1. Recycled Organics Unit
Composting Science for Industry Mr Angus Campbell

2. Lecture Overview Composting Science Part 1 Composting Science Part 2
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

3. 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”

4. Introduction www.recycledorganics.com
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.

5. Introduction... www.recycledorganics.com
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

6. Process diagram: composting systems

7. 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) > 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 (<1-2 m3) may not heat up because the heat generated by the microbial population is lost quickly to the atmosphere (mainly convective losses).

8. Temperature changes during composting

9. 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.

10. 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.

11. Compost microbiota www.recycledorganics.com
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).

12. 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.

13. Temperature profiles www.recycledorganics.com
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.

14. Temperature development in composting systems
In-vessel
Turned windrow

15. 2) Importance of oxygen www.recycledorganics.com
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.

16. Mechanism of aeration - turned windrows
In turned windrows, much of the aeration is achieved by convection and diffusion mechanisms.
High level of porosity (>20% v/v) is required to assist in ‘natural aeration’.
Convective air flow in a turned windrow

17. 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.

18. Oxygen profiles - turned windrow

19. Oxygen profiles... www.recycledorganics.com
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.

20. 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.

21. Odour formation during composting...
“The most problematic odour is ammonia NH3”

22. Odour treatment www.recycledorganics.com
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.

23. www.recycledorganics.com 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.

24. Decomposition model www.recycledorganics.com
“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.”

25. Impact of excess water www.recycledorganics.com
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.

26. 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.

27. 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 mm diameter particles.

28. Porosity & air flow resistance

29. 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

30. 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 parts of carbon by weight for each part of nitrogen used for the production of protein (C:N :1).
Preparing feedstock to an optimum C:N ratio results in the fastest rate of decomposition- assuming other factors are not limiting.

31. C:N ratios of different feedstocks
Food organics C:N ~ 15:1
Wood chips C:N ~ :1
Garden organics C:N ~ :1
Manure C:N ~ :1

32. C:N ratio of common feedstocks

33. C:N ratio and other nutrients
A C:N ratio of between 20 and 40:1 is often suitable for composting depending on the make-up of the feedstock. As composting proceeds, the C:N ratio gradually decreases to between 10 and 20:1.
Feedstocks of low C:N ratios (<15:1) 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.

34. 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.

35. 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).

36. How organic materials break down...

37. 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).

38. 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.

39. pH changes during composting

40. 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 C:P ratio of between 75 and 150:1 is required.

41. 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.

42. Turned windrows

43. Passively aerated windrow

44. Aerated static pile

45. Aerated covered windrow

46. Rotating drums

47. Agitated bed or channel

48. In-vessel (horizontal configuration)

49. In-vessel (vertical configuration)

50. 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 C:N ratio and good aeration is required.

51. Processing time for different systems

52. Curing www.recycledorganics.com
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.

53. Curing... www.recycledorganics.com
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 (>70%).
Larger piles required forced aeration to remain in an aerobic state.

54. Conclusions www.recycledorganics.com
Conversion of organic materials into quality composted products that can improve soils and the environment is a central component of the NSW Government’s 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, under “publications”!