Session 2: Fundamentals of the Session 2: Fundamentals of the Composting ProcessComposting Process

Cary OshinsUSCC

Learning ObjectivesLearning 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?Why Biology?Because composting is a biologically driven and

mediated process

The Composting Process

Compost Pile

Feedstocks

microorganisms

oxygenwater

Compost

Odors?CO2Water Heat

Why does composting happen?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?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 respirationAerobic 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

Time

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Mesophilic

ThermophilicCuring &Maturation

Date

28Nov 14Dec 1Jan 19Jan 8Feb 28Feb 20Mar 9Apr

Tem

pera

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, F

100

105

110

115

120

125

130

135

140Actual Compost Temperature DataActual Compost Temperature Data

Phases of aerobic compostingPhases 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?Who are the decomposers?

Scientific classificationAerobes vs anaerobesObligate vs. facultative

Psychrophiles – mesophiles – thermophiles

Microorganisms involved in the Microorganisms involved in the composting processcomposting process

• Bacteria• Fungi• Actinomycetes

How many microbes?How many microbes?

Yard debris Spent mushroom substrate

Succession of microbial communities Succession of microbial communities during compostingduring 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

Lo

g #

CF

U's

/g

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8

6

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18

Time0

Bacteria

Actinomycetes

Fungi

x

Temperature

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.

Generalized Microbial Population Dynamics During Composting

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Session 2 Session 2 Fundamentals of CompostingFundamentals of Composting

Part 2: Key Process Variables

The Composting Process

Compost Pile

Feedstocks

microorganisms

oxygenwater

Compost

CO2WaterOdors?

Heat

The Key Process Variables for Control of The Composting Process

1. Initial feedstock mix2. Pile moisture3. Pile aeration4. Pile shape and size5. Pile temperature6. Composting retention time

The Key Process Variables for Control of The Composting Process

1. Initial feedstock mix2. Pile moisture3. Pile aeration4. Pile shape and size5. Pile temperature6. Composting retention time

Feedstocks: Your raw materialsFeedstocks: Your raw materials

Chemical composition • Organic Matter, Nutrients, Degradability

Physical characteristics• Moisture, Bulk density, Heterogeneity

Other• Contamination, Cost, Availability, Regulations

What is organic matter?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 carbonTypes of organic carbon

• Sugars, starches• Proteins, fats• Cellulose, hemicellulose, chitin• Lignin and lignocellulose

NitrogenNitrogen

• 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)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 ratioC: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 ratiosExample of Feedstock C:N ratiosHigh Nitrogen (low C:N) C:N ratio

Grass clippings 15-25Manure 5-25

Vegetable wastes 15-20

High Carbon (high C:N)Fall leaves 30-80

Straw 40-100Wood chips 100-500

Bark 100-130Mixed paper 150-200Newspaper 560

Other nutrient rangesOther 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 factorsPhysical factors

• Particle Size• Structure• Porosity• Free Air Space• Permeability• Bulk Density

Particle size and shapeParticle size and shape

• Decomposition happens on surface• Smaller particles = more surface area• Very fine particles prevent air flow• Rigid particles provide structure

Particle size and porosity effects on aeration

Loosely packed, well structured

Loosely packed, uniform particle size

Tightly packed, uniform particle size

Tightly packed, mixed particle sizes

Adapted from T. Richard

Porosity and Free Air SpacePorosity 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%

30%

30%

FAS 20%

40%

40%

Solids

Water

Water

Solids

Pile Structure/Porosity

airflow

free air spaceliquid film

compost particles

Pore space

Bulk DensityBulk 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-compactedLow bulk density

CompactedHigh bulk density

Lost pore volume

Initial Bulk Density & FASInitial 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 summaryFeedstock 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 Variablesfor Control of The Composting Process

1. Initial feedstock mix2. Pile moisture3. Pile aeration4. Pile shape and size5. Pile temperature6. Composting retention time

