Municipal Solid Waste Composting

JICA Ex Participants Association of Solid Waste Management One day workshop, Colombo , 18th February 2016

Dr Anurudda Karunarathna

Senior Lecturer in Environmental Engineering

Department of Agricultural Engineering

University of Peradeniya

077 297 6234 anujica@yahoo.com

• Decomposition of heterogeneous organic matter by a mixed microbial population in a moist, warm, aerobic environment to a state sufficiently stable for nuisance-free storage and utilization (Gray & Biddlestone, 1974).

• Compost systems can be classified on three general bases:

1. Oxygen status (aerobic & anaerobic)

2. Temperature (Mesophilic 15-40 OC & Thermophilic 45-65 OC)

3. Technological approach (static pile or windrow, and mechanical or "enclosed" composting)

Definition of composting process

Compostingprocess

Rapidly degradable organic matter (carbohydrate, protein, lipids, etc. + chemical energy)

Slowly degradable organic matter (cellulose, lignin, wax etc. + chemical energy)

Water

Microorganisms (decomposers/ saprotrophs, pathogens, worms)

Organic matter (carbon,humus)

Slowly degradable organic matter

Minerals (N, P, K, S, etc.)

Water

Microorganisms (dead, live)

Heat CO2Water

W

V

~0.2W

~0.5V

Feedstock Compost

O2O2

1) Moisture

2) Density (porosity)

3) Physical state; solid, semi-solid, liquid & gases

4) Mechanical properties e.g. shear strength

5) Particle size, shape, size distribution

6) Physical composition of wastes:

Categories

Sub-classes

Feedstock characteristics - physical

• For most organic waste, it will vary from 60 to 80%, depending on the composition of the wastes, the season of the year, and the humidity and weather conditions

Moisture content

Ash

Dry matter

Biodegradable/ combustible

Moisture

Change of MC & FC during composting

0

10

20

30

40

50

60

70

80

0 14 28 42 56 70 84 98 112

Mo

iatu

re C

on

ten

t (W

/W)

Days

Available MC % MC at Field Capacity

Particle size and size distribution

Proximate analysis: determination of combustible components of waste includes analysis of moisture content, volatile combustible matter, fixed carbon and ash.

Ultimate analysis: involves the determination of the percent of C, H, O, N, S, ash and halogens. ◦ to characterize the chemical composition of the organic matter in waste ◦ to define the proper mix of waste materials to achieve suitable C/N ratios for

biological conversion processes.

Chemical properties

8

• The principal organic waste components in MSW are often classified as rapidly and slowly decomposable

Biodegradability

9

Component Volatile solids (VS) % of total

solids (TS)

Lignin content (LC) % of VS

Biodegradable fraction (BF)

Food wastes 85 - 90 0.4 0.82

Paper Newsprint Office paper Cardboard

94 96.4 94

21.9 0.4

12.9

0.22 0.82 0.47

Yard wastes 50 - 90 4.1 0.72

Biodegradation- Volatile solid (VS)

0

10

20

30

40

50

60

70

80

0 20 40 60 80 100 120

VS

(%)

Days

• In Germany, the biodegradability is defined either with the respiration activity (AT4) or gas formation potential (GB21).

Solid biodegradability- measurement

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Organic Material Breakdown Rate

Group I

Sugars

Starches, glycogen, pectin, fatty acids and

glycerol

Lipids, fats, and phospholipids

Amino acids

Nucleic acids

Protein

Readily degradable

Group II

Hemicellulose and cellulose

Chitin

Low molecular weight

Aromatics and aliphatics

Slower to degrade

(more relevant during

maturation)

Group III

Lignocellulose

Lignin

Usually resistant

The Relative Microbiological Breakdown Rates

1) pH

2) Moisture content

3) Aeration (oxygen consumption) rates

4) Temperature & Heat evolution

evaporative cooling, etc., are governed by biochemical properties of the substrate and the type of microbial activities taking place during the degradation processes.

5) Degradation processes, which in turn depend on particle size and pile height.

6) Permeability

7) Composition

Parameters governing the composting

• Combined activities of a rapid succession of microbial populations, each suited to a particular environment for a relatively limited duration of time and being active in the degradation process of one particular type or group of organic material.

