Chapter 3 Composting technology – Basic concept
3.

Composting technology – Basic concept

3.1

History of composting

Humans have applied composting to plant culture, over a long history. However, composting in the ancienttimes was only a result of natural composting of organic substances, or an attempt by humans based on therule of thumb as they observed the natural phenomena. It was Albert Howard who systematically organizedthese experiences as a composting technology. Howard dedicated himself to a study on composting in theIndore region of India between 1924 and 1934, and summarized the composting technology he hasdeveloped under the title “Indore-style composting method.” The Indore-style composting method has been
appraised highly as the origin and the fundamental law of composting.
In Japan, composting started when the division of internal affairs of the Kanagawa Prefectural government
published in 1920 a booklet titled “Taihi no Susume (Everyone is Advised to Try Composting).” Thecontents of the booklet are deemed appropriate even today, and raw garbage was gathering attention as asource material of composting at the time already.
The booklet states as follows in one of its sections.
“Raw garbage generated in a large volume in cities is a good material for composting. These days, however,people in cities are having trouble disposing of raw garbage, and have reluctantly landfilled these wastes. It
is truly regretful that the precious materials for composting are being buried in the ground. Thus, farming
families near cities are recommended to form an association for collecting raw garbage in cities and use it
for producing composts.”
Raw garbage composting has been an issue, in the past and in recent years alike.
3.2

Basic concept of composting“Composting” is explained in many cases as “having organic components of wastes fermented by
microorganisms under an aerobic condition.” However, technologies that use life forms other than
microorganisms for composting, such as earthworm composting, have also disseminated widely. Taking
into consideration also the notable effects of composting, namely “improvement of physical properties(ease of draining, air permeation, water retention as soil crumbs are formed), chemical properties (nutrient
supply and retention, buffer capacity to hazardous substances and pH) and biological properties (preventionof diseases in soil, extraction of useful bacteria) of the soil,” the basic concept of composting may berepresented as follows.
“Composting is to convert organic substances into a soil improvement material that yields components forhelping plants grow and promotes soil crumb formation, through aerobic fermentation by microorganisms
and dietary effects brought about by life forms in the soil.”
Here, the working of microorganisms is represented as “fermentation,” for the reasons described below.

1© 2019 Koji TakakuraChapter 3 Composting technology – Basic concept

Utilizing
microorganisms
Raw garbage
composting

Aerobic fermentation

Anaerobic fermentation*1

Utilizing life forms

Earthworm composting

Utilizing insect larvae*2*1: The compost may be used after mixing with the soil and humus and going through aerobic
fermentation as the secondary processing, or fermented aerobically in the soil when applied
directly as a fertilizer without the secondary processing.
*2: Insects used include larvae of house flies (already in practical application) and larvae of soldier
flies.
Fig. 3-1: Raw garbage composting methods

3.2.1 Composting process involves “fermentation”
The decomposition process by microorganisms may be divided broadly into “fermentation” and “decaying,”
but how are they differentiated? Microorganisms discharge diverse substances as metabolites while
decomposing organic substances. For microorganisms, however, the process is nothing but a metabolic
activity using organic substances. On the other hand, humans distinguish the metabolites into “useful andgood ones” and “harmful and bad ones,” which have been named “fermentation” and “decaying”
respectively. What is more, the distinction is being made based on preference of each person.
Let us take “natto” as an example.
Natto is one of the fermented foods thathas traditionally been incorporated in
the dietary life of Japanese people.

Looks good.
It’s fermenting!

It stinks and hasdecayed.

However, many people feel that natto
has “decayed,” because of its peculiar
odor reminding of ammonia. Foodmaterials form threads in many cases
when decayed, and some people,

Each person feels differently about an odor, and some think theespecially those from abroad, judges

substance is fermenting while others think it is decaying.

that natto has “decayed” because of theFig. 3-2: Each person feels differently about odor

“odor” and the “forming of threads.”

