Aerobic Composting

Question 1

What is MBT?

Answer 1

Mechanical and biological processes for residual MSW management

Question 2

What are the three main stages of MBT?

Answer 2

Mixed waste input, mechanical sorting and pre-treatment, and biological treatment

Question 3

What are the two main processes within the mechanical stage of MBT, and what do they consist of?

Answer 3

1) Waste preparation - removal of objects that could cause problems, prepare materials for separation.
2) Waste Separation - sort waste into different fractions, separates materials for different end uses such as recycling, energy recovery etc.

Question 4

Name the 6 waste preparation techniques. (write the principle and key concerns on paper).

Answer 4

1) Hammer Mill
2) Shredder
3) Rotating drum
4) Ball mill
5) Wet rotating drum with knives
6) Bag splitter

Question 5

Name the 8 waste separation techniques. (write what they separate, target materials and key concerns on paper).

Answer 5

1) Trommels/screens
2) Manual Separation
3) Magnetic separation
4) Eddy current separation
5) Wet separation technology
6) Air classification
7) Ballistic separation
8) Optical separation

Question 6

What are the 5 different types of composting?

Answer 6

1) Home, community
2) centralised
3) on-farm composting
4) in-situ composting

Question 7

What are the advantages of MBT?

Answer 7

1) Captures recyclable material
2) reduces the volume of waste and therefore increases landfill space
3) reduces biodegradability of waste (methane and leachate production)
4) hazardous waste does not reach landfill (batteries, solvents, paints etc)
5) plants are modular

Question 8

What are the disadvantages of MBT?

Answer 8

1) Large MBT plants attract waste from wider radius which defeats the principle ‘reducing the distance waste has to travel’.
2) Long term MBT plant contracts ‘tie hands’ of LAs (fixed tonnage etc).

Question 9

What is Aerobic Composting?

Answer 9

The breakdown of waste by micro-organisms in the presence of oxygen

Question 10

What are the three main stages of Aerobic Composting?

Answer 10

Mesophilic growth stage, Thermophilic stage and Maturation stage.

Question 11

What does the first phase of Aerobic Composting consist of?

Answer 11

Bacteria grows at temps 25-40deg.
Mesophilic organisms multiply rapidly on sugars and amino acids.
Process generates heat making temps rise until organisms can no longer multiply.
Lasts a couple of days.

Question 12

What does the second phase of Aerobic Composting consist of?

Answer 12

Temps raised from phase 1 to 50-70deg. BACTERIA, FUNGI and ACTINOMYCETES break down cellulose, lignin and other resistant materials.
Temps held at upper limit for 1 day to destroy pathogens and contaminants.
Lasts from a few days to months.

Question 13

What does the third phase of Aerobic Composting consist of?

Answer 13

Temps stabilise. Identified when turning no longer reheats the pile.
Some fermentation occurs - material converted to humus through NITRIFICATION.
Temps decrease to ambient. Lasts several months.

Question 14

What are the operational controlling parameters?

Answer 14

Before: water content, structure, substrate composition and nutrient content.

During: Temp, water content, oxygen content, structure/porosity and pH.

After: structure, compost properties.

Question 15

Outline some key points about Home Composting.

Answer 15

• Waste used where it is created - little environmental impact.
• Cuts transport cost.
• Methods - compost heap, compost/worm bin, container made from wood, bricks etc.

Question 16

Outline some key points about Community Composting.

Answer 16

• Operated by non-profit organisations. Methods similar to HC.
• Mainly in areas where HC and centralised not possible.

Question 17

Outline some key points about Centralised Composting.

Answer 17

1.71Mt - majority of composting.

Involves collection and sent to centralised site.

Question 18

Outline some key points about Farm Composting.

Answer 18

85% of farm waste. Exempt from waste management licensing. Agricultural companies and individual farmers.

Question 19

What are the two processes within Large Scale Composting?

Answer 19

Open-air (windrow) and closed processes (in-vessel/reactor).

Question 20

Outline the windrow composting process.

Answer 20

1) Waste pre-treated and heaped into piles - turned at intervals
2) Mixed and turned for 12-16weeks
3) Concrete pads and collection lagoons for any liquid runoff.

Question 21

What are some issues of windrow composting?

Answer 21

1) Odour
2) dust
3) noise
4) leachate collection effectiveness
5) litter treatment
6) wet/dirty roads.

Question 22

What are the three processes within closed composting?

Answer 22

1) Non-flow (Batch)
2) vertical flow
3) horizontal flow.

Question 23

Outline some key points about batch and vertical flow composting.

Answer 23

Batch - simplest form and windrow is enclosed in a hall, easier for operator to track batches and meet ABP regs.

Vertical Flow - feedstock sorted & mixed with waxed cardboard waste. Provided correct liquid-solid balance.

Question 24

What are the two processes within horizontal flow in-reactor composting?

Answer 24

Aerated in-vessel and Agitated bins.

Question 25

Outline the aerated in-vessel process.

Answer 25

1) Material is piled at roughly 2m and moved uninterrupted through a container
2) Reaches first composting zone where it is aerated and vented at optimum temp for 6 days.
3) Mixing zone where material is scooped and thrown which aerated it further and adds moisture. Remains in container for 15 days.

Question 26

Outline the agitated bins process.

Answer 26

1) Shredded and blended waste moistened and loaded into bin where temp, CO2 and air controlled.
2) Air enters through base vents and leaves through roof filter.
3) 7-14 days followed by 12 week maturation.

Question 27

What method is there for in-situ composting?

Answer 27

Bioreactors

Question 28

Outline the process within an aerobic bioreactor.

Answer 28

1) Leachate injected into waste - accelerates decomposition actively moistening and introducing air to waste.
2) Reduces landfill construction and engineering.

Question 29

What are the advantages of a bioreactor?

Answer 29

Rapid waste stabilisation & settlement, improved leachate quality, reduced methane production, reduced odour.

Question 30

What is the one disadvantage of a bioreactor?

Answer 30

It still uses landfill.