Bacteria (mesophyllic and thermophyllic)

Actinomycetes

Fungi

Protoza

Rotifers

Biological Aspects of Composting

Food Web of a Compost pile

Organic Residue

Primary Consumers

bacteria, fungi, actinomycetes

Secondary Consumers

nematodes, protozoa, rotifera,

Tertiary Consumerscentipedes, mites, beetles

Nutrients required for micro-organisms

Nitrogen

Phosphorous

Potassium

Magnesium

Sulphur

Calcium

Other elements

• The ideal C/N ratio for composting of MSW generally falls in the range of 20:1 to 25:1 The experimental values obtained in the Sri Lankan studies varied between 20-33:1

• Feedstock, gradually reducing to between 9-10:1 after 60 days of composting

• C/N ratio of 10 to be ideal for mature compost

Carbon: Nitrogen ratio

pH

0

1

2

3

4

5

6

7

8

9

0 1 0 2 0 3 0 4 0

D ay s

pH

Typical Values of pH Variation with Time

• Experiment conducted on an engineering model, a vertical reactor obtained values of 0.36 to 0.39 m3 of air per day per kg of volatile solids

Aeration Rates

Natural Air Circulation

in a Compost Windrow

The Temperature time Pattern in relation to Biological Activities

0

10

20

30

40

50

60

0 5 10 15 20 25

Time in days

Tem

pera

ture

0 C

a b c d

Temperature Variations with Time with Indicating the

Stages of the Composting Process

1. Psychrophilic-mesophilic-- Microbes multiply exponentially; give off heat; pile warms up.

2. Thermophilic or “hot” phase -- Bacteria consume ‘easy’ food; pile heats rapidly; H2O, heat, CO2 given off in large amounts.

3. Cool-down – Microbes run out of food and/or oxygen; pile cools; may be re-activated by turning, introducing air, and/or adding water.

4. Curing – At ambient temps; actinomycetes and fungi multiply; slow breakdown of cellulose & lignins. Humic compounds form. Mineralization of N to NO3.

Phases

• 300 components were evaluated and classified and 18 chemicals were found to be of special importance and thus well-suited as control parameters in emissions.

• Methane

• Ammonia

• Benzene

• Xylole

• Trichlorethene

• Tetrachorethene

• limonene and

• Ethanol

Gas Emissions During composting MSW

• Highest emissions occur during the self-heating stage in the first days of the rotting process. At this stage, the waste gas emits a range of volatile pollutants taking about two weeks to complete.

• During the whole composting process, readily degradable carbon compounds such as alkanes, aldehydes, alcohols, etc., are present in the waste air. These are volatile metabolites, which can be removed through biological air filters.

• Referring to its efficiency, a comparison of the content of the pollutants before and after the bio-filters indicate a reduction of about 60% as a mean value with cleaning efficiencies varying between 0% and 100% depending on the substance.

• Dioxine and most heavy metals are in the concentration range of the natural environment.

Type of emmisions

Compost production

Aspect Preferred condition Remarks

MSW characteristics Organic fraction of MSW Kitchen waste, food waste, garden waste, paper and small amount

of non-degradable/ long-term degradable materials may present

Particle size 25-75 mm for optimum results Smaller particles increases the density thus reduce the air

movement inside the pile, larger particles takes long time to

disintegrate and decompose

C:N ratio 25 to 50 C:N ratio above the limit prolong the decomposition and very low

C:N ratio of feedstock may impend anaerobic condition and

ammonia leaching

Blending & seeding 1-5 % Addition of partly decomposed matter accelerate the

decomposition at early stages, especially when the feedstock is

garden waste

Moisture content 55% (optimum) Higher moisture increases leachate generation and reduces

aeration, dryness ceases the decomposition

Windrow size 2 m width and 1.5 m height

and 3 m long

Wide and tall windrows restrict the air movement inside the pile

Mixing/turning 4-5 days (optimum)

Weekly (recommended)

Low turning frequency reduces the aeration, mixing and

disintegration and slower the decomposition

Maturing/curing Weekly mixing/turning

(optimum)

