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 Compost Pile Feedstocks microorganisms oxygen water Compost Odors? CO 2 WaterHeat

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 + O 2  energy + CO 2 + H 2 0 Anaerobic respiration : without oxygen carbohydrate  energy + H 2 O + 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

Time Temperature °C °F Mesophilic Thermophilic Curing & Maturation

Actual Compost Temperature Data

Phases of aerobic composting Mesophilic – ambient to 110 o F – lasts a few days to weeks Thermophilic – 110 o to 170 o 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 debrisSpent mushroom substrate

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

Log # CFU's/g Time 0 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 Temperature °C °F

Session 2 Fundamentals of Composting Part 2: Key Process Variables

The Composting Process Compost Pile Feedstocks microorganisms oxygen water Compost CO 2 Water Odors? Heat

The Key Process Variables for Control of The Composting Process 1.Initial feedstock mix 2.Pile moisture 3.Pile aeration 4.Pile shape and size 5.Pile temperature 6.Composting retention time

The Key Process Variables for Control of The Composting Process 1.Initial feedstock mix 2.Pile moisture 3.Pile aeration 4.Pile shape and size 5.Pile temperature 6.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 Example of Feedstock C:N ratios High Nitrogen (low C:N)C:N ratio Grass clippings15-25 Manure5-25 Vegetable wastes15-20 High Carbon (high C:N) Fall leaves30-80 Straw Wood chips Bark Mixed paper Newspaper560

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

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 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% FAS 20% 40% Solids Water Solids

Pile Structure/Porosity airflow free air space liquid film compost particles Pore space

Bulk Density Measure of mass (weight) per unit volume – pounds/cubic foot, tons/cubic yard, kg/L – Examples Water: 62 lb/ft 3, 1.44 ton/yd 3 Topsoil (dry): ~75 lb/ft 3, ~1.73 ton/yd 3 Compost : ~44 lb/ft 3, ~1200 lb/yd 3 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/m 3 ) – May not hold heat Above 1000 (600 kg/m 3 ) – increasing difficult to aerate Above 1200 (700 kg/m 3 ) – 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.Initial feedstock mix 2.Pile moisture 3.Pile aeration 4.Pile shape and size 5.Pile temperature 6.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 airflow free air space liquid film compost particles

O2O2 CO 2

O2O2

O2O2 Odors

Anaerobic ConditionsAerobic Conditions airflow Water-filled pores

Anaerobic Conditions Water-filled poresLow 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.Initial feedstock mix 2.Pile moisture 3.Pile aeration 4.Pile shape and size 5.Pile temperature 6.Composting retention time

Aeration Supplies oxygen Ambient air is 21% oxygen Below 16% bacteria start switching to anaerobic respiration O 2 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 Oxygen Percent Odor Threshold Odor Saturation Threshold of predominant aerobic conditions at about 16% pile O 2 Threshold of predominant anaerobic conditions at about 6% pile O 2 Transition between about 6 and 16% pile O 2

Aeration Controlled by – Porosity (particle size) – Compaction (pile height and density) – Moisture Without mechanization (blowers) relies on diffusion and convection

Convective aeration warmair Cooler Ambient air Cooler Ambient air Cooler Ambient 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 Turning increases porosity Turning cools the pile Turning speeds decomposition MYTH! FACT!

The Key Process Variables for Control of The Composting Process 1.Initial feedstock mix 2.Pile moisture 3.Pile aeration 4.Pile shape and size 5.Pile temperature 6.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

Windrow size matches equipment

The Key Process Variables for Control of The Composting Process 1.Initial feedstock mix 2.Pile moisture 3.Pile aeration 4.Pile shape and size 5.Pile temperature 6.Composting retention time

Temperature Higher temps result in faster breakdown, up to 140 o F At temps > 160 o F lose microbial diversity, composting actually slows Most weeds and pathogens killed at temps > 130 o F (55 o C) – 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: – 55 o C for 3 days Turned windrow: – 55 o C for >15 days with 5 turnings

Time Temperature °C °F Mesophilic Thermophilic Curing & Maturation 55 o C

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! NOTE: Not all markets require compost to be mature!

Summary Key initial parameters for thermophilic composting