MoistureMoisture

• 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

airflow

free air spaceliquid film

compost particles

O2CO2

O2CO2

O2CO2

Odors

Anaerobic ConditionsAerobic Conditions

airflowWater-filled pores

Anaerobic Conditions

Water-filled pores Low pore space

MoistureMoisture

• 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 Variablesfor Control of The Composting Process

1. Initial feedstock mix2. Pile moisture3. Pile aeration4. Pile shape and size5. Pile temperature6. Composting retention time

AerationAeration

• 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

Odor Saturation %

Pile

Oxy

gen

Perc

ent

0

1

2

3

4

5

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7

8

910

1112

1414

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1617

18

1920

21

0 10 20 30 40 50 60 70 80 90 100

Odor Threshold

Odor Saturation

Threshold of predominant aerobic conditions at about 16% pile O2

Threshold of predominant anaerobic conditions at about 6% pile O2

Transition between about 6 and 16% pile O2

AerationAeration

• Controlled by– Porosity (particle size)– Compaction (pile height and density)– Moisture

• Without mechanization (blowers) relies on diffusion and convection

Convective aerationConvective aerationwarm air

CoolerAmbient

air

CoolerAmbient

air

CoolerAmbient air

CoolerAmbient air

Forced Aeration: PositiveForced Aeration: Positive

Forced Aeration: NegativeForced Aeration: Negative

Variables are related!Variables are related!

↑ Bulk Density = ? Porosity

Variables are related!Variables are related!

↑ Bulk Density = ↓ Porosity

Variables are related!Variables are related!

↑ Bulk Density = ↓ Porosity ↑ Moisture = ? Aeration

Variables are related!Variables are related!

↑ Bulk Density = ↓ Porosity ↑ Moisture = ↓ Aeration

Variables are related!Variables are related!

↑ Bulk Density = ↓ Porosity ↑ Moisture = ↓ Aeration

↑ Free Air Space = ? Aeration

Variables are related!Variables are related!

↑ Bulk Density = ↓ Porosity ↑ Moisture = ↓ Aeration

↑ Free Air Space = ↑ Aeration

Turning compost pilesTurning compost pilesmyths and factsmyths and facts

• Turning = aeration

• Turning increases porosity

• Turning cools the pile

• Turning speeds decomposition

MYTH!

MYTH!

MYTH!FACT!

The Key Process Variablesfor Control of The Composting Process

1. Initial feedstock mix2. Pile moisture3. Pile aeration4. Pile shape and size5. Pile temperature6. Composting retention time

Pile typesPile types

• Static pile• Windrow• Trapezoidal or extended windrow• In-vessel

Pile size and shapePile 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 waterCan use shape to capture or shed water

Windrow size matches equipmentWindrow size matches equipment

The Key Process Variablesfor Control of The Composting Process

1. Initial feedstock mix2. Pile moisture3. Pile aeration4. Pile shape and size5. Pile temperature6. Composting retention time

TemperatureTemperature

• 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

PFRPPFRP

• 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

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Mesophilic

ThermophilicCuring &Maturation

55oC

TimeTime

• 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?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!

NOTE:Not all

markets require

compost to be mature!

SummarySummaryKey initial parameters for thermophilic compostingKey initial parameters for thermophilic compostingCondition Reasonable range Preferred range Moisture % 40 — 65 50 — 60 C:N 20:1 — 60:1 25:1 — 40:1 Oxygen % Greater than 5 Greater than 10 Temperature oF oC

113 — 160 45 -- 71

120 — 150 49 -- 66

pH 5.5 — 9.0 6.5 — 8.0 Particle size 1/8 to 2 inches

.3-5 cm Depends on feedstocks

and use for compost Porosity:

Bulk density lbs/ yd3

(kg/l)

Free Air Space %

Less than 1200

(.7) 40-60

800-1000

(.45-.6) 50-60