Transcript Green Waste technology Unit

Recycled Organics Unit
Composting Science for Industry
Mr Angus Campbell
www.recycledorganics.com
Lecture Overview
 Composting Science Part 1
1)
2)
3)
4)
5)
Introduction
Temperature management
Importance of oxygen
Water availability
Physical properties of the compost mix
 Composting Science Part 2
1)
2)
2)
4)
Nutrients required for rapid composting
Role of pH and other nutrients
Commercial composting systems
Processing time and curing
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Composting Science Part 1
“An understanding of the underlying principles of
microbiology, chemistry, biochemistry and engineering
give us the ability to manipulate and manage the
composting processes”
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Introduction
 Aerobic composting is a biological process governed by the activity of
naturally occurring microorganisms.
 Understanding the fundamentals = ability to manipulate process.
 Aerobic microorganisms require suitable environmental conditions to
grow and multiply - needed for rapid breakdown of the organic
fraction during composting.
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Introduction...
 These conditions relate to the availability of:
• oxygen (~21% in air)
• water
• food (carbon, nitrogen and other nutrients)
• suitable environmental conditions – mainly warmth or heat
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Process diagram: composting systems
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1) Temperature management
 Why do temperatures rise above ambient in composting systems?
….Heat is released by microorganisms during the aerobic metabolism
of an organic substrate, e.g. glucose:
C6H12O6 (s) + 6O2 (g) -----> 6CO2 (g) + 6H2O (l) + HEAT!
 Heat builds up when the insulating properties of the mass results in
the rate of heat gain being greater than the rate of heat loss.
 Small volumes of organic materials (<1-2 m3) may not heat up
because the heat generated by the microbial population is lost quickly
to the atmosphere (mainly convective losses).
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Temperature changes during composting
80
Temperature (ºC)
70
intensive decom position
curing
60
50
therm ophilic stage
40
m esophilic stage
30
20
pasteurised or
fresh com post
10
Tim e
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stable
& m ature
com post
Temperature changes during composting
 Temperature has a self-limiting effect on microbial activity and thus
the rate of degradation of organic materials.
 The highest rates of decomposition of organic materials usually occur
at temperatures between 35 and 55ºC.
 Thermophilic conditions begin at temperatures above 45ºC.
 Temperature can also indicate when a compost product is stable or
mature.
 Temperatures above 55ºC are ESSENTIAL for pasteurisation
(sanitation) - a process involving the thermal deactivation of plant
seeds and cuttings, plant pathogens, animal pathogens and human
pathogens.
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Temperature development and microbial successions
 Temperature affects the rate of decomposition of organic materials by
directly influencing the make-up of the microbial population.
 Bacteria, fungi and actinomycetes all play a major role in the
decomposition of organic materials during aerobic composting.
 The initial period of composting, which is characterised by a rapid
increase in microbial activity and the first signs of a rise in
temperature, is mainly due to the activity of mesophilic bacteria
consuming freely available compounds.
 As the temperature begins to rise, mesophilic organisms begin to die
off and thermophilic organisms then begin to dominate.
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Compost microbiota
 Scanning electron micrograph of thermophilic Bacillus sp. bacteria commonly found in
composting systems (left). Note their characteristic ‘rod’ shape. A phase-contrast light
microscope picture of Bacillus sp. bacteria in chain form (right). These bacteria are in
a spore generating phase. Heat resistant spores are produced when temperatures
exceed that tolerable by the cells (e.g. temperatures above 65C).
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Temperature development and microbial successions...
 If temperatures in the composting mass reach 65-70ºC, the activity of
thermophilic organisms also begins to be inhibited, and only some
spore forming bacteria can survive. At this point, the rate of
decomposition slows.
 During the curing phase, after temperatures begin to fall, fungi and
