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5. COMPOSTING

To further treat the
biowaste the digestate produced during the anaerobic digestion process is
composted with the garden waste produced in Biocity. Using composting after
anaerobic digestion allows us to further treat residues, like the digestate produced.
Moreover, good quality compost is obtained, which can substitute mineral
fertilizers and allows the recirculation of organics into the natural cycle.

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5.1. FUNDAMENTALS OF COMPOSTING

Composting
is defined as the biological decomposition and stabilization of organic
substrates under aerobic conditions. Organic matter is convertes into a
humus-like product. Finished compost is a stabilized and sanitized material (Sunar
et all, 2014).

The
composting process can be divided into four different stages (see figure
xx).  First of all, the mesophilic
microorganisms decompose rapidly the easily degradable compounds. Heat is
produced and therefore the temperature rises. Mesophilic microorganisms are
replaced by termophilic microorganisms, which degrade proteins, fats, and
complex carbohydrates. In this stage, temperature must be under 65°C, to avoid
the elimination of microorganisms. To mantain the temperature it is important
to aerate and turn the material. After the thermophilic phase temperature
decreases and mesophilic microorganisms take part in the maturation of the
compost.

Figure xx:
stages of composting: (1) mesophilic phase, (2) termophilic phase, (3) cooling
phase, (4) maturing phase. (Shileve et all., 2006).

Apart fom temperature, other process parameters are of
importance in the composting process. Those are: water content, pH, C/N ratio
and air pore volume.

Water content

Microorganisms
need to be in an aqueous solution in order to decompose the organic matter. When
no water is present, they become inactive. On the other hand, a very high water
content can affect the availability of the oxygen, making the process
anaerobic.  Values from 45% to 60% are
optimal for the composting of biowaste (Amlinger et al.,
2009).

C/N ratio

Organic
carbon and nitrogen should have a balanced proportion. High C/N ratios lead to
an increase of ammonia and low ratios can make the decomposition process slow.
An ideal ratio would be 30:1. Values from 25:1 to 35:1 are considered optimal.
(Amlinger et al., 2009;  Shileve et all., 2006)

pH value

The
optimal pH should be neither too acidic neither too alkaline. A value in
between 5,5 and 8,5 is ideal (Shileve et all., 2006)

Bulk density

It
is the mass divided by the volume. It gives information about the pore distribution
in the compost material. If it is low, it will be difficult for air to pass
through the compost pile. If it is too high, the volume of material increases,
having to treat larger quantities. An optimal value is in between 400 and 700.
(learn2compost, 2018)

5.2. PROCESS
DESCRIPTION

In the
following section, the steps that take place during the composting process are
presented. In figure xx a diagram with these steps is shown.

5.2.1. Digestate dewatering

The digestate
obtained from the anaerobic digestion has a high water content because of the
technology used (Dranco technology). Because of that the water content might be
too high .As stated before, the ideal water content for composting is in
between 45% and 60% (Amlinger et al.,
2009). It is important to dewater the digestate prior to the compost stage.
The digestate will be dewatered until it has a dry solids content of 40%, which
is an optimal value. The technology used to dewater the digestate would be a
screw-press machine.

 

In the table
xx the quantities of digestate before and after dewatering (example for autumn
months) can be seen.

Total  (ton/mth)

Water (ton/mth)

DS (ton/mth)

Water
fraction

DS
fraction

Digestate

6737,54

5464,75

1273,78

0,81

0,19

40%DS digestate

3184,45

1910,67

1273,78

0,6

0,4

Table xx: digestate before and after dewatering (example for autumn)

5.2.2. Mixing

The 40%DS
digestate will be composted with the shredded garden waste and newspapers as an
additive. The digestate, garden waste, water and newspapers will be mixed
before entering the composting units in a feedstock mixer.

5.2.3. Composting stage

The composting
process takes place in a series of tunnel. In the next section this process
will be explained in detail.

5.2.4. Screening and sieving

When the
composting process is over the material is screened in a rotating drum to
eliminate impurities. Usually the impurities are smaller than 1% of the total
compost produced (REFERENCE). In the calculations we won’t take this value into
consideration as the majority of the impurities were separated in the
pre-treatment of the waste. After screening, the compost will be sieved to
ensure a homogeneous product. The big particles are reticulated back to the
composting stage.

5.2.5. Packaging and storage of compost

We have to
consider that the compost must be stored, especially in the period from October
to March. For that, we will have a special area in the plant where the compost
will be stored. It will have a garage door to the outside of the plant so
farmers can pick the compost directly.

