple molecules of amino acids, fatty acids and sugars

Question 6

Characteristics of acidification

Answer 6

the simple molecules, which are now organic are broken down into basic compounds such as organic acids

Question 7

Characteristics of acetic acid formation

Answer 7

bacteria works on the basic compounds and organic acids to produce acetic acids, as well as hydrogen monohydride, ammonium and carbon dioxide

Question 8

Why is AD an integrate process

Answer 8

• A waste treatment process
• Biogas, renewable energy production
• Nutrient recovery from digestate
• Social, financial and environmental benefits

Question 9

Examples of AD substrates

Answer 9

• Yield of biogas depends on composition and biodegradability of the feedstock
• Waste from agriculture
• Waste from food processing industries
• Food waste from restaurants
• Waste from slaughterhouse
• Sewage sludge

Question 10

AD reactor types

Answer 10

-Suspended growth

-Fixed Film

-Granular Growth

-Hybrids

Question 11

SRT

HRT

OLR

Answer 11

o SRT= Solids Retention Time- average time that bacteria are in the anaerobic digestor

o HRT= Hydraulic Retention Time- average time that the waste/sludge is in the anaerobic digestor

• OLR= Organic Loading rate

Question 12

Disadvantages of AD

Answer 12

• Feedstock availability, proximity of the grid
• Anaerobic bacteria grow much more slowly than aerobes.
• Slow growth means longer reaction times and therefore higher capital costs.
• Worst case retention time (HRT) aerobic treatment 10 hours anaerobic treatment 10 days.
• What to do with digestate

Question 13

Important AD parameters

Answer 13

• Total solids (TS) and Volatile Solids (VS) of Substrate
• Organic Loading Rate
• Hydraulic Retention Time
• Biogas Yield

Question 14

Economic and Environmental Benefits of AD

Answer 14

• Economic
  o Reduced energy expenses
  o Increased crop yields- fertiliser
  o Stimulate national economy- job creation
• Environmental
  o Reduction of the biomass resource depletion
  o Reduction of the GHG emissions
  o Nutrient recovery