actinomycetes begin to colonise and decompose the more resistant
materials such as cellulose and lignin.
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Temperature profiles
 Temperatures attained in composting systems are rarely uniform
throughout the entire mass.
 Gradients of between 20 and 45C can exist between the surface and
the centre of a windrow.
 Such temperature differences may be as small as 2-5C in a well
insulted in-vessel composting system.
 Exposure of the entire mass to temperatures above 55C for at least 3
days is required for pasteurisation to occur.
 Pasteurisation is a key RISK MINIMISATION step in composting.
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Temperature development in composting systems
In-vessel
80
80
70
70
60
Temperature (°C)
Temperature (°C)
Turned windrow
centre
50
40
30
outer surface (10 cm deep)
20
60
centre
50
40
30
outer surface (10 cm deep)
20
10
10
0
2
4
6
8
10
12
Time (weeks)
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14
0
2
4
6
8
Time (weeks)
10
12
14
2) Importance of oxygen
 When microorganisms feed on the carbon component of organic
materials for their energy, oxygen (O2) is consumed and carbon
dioxide (CO2) is produced.
 The oxygen concentration in air is about 21%, but aerobic
microorganisms cannot function effectively at concentrations below
about 5% in compost.
 Ideally, oxygen concentrations of about 10-14% are required for
optimum composting conditions.
 The anaerobic microbiota at low oxygen concentrations are
responsible for much of the odour production.
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Mechanism of aeration - turned windrows
 In turned windrows, much of the aeration is achieved by convection
and diffusion mechanisms.
 High level of porosity (>20% v/v) is required to assist in ‘natural
aeration’.
Convective
air flow in a
turned
windrow
HOT!
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Mechanism of aeration - aerated static piles
 Forced aeration is a feature
of aerated static pile or invessel systems.
 In the case of static piles,
forced aeration by blowing
also has the advantage of
delivering warm air to the
cooler outer layers.
 Insulating layer of compost
on outside is needed to
maintain uniform
temperatures.
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Oxygen profiles - turned windrow
2
0
20
18
1
5
16
14
12
1
0
10
8
6
Oatxfypgeln(c%o,v/)trai
Oxygen concentrations three days
after turning (%, v/v)
22
5
4
T
T
T
2
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0
0
2
4
6
8
1
0
1
2
1
4
Distance from exterior surface of pile (m)
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T
i
m
e
(
d
a
y
s
)
Oxygen profiles...
 As with temperature, the concentration of oxygen is not uniform
throughout the composting mass.
 Turning or the forced delivery of air into a composting mass is
necessary to ensure that the entire mass is kept in an aerobic state.
 Aeration is necessary to maintain high decomposition rates and to
minimise odour production.
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Odour formation during composting
 Odour formation is strongly associated with the development of
anaerobic conditions in composting systems.
 These odours are produced through the decomposition of organic
matter.
 Composting odours are mostly produced as vapours, though
particulate (i.e. aerosol) odours can be produced.
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Odour formation during composting...
Threshold
(nL/L)
Compound
Formula
Characteristic odour
Ethanal
CH3CHO
Pungent
2
Butanoic acid
CH3CH2CH2COOH
Rancid
0.28
Ammonia
NH3
Pungent
37
Trimethyl amine
(CH3)3N
Pungent
4
3-methylindole (skatole)
C6H5C(CH3)CHNH
Faecal
Hydrogen sulfide
H2S
Rotten egg
Carbon oxysulfide
COS
Pungent
Dimethyl sulfide
CH3SCH3
Foul
20
Dimethyl disulfide
CH3SSCH3
Foul
-
Diethyl sulfide
CH3CH2SCH2CH3
Foul
0.25
Methanethiol
CH3SH
Decaying cabbage
1.1
Ethanethiol
CH3CH2SH
Decaying cabbage
0.016
1-Propanethiol
CH3CH2CH2SH
Unpleasant
0.075
1-Butanethiol
CH3CH2CH2CH2SH
Skunk like
1.4
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7.5x10-5
1.1
-
“The most
problematic odour
is ammonia NH3”
Odour treatment
 Odours can easily be treated in systems that permit the collection of
process air from a composting system. Examples include in-vessel
systems with forced aeration, or an aerated static pile with a suctiontype aeration system.
 Process air produced by these systems can be directed to a biofilter
— a vessel containing mature compost — to remove the odorous
compounds from the air.
 Bacteria present in the biofilter decompose the odorous compounds
and use them as a food source, thereby removing the smell from the