Another
special area is dedicated to the packaging of the compost in small bags and
storage of the bags. These compost bags would be ready to sell to suppliers and
individuals.

The dimensions
of these facilities will be presented after the dimensioning.

 

 

5. 3. TUNNEL COMPOSTING

For the
composting process the technology of tunnel composting was selected. A
composting tunnel is an enclosed composting system. An enclosed system allows
to control parameters like oxygen supply, temperature and humidity. Apart from
that, the decomposition rate is higher than in open composting systems.

The composting
tunnel functions in a batch-wise manner. It has a series of aeration plates in
the floor, where the oxygen is supplied. On the ceiling of the tunnel a series
of spraying nozzles are distributed. With this system it is possible to apply
moisture to the material. Apart from
forced aeration and wetting, periodic turning is needed. A weekly to
fortnightly turning is done in order to re-structure the piles (Amlinger
et al., 2009).

Composting
tunnels are usually made of reinforced concrete and have a sliding door to fill
or empty the tunnel. In figure xx two images of composting tunnels can be seen.

Figure xx: images of a composting tunnel (“Tunnel composting systems”,
2018; (“Linde
KCA – Waste management – Christiaens Group”, 2018))

 

Figure xx:
Schema of tunnel composting (Yoshida, 2017).

In tunnel
composting systems, the process water used to add moisture through the spray
nozzles can be recirculated. Air recirculation is as well possible. The exhaust
air, which composes excess air, carbon dioxide and water is treated in the
biofilter.

A big
advantage of tunnel composting systems is that each tunnel can be controlled
individually. This offers a high flexibility in the operation and control. The
substrates fed to the tunnels can differ and the composting parameters can be
adapted.

The composting
area would consist in a series of tunnels in a hall in parallel, similarly as
what it is shown in figure xx. The material is feed automatically into the
tunnels.

 

Figure xx: composting tunnels in a hall (“Linde KCA – Waste
management – Christiaens Group”, 2018)

 

5.4. DIMENSIONING OF THE COMPOSTING TUNNELS

As stated before,
the technology used in the composting stage is the composting tunnel. In the
following section, the number and dimensions of tunnels, and the retention time
will be presented. For the dimensioning of the process, the online tool learn2compost, from the Institute of
Environmental Technology and Energy Economics form TUHH is used.

In order to
use the learn2compost tool, a series of input parameters have to be defined. In
the first place, the input streams have to be defined in tones per month. The
season with the highest waste input (autumn) was used to dimension the
equipment.

AUTUMN

Total mass ton/month

Water ton/month

DS ton/month

oDS ton/month

Inorganic ton/month

40%DS digestate

3184,45

1910,67

1273,78

661,30

612,47

Garden waste

85

16,15

68,85

54,59

14,25

Newspapers

56

3,36

52,64

49,48

3,15

Total

3325,45

1930,18

1395,27

765,38

629,88

Table xx: input wastes in ton/month in autumn months

Because the
garden waste defined in the contract notice has a different dry solid content
as the garden waste defined in the application learn2compost, a new substrate
is defined. This will have the same C/N ratio, pH and BD as the input defined
in the application but different water content.

Figure xx:
garden waste substrate definition (learn2compost, 2018).

Figure xx: learn2comopst online tool, substrate selection (learn2compost,
2018).

Figure xx: learn2comopst online tool, tunnel dimmensioning (learn2compost,
2018).

We can see in figure XX that we will need 19 tunnels in our plant. The
available capacity is 9500 m3, which will be distributed in 19
tunnels of dimensions 4x4x32 m3

Capacity (m3)

Tunnels

Height (m)

Width (m)

Longitude(m)

9500

19

4

4

32

Table xx: number and dimensions of tunnels

5.4. MASS BALANCE

5.4.1. Input of waste

 In the next section the mass balance in the
composting system is explained. First of all the input streams have to be
determined. We have garden waste, the dewatered digestate obtained from the
anaerobic digestion process (40% DS) and newspapers as an amendment. The water
content of the newspapaers is 6%, as stated in the online tool learn2compost,
and the volatile solid content (or organic dry matter) is 94% (Rada,
2015). In the following tables the input substrates, their water, dry solid
and organic solid fraction can be seen.

