Biodegradable Solid Waste and Sludge Treatment Processes​

Question 1

What are the stages of wastewater treatment?

Answer 1

1. Preliminary treatment
2. Primary treatment
3. Secondary treatment
4. Tertiary treatment
5. Sludge treatment

Question 2

What happens in the secondary treatment?

Answer 2

Removes soluble organic contaminants in the wastewater and biological treatment

Question 3

What’s involved in the primary treatment?

Answer 3

• Primary clarifyer
• Flotation

Question 4

What is a significant by-product of anaerobic degradation of biodegradable materials, and why is it important?

Answer 4

Generates methane (CH4), which contributes to climate change but can be captured and used as a renewable energy source

Question 5

What are the applications of anaerobic degradation in waste management?

Answer 5

Crucial for:
- Landfill processes
- The principal treatment for sewage sludge
- The treatment of other biodegradable wastes such as food waste and livestock manure

Question 6

What is the result of anaerobic degradation regarding the residual material?

Answer 6

The residual material from anaerobic degradation is stabilized and can be used as a fertilizer or soil conditioner when spread on land

Question 7

How does secondary treatment in wastewater management function?

Answer 7

• Provides oxygen for bacteria to reproduce
• Captures residual gas
• Increases pathogen removal without heating or mixing

Question 8

What percentage of sewage sludge produced in the UK is treated by anaerobic digestion?

Answer 8

80%

Question 9

What is the total production of sewage sludge in the UK, and how is it managed?

Answer 9

1.4 million tonnes of dry solids (DS) of sewage sludge annually, with almost 100% applied to land after treatment

Question 10

What are the benefits of sewage sludge treatment?

Answer 10

• Increases stability of matter
• Reduces odour
• Reduces vector attraction
• Reduces or eliminates pathogen content
• Improves physical properties as a product for land application

Question 11

What is hydraulic retention time?

Answer 11

The average amount of time that liquid and soluble compounds stay in a reactor or tank

Question 12

What is the formula for HRT?

Answer 12

Reactor volume/flow rate

Question 13

What is the HRT for primary stage?

Answer 13

12 days

Question 14

Main advantage of biogas being produced during sludge anaerobic digestion?

Answer 14

Energy production

Question 15

What is the typical residence time in a digester for sludge?

Answer 15

Days

Question 16

What role do enzymes play in the digestion process?

Answer 16

Catalyse biochemical reactions

Question 17

What is the initial step in the biochemistry of anaerobic decomposition and how does it occur?

Answer 17

The initial step is hydrolysis, which is facilitated by extracellular enzymes that break down complex organic compounds

Question 18

What occurs during the fermentation stage of anaerobic decomposition?

Answer 18

Volatile acids are produced through acidogenic reactions. One key product is acetate, which can subsequently be converted to methane (CH4) by methanogenic bacteria.

Question 19

What is acetogenesis in anaerobic decomposition?

Answer 19

Acetogenesis involves Obligatory Hydrogen Producing Acetogenic bacteria, which play a key role in the bioconversion processes leading to methane production

Question 20

How is ammonia related to anaerobic decomposition?

Answer 20

In anaerobic decomposition, ammonia can be degraded from complex polymers, contributing to the breakdown and stabilization of organic material

Question 21

What are methanogenesis reactions?

Answer 21

Methanogenesis reactions involve the liberation of methane from acetate by specialized bacteria and the oxidation of hydrogen (H2) using carbon dioxide (CO2) as an electron acceptor.

Question 22

How much more methane created from acetate than from CO3 reduction?

Answer 22

x2

Question 23

Why is the growth of fermentative bacteria encouraged in anaerobic decomposition?

Answer 23

The growth of fermentative bacteria is encouraged to produce hydrogen, which is a critical intermediate in the process of methane generation through subsequent biochemical reactions.

Question 24

What is the primary feedstock used in compost production?

Answer 24

• Green waste
• Sewage sludge
• Municipal solid waste (MSW)

Question 25

What are the main composting processes used in producing organic fertilizers?

Answer 25

• A Windrow Composting Process
• An Invessel Composting Process

Question 26

What type of decomposition is utilized in compost production, and what does it involve?

Answer 26

Compost production utilizes autothermophilic aerobic decomposition, which involves stacking solid substrates that undergo biological degradation with controlled aeration.

Question 27

How is waste managed during the composting process?

Answer 27

Waste is aerated and mixed several times to ensure proper decomposition and nutrient distribution. This process also helps to generate heat necessary for the degradation process.

Question 28

How is heat generated during degradation process?

Answer 28

Mesophilic activity

Question 29

What is the final step in compost production and what does it involve?

Answer 29

The final step is the screening of the final product to ensure its quality before it is marketed as a soil conditioner

Question 30

What is the optimal nitrogen to carbon (N/C) ratio for composting?

Answer 30

1:25 to 1:30

Question 31

What is the ideal particle size for agitated composting systems?

Answer 31

12.5 mm

Question 32

What should the free air space percentage be in a composting system?

Answer 32

30%

Question 33

What is the optimal temperature for composting?

Answer 33

55˚C

Question 34

What are the dimensions for an effective compost heap?

Answer 34

1.5 m high and 2.5 wide

Question 35

How many stages are there in the microbial succession of composting and what are they?

Answer 35

• 4 stage microbial succession
  
  • Mesophilic
  • Thermophilic
  • Cooling
  • Maturation/curing

Question 36

At what temperature does mesophilic activity decline in composting, and which microbial population takes over?

Answer 36

Mesophilic activity declines at temperatures above 40 degrees Celsius, after which thermophilic bacteria take over.

Question 37

How does the population of microbes vary with temperature in composting?

Answer 37

Mesophilic populations are higher at 40 degrees Celsius and below, while thermophilic populations are higher between 40 and 70 degrees Celsius

Question 38

What indicates that a compost pile is functioning properly in terms of microbial activity?

Answer 38

An increasing diameter of the pile, indicating proper nutrient and food storage, along with rising temperatures, suggests thermophilic activity is functioning properly

Question 39

What happens during the cooling and maturation phase of composting?

Answer 39

• Reaction rate slows as easily degradable substrates are depleted
• Heat loss occurs and the pile cools, allowing other bacteria and fungi to take over
• This supports the succession of higher organisms like nematodes and protozoa

Question 40

What is a key chemical transformation that occurs during the maturation phase of composting?

Answer 40

Ammonia is converted into nitrate

Question 41

Whats involved in the preliminary treatment?

Answer 41

• Screening
• Grit chamber
• Equalization tank

Question 42

What is the HRT for secondary stage?

Answer 42

14 days