Session 2: Fundamentals of the
Composting Process
Cary Oshins
USCC
Learning Objectives
• Part 1) Understand the biology of the compost
pile
• Part 2) Learn the six factors used to control
the composting process, the
KEY PROCESS VARIABLES
Why Biology?
Because composting is a
biologically driven and
mediated process
The Composting Process
Water
CO2
Heat
Odors?
Feedstocks
microorganisms
oxygen
water
Compost
Pile
Compost
Why does composting happen?
• Microbes need to consume feedstocks to
– Obtain energy
– Obtain nutrients
• Heat gets trapped in pile
– Accelerates process
How do microbes obtain energy?
• Aerobic respiration uses oxygen
carbohydrate + O2  energy + CO2 + H20
• Anaerobic respiration: without oxygen
carbohydrate  energy + H2O + partial
breakdown products
• Fermentation: special form of anaerobic
respiration that produces acetic acid, lactic
acid, ethanol, methane
Aerobic respiration
• Most efficient in terms of energy yield
• Quickest way to achieve biological stability
• Generates heat as a by-product of
metabolism
• Offensive odors are minimal
°C
158
70
140
60
122
50
104
Thermophilic
Curing &
Maturation
40
86
30
68
20
50
10
Mesophilic
32
0
Time
Temperature
°F
Actual Compost Temperature Data
140
135
Temperature, F
130
125
120
115
110
105
100
28Nov
14Dec
1Jan
19Jan
Date
8Feb
28Feb
20Mar
9Apr
Phases of aerobic composting
• Mesophilic
– ambient to 110o F
– lasts a few days to weeks
• Thermophilic
– 110o to 170o F
– few weeks to several months
• Curing and maturation
– moderate to ambient temps
– 1 to many months
Who are the decomposers?
Scientific classification
Aerobes vs anaerobes
Obligate vs. facultative
Psychrophiles – mesophiles – thermophiles
Microorganisms involved in the
composting process
• Bacteria
• Fungi
• Actinomycetes
How many microbes?
Yard debris
Aerobic bacteria
Spent mushroom substrate
Anaerobic
bacteria
yeasts and molds
actinomycetes
Succession of microbial communities
during composting
• Mesophilic bacteria break down soluble, readily
degradable compounds (sugars and starches)
• Thermophilic bacteria break down proteins, fats.
Work with actinomycetes to begin breaking down
cellulose and hemicellulose
• Actinomycetes and fungi important during curing
phase in attacking most resistant compounds
Generalized Microbial Population
Dynamics During Composting
70 158
14
60 140
Bacteria
Temperature
10
50 122
40 104
8
Actinomycetes
6
30 86
x
4
20 68
Fungi
2
10 50
0
0 32
0
Temperature
12
Log # CFU's/g
°C °F
Time
A simulation by Phil Leege based on:
Personal observations, Beffa, Blanc, Marilley, Fischer, Lyon and Aragno “Taxonomic and
Metabolic Diversity during Composting” 1995; Jeong and Shin “Cellulosic Degradation in
Bench-Scale Composting of Food Waste and Paper Mixture” 1997; Whitney and Lynch “The
Importance of Lignocellulosic Compound in Composting” 1995, and others.
18
Session 2
Fundamentals of Composting
Part 2:
Key Process Variables
The Composting Process
Water
CO2
Heat
Odors?
Feedstocks
microorganisms
oxygen
water
Compost
Pile
Compost
The Key Process Variables
for Control of The Composting Process
1.
2.
3.
4.
5.
6.
Initial feedstock mix
Pile moisture
Pile aeration
Pile shape and size
Pile temperature
Composting retention time
The Key Process Variables
for Control of The Composting Process
1.
2.
3.
4.
5.
6.
Initial feedstock mix
Pile moisture
Pile aeration
Pile shape and size
Pile temperature
Composting retention time
Feedstocks: Your raw materials
Chemical composition
• Organic Matter, Nutrients, Degradability
Physical characteristics
• Moisture, Bulk density, Heterogeneity
Other
• Contamination, Cost, Availability, Regulations
What is organic matter?
•
•
•
•
Derived from living organisms
Always contains carbon
Source of energy for decomposers
Contains various amounts of other elements
–
–
–
–
–
Nitrogen
Phosphorous
Oxygen, Hydrogen
Sulfur
K, Mg, Cu, Cl, etc.
