Recycled Organics Unit

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• Recycled Organics Unit

Composting Science for Industry Mr Angus
Campbellwww.recycledorganics.com
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Lecture Overview

• Composting Science Part 1
• 1) Introduction
• 2) Temperature management
• 3) Importance of oxygen
• 4) Water availability
• 5) Physical properties of the compost mix
• Composting Science Part 2
• 1) Nutrients required for rapid composting
• 2) Role of pH and other nutrients
• 2) Commercial composting systems
• 4) Processing time and curing

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Composting Science Part 1

• An understanding of the underlying principles
  of microbiology, chemistry, biochemistry and
  engineering give us the ability to manipulate and
  manage the composting processes

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Introduction

• Aerobic composting is a biological process
  governed by the activity of naturally occurring
  microorganisms.
• Understanding the fundamentals ability to
  manipulate process.
• Aerobic microorganisms require suitable
  environmental conditions to grow and multiply -
  needed for rapid breakdown of the organic
  fraction during composting.

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Introduction...

• These conditions relate to the availability of
• oxygen (21 in air)
• water
• food (carbon, nitrogen and other nutrients)
• suitable environmental conditions mainly warmth
  or heat

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Process diagram composting systems
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1) Temperature management

• Why do temperatures rise above ambient in
  composting systems?
• .Heat is released by microorganisms during the
  aerobic metabolism of an organic substrate, e.g.
  glucose
• C6H12O6 (s) 6O2 (g) -----gt 6CO2 (g) 6H2O
  (l) HEAT!
• Heat builds up when the insulating properties of
  the mass results in the rate of heat gain being
  greater than the rate of heat loss.
• Small volumes of organic materials (lt1-2 m3) may
  not heat up because the heat generated by the
  microbial population is lost quickly to the
  atmosphere (mainly convective losses).

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Temperature changes during composting
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Temperature changes during composting

• Temperature has a self-limiting effect on
  microbial activity and thus the rate of
  degradation of organic materials.
• The highest rates of decomposition of organic
  materials usually occur at temperatures between
  35 and 55ºC.
• Thermophilic conditions begin at temperatures
  above 45ºC.
• Temperature can also indicate when a compost
  product is stable or mature.
• Temperatures above 55ºC are ESSENTIAL for
  pasteurisation (sanitation) - a process involving
  the thermal deactivation of plant seeds and
  cuttings, plant pathogens, animal pathogens and
  human pathogens.

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Temperature development and microbial successions

• Temperature affects the rate of decomposition of
  organic materials by directly influencing the
  make-up of the microbial population.
• Bacteria, fungi and actinomycetes all play a
  major role in the decomposition of organic
  materials during aerobic composting.
• The initial period of composting, which is
  characterised by a rapid increase in microbial
  activity and the first signs of a rise in
  temperature, is mainly due to the activity of
  mesophilic bacteria consuming freely available
  compounds.
• As the temperature begins to rise, mesophilic
  organisms begin to die off and thermophilic
  organisms then begin to dominate.

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Compost microbiota

• Scanning electron micrograph of thermophilic
  Bacillus sp. bacteria commonly found in
  composting systems (left). Note their
  characteristic rod shape. A phase-contrast
  light microscope picture of Bacillus sp. bacteria
  in chain form (right). These bacteria are in a
  spore generating phase. Heat resistant spores are
  produced when temperatures exceed that tolerable
  by the cells (e.g. temperatures above 65?C).

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Temperature development and microbial
successions...

• If temperatures in the composting mass reach
  65-70ºC, the activity of thermophilic organisms
  also begins to be inhibited, and only some spore
  forming bacteria can survive. At this point, the
  rate of decomposition slows.
• During the curing phase, after temperatures begin
  to fall, fungi and actinomycetes begin to
  colonise and decompose the more resistant
  materials such as cellulose and lignin.

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Temperature profiles

• Temperatures attained in composting systems are
  rarely uniform throughout the entire mass.
• Gradients of between 20 and 45?C can exist
  between the surface and the centre of a windrow.
• Such temperature differences may be as small as
  2-5?C in a well insulted in-vessel composting
  system.
• Exposure of the entire mass to temperatures above
  55?C for at least 3 days is required for
  pasteurisation to occur.
• Pasteurisation is a key RISK MINIMISATION step in
  composting.

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Temperature development in composting systems
In-vessel
Turned windrow
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2) Importance of oxygen

• When microorganisms feed on the carbon component
  of organic materials for their energy, oxygen
  (O2) is consumed and carbon dioxide (CO2) is
  produced.
• The oxygen concentration in air is about 21, but
  aerobic microorganisms cannot function
  effectively at concentrations below about 5 in
  compost.
• Ideally, oxygen concentrations of about 10-14
  are required for optimum composting conditions.
• The anaerobic microbiota at low oxygen
  concentrations are responsible for much of the
  odour production.

