Introduction

Composting is the biological decomposition of the organic compounds of wastes under controlled aerobic conditions. In contrast to uncontrolled natural decomposition of organic compounds, the temperature in waste heaps can increase by self-heating to the ranges of mesophilic (25-40 °C) and thermophilic microorganisms (50-70 °C). The end product of composting is a biologically stable humus-like product for use as a soil conditioner, fertilizer, biofilter material, or fuel.

The objectives of composting can be stabilization, volume and mass reduction, drying, elimination of phytotoxic substances and undesired seeds and plant parts, and sanitation. Composting is also a method for decontamination of polluted soils. Almost any organic waste can be treated by this method. The pretreatment of organic waste by composting before landfilling can reduce the emissions of greenhouse gases.

In any event, composting of wastes is conducted with the objective of high economic effectiveness and has the goal of compost production with the lowest input of work and expenditure. The consequence of this approach is the effort to optimize the biological, technical, and organizational factors and elements that influence the composting process. The factors that influence the composting process are well known and have been published in numerous reviews and monographs. The period since 1970 has been characterized by the development of new strategies, composting processes, and technologies and the optimization of existing processes against the background of an expanding market for composting technology. Among others, reasons for these developments are rising costs for sanitary landfills, improved environmental protection requirements, as well as new laws, ordinances, and regulations. The realization that resources are limited and the idea of recycling refuse back to soil have also provided important impetuses for developments in this field.

Numerous publications and reviews on composting are published in specific journals such as Compost Science and Utilization, Bioresource Technology, and Biosystems Engineering. Some monographs give an overview of the field of composting [1-8].

334 | 13 Composting of Organic Waste 13.2

Waste Materials for Composting

The origins of organic waste for composting are households, industry, wastewater treatment plants, agriculture, horticulture, landscapes, and forestry (Table 13.1). The amount, composition, and physical characteristics of plant wastes are influenced by numerous factors such as the origin, production process, preparation, season, collecting system, social structure, and culture. The wide range in the amount and composition of waste requires analyses for planning a composting plant and for estimating the compost quality in each individual situation.

Table 13.1 Nutrient content per dry weight of some wastes for composting [3; own analyses].

Waste

VS"

C/N

N

P2Os

k2o

CaO

MgO

[%]

[-]

%%]

[%]

[%]

[%]

[%]

Kitchen waste

20-80

12-20

0.6-2.2

0.3-1.5

0.4-1.8

0.5-4.8

0.5-2.1

Biowaste

30-70

10-25

0.6-2.7

0.4-1.4

0.5-1.6

0.5-5.5

0.5-2.0

Garden and green waste

15-75

20-60

0.3-2.0

0.1-2.3

0.4-3.4

0.4-12

0.2-1.5

Garbage

25-50

30-40

0.8-1.1

0.6-0.8

0.5-0.6

4.4-5.6

0.8

Feces (human)

15-25

6-10

2

1.8

0.4

5.4

2.1

Wastewater sludge (raw)

20-70

15

4.5

2.3

0.5

2.7

0.6

Wastewater sludge

(anaerobic stabilized)

