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ABE 482Environmental

Engineering in Biosystems

September 15, 2008Lecture 5

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Today

Composting

Definition, types of composting,pros/cons

Required conditions

C/N, oxygen, moisture, temperature, pH

Physical properties

Particle size distn, density, porosity andairspace, moisture content

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Definition

Composting: A controlled process in which

aerobic, thermophilic microorganismsconvert organic material into a hygienic,biostable, humus-like product.

Organic Matter Compost

Microbes Water Oxygen

Heat Water CO2

What is organic matter made up of??

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Types of Composting

Natural

Leaves, grass, twigs on forest floor

Planned

Static piles/windrows Aerated piles

Channels

In-vessel or bin

What is the difference between decomposing and composting?

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Natural composting

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Static piles/windrows

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Aerated static pile/windrow

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Channel composting

In-vessel or bincomposting

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Pros/Cons of Composting

Pros

Mass/volume reduction (approx 50%)

Pathogen and weed seed reduction (ifthermophilic for >4 days)

Useful/marketable product with fertilizervalue, good water holdling capacity,reduced bulk density and erosion control

value Stabilize putrescible material by reducing

BOD, smell, pollutants

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Pros/Cons of Composting

Cons

Requires time, money and space

Lack of site availability

Lack of suitable materials

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Conditions Required for Composting

C:N ratio for microorganism activityand growth

Oxygen levels that support aerobic

organisms Moisture to permit biological activity

without hindering aeration

Temperature for vigorous microbialactivity from thermophilic organisms

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Carbon and Nitrogen

Carbon

Needs to be available to microbes

1/3:1/3:1/3 organisms, metabolism, heat

brown, dry ingredients

Nitrogen Major nutrient needed by microbes for

assimilating carbon (protein and reproductivity)

Takes 25-30 units of C for every unit of N

green, wet ingredients

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C:N Ratio

Optimum C:N = 25-30

If C:N too low, not all N is utilized ammonia given off smelly

If C:N too high, process is slowed left with undecomposed substrate

Organic carbon and nitrogen contents

of ingredients can be analyzed in lab

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Oxygen

Required for aerobic microbes to live

Require approximately 5-20% oxygencontent to sustain aerobic microbes

Can be measured using oxygen probeor calculate from airspace content

What percentage of air is made of up oxygen?

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Moisture (cont)

If pile too wet, can turn anaerobic

Pore spaces fill with water instead of air Structure degrades, leads to excessive compaction Excessive moisture leachate nitrate and

pollutant runoff

If pile becomes too dry, cannot support microbial

populations, activity stops TMECC (Test Methods for the Examination of

Compost and Composting) suggests drying samples(approx 500 g) in oven at 705C for 24 hours todetermine gravimetric moisture content

ASTM specifies 105C for 16 hours

What are some other ways to measure moisture content?

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pH

Want preferred range of 6.5-8.0

Tends to fall at start due to organic acid production If pH too high, tends to encourage ammonia

volatilization (loss of N and smelly)

Let pile go anaerobic for awhile (really??)

If pH too low, pile becomes acidic and microbe activityis slowed

Add lime

Measure pH by making a slurry and use pH meter(use 2 slurry dilutions and extrapolate to zerodilution)

What does the pH versus time relationship look like?

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Temperature

Best indicator of activity

Prefer pile to be active (>40C) forabout a week

Compost temperatures can reachthermophilic range even in -30Cweather

20C-40C Mesophilic range

40C-60C Thermophilic range

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20C

40C

60C

Temp

Time2-3 days 1 week

Thermophilic

Mesophilic

Mesophiles die off

High rate composting

Active Curing

Compost Temperature Evolution with Time

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Physical Properties of Compost

Properties: particle size, density,porosity, etc.

Proper management will ensure

Microbes can access sufficient substratesto satisfy metabolism and multiply

A suitable environment exists (enoughO

2

)

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Next Day

Physical properties of compost

Density

Porosity and airspace

Composting material andamendments

Calculation examples

Compost maturity