air.
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3) Importance of water
 Moisture, or water, is essential to all living organisms. Moisture is lost
during composting by evaporation.
 This has the benefit of cooling the compost to prevent overheating
and a reduction in microbial activity.
 The optimum moisture content for composting is generally between
50 and 60% (w/w).
 Below about 30%, microbial activity virtually stops. Moisture contents
above 50% are critical for effective pathogen and weed control during
the thermophilic stage of composting.
 With turned windrows, water can be added by soaker hoses, or by
injection during turning.
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Decomposition model
“Decomposition
model for solid
particles in a
composting
system.
Decomposition is
performed by
microorganisms
present within the
liquid film and on
the surface of
particles.”
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Impact of excess water
 As moisture content increases, the thickness of the layer of water
surrounding each compost particle increases.
 Secondly, water fills the smallest pores (the space between particles)
first, creating water filled zones between particles.
 Above about 60% moisture content, the rate of diffusion of oxygen is
too slow to replenish the oxygen utilised. Odorous compounds then
build up in the anaerobic zone and can become detectable in the
atmosphere.
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4) Physical properties of the composting mix
 Porosity, structure and texture relate to the physical properties of the
materials such as particle size, shape and consistency.
 They affect the composting process by their influence on aeration.
 The physical properties of a composting mix can be adjusted by
selecting suitable raw materials and by grinding or mixing.
 Materials added to adjust these properties are referred to as bulking
agents.
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Porosity, structure & texture
 Porosity is a measure of the air space within the composting mass
and determines the resistance to airflow. Determined by particle size,
the size gradation of the materials, and the continuity of the air
spaces.
 Structure refers to the rigidity of particles — that is, their ability to
resist settling and compaction.
 Good structure prevents the loss of porosity in the moist environment
of a compost pile.
 Texture refers to the available surface area for microbial attack.
 Optimum particle size: mixture of 3 - 50 mm diameter particles.
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Porosity & air flow resistance
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Composting Science Part 2
 Overview
1)
2)
2)
4)
Nutrients required for rapid composting
Role of pH and other nutrients
Commercial composting systems
Processing time and curing
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1) Nutrients required for rapid composting
 Carbon (C) in organic matter is the energy source and the basic
building block for microbial cells.
 Nitrogen (N) is also very important and along with C, is the element
most commonly limiting.
 Microorganisms require about 25-30 parts of carbon by weight for
each part of nitrogen used for the production of protein (C:N 2530:1).
 Preparing feedstock to an optimum C:N ratio results in the fastest
rate of decomposition- assuming other factors are not limiting.
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C:N ratios of different feedstocks
Food organics
C:N ~ 15:1
Garden organics
C:N ~ 50 - 80:1
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Wood chips
C:N ~ 200 - 300:1
Manure
C:N ~ 5 - 10:1
C:N ratio of common feedstocks
Feedstock
Moisture
Structure
C:N
%N
Mixed tree and shrub prunings
dry to moist
good
70-90
0.5-1
Eucalyptus bark
dry
good
250
0.2
Eucalyptus sawdust
dry
average
500
0.1
Pinus radiata bark
dry
good
500
0.1
Pinus radiata sawdust
dry
average
550
0.09
Grass clippings
moist to wet
poor
9-25
2-6
Food organics
moist to wet
average
14-16
1.9-2.9
Vegetable produce
wet
poor
19
2.7
Fruit
wet
poor
20-49
0.9-2.6
Fish
moist to wet
poor
2.6-5
6.5-14.2
Mixed solid waste
-
average
34-80
0.6-1.3
Biosolids
moist to wet
poor
5-16
2-6.9
poor
poor
13.8
0.81
(2) raw flocculated sludge
moist
moist
19
1.61
Tannery waste (hair)
dry to moist
average
3.1-4.3
11.7-14.8
Mixed abattoir wastes
moist to wet
poor
2-4
7-10
Chicken manure (layers)
dry to moist
poor
3-10
4-10
Chicken manure (broiler)
dry to moist
poor
12-15
1.6-3.9
Newsprint
dry
poor
398-852
0.06-0.14
Paper
dry
poor
127-178
0.2-0.25
Wheaten straw
dry
good
100-150
0.3-0.5