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Mechanism of aeration - turned windrows

• In turned windrows, much of the aeration is
  achieved by convection and diffusion mechanisms.
• High level of porosity (gt20 v/v) is required to
  assist in natural aeration.

Convective air flow in a turned windrow
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Mechanism of aeration - aerated static piles

• Forced aeration is a feature of aerated static
  pile or in-vessel systems.
• In the case of static piles, forced aeration by
  blowing also has the advantage of delivering warm
  air to the cooler outer layers.
• Insulating layer of compost on outside is needed
  to maintain uniform temperatures.

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Oxygen profiles - turned windrow
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Oxygen profiles...

• As with temperature, the concentration of oxygen
  is not uniform throughout the composting mass.
• Turning or the forced delivery of air into a
  composting mass is necessary to ensure that the
  entire mass is kept in an aerobic state.
• Aeration is necessary to maintain high
  decomposition rates and to minimise odour
  production.

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Odour formation during composting

• Odour formation is strongly associated with the
  development of anaerobic conditions in composting
  systems.
• These odours are produced through the
  decomposition of organic matter.
• Composting odours are mostly produced as vapours,
  though particulate (i.e. aerosol) odours can be
  produced.

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Odour formation during composting...
The most problematic odour is ammonia NH3
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Odour treatment

• Odours can easily be treated in systems that
  permit the collection of process air from a
  composting system. Examples include in-vessel
  systems with forced aeration, or an aerated
  static pile with a suction-type aeration system.
• Process air produced by these systems can be
  directed to a biofilter a vessel containing
  mature compost to remove the odorous compounds
  from the air.
• Bacteria present in the biofilter decompose the
  odorous compounds and use them as a food source,
  thereby removing the smell from the air.

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3) Importance of water

• Moisture, or water, is essential to all living
  organisms. Moisture is lost during composting by
  evaporation.
• This has the benefit of cooling the compost to
  prevent overheating and a reduction in microbial
  activity.
• The optimum moisture content for composting is
  generally between 50 and 60 (w/w).
• Below about 30, microbial activity virtually
  stops. Moisture contents above 50 are critical
  for effective pathogen and weed control during
  the thermophilic stage of composting.
• With turned windrows, water can be added by
  soaker hoses, or by injection during turning.

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Decomposition model
Decomposition model for solid particles in a
composting system. Decomposition is performed by
microorganisms present within the liquid film and
on the surface of particles.
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Impact of excess water

• As moisture content increases, the thickness of
  the layer of water surrounding each compost
  particle increases.
• Secondly, water fills the smallest pores (the
  space between particles) first, creating water
  filled zones between particles.
• Above about 60 moisture content, the rate of
  diffusion of oxygen is too slow to replenish the
  oxygen utilised. Odorous compounds then build up
  in the anaerobic zone and can become detectable
  in the atmosphere.

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4) Physical properties of the composting mix

• Porosity, structure and texture relate to the
  physical properties of the materials such as
  particle size, shape and consistency.
• They affect the composting process by their
  influence on aeration.
• The physical properties of a composting mix can
  be adjusted by selecting suitable raw materials
  and by grinding or mixing.
• Materials added to adjust these properties are
  referred to as bulking agents.

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Porosity, structure texture

• Porosity is a measure of the air space within the
  composting mass and determines the resistance to
  airflow. Determined by particle size, the size
  gradation of the materials, and the continuity of
  the air spaces.
• Structure refers to the rigidity of particles
  that is, their ability to resist settling and
  compaction.
• Good structure prevents the loss of porosity in
  the moist environment of a compost pile.
• Texture refers to the available surface area for
  microbial attack.
• Optimum particle size mixture of 3 - 50 mm
  diameter particles.

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Porosity air flow resistance
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Composting Science Part 2

• Overview
• 1) Nutrients required for rapid composting
• 2) Role of pH and other nutrients
• 2) Commercial composting systems
• 4) Processing time and curing

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1) Nutrients required for rapid composting

• Carbon (C) in organic matter is the energy source
  and the basic building block for microbial cells.
• Nitrogen (N) is also very important and along
  with C, is the element most commonly limiting.
• Microorganisms require about 25-30 parts of
  carbon by weight for each part of nitrogen used
  for the production of protein (CN 25-301).
• Preparing feedstock to an optimum CN ratio
  results in the fastest rate of decomposition-
  assuming other factors are not limiting.