Continue pile mixing/turning until temperature drops to 40 °C or

less, and add water while mixing/turning

Compost production- Operation

Aspect Preferred condition Remarks

Monitoring parameters

Temperature 50-55 °C for first few days

55-65 °C for reminder period

Temperature below 50 °C during the composting caused by low

C:N ratio, excessive moisture, dryness, compaction due to large

pile size and toxins

pH 7 – 7.5 (optimum) Not above 8.5 to minimize nitrogen losses via ammonia

volatilization, very low pH at early stages indicates excessive, fast

decomposable carbohydrates in feedstock

Maturity indices

Stability Simple tests

C:N ratio <25

Self heating test

Advance tests

Specific O2 uptake rate

CO2 evolution

Simple tests are cheap and easy but low accuracy

Advance test are more accurate but requires special equipments

and skills to perform

Toxicity assessment

(Phytotoxicity)

NH4-N:NO3-N ratio,

Compost bioassays

Caused by the presence of organic chemicals such as lactic and

acetic acid, herbicides, heavy metals, and pathogens

• Advantages;

Easy to construct and minimum operation & maintenance requirement

No or minimum cost of construction

• Disadvantages:

Requires relatively large area

Use of meat & fish waste is not recommended

Requires relatively long time to decompose

Small scale on-site composting - pit/heap method

• Advantages;

Easy to construct and minimum operation & maintenance requirement

Relatively low cost of construction

• Disadvantages:

Requires relatively large area

Use of meat & fish waste is not recommended

Small scale on-site composting - Jeewakotuwa

• Advantages;

Composters are available at subsidized price & in markets

Small land area requirement

• Disadvantages:

Purchasing and transport cost

Requires basic skills to operate

Difficult to fabricate in-house

Small scale on-site composting - Bin

MSW composting

1. Dry matter/volume reduction ~ 50%

2. Cheaper transportation

3. Land apply at farmer’s convenience

4. Reduced odor

5. Reduces fly and disease problems

6. Environmental conservation

7. Prolong landfill life

Aims

High-nitrogen material [“greens”]—

Manure– Dairy, beef, horse, poultry, swine

Hatchery waste– shells, washing, rejects

Food processing waste (vegetable or fruit)

Fish processing waste

High carbon material [“browns”]---

Yard trimmings, leaves, woodchips

Sawdust, shavings, straw, old hay, seed cleaning waste

Shredded Paper and Un-waxed Cardboard

Composting feedstock

Windrow method

Windrow composting- indoor

Continuous turning, Supersoils, NC

Composting under roof

Passive aeration - Static piles

Passive aeration, Vermont

Forced aeration

Active aeration- Composting reactors

Active aeration- Composting reactors

Is a few days enough?

Odor

Odor management- Filters

Municipal Solid Waste Composting - University research

Open Windrow Construction with Dome Aeration Equipment

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* Heterogeneous MSW

1. Sorted quickly to remove hazardous non-degradable components

2. Sorted waste was mixed well and used for the experiment

* SP and SG were heaped up to 3m and 2.5m respectively

SP -3m (height) SG - 2.5m(height),1m (width)

Appearance after filling with MSW Appearance after filling with MSW

Schematic Diagram of Aerated Static Pile Composting System (Source:

Tchobanoglous et al., 1993)

Inclined Step-Grate (ISG)

Solar Aeration Chimney Composting System

Ventilator

6"height panel

4"dia PVC pipeWASTEPILE

Chimney

Plastic coated wiremesh & porous pipe

concreted floor

Cone shaped mesh cage

inside the waste pile

MSW composting- Local Authorities

Waste receiving and unloading

Waste sorting

Making windrow piles

Windrow piles

Leachate control and treatment

Turning and maturing windrow piles

Sieving and processing

Final Product – Compost Fertilizer

Material flow in windrow composting (20TPD of MSW)

Compost

Raw Raw waste

1 2 3 4 5 6 7

14 13 12 11 10 9 8

15 16 17 18 19 20 21

22 23 24 25 26 27 28

Recyclables

Pre- Rejects

Screening

Leachate

Volatile losses

Post- Rejects

Sorting

49%

2%

49%

13%

Water

700 L/D

16%

20%

END