WINTER

Total  (ton/mth)

Water
(ton/mth)

DS
(ton/mth)

oDS
(ton/mth)

Inorganic (ton/mth)

DS fraction

oDS fraction

Digestate

2145,22

1287,13

858,09

445,84

412,24

0,4

0,51

Garden waste

34,58

6,5702

28,00

22,21

5,79

0,81

0,79

Newspapers

37

2,22

34,78

32,69

2,08

0,94

0,94

Total input

2216,80

1295,92

920,87

500,74

420,12

0,41

0,54

Table xx: input of waste in ton/month in winter

SPRING

Total  (ton/mth)

Water
(ton/mth)

DS
(ton/mth)

oDS
(ton/mth)

Inorganic (ton/mth)

DS fraction

oDS fraction

Digestate

2182,08

1309,25

872,83

453,51

419,32

0,4

0,51

Garden waste

48,31

9,17

39,13

31,03

8,10

0,81

0,79

Newspapers

38

2,28

35,72

33,57

2,14

0,94

0,94

Total input

2268,39

1320,71

947,68

518,11

429,56

0,41

0,54

Table xx: input of waste in ton/month in spring

SUMMER

Total  (ton/mth)

Water
(ton/mth)

DS
(ton/mth)

oDS
(ton/mth)

Inorganic (ton/mth)

DS fraction

oDS fraction

Digestate

2895,48

1737,29

1158,19

601,77

556,42

0,4

0,51

Garden waste

115,95

22,03

93,91

74,47

19,44

0,81

0,793

Newspapers

51

3,06

47,94

45,06

2,87

0,94

0,94

Total input

3062,43

1762,38

1300,05

721,31

578,74

0,42

0,55

                                 Table xx: input
of waste in ton/month in summer       

 

 

AUTUMN

Total  (ton/mth)

Water
(ton/mth)

DS
(ton/mth)

oDS
(ton/mth)

Inorganic (ton/mth)

DS fraction

oDS fraction

Digestate

3184,45

1910,67

1273,78

661,30

612,47

0,4

0,51

Garden waste

85

16,15

68,85

54,59

14,25

0,81

0,79

Newspapers

56

3,36

52,64

49,48

3,15

0,94

0,94

Total input

3325,45

1930,18

1395,27

765,38

629,88

0,42

0,55

Table xx: input of waste in ton/month in autumn

5.4.2. Specifications

To calculate
the compost produced we need the degree of degradation of the organic material
that takes place in the process. Usually the decomposition of organic
substances is 55 – 60% (Bilitewski et all., 1997).
Because of our technology we expect a degradation of the biomass of 60%

The
water content of finished compost should be in between 35 – 45% (Christensen,
2011). We specify a value of 40% moisture for the finished
product, which will be achieved with proper controlling of the process.

5.4.3. Example of calculation

As
an example for the autumn season the calculations done for the mass balance
process are shown in this section.

Inorganic matter or ash content

The
inorganic solid part of the waste will remain constant during the composting
process.

Organic matter

Using 60% degradation of organic
matter, the amount of organic matter not degraded is:

 

The
amount of water produced by the decomposition of organic matter can be
calculated with a conversion factor of 0,7 times the decomposed matter (Robinzon
et all., 2000)

Oxygen

About
1,75 kg of oxygen are required for the decomposition of 1 kg of waste organic
matter (Robinzon
et all., 2000). Using the dry mass decomposed calculated before:

Water balance

Since
we specified the water content in our final product (40 %) we can calculate the
mass of finished compost and the amount of water evaporated can be calculated using
the water balance.

Global balance

The CO2 emissions
can be calculated with a total balance.

5.4.4. Results

In
the following section the results of the mass balance is presented for each
season. The quantities presented are monthly values. The average throughout the
year is as well presented.