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CN ratios of different feedstocks
Food organics CN 151
Wood chips CN 200 - 3001
Manure CN 5 - 101
Garden organics CN 50 - 801
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CN ratio of common feedstocks
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CN ratio and other nutrients

• A CN ratio of between 20 and 401 is often
  suitable for composting depending on the make-up
  of the feedstock. As composting proceeds, the CN
  ratio gradually decreases to between 10 and 201.
• Feedstocks of low CN ratios (lt151) may
  decompose rapidly, but odours can become a
  problem because of the complete and rapid usage
  of oxygen without replenishment, resulting in the
  production of odourous sulphur compounds such as
  thiols.
• Microorganisms also require adequate phosphorus,
  sulfur and micronutrients for growth and enzyme
  function, but their role in composting is less
  well known.

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How organic materials break down

• Compost feedstock is a complex mix of organic
  materials ranging from simple sugars and starches
  to more complex and resistant molecules such as
  cellulose and lignin.
• In general terms, composting microbes first
  consume compounds that are more 'susceptible' to
  degradation in preference to compounds that are
  more resistant.
• The breakdown of organic matter is therefore a
  step-wise reduction of complex substances to more
  simpler compounds.

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How organic materials break down...

• During the intensive phase of composting, the
  more easily degradable compounds are broken down
  first.
• Feedstocks that contain a high proportion of
  compounds that are difficult to break down, such
  as lignin, require longer periods of composting
  decomposition of lignin occurs more rapidly
  during the curing phase, at mesophilic
  temperatures.
• For many organic materials, a period of
  maturation is also essential to eliminate
  compounds that are toxic to plant growth
  (phytotoxic).

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How organic materials break down...
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2) pH and other nutrients

• Optimum pH range for composting is somewhere in
  the range of 5.5 to 9.
• It is important to note that composting is likely
  to be less effective at 5.5 or 9 than it is at a
  pH near neutral (pH 7).
• pH does become important with raw materials that
  have a high percentage of nitrogen (e.g. manure
  and biosolids).

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pH and role in composting

• A high pH, above 8.5, encourages the conversion
  of nitrogen compounds into ammonia, which further
  adds to alkalinity.
• Loss of nitrogen in the form of ammonia to the
  atmosphere not only causes nuisance odours, but
  also reduces the nutrient value of the compost.
• Adjusting the pH downward below 8.0 reduces
  ammonia loss. This can be achieved by adding an
  acidifying agent, such as superphosphate or
  elemental sulfur.

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pH changes during composting
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Other nutrients required for composting

• Apart from C and N, compost microorganisms
  require an adequate supply of other nutrients
  such as phosphorus, sulphur, potassium and trace
  elements (e.g. iron, manganese, boron etc).
• These nutrients are usually present in ample
  concentrations in compost feedstock, though
  phosphorus (P) can sometimes be limiting. A CP
  ratio of between 75 and 1501 is required.

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3) Commercial composting systems

• At least eight different forms of composting
  systems are available for processing a wide range
  of organic materials.
• Turned windrow systems have been the predominant
  form of composting in Australia, particularly for
  garden organics.
• Higher technology composting systems are now
  being implemented for processing materials that
  have traditionally been difficult to process in
  outdoor turned windrow systems, such as food
  organics.
• All systems aim to control compost production by
  manipulating temperature, oxygen and moisture
  during composting. This varies from system to
  system.

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Turned windrows
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Passively aerated windrow
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Aerated static pile
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Aerated covered windrow
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Rotating drums
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Agitated bed or channel
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In-vessel (horizontal configuration)
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In-vessel (vertical configuration)
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4) Processing time curing

• The length of time it takes to convert raw
  materials into mature compost depends upon many
  factors, including
• Types of raw materials being processed
• Compost recipe (feedstock) prepared
• Temperature
• Moisture, and
• Frequency of aeration.
• To achieve the shortest possible composting
  period, sufficient moisture, an adequate CN
  ratio and good aeration is required.

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Processing time for different systems
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Curing

• Curing is a critical and often neglected stage of
  composting during which the compost matures.
• Curing occurs at low, mesophilic temperatures for
  periods of up to 6 months, depending on the
  material composted.
• In this process, the rate of oxygen consumption,
  heat generation, and moisture evaporation are
  much lower than in the active composting phase.

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Curing...

• Because curing continues the aerobic
  decomposition process, adequate aeration in
  necessary.
• If piles are to be naturally aerated (i.e. no
  active means of aeration), pile size needs to be
  relatively small (height 1 m) and moisture
  cannot be excessive (gt70).
• Larger piles required forced aeration to remain
  in an aerobic state.

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Conclusions

• Conversion of organic materials into quality
  composted products that can improve soils and the
  environment is a central component of the NSW
  Governments strategy to reduce waste disposal to
  landfill.
• An understanding of the basic principles of
  composting science will allow solid waste
  managers to select and implement appropriate
  composting solutions.
• Other supporting info on licensing and
  establishing a composting facility in NSW can be
  obtained for free from our web site,
  http//www.recycledorganics.com under
  publications!