Session 2: Fundamentals of the Composting Process Gary Felton University of Maryland
Learning Objectives Part 1: Understand BIOLOGY of compost pile Part 2: Learn factors used to control composting process: 6 KEY PROCESS VARIABLES
Why Biology? Because composting is a biologically driven and mediated process
The Composting Process Feedstocks Water CO 2 Heat Odors? Microorganisms Oxygen Water Compost Pile Compost
Why Does Composting Happen? Microbes consume feedstocks to obtain energy & nutrients Their activity creates heat Heat gets trapped in pile Accelerates process
The needs of composting organisms are the same as those of all living things: Food (energy) Oxygen (something to respire) Nutrients Water A hospitable environment -Temperature -Neutral chemical (ph) conditions
Types of Organic Carbon Sugars, starches (1 st to break down) Proteins, fats (readily decomposable) Cellulose, hemicellulose, chitin (slower to degrade) Lignin and lignocellulose (most resistant to decay)
Microorganisms Involved in the Composting Process Bacteria Fungi Actinomycetes
Bacteria Smallest living organisms 250,000 500,000 fit inside. 80-90% of microbes in compost pile Responsible for most of decomposition and heat generation in compost
Fungi Molds, yeasts, mushrooms Numerous during mesophilic phases When temperatures are high, most fungi live in outer layer of compost Break down tough organic debris cellulose, hemicellulose, lignin Can decompose materials too dry, acidic, or low in nitrogen for bacterial activity
Actinomycetes Cause earthy smell Bacteria with filiments (resemble fungi) Look like gray spider webs Degrade cellulose, lignin, chitin, proteins Bark, woody stems, paper Live in wider range of ph than other bacteria Some species in thermophilic phase, others in curing phase
How Many Microbes? ~3 trillion!
Three Temperature Phases of Composting First: Mesophilic (68 104 F; 20-40 C) Second: Thermophilic (105-150 F; 40.6 65.6 C) Third: Mesophilic (<105 F; <40.6 C)
Temperature F 158 Stages of Composting C 70 140 60 122 104 86 Thermophilic Curing & Maturation 50 40 30 68 20 50 32 Mesophilic Time 10 0
Typical Compost Pile Temperature Profile Good bugs killed off Weed seeds killed off Most pathogens killed
Temperature ( F) Daily Temperature Variance in a Composter 170 160 150 140 130 120 Turned 110 100 0 5 10 15 20 25 30 35 40 Day
Phase 1 Microbe Activity Bacteria break down cellulose into glucose Sugars, protein, starch Makes temperatures in pile rise Produce endospores as pile heats up
Endospores Bacteria develop tough coating Resists heat, drying, UV radiation, chemicals Can survive next, hotter phase then return to active state when cool again
Phase 2 Activity Thermophilic bacteria, fungi take over Heat-intolerant microbes go dormant Pathogens (human, plant) destroyed Complex carbohydrates fully broken down Some proteins are decomposed Hemicelluloses (more resistant) decay All this activity makes temperature continue to rise!
Phase 3 Activity Mesophilic microbes return to active state Proteins and carbs diminish Metabolic activity decreases Temperatures in pile drop Lignin (most resistant plant component) decayed by actinomycetes, fungi
Phase 3 cont. Physical decomposers support microbes Matter gets exposed to bacteria as arthropods forage Allows microbial populations to increase Arthropods: earthworms, mites, spiders, ants, snails, sow bugs, slugs, nematodes, springtails, centipedes, etc.
Log # CFU's/g (# microbes) Temperature 14 Generalized Microbial Population Dynamics During Composting C 70 F 158 12 10 Bacteria Temperature 60 50 140 122 8 40 104 6 Actinomycetes 30 86 4 x 20 68 2 Fungi 10 50 0 0 Time 0 32 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. 23
Summary: 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 are important during curing phase in attacking most resistant compounds
Session 2 Fundamentals of Composting Part 2: Key Process Variables
How Do We Control the Composting Process? Feedstocks Water CO 2 Heat Odors? Microorganisms Oxygen Water Compost Pile Compost
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
1. 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 (1 st to break down) Proteins, fats (readily decomposable) Cellulose, hemicellulose, chitin (slower to degrade) Lignin and lignocellulose (most resistant to decay) Do you remember which microbes break them down?
