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Type of Waste to be treated
Source Separated Organic Waste
Source Separated Packaging
Mixed MSW
Min Capacity (in tonnes)
>5.000
>12.500
>50.000
ALL
Secondary Products or Energy Production
FE
AL
Plastic
Paper
Glass
Wood
RDF
SRF
Compost
Biogas
Max Residues (%)
0-10%
10-20%
20-30%
30-40%
Capital Expenditure € tn of total feedstock on wet basis
0-100
101-200
301-400
601-700
Annual operation and maintenance cost € tn of total feedstock on wet basis
21-40
41-60
61-80
81-100
Land requirement m2 per tn of total feedstock
0-0.2
0.21-0.4
0.41-0.6
0.61-0.8
0.81-1
Water consumption m3 per tn of total feedstock
0-0.05
0.051-0.1
0.11-0.15
Composting
1.  Introduction to composting process
2.  Biology of composting
3.  Factors affecting the composting process
4.  General classification of composting systens
5.  Post-processing
6.  Mass and energy balances
7.  Market potential for products
8.  Environmental impacts
9.  Economic data
10.Applicability
References

1. Introduction to composting process

Composting is defined as the aerobic, or oxygen requiring process during which the organic matter is decomposed by micro-organisms under controlled conditions to a biologically stable end product. During composting the microorganisms consume oxygen for the