Seaweed (kelp)
dry to moist
average
25
1.5
Sawdust
dry
poor
200-750
0.06-0.8
Wool scour waste:
(1) raw decanter sludge
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C:N ratio and other nutrients
 A C:N ratio of between 20 and 40:1 is often suitable for composting
depending on the make-up of the feedstock. As composting proceeds,
the C:N ratio gradually decreases to between 10 and 20:1.
 Feedstocks of low C:N ratios (<15:1) may decompose rapidly, but
odours can become a problem because of the complete and rapid
usage of oxygen without replenishment, resulting in the production of
odourous sulphur compounds such as thiols.
 Microorganisms also require adequate phosphorus, sulfur and
micronutrients for growth and enzyme function, but their role in
composting is less well known.
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How organic materials break down
 Compost feedstock is a complex mix of organic materials ranging
from simple sugars and starches to more complex and resistant
molecules such as cellulose and lignin.
 In general terms, composting microbes first consume compounds that
are more 'susceptible' to degradation in preference to compounds
that are more resistant.
 The breakdown of organic matter is therefore a step-wise reduction of
complex substances to more simpler compounds.
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How organic materials break down...
 During the intensive phase of composting, the more easily degradable
compounds are broken down first.
 Feedstocks that contain a high proportion of compounds that are
difficult to break down, such as lignin, require longer periods of
composting — decomposition of lignin occurs more rapidly during the
curing phase, at mesophilic temperatures.
 For many organic materials, a period of maturation is also essential to
eliminate compounds that are toxic to plant growth (phytotoxic).
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How organic materials break down...
Table 1. Susceptibility of organic compounds found in compost feedstock to decomposition.
Organic compound
Susceptibility
Sugars
Very susceptible
Starches, glycogen, pectin
Fatty acids, lipids, phospholipids
Amino acids
Protein
Usually susceptible
Hemicellulose
Cellulose
Lignocellulose
Lignin
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Resistant
2) pH and other nutrients
 Optimum pH range for composting is somewhere in the range of 5.5
to 9.
 It is important to note that composting is likely to be less effective at
5.5 or 9 than it is at a pH near neutral (pH 7).
 pH does become important with raw materials that have a high
percentage of nitrogen (e.g. manure and biosolids).
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pH and role in composting
 A high pH, above 8.5, encourages the conversion of nitrogen
compounds into ammonia, which further adds to alkalinity.
 Loss of nitrogen in the form of ammonia to the atmosphere not only
causes nuisance odours, but also reduces the nutrient value of the
compost.
 Adjusting the pH downward below 8.0 reduces ammonia loss. This
can be achieved by adding an acidifying agent, such as
superphosphate or elemental sulfur.
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pH changes during composting
9
.
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-
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+
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=
=
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Other nutrients required for composting
 Apart from C and N, compost microorganisms require an adequate
supply of other nutrients such as phosphorus, sulphur, potassium and
trace elements (e.g. iron, manganese, boron etc).
 These nutrients are usually present in ample concentrations in
compost feedstock, though phosphorus (P) can sometimes be
limiting. A C:P ratio of between 75 and 150:1 is required.
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3) Commercial composting systems
 At least eight different forms of composting systems are available for
processing a wide range of organic materials.
 Turned windrow systems have been the predominant form of
composting in Australia, particularly for garden organics.
 Higher technology composting systems are now being implemented
for processing materials that have traditionally been difficult to
process in outdoor turned windrow systems, such as food organics.
 All systems aim to control compost production by manipulating
temperature, oxygen and moisture during composting. This varies
from system to system.
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Turned windrows
Turned windrow