Nitrogen Second most important element 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, which is affected by: Degradability Surface area Particle size C:N
C:N Ratio Ideal starting range: 25:1-35:1 High C:N > 40:1 slows composting process (N limited) Low C:N < 20:1 results in net N release (as ammonia)
Example of Feedstock C-N Ratios Carbon Sources (est.) Bark 100-130:1 Cardboard 200-500:1 Leaves 30-80:1 Mixed paper 150-200:1 Newspaper 560:1 Peanut shells 35:1 Peat moss 30-65:1 Pine needles 250:1 Sawdust 100-230:1 Triticale, oat, rye straw 70-90:1 Wood chips 200-700:1 Wheat straw 140-150:1 Nitrogen Sources (est.) Alfalfa 13:1 Clover 23:1 Coffee grounds 20:1 Food scraps 15-25:1 Garden debris 20-60:1 Grass clippings 15-25:1 Alfalfa, timothy hay 15-25:1 Cow manure 20:1 Pig manure 5-7:1 Poultry manure 5-10:1 Horse, llama, donkey, alpaca manure 15-25:1 Blood or bone meal 3-4:1 Ideal Starting Range is 25:1-35:1
Physical Factors Affecting Decomposition 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 & help aerate
Particle Size Source: Mid-Scale Composting Manual
Particle Size and Porosity Effects on Aeration Adapted from T. Richard Loosely packed, well structured Loosely packed, uniform particle size Tightly packed, uniform particle size Tightly packed, mixed particle sizes
Porosity and Free Air Space Porosity = non-solid portion of pile Determined by size and type of particles, and height of pile Free Air Space (FAS) = portion of pore space not occupied by liquid May vary in pile Start > 50% porosity
Relationship of FAS to Pile Depth FAS 40% Water Solids 30% 30% FAS 20% Free air space changes in pile due to compaction Water Solids 40% 40%
Pile Structure/Porosity liquid film free air space compost particles Pore space airflow
Bulk Density Highly correlated with Free Air Space (FAS) 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 lbs/cy (600 kg/m 3 ) increasingly difficult to aerate Above 1200 lbs/cy (700 kg/m 3 ) Too dense Starting FAS: above 50% will assure good airflow
Feedstock Summary Each feedstock has certain attributes: Carbon, nitrogen, moisture, bulk density, rigidity, ph, homogeneity, consistency, putrescibility, pathogenicity, tip fee The RECIPE is how feedstocks are combined Composting system & site are designed and managed based on types, amounts of 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
2. Pile Moisture Required by microbes for life processes, heating and cooling, place to live Optimum is 45-60% moisture > 65% means pore spaces filled anaerobic conditions < 40% fungus dominates difficult to re-wet < 35% dust problems
Moisture Composting consumes water Better to start on high end Adding water is difficult 25 gallons per ton raises moisture content ~10% Example: 3,000 yards of leaves = 1,000 tons If they are at 40% moisture, need 25,000 gallons to raise them to 50% moisture 25 trips with a 1,000 gallon water truck!
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
3. Pile Aeration Aeration supplies oxygen Ambient air is 21% oxygen O 2 consumption increases with temperature Compost organisms can survive 5% oxygen Below 10% oxygen in pile, bacteria can start switching to anaerobic respiration Produces hydrogen sulfide (rotten egg smell) Maintaining adequate oxygen will reduce odor complaints!
Aeration Controlled by Porosity (particle size) Compaction (pile height and density) Moisture Without blowers, rely on diffusion and convection
Convective Aeration Warm Air Cooler Ambient air Cooler Ambient air
Forced Aeration: Positive
Forced Aeration: Negative
Variables are Related Question: As Bulk density goes up, what happens to Porosity? Bulk Density =? Porosity
Variables are Related Answer: As Bulk Density goes up, Porosity goes down Bulk Density = Porosity
Variables are Related Question: As Moisture goes up, what happens to Aeration? Moisture =? Aeration
Variables are Related Answer: As Moisture goes up, Aeration goes down Moisture = Aeration
Variables are Related Question: As Free Air Space goes up, what happens to Aeration? Free Air Space =? Aeration
Variables are Related Answer: As Free Air Space goes up, Aeration goes up Free Air Space = Aeration
Turning Compost Piles: Myths and Facts Turning = Aeration MYTH! Oxygen introduced by turning may be consumed within minutes in active pile Passive or forced aeration is needed to sustain oxygen levels after turning
Turning Compost Piles: Myths and Facts Turning increases porosity MYTH! Depending on feedstocks and type of turning equipment, turning may reduce particle size and pore space Pore space can increase after turning but it gradually decreases as materials resettle and compact
Turning Compost Piles: Myths and Facts Turning cools the pile MYTH! Cools it temporarily, followed by increase in temperature beyond pre-turning point Exposes fresh surfaces for decomposition Breaks up anaerobic pockets Turning can have permanent cooling effect during later phase of composting
Turning Compost Piles: Myths and Facts Turning speeds decomposition FACT! Releases trapped gases Exposes fresh surfaces Breaks apart anaerobic lumps Redistributes moisture, nutrients, microbes
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
4. Pile Shape and Size Smaller piles allow for greater air flow, especially to center of pile Larger piles retain temperatures Too large compacts bottom of pile You can have a Bigger pile if: Have good structure (sticks) Higher C:N Lower 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. Initial feedstock mix 2. Pile moisture 3. Pile aeration 4. Pile shape and size 5. Pile temperature 6. Composting retention time
5. Temperature Higher temps result in faster breakdown > 160 o F (71 C) lose microbial diversity, composting actually slows Most weeds and pathogens killed at temps 131 o F (55 o C) or higher
PFRP Process to Further Reduce Pathogens Time and Temperature requirements to assure pathogen reduction Aerated Static Pile and In-vessel: 131 F (55 C) or higher for 3 days Turned windrow: 131F (55 C) or higher for 15 days or more with 5 turnings
Temperature F 158 PFRP C 70 140 131 o F 60 122 104 86 Thermophilic Curing & Maturation 50 40 30 68 20 50 32 Mesophilic Time 10 0
6. 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 Pile Done? Temperature of pile is <10 warmer than ambient temperature About 1/3 of its original volume Can t recognize the original materials It is a dark color Smells earthy (not like ammonia or rotten eggs) Stability (activity diminished) vs maturity (will grow plants) Testing for doneness (lab and facility tests)
Summary Key Initial Parameters for Thermophilic Composting Condition 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 o F 113 160 120 150 o C 45 -- 71 49 -- 66 ph 5.5 9.0 6.5 8.0 Particle size Porosity: Bulk density lbs/ yd 3 (kg/l) Free Air Space % 1/8 to 2 inches.3-5 cm Less than 1200 (.7) 40-60 Depends on feedstocks and use for compost 800-1000 (.45-.6) 50-60