2© 2019 Koji TakakuraChapter 3 Composting technology – Basic concept
The “odor” generated from metabolic activities of microorganisms relates delicately with fermentation anddecaying in the composting process, and is related deeply with the types of microorganisms used incomposting, as explained later.
Let us now take a look at composting. Compost is a product made through metabolic activities of
microorganisms, and brings about common benefits for humans, such as helping plants grow. When viewed
this way, it seems more appropriate to think of composting as a “fermentation” process, rather than just“decomposition.” The term “fermentation” is used hereinafter for representing the composting process, but
the term “decomposition” is also used from time to time. However, please keep in mind that the term
“decomposition” also refers to “fermentation.”
3.3

Basic process flow of composting

The basic process flow of composting may be divided into three steps, namely “pre-treatment,”“fermentation” and “preparation as a compost product.”
3.3.1

Pre-treatment process(1) Removing foreign matters
Raw garbage is usually discharged, collected and transported in mixture with other types of wastes. As such,the mixing ratio of foreign matters is high, and mechanical or manual foreign matter removal process isnecessary as a pre-treatment. However, the removal ratio would be limited naturally if foreign matters wereremoved from a mixture of wastes, leading to incomplete removal of foreign matters and decline in thequality of compost products.

Photograph 3-1: Removing foreign mattersmechanically and manually

Photograph 3-2: Only metals are removed inmagnetic sorting

Looking at the situation from an opposite angle, a method was adopted in some cases that only raw garbage
that may be composted was sorted and collected from the mixture of wastes. This led to production ofcompost with extremely small mixing ratio of foreign matters that were evaluated highly. In any case, it isinefficient to extract the targeted waste from mixture of wastes. Therefore, it is essential to establish asystem in which “raw garbage is sorted and stored” at the source of wastes and “only the raw garbage iscollected using exclusive vehicles” for transportation.

3© 2019 Koji TakakuraChapter 3 Composting technology – Basic concept

Photograph 3-3: Only raw garbage is pickedup from mixture of wastes

Photograph 3-4: Raw garbage is sorted at thesource of wastes and collected by an
exclusive vehicle(2) Limitation of mechanical foreign matters removal
A large volume of raw garbage is generated in urban areas, where a large-scale composting plant is built forcomposting the raw garbage, in many cases. It is desirable if the raw garbage was sorted and collected, but
in reality, this is extremely difficult and mechanical sorting of waste mixture is adopted in many cases.
However, there is a limitation in the accuracy of sorting, and the accuracy affects the quality of compostproduced.
Here, let us look at the foreign matter removal process assuming that a large-scale composting plant
operated by the conventionally adopted method is built next to a landfill disposal site, for simulating theprocess flow of composting.
Processing capacity assumed: 1,000 ton/day of mixed waste
Sorting method:bag tearing process  manual sorting  trommel sieving machine (sievemesh of 70 mm)  specific gravity separating apparatus (for removing
sand, glass and metals)  (a magnetic sorting apparatus is used in somecases)  vibration sieving machine (sieve mesh of 7 mm)

Fermentation method:indoor aeration standing fermentation method

Shipment mode:

packaged in plastic bags

[1] Reception of mixed waste
A part of or the whole of the mixed wastes carried into the landfill disposal site is to be composted, and iscarried into the composting material storage yard (indoor or outdoor) and stored for a while. The raw
garbage is decomposed while decaying, and generates odor and wastewater. If the waste was stored for along time, decaying and decomposition of the raw garbage progresses, lowering the ratio of raw garbage inthe mixed waste that is to be processed in the next step for foreign matter removal. One might erroneously
imagine that filthy and odorous plastic bags are being handled.

4© 2019 Koji TakakuraChapter 3 Composting technology – Basic concept

Photograph 3-5: Temporary storage of mixed
waste (outdoor)

Photograph 3-6: Temporary storage of mixed
waste (indoor)[2] Removal of foreign matters
Plastics might get finely broken if mixed using a heavy machine or by an influence of ultraviolet rays.
Plastic bags containing mixed waste go through a bag tearing process, have foreign matters removed andrecyclable wastes collected on a manual sorting line, and processed by a huge trommel sieving machine(with a sieve mesh of 70 mm, for example) for removing large foreign matters.
A method that cuts (tears) the bags with a blade is usually adopted for the bag tearing process, but there isalso a method that places the plastic bags on a belt conveyor and has the bags come in contact with aheating wire, for having them dissolved with heat. Either way, bags that were not completely torn areremoved by the sieve.