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Most common system for waste of low odour generating potential
Low capital costs unless concrete pads are installed
High operating costs
Very flexible system - a range of organic materials can be composted
and adjustments can be made within a composting cycle
Aeration by turning with front-end loader or specialised machine
Slow rate of decomposition due to varying conditions in pile
Stable compost in 3-12 months
Windrows can be outdoors or formed under a roof (no sides)
Great care needed for effective odour and leachate control
Passively aerated windrow
Passively aerated windrow






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Cheapest system; no turning
Windrows must be covered with finished compost to reduce odours
May be more space efficient than turned windrows
Reduced flexibility - careful preparation of starting materials
essential
Little control of temperature and aeration during composting
Compost in 10-12 weeks; further curing usually required
Aerated static pile
Aerated static pile







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Medium capital costs
Medium operating costs
Forced aeration
Reduced flexibility - careful preparation of feedstock is essential
Space efficient
Piles usually must be covered (e.g. with compost) to reduce odours
Some control of temperature and aeration resulting in faster
composting (6-12 weeks); further curing usually required
Aerated covered windrow
Aerated covered windrow







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Medium capital costs
Medium operating costs
Cover for windrows reusable
Forced aeration; computer control of composting possible
Reduced flexibility - careful preparation of feedstock essential
Space efficient
Improved control of temperature and aeration resulting in faster
composting (3-6 weeks); further curing usually required
Rotating drums
Rotating drum






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High capital cost
Medium operating costs
Less preparation of starting materials required due to constant
mixing and size reduction
Rapid initial decomposition in drum (up to seven days)
Further decomposition required in windrows or aerated static piles
Provides mixing and aeration by means of drum rotation and forced
aeration
Agitated bed or channel
Agitated bed or channel







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High capital cost
Medium operating costs
Flexible system – both forced aeration and mechanical mixing used
Space efficient
Beds are covered in a fully enclosed building or roof
Good capacity for odour and leachate control
Rapid composting: 2-4 weeks; further curing usually required
In-vessel (horizontal configuration)
In-vessel (horizontal configuration)








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High capital cost
Automated system
Uniform temperature and oxygen profile throughout contents of
vessel
Composting vessels can be housed in a building or outdoors
Excellent control of odours and leachate
Can be located with minimal buffer distances
Very fast composting (7-14 days)
Further curing in windrows or in-vessel usually required
In-vessel (vertical configuration)
In-vessel (vertical configuration)








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High capital cost
Automated system
Uniform temperature and oxygen profile throughout contents of
vessel
Composting vessels can be housed in a building or outdoors
Excellent control of odours and leachate
Can be located with minimal buffer distances
Very fast composting (7-14 days)
Further curing in windrows or in-vessel usually required
4) Processing time & curing
 The length of time it takes to convert raw materials into mature
compost depends upon many factors, including:
• Types of raw materials being processed
• Compost recipe (feedstock) prepared
• Temperature
• Moisture, and
• Frequency of aeration.
 To achieve the shortest possible composting period, sufficient
moisture, an adequate C:N ratio and good aeration is required.
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Processing time for different systems
Active composting
time
Method
Materials
Windrow –
infrequent turning
a
Windrow –
frequent turning
b
Garden organics
Manure +
amendments
Garden organics +
manure
Range
Typical
Curing
(weeks
)
(weeks)
(weeks)
26 – 52
12 – 32
36
24
16
4–8
4 – 16
8
4–8
10 – 12
8 – 10
–
–
4–8
4–8
Passively aerated
windrow
Manure + bedding or
Food organics +
garden organics
Aerated static pile
Biosolids +
woodchips
3–5
4
4–8
Rectangular
agitated bay
Biosolids + garden
organics or manure +
sawdust
2–4
3
4–8
Rotating drums
Biosolids / food
organics + garden
organics
0.5 – 2
–
8
c
In-vessel (vertical
configuration)
Biosolids / food
organics + garden
organics
1–2
–
8
c
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Curing
 Curing is a critical and often
neglected stage of composting
during which the compost matures.
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70
Temperature (ºC)
 Curing occurs at low, mesophilic
temperatures for periods of up to 6
months, depending on the material
composted.
 In this process, the rate of oxygen
consumption, heat generation, and
moisture evaporation are much
lower than in the active
composting phase.
80
intensive decom position
curing
60
50
therm ophilic stage
40
m esophilic stage
30
20
pasteurised or
fresh com post
10
Tim e
stable
& m ature
com post
Curing...
 Because curing continues the
aerobic decomposition process,
adequate aeration in necessary.
 If piles are to be naturally
aerated (i.e. no active means of
aeration), pile size needs to be
relatively small (height ~1 m)
and
moisture
cannot
be
excessive (>70%).
 Larger piles required forced
aeration to remain in an aerobic
state.
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Conclusions
 Conversion of organic materials into quality composted products that
can improve soils and the environment is a central component of the
NSW Government’s strategy to reduce waste disposal to landfill.
 An understanding of the basic principles of composting science will
allow solid waste managers to select and implement appropriate
composting solutions.
 Other supporting info on licensing and establishing a composting
facility in NSW can be obtained for free from our web site,
http://www.recycledorganics.com under “publications”!
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