Types of organic carbon
•
•
•
•
Sugars, starches
Proteins, fats
Cellulose, hemicellulose, chitin
Lignin and lignocellulose
Nitrogen
• Found in
– Amino acids
– Proteins
• Sources include
– fresh plant tissue (grass clippings, green leaves,
fruits and vegetables)
– animals wastes (manure, meat, feathers, hair,
blood, etc)
Carbon to Nitrogen ratio (C:N)
• Ratio of total mass of elemental carbon to
total mass of elemental nitrogen
• Expressed as how much more carbon than
nitrogen, with N = 1
• Does NOT account for availability
– Degradability
– Surface area
– Particle size
C:N ratio
• High C:N
– more carbon relative to nitrogen
– C:N > 20:1 results in net N immobilization
– if > 40:1 slows composting process (N limited)
• Low C:N
– still more carbon relative to nitrogen, but less so
– C:N < 20:1 results in net N release (as ammonia)
• “Ideal” starting range: 25:1 to 35:1
Example of Feedstock C:N ratios
High Nitrogen (low C:N)
Grass clippings
Manure
Vegetable wastes
C:N ratio
15-25
5-25
15-20
High Carbon (high C:N)
Fall leaves
Straw
Wood chips
Bark
Mixed paper
Newspaper
30-80
40-100
100-500
100-130
150-200
560
Other nutrient ranges
• Carbon to Phosphorus (C:P)
– 75:1 to 250:1
• Carbon to Potassium (C:K):
– 100:1 to 150:1
• Carbon to Sulfur (C:S)
– greater than 100:1
Physical factors
•
•
•
•
•
•
Particle Size
Structure
Porosity
Free Air Space
Permeability
Bulk Density
Particle size and shape
•
•
•
•
Decomposition happens on surface
Smaller particles = more surface area
Very fine particles prevent air flow
Rigid particles provide structure
Adapted from T. Richard
Particle size and porosity effects
on aeration
Loosely packed,
well structured
Tightly packed,
uniform particle size
Loosely packed,
uniform particle size
Tightly packed, mixed
particle sizes
Porosity and Free Air Space
• Porosity=non-solid portion of pile
• Free Air Space (FAS) = portion of pore space
not occupied by liquid
• May vary in pile
• Start > 50%
FAS
40%
Water
30%
Solids
30%
FAS
20%
Water
40%
Solids
40%
Pile Structure/Porosity
liquid film
Pore
space
free air space
airflow
compost
particles
Bulk Density
• Measure of mass (weight) per unit volume
– pounds/cubic foot, tons/cubic yard, kg/L
– Examples
• Water: 62 lb/ft3, 1.44 ton/yd3
• Topsoil (dry): ~75 lb/ft3, ~1.73 ton/yd3
• Compost : ~44 lb/ft3, ~1200 lb/yd3
• Lower bulk density usually means greater
porosity and free air space
Non-compacted
Low bulk density
Compacted
High bulk density
Lost pore volume
Initial Bulk Density & FAS
Rule of thumb for starting mix:
• Below 800 lbs/cubic yard (475 kg/m3)
– May not hold heat
• Above 1000 (600 kg/m3)
– increasing difficult to aerate
• Above 1200 (700 kg/m3)
– Too dense
Starting FAS: above 50% will assure good airflow
Feedstock summary
•
•
•
•
Each feedstock has certain attributes
The RECIPE is how feedstocks are combined
Composting system designed for feedstocks
Regulations are always partly based on
feedstock
The Key Process Variables
for Control of The Composting Process
1.
2.
3.
4.
5.
6.
Initial feedstock mix
Pile moisture
Pile aeration
Pile shape and size
Pile temperature
Composting retention time
Moisture
• Required by microbes for life processes,
heating and cooling, place to live
• > 65% means pore spaces filled
– anaerobic conditions
• < 40% fungus dominates
– difficult to re-wet
– < 35% dust problems
Pile Structure/Porosity
liquid film
free air space
airflow
compost
particles
CO2
O2
CO2
O2
CO2
O2
Odors
Anaerobic
Aerobic Conditions
airflow
Water-filled pores
Anaerobic Conditions
Water-filled pores
Low pore space
Moisture
• Optimum is 45-60% moisture
• Composting consumes water
– Better to start on high end
– Adding water is difficult
– 25 gallons per ton raises moisture content ~10%
The Key Process Variables
for Control of The Composting Process
1.
2.
3.
4.
5.
6.
