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Environmental Biotechnology - Jordening and Winter

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336 13 Composting of Organic Waste

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 undegradable 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

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

13.3 Fundamentals of Composting Process 337

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

 

 

 

 

338 13 Composting of Organic Waste

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

Fig. 13.3 Factors influencing the composting process.

13.3 Fundamentals of Composting Process 339

Fig. 13.4 Characteristic temperature curve during composting process.

of the waste, the surface/volume ratio of the heap, air temperature, wind velocity, aeration rate, C/N ratio, processing technique, and mixing frequency.

The first phase of the composting process, up to a temperature up to about 60 °C, is called the preand main composting; the second phase is called the post-composting or mature phase. Both phases are characterized by different processes (Table 13.4).

Frequently, the designers of a composting facility must consider both phases by dividing the entire composting process into different technical stages, especially when the wastes have a risk of strong odor emissions:

Preand main composting occurs in closed reactors or in roofed facilities, and in frequently-mixed or forced-aerated windrows.

The post-composting/mature phase is done in windrows.

The consequences for the composting process are basically to optimize the factors that influence the rotting process. The most important factor is, for a given composition of waste, to ensure gas exchange in the heap. This can be done by taking the following measures:

adapting the height of the heap to the structure, water content, and oxygen demand (high during preand main composting, low during the mature phase)

turning (mixing, loosening) the windrows

constructing windrows in thin, ventilatable layers

Table 13.4 Phases and characteristics of the composting process.

Preand Main Composting

Postcomposting, Mature Phase

Degradation of easily degradable compounds: sugar, starch, pectin, protein

Degradation of difficult-to-decay degradable compounds: hemicellulose, wax, fat, oil, cellulose, lignin

Inactivation of pathogenic micro-

Composition of high molecular weight compounds

organisms and weed seeds

(humus)

High oxygen demand

Low oxygen demand

Emissions of odor and drainage water

Low emissions

Time: 1–6 weeks

Time: 3 weeks to 1 year

 

 

34013 Composting of Organic Waste

mixing and loosening the rotting material in reactors (in rotating drums, with tools)

using forced aeration

decreasing the streaming resistance by adding bulking material having a rough structure or in the form of pellets

13.4

Composting Technologies

The production of compost consists of preparing and conditioning the raw material, followed by the actual composting (Fig. 13.5). To produce a marketable product it is necessary to convert the compost to an end product. The aim of raw material preparation and conditioning is to optimize conditions for the following composting process, to remove impurities so as to protect the technical equipment, to reduce the input of heavy metals and hazardous organic components (if the impurities contain these components), and to meet quality requirements for the finished compost. The basic steps of raw material preparation and conditioning are:

disintegration of rough wastes (e.g., wood scraps, trees, brush, long grass) by chopping, crushing, or grinding to increase the surface area available for microbial activity

dehydration or (partial) drying of water-rich, structureless wastes (e.g., sludge, fruit remains) if they are too wet for the composting process

addition of water (fresh water, wastewater, sludge) if the wastes are too dry for the composting process

Fig. 13.5 Basic flow sheet of compost production.

13.4 Composting Technologies 341

mixing of components (e.g., wet and dry wastes, N-rich and C-rich wastes, wastes with rough and fine structure)

manual or automatic separation of impurities (glass, metals, plastics)

The products of preparation and conditioning of the wastes are waste air (depending on the composition and the conditions of storage, it may include bad smells and dust) and possibly drainage water beneath the raw material. The basic steps of the subsequent composting process may be:

aeration to exchange the respiration gases oxygen and carbon dioxide and to remove water (the only essential step during composting)

mixing to compensate for irregularities in the compost heap (e.g., dry zones at the surface, wet zones at the bottom, cool zones, hot zones) and to renew the structure for better aeration

moistening of dry material to improve microbial activity

drying of wet material by aeration or/and mixing to increase the free air pore space for microbial activity or to improve the structure of the compost for packaging

manual removal of impurities

The products of the composting process are a biologically stabilized compost, waste air, and drainage water (when the material is very wet). It may be necessary to prepare the compost for transport, storage, sale, and its application. When post-prepar- ation is needed, the basic steps can be:

sieving the compost to obtain different fractions for marketing or to remove impurities

manually or automatically removing impurities

drying wet compost to prevent formation of a clumpy, muddy product and drainage of water during storage

disintegrating clumps in the compost by crushing or grinding to prevent problems that may occur when the fertilizer is packaged

mixing the compost with additives (soil, mineral fertilizer) to produce potting mixes or gardening soils.

Disintegration (crushing, chopping, grinding), especially of bulky wastes containing wood pieces, is necessary to increase the surface area available for the microorganisms and to ensure the functioning of the machines and equipment used in subsequent stages of the process (e.g., turning machine or tools, screens, belt conveyor). The intensity of disintegration depends on the velocity of the biodegradation of the waste, the composting process, the dimensions of the heap, the composting time, and the intended application of the final product. For disintegration of organic wastes, chopping machines or various kinds of mills (cutting, cracking, hammer, screw) are mainly used [1].