Photograph 3-7: Bag tearing and manual
sorting line

Photograph 3-8: Collecting recyclable wastes5© 2019 Koji TakakuraChapter 3 Composting technology – Basic concept

Photograph 3-9: Old wastes in the red frameand new wastes in the yellow frame

Photograph 3-10: Huge trommel sieving
machine[3] Fermentation of raw garbage
After removing foreign matters, the raw garbage is carried indoor for conducting standing fermentation
while adjusting the moisture content, controlling the temperature, and performing aeration. The
fermentation product contains a large amount of foreign matters. Especially to note is that a plastic bag
piece that measures 70 mm or more would pass through the sieve mesh of 70 mm, if pressed with force.

Photograph 3-11: Indoor standing
fermentationPhotograph 3-12: Fermentation productcontains many foreign matters[4] Removal of foreign matters
After fermenting for the set duration of time, the fermentation product is thrown into a vibration sieving
machine (with a sieve mesh of 7 mm, for example) for removing small foreign matters. Organic matterssuch as raw garbage have turned into dark brown to black color, on which white foreign matters such as
plastics stand out.

6

© 2019 Koji TakakuraChapter 3 Composting technology – Basic concept

Photograph 3-13: Fermentation productbecomes brownish to blackish in colorPhotograph 3-14: Vibration sieving machine
removes small foreign matters[5] Storage of compost
After sieving, the compost waits to be shipped in a storage yard. The compost produced inevitably contains
foreign matters (mainly plastics) that have passed through the 7-mm sieve mesh.

Photograph 3-15: Compost product waits tobe shipped in a storage yard

Photograph 3-16: Whitish particles areforeign matters that passed through the 7-mm
sieve, mainly plastics[6] Preparing the compost for shipment
The compost product is either “pulverized,” “pelletized” or “packaged in a bag” for shipment, to suit thecustomers’ needs.

7© 2019 Koji TakakuraChapter 3 Composting technology – Basic concept

Photograph 3-17: Compost is sometimes“pelletized” to suit the customers’ needs

Photograph 3-18: Packaged compost is being
shippedPlastics become finely broken down as time goes by, and inevitably pass through the sieve mesh and get
mixed into the compost product. Wind sorting could be performed prior to fermentation, but plastics thatcannot be wind-sorted remain, depending on the moisture content, material form, etc. If wind sorting wasperformed after fermentation, the compost is also blown away with the wind, causing the product loss toincrease.
It is difficult to completely remove foreign matters from the mixed wastes, especially plastics, regardless ofwhether manual or mechanical sorting was performed. It is important to promote sorting at the place oforigin of wastes, in order to produce compost with a small mixing ratio of foreign matters.
3.3.2

Fermentation process

The fermentation process is the most important process that determines the product quality, and is usuallydivided into the primary fermentation and secondary fermentation (maturing) steps.
 Primary fermentation: easily decomposed organic substances are mainly decomposed rapidly,
generating a large amount of fermentation heat that raises the temperature.
 Secondary fermentation (maturing): decomposition and decaying of easily decomposed organic
substances progress and the process stabilizes.
Basically, the air is supplied as appropriate to realize aerobic fermentation, and water is also added if
deficient, for adjusting the moisture content. At this time point, seed compost (made by culturing
microorganisms) and auxiliary materials (rice hull, sawdust, etc.) are added for leading to satisfactoryfermentation under the desirable conditions, which may be realized by adding appropriate microorganisms,promoting ventilation through the raw materials, adjusting the moisture content and pH, etc.
It would be costly if new seed compost was prepared each time to be added. Since the compost after
completion of the secondary fermentation also contains a large amount of high-quality fermentation
microorganisms, a part of it is used as the seed compost for reducing the costs. Such seed compost isreferred to as returned compost. Also, opportunities are sought for obtaining the auxiliary materials at a low
cost, for example, by obtaining those generated as wastes in the neighborhood.
8© 2019 Koji TakakuraChapter 3 Composting technology – Basic concept(1) Concept of returned compost
Composting is done after mixing the seed compost into organic wastes, but not all of the mixture is shipped
as a compost product. Since the seed compost is added for promoting satisfactory composting of organic
wastes, it is collected and used repeatedly. Since the product of secondary fermentation is returned to theprimary fermentation process, it is referred to as returned compost.
Since organic wastes that have been input are composted as they are decomposed by microorganisms, thedecomposition ratio of organic substances in the waste may be clarified for each type of organic wastes.
Therefore, the composting ratio (amount of compost produced/amount of organic wastes input × 100) may
be estimated.
Example 1:

Calculation of the amount of compost that may be produced from 1 ton of raw
garbage

Conditions: moisture content of the raw garbage: 80%; decomposition ratio of the raw
garbage: 75%; moisture content of the compost product: 50%
1 ton × (1 - 0.8) × (1 - 0.75) / 0.6 = 0.1 tons (Composting ratio: 10%)
0.1 tons of compost product is derived from the mixture of seed compost and raw garbage compost.

1 ton of raw garbage is
input into seed compost

Mixture of seed compost
and raw garbage compost

Shipped as a compost
product: 0.1 tons

Re-used as returned compost
Since a part of the seed compost may
be lost, auxiliary materials are addedfor increasing the volumeFig. 3-3: Concept of returned compost9© 2019 Koji TakakuraChapter 3 Composting technology – Basic concept
The table below shows examples of decomposition ratio of composting raw materials. Using the figures asa reference, an approximate amount of compost that may be taken out as a compost product may be
calculated.

Table 3-1: Decomposition ratio of compost raw materials (examples)
Raw materials

RawgarbageSludge in a
septic tank

CattlemanurePoultry
manurePorcinemanureRicehull

Sawdust

Rice
straw

Decomposition
ratio (%)

7530403535

10530

3.3.3

Product preparation processThis is the process for preparing the fermented compost as a commercial product. The compost is sieved
for removing foreign matters, for finishing it as a product. Then, processes such as sieving for collecting
particles of the specified sizes, granulation, and packaging are performed for making the productconvenient for storage prior to shipment, stockpiling, transport, and use.
Not all the compost sieved for removing foreign matters is made into products, but instead, the seed
compost that was added for promoting satisfactory fermentation is collected as returned compost.
A big chunk of materials removed by sieving is returned as a raw material, as it could function as thereturned compost also. However, if materials that remained on the sieve were used as the returned compost,
foreign matters are also returned at the same time. As such, foreign matters on the sieve become
concentrated as time goes by. For this reason, materials that are judged to contain a large amount of foreign
matters are discarded.
The diagram below illustrates the basic processes of composting discussed so far.

Returned compost

Organic waste

Pre-treatment

Fermentation

Preparation asproducts

Removal of foreign
matters

Compost productDiscarded materials
that are unfit forcompostingFig. 3-4: Basic processes of composting10© 2019 Koji TakakuraChapter 3 Composting technology – Basic concept
References: Koji Takakura: Kaigai Gijutu Kyoryoku wo Tsuujita Takakura-shiki Konposuto no Gijutsu Iten ni
Kansuru Kenkyu (A Study on Technical Transfer for Takakura Composting Method through Overseas
Technological Cooperation), doctoral thesis, Graduate School of Life Science and Systems Engineering,
Kyushu Institute of Technology, May 2016
 Institute for Global Environmental Strategies: Nama Gomi Konposutotte Sugoi! (Raw Garbage
Composting is Great!), 2013
 Institute for Global Environmental Strategies: Composting for Waste Reduction; INFORMATION KIT,2010
 Minami-Boso City Government: III. Taihi no Seizo Hoho (III. Compost Production Method) Kanagawa Prefectural Government: Taihi no Susume, Taisho Jidai no Taihi no Tsukurikata (Everyone isAdvised to Try Composting – Production Method of Compost during the Taisho Era)11

© 2019 Koji Takakura