Initial feedstock mix
Pile moisture
Pile aeration
Pile shape and size
Pile temperature
Composting retention time
Aeration
• Supplies oxygen
• Ambient air is 21% oxygen
• Below 16% bacteria start switching to
anaerobic respiration
• O2 consumption increases with temperature
Pile Oxygen vs. Odor from Sulfur, Volatile Fatty
Acids and Other Compounds
Composting Pile Oxygen Percent, measured 18”
below surface, versus Odor Saturation
21
20
19
18
Odor Threshold
17
16
Threshold of predominant aerobic
conditions at about 16% pile O2
Pile Oxygen Percent
15
14
14
12
11
Transition between about 6 and 16% pile O2
10
9
8
7
6
5
Threshold of predominant anaerobic
conditions at about 6% pile O2
4
3
2
Odor Saturation
1
0
0
10
20
30
40
50
Odor Saturation %
60
70
80
90
100
Aeration
• Controlled by
– Porosity (particle size)
– Compaction (pile height and density)
– Moisture
• Without mechanization (blowers) relies on
diffusion and convection
Convective aeration
warm
Cooler
Ambient air
air
Cooler
Ambient air
Forced Aeration: Positive
Forced Aeration: Negative
Variables are related!
↑ Bulk Density = ? Porosity
Variables are related!
↑ Bulk Density = ↓ Porosity
Variables are related!
↑ Bulk Density = ↓ Porosity
↑ Moisture = ? Aeration
Variables are related!
↑ Bulk Density = ↓ Porosity
↑ Moisture = ↓ Aeration
Variables are related!
↑ Bulk Density = ↓ Porosity
↑ Moisture = ↓ Aeration
↑ Free Air Space = ? Aeration
Variables are related!
↑ Bulk Density = ↓ Porosity
↑ Moisture = ↓ Aeration
↑ Free Air Space = ↑ Aeration
Turning compost piles
myths and facts
• Turning = aeration MYTH!
• Turning increases porosity MYTH!
• Turning cools the pile MYTH!
• Turning speeds decomposition FACT!
The Key Process Variables
for Control of The Composting Process
1.
2.
3.
4.
5.
6.
Initial feedstock mix
Pile moisture
Pile aeration
Pile shape and size
Pile temperature
Composting retention time
Pile types
•
•
•
•
Static pile
Windrow
Trapezoidal or extended windrow
In-vessel
Pile size and shape
• Smaller piles allow for greater air flow,
especially to center of pile
• Larger piles retain temperatures
• Too large compacts bottom of pile
• Bigger piles if
– Better structure
– Higher C:N
– Lower moisture, bulk density
• Equipment should match pile size
Can use shape to capture or shed water
The Key Process Variables
for Control of The Composting Process
1.
2.
3.
4.
5.
6.
Initial feedstock mix
Pile moisture
Pile aeration
Pile shape and size
Pile temperature
Composting retention time
Temperature
• Higher temps result in faster breakdown, up to
140oF
• At temps > 160oF lose microbial diversity,
composting actually slows
• Most weeds and pathogens killed at temps >
130oF (55oC)
– PFRP=Process to Further Reduce Pathogens
• Moisture moderates temperature fluctuation
PFRP
• Time and Temperature requirements to
assure pathogen reduction
• Aerated Static Pile and In-vessel:
–55oC for 3 days
• Turned windrow:
–55oC for >15 days with 5 turnings
°F
°C
158
70
55oC
140
122
50
Thermophilic
Curing &
Maturation
40
86
30
68
20
50
10
Mesophilic
32
0
Time
Temperature
104
60
Time
• Mesophilic
– a few days to 2 weeks
• Thermophilic
– 3 weeks to several months
• Curing and maturation
– 1 to several months
– eliminates inhibitors to seed germination and crop
growth
When is it done?
• AFTER CURING!
• Stability vs maturity
– Stable: activity diminished
– Mature: will grow plants
• Testing for doneness
– Lab tests
– Facility test
NOTE:
Not all
markets
require
compost to
be mature!
Summary
Key initial parameters for thermophilic composting
Condition
Moisture %
C:N
Oxygen %
Temperature oF
o
C
pH
Particle size
Porosity:
Bulk density lbs/ yd3
(kg/l)
Free Air Space %
Reasonable range
40 — 65
20:1 — 60:1
Greater than 5
113 — 160
45 -- 71
5.5 — 9.0
1/8 to 2 inches
.3-5 cm
Preferred range
50 — 60
25:1 — 40:1
Greater than 10
120 — 150
49 -- 66
6.5 — 8.0
Depends on feedstocks
and use for compost
Less than 1200
(.7)
40-60
800-1000
(.45-.6)
50-60