The raw waste or compost is screened to separate particles with a required granule size. These particles can be the organic raw material for composting, the compost itself, or impurities. In practice, drumand plain-screens (with hole plates, wire

342 13 Composting of Organic Waste

grates, stars, or profile iron) are usually used. The size of the sieve holes depends on the subsequent use of the compost or on whether impurities are being removed (>80 mm: removal of impurities; 80 to 40 mm: production of mulching material; 10 to 25 mm, production of compost for landscaping, agriculture, and gardening [1].

13.5

Composting Systems

Composting systems can be classified into nonreactor systems and reactor or vessel systems (Fig. 13.6) [1, 2, 4–6]

13.5.1

Nonreactor Composting

Figure 13.7 shows the types of nonreactor composting systems.

Field composting: During field composting, which is the simplest way of composting organic wastes, all microbial activity takes place in a thin layer at the soil surface or within a few centimeters of the soil surface (arable land or grassland). This system is useful for treating both sludge and green wastes (grass, straw, brushwood). To ensure rapid and uniform decomposition, green wastes need to be chopped. Mulching machines can be used if the wastes are growing in the same area (e.g., vineyard prunings); otherwise, collected wastes are spread out with a manure spreader after chopping. Because the waste material surface exposed to the atmosphere is large,

Fig. 13.6 Classification of composting systems.

13.5 Composting Systems 343

Fig. 13.7 Classification of nonreactor composting systems.

self-heating does not occur, and therefore neither do thermal disinfecting or killing of weed seeds. Therefore, only wastes without problems of hygiene or weed seeds can be utilized in this kind of composting. In the narrower sense of the definition of composting, field composting is not composting, because there is no self-heating and no real process control.

Windrow composting: The main characteristic of nonreactor windrow composting is direct contact between the waste material and the atmosphere and, therefore, interdependence between the two. The composting process influences the atmos-

344 13 Composting of Organic Waste

phere by emitting odors, greenhouse gases, spores, germs, and dust. The atmosphere, which carries the respiration gas oxygen, can influence the composting process by

supplying rain water

advantage: adds water, which is needed if the material for composting is or has become too dry, thus resulting in more rapid biodegradation

disadvantages: blocks free airspace, favors anaerobic conditions and associated odor emissions, decreases compost quality, increases drainage water

changes in air temperature

advantages: high air temperatures can increase the evaporation rate of very wet wastes, increasing the amount of free air space; high temperatures can shorten the lag phase at the start of the process

disadvantages: high air temperatures can increase the evaporation rate, leading to insufficient moisture; low air temperatures can delay or inhibit self-heating

changes in air humidity

advantages: low air humidity can increase the evaporation rate of very wet wastes; high air humidity reduces the evaporation rate

disadvantages: low air humidity can increase the evaporation rate, leading to insufficient moisture; high air humidity can decrease the evaporation rate, leading to too much moisture

supplying wind

advantages and disadvantages: wind can intensify the effects of air temperature and humidity

The extent of contact between waste material and atmosphere can be influenced by covering the piles with mature compost material, straw, or special textile or fleece materials that allow gas exchange but reduce the infiltration of rain water. The cross section shape of a windrow compost pile can be triangular or trapezoidal. The height, width, and shape of a windrow depend on the waste material, climatic conditions, and the turning equipment.

Natural aeration in windrows can be supported by (1) addition of bulking material to the waste, (2) using bulking material as an aeration layer at the bottom of the windrow (20–30 cm), (3) aeration pipes from the bottom of the windrow, and (4) perforated floor (Fig. 13.8).

To ensure a high quality of the compost, windrows are disturbed from time to time by turning. The effects of turning are (1) mixing of the material for homogenization (dry or wet zones at the surface, wet zones at the bottom) and for killing pathogenic microorganisms and weed seeds, (2) renewing the structure and free airspace, and (3) increasing evaporation to dry the waste material or the mature compost. The turning frequency depends on the kind and structure of the waste and the quality requirements of the finished compost. It can vary from several times a day (at the start of the process when the oxygen demand is high or for drying mature compost) to once every several weeks.

Machines and equipment for turning include tractor mounted front-end loaders, wheel loader shovels, manure spreaders, tractor-driven windrow turning machines,

13.5 Composting Systems 345

Fig. 13.8 Ways to improve natural aeration in compost windrows.

and self-driven windrow turning machines (Fig. 13.9). The mixing quality of frontend loaders and wheel loaders is relatively poor and requires an experienced driver. Compression of the (wet) wastes by the weight of the machinery can be a disadvantage.

An example of a simple open-windrow composting plant with a tractor-driven turning machine is shown in Figures 13.10 and 13.11. It consists of a concrete or asphalt floor area with an open shed for storing the finished compost. All the drainage and rain water are collected in a tank or basin. Large pieces of waste (branches, trees) are chopped periodically by a machine that is driven from one place to another. After separating the impurities from the biowaste, windrows are formed from both components with a wheel loader. The windrows are turned frequently with a tractor-driv- en turning machine (or a self-driven machine). The finished compost is screened with a mobile screening device. The oversize fractions from the screening are