15-30

15

2.3

1.5

0.5

5.7

1.0

Dung

Cattle

20.3

20

0.6

0.4

0.7

0.6

0.2

Horses

25.4

25

0.7

0.3

0.8

0.4

0.2

Sheep

31.8

15-18

0.9

0.3

0.8

0.4

0.2

Pigs

18.0

15-20

0.8

0.9

0.5

0.8

0.2

Liquid manure

Cattle

10-16

8-13

3.2

1.7

3.9

1.8

0.6

Pigs

10-20

5-7

5.7

3.9

3.3

3.7

1.2

Chickens

10-15

5-7

9.8

8.3

4.8

17.3

1.7

Beet leaves

70

15

2.3

0.6

4.2

1.6

1.2

Straw

90

100

0.4

2.3

2.1

0.4

0.2

Bark, fresh

90-93

85-180

0.5-1.0

0.02-0.06

0.03-0.06

0.5-1

0.04-0.1

Bark mulch

60-85

100-130

0.2-0.6

0.1-0.2

0.3-1.5

0.4-1.3

0.1-0.2

Wood chips

65-85

400-500

0.1-0.4

1.0

0.3-0.5

0.5-1.0

0.1-0.15

Leaves

80

20-60

0.2-0.5

Reed

75

20-50

0.4

Peat

95-99

30-100

0.6

0.1

0.03

0.25

0.1

Paunch manure

8.5-17

15-18

1.4

0.6

0.9

2.0

0.6

Grape marc

81

50

1.5-2.5

1.0-1.7

3.4-5.3

1.4-2.4

0.2

Fruit marc

90-95

35

1.1

0.6

1.6

1.1

0.2

Tobacco

85-88

50

2.0-2.4

0.5-6.6

5.1-6.0

5.0

0.1-0.4

Paper

75

170-180

0.2-1.5

0.2-0.6

0.02-0.1

0.5-1.5

0.1-0.4

a VS Volatile solids.

a VS Volatile solids.

Table 13.2 Heavy metal content per dry weight of some wastes for composting [4].

Waste

[mg kg-']

[mg kg-']

[mg kg-']

[mg kg-']

[mg kg-']

[mg kg-']

[mg kg-']

Biowaste

50-470

8-81

0.1-

1

5-130

10-183

6-

59

0.01-0.8

Green waste

30-

138

5-31

0.2-

0.9

28-86

24-138

9-

27

0.1-3.5

Paper

93

60

0.2

4

20

1

0.08

Paper, printed

112

66

0.2

31

78

3

0.04

Paper, collected

40

21

0.2

3

12

1

0.06

Paper sludge

150-

1500

15-100

0.1-

1.5

30-300

70-90

5-

15

0.2-0.5

Bark

150-

300

40-60

0.6-

2.1

30-63

20-57

12-

20

0.1-0.5

Bark mulch

40-

500

10-30

0.1-

2

500-1000

50-100

30-

60

0.1-1

Wood chips

58-

137

8-11

0.1-

0.2

6-8

13-53

4

0.1

Grapes marc

60-

80

100-200

0.5

2.5-7.4

10

1-4

0.02-0.04

Fruit marc

20-

30

9.5

0.2

0.02-1

0.3-1

2-4

0.03

Brewers' grains

13

6

0.3

16

10

16

0.04

Oil seed residues

4

1

0.03

1-0.05

0.1

0.1-0.4

1-

3

0.01

Cacao hulls

89

7-12

0.25

0.5

0.4

0.

3

0.02

The content of heavy metals and organic compounds in the waste is of great importance, particularly with regard to the use of compost as soil conditioner and fertilizer (Table 13.2). Ways to reduce the heavy metal content in compost from bio-waste are to collect the waste separately and to obtain detailed information of the producers of the waste. The fact that the concentrations of heavy metals increase as the organic compounds are degraded also must be taken into consideration.

Analyses of biowaste show a content of impurities (e.g., plastics, glass, metals, rocks) between 0.5% and 5.0%, depending on the social structure, buildings, and public relations work in the area [2]. In densely built-up areas the contamination is higher than in others. More than 90% of the impurities have a size >60 mm, and 90% of the biowaste materials have a size <60 mm [1].

Depending on climatic and cultural conditions (e.g., growing and harvest times, holidays, traditions) some plant wastes, like leaves or branches, are not available during the whole year. An important factor for the operation of a composting plant can be the fluctuation in the composition of the wastes, in particular, the water content [5].

13.3

Fundamentals of Composting Process

Degradation of the organic compounds in waste during composting is initiated predominately by a very diverse community of microorganisms: bacteria, actinomyctes, and fungi [7, 9, 10, 18, 23, 24, 28, 33, 36]. Just as in biological wastewater treatment, an additional inoculum for the composting process is not generally necessary, because of the high number of microorganisms in the waste itself and their short gen eration time. Invertebrate animals play no role in the rotting process during the first phase at a high temperature level. Nevertheless, earthworms are sometimes used in waste management and to produce a high-value compost [1, 8, 20, 27]

Rotting waste material, even during well aerated composting, is characterized by aerobic and anaerobic microbial processes at the same time (Fig. 13.1) [13]. The relation between aerobic and anaerobic metabolism depends on the physical properties of the waste/compost [31], including the structure of the heap, its porosity, its water content and capacity, its free air space, and the availability of nutrients.

The aerobic microorganisms in the rotting material need free water and oxygen for their activity. End products of their metabolism are water, carbon dioxide, NH4 (or, at higher temperature and pH >7, NH3), nitrate, nitrite (nitrous oxide as a product of nitrification), heat, and humus or humus-like products. The waste air from the aerobic metabolism in compost heaps contains evaporated water, carbon dioxide, ammonia, and nitrous oxide. The end products of the anaerobic microorganisms are methane, carbon dioxide, hydrogen, hydrogen sulfide, ammonia, nitrous oxide, nitrogen gas (both from denitrification) and water as liquid [5, 12, 14-17, 19, 21, 25, 26, 30, 34]

Mature compost consists of components that are difficult to digest or undegrad-able components (lignin, lignocellulosics, minerals), humus, microorganisms, water, and mineral nitrogen compounds. The organisms that take part in the composting process are microorganisms (bacteria, actinomyces, mildews) in the first phase aerobic conversions

(rainwater)

anaerobic conversions oxygen aerobic conversions

(rainwater)

anaerobic conversions oxygen

Fig. 13.1 Substrates and products of microbial activity in a compost heap.

of composting. They each have optimal growing conditions at different temperatures: psychrophilics between 15 and 20 °C, mesophilics between 25 and 35 °C, and thermophilics between 55 and 65 °C. In mature compost at temperatures below 30-35 °C, other organisms such as protozoa, collembolans, mites, and earthworms join in the biodegradation.

A pile of organic wastes consists of solid, liquid, and gaseous phases, and the microorganisms depend on free water for their metabolism (Fig. 13.2). Dissolved oxygen, from the gas phase in the heap, must be available for the activity of aerobic microorganisms. To make sure that oxygen transfer from the gas phase to the liquid phase and carbon dioxide transfer from the liquid phase to the gas phase occur, a permanent partial-pressure gradient must be maintained, which is possible only by a permanent exchange of the gas phase by forced or natural aeration [2, 22]. Specifications about the optimal water content for composting are meaningful only in combination with the knowledge of the specific type of waste to be composted, its structure and volume of air pores (Table 13.3) [35]. In general, the water content can be higher when the waste structure, air pore volume, and water capacity are higher and more stable (also during the rotting process). Theoretically, the water content for composting can be 100%, provided the oxygen supply is sufficient for microbial activity.

Fig. 13.2 Metabolism of aerobic microorganisms at the gas/water interface.
Table 13.3 Optimal water content and structure of wastes for composting.

Waste

Water Content

Structure

Air Pore Volume

%]

Woodchips, cut trees, and brushwood

75-90

good

>70

Straw, hay, cut grass

75-85

good

>60

Paper

55-65

middle

<30

Kitchen waste

50-55

middle/bad

25-45

Sewage sludge

45-55

bad

20-40

In addition to a sufficient content of free water, the microorganisms need a C/N ratio in the substrate of 25-30 for optimal development and fast enough rotting process, and the carbon should be readily available. At C/N ratios below the optimum, the danger of nitrogen loss as ammonia gas increases (especially when the temperature rises and the pH is >7). If the C/N ratio is higher than optimum, the composting process needs a longer time to stabilize the waste material.

Figure 13.3 shows the relations between the factors influencing the rotting process. The structure of the waste, i.e., its consistency and the configuration and geometry of the solids, determines the pore volume (whether filled with water or air) and the air flow resistance of a compost heap. These in turn influence the gas exchange and the oxygen and carbon dioxide concentrations in the air pores and liquid phase. When these factors are optimum, exothermic microbial activity is rapid, leading to increasing temperature by build up of heat within the heap. Microbial activity is affected by the water content, nutrients (C/N ratio, availability), and pH. The mass and volume of the heap influence the temperature according to the heat capacity and heat losses by irradiation. Heat convection within the heap, which is conditioned by the temperature difference between the material and the atmosphere, affects the gas exchange. The gas exchange and the temperature influence the evaporation of water and thus also the proportion of water-filled pores [3, 4, 6, 11, 22, 29].

One effect of the activity of the different microbial groups is a characteristic temperature curve during composting (Fig. 13.4). After a short lag, the temperature increases exponentially to 70-75 °C. At 40 °C there is often a lag during the changeover from mesophilic to thermophilic microorganisms. After reaching a maximum, the temperature declines slowly to the level of the atmosphere. The progression of the temperature curve depends on numerous factors such as the kind and preparation structure Ï

water

free air space

nutrients. C/N

PH

mass, volume

An air flow resistance gas exchange >-<

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