WINTER

Waste input

Air input

Compost output

Water output

CO2 output

Water

1295,92

0

413,61

1092,62

0

oDS

500,74

0

200,29

0

0

Inorganic

420,12

0

420,12

0

0

O2

0

525,78

0

0

0

CO2

0

0

0

0

615,92

Total

2216,80

525,78

1034,04

1092,62

615,92

Table
xx: Mass balance composting unit per month in winter

SPRING

Waste input

Air input

Compost output

Water output

CO2 output

Water

1320,71

0

424,54

1113,77

0

oDS

518,11

0

207,24

0

0

Inorganic

429,56

0

429,56

0

0

O2

0

544,02

0

0

0

CO2

0

0

0

0

637,28

Total

2268,39

544,02

1061,35

1113,77

637,28

Table
xx: Mass balance composting unit per month in spring

SUMMER

Waste input

Air input

Compost output

Water output

CO2 output

Water

1762,38

0

578,17

1487,15

0

oDS

721,31

0

288,52

0

0

Inorganic

578,74

0

578,74

0

0

O2

0

757,37

0

0

0

CO2

0

0

0

0

887,21

Total

3062,43

757,37

1445,44

1487,15

887,21

Table
xx: Mass balance composting unit per month in summer

AUTUMN

Waste input

Air input

Compost output

Water output

CO2 output

Water

1930,18

0

624,02

1627,61

0

oDS

765,38

0

306,15

0

0

Inorganic

629,88

0

629,88

0

0

O2

0

803,65

0

0

0

CO2

0

0

0

0

941,42

Total

3325,45

803,65

1560,06

1627,61

941,42

Table
xx: Mass balance composting unit per month in autumn

Average per month

Waste input

Air input

Compost output

Water output

CO2 output

Water

1577,29

0

510,09

1330,29

0

oDS

626,39

0

250,55

0

0

Inorganic

514,58

0

514,58

0

0

O2

0

657,71

0

0

0

CO2

0

0

0

0

770,46

Total

2718,27

657,71

1275,22

1330,29

770,46

Table xx: Mass balance composting unit (monthly
average)

Average per year

Waste input

Air input

Compost output

Water output

CO2 output

Water

18927,59

0

6121,10

15963,50

0

oDS

7516,69

0

3006,67

0

0

Inorganic

6174,97

0

6174,97

0

0

O2

0

7892,52

0

0

0

CO2

0

0

0

0

9245,53

Total

32619,27

7892,52

15302,75

15963,50

9245,53

Table
xx: Mass balance composting unit (yearly average)

For a better visualization of the
quantities formed in the composting process the mass balance for each season is
presented in graphic format (see figures xx, xx, xx).

(a)

(b)

(c)

(d)

Figure x: mass
balance in ton/month in  the composting
stage for winter (a), spring (b), summer (c) and autumn (d). (1=in, 2=out)

Figure x:
average mass balance in ton/month (1=in, 2=out).

Figure x:
average mass balance in ton/year (1=in, 2=out).

Compost mass (ton/year)

15301

Compost volume (m3/year)

11475

Figure xx: compost production per year

5.4.4. Space requirements

Apart
from the area dedicated to composting, we must ensure as well storage for the
finished compost for 6 months (October to March).

Compost tunnel area

Tunnel vol (m3)

Tunnels

Height (m)

Width (m)

Longitude(m)

9500

19

4

4

32

Table xx: space requirements for composting

Compost tunnel area

Compost vol (m3)

Height (m)

Width (m)

Longitude(m)

5737

 

 

 

Table
xx: Space requirements for compost storage

5.4.4. References

Amlinger,
F., Peyr, S., Hildebrandt, U., Müsken, J., Cuhls, C., & Clemens, J. (2009).
The State of the Art of Composting –A Guide to Good Practice.  AUSTRIA

Kutzner, H. J.
(2008). Microbiology of Composting.
35-100. MASS BALANCE

Sunar, M., Stentiford,
E., Stewart, D., & Fletcher, L. (2014). The Process and Pathogen Behavior in
Composting: A Review.  TEORIA

Institut für
Energietechnik, Technische Universität Hamburg. (2018) . Learn2compost.

Shilev, S.,
Naydenov, M., Vancheva, V., 
Aladjadjiyan, A. (2006). Composting of Food and Agricultural Wastes.
Utilization of By-Products and Treatment of Waste in the Food Industry.

Yoshida, T. (2017). Applied
Bioengineering: Innovations and Future Directions.

Tunnel composting systems.
(2018). Sutco RecyclingTechnik GmbH. Retrieved 21 January 2018, from https://www.sutco.de/en-de/plant-technology/tunnel-composting-systems/

Linde KCA – Waste management |
Christiaens Group. (2018). Christiaenswastemanagement.com.
Retrieved 21 January 2018, from http://www.christiaenswastemanagement.com/germany/linde-kca

Rada, E. (2015). Biological
treatment of solid waste.

Bilitewski, B.,Härdtle, G.,
Marek,K. (1997). Waste Management.

Christensen, T. (2011). Solid
waste technology & management. Chichester, West Sussex, U.K.: Wiley.

R. Robinzon, E. Kimmel, &
Y. Avnimelech. (2000). ENERGY AND MASS BALANCES OF WINDROW COMPOSTING SYSTEM. Transactions
Of The ASAE, 43(5), 1253-1259. http://dx.doi.org/10.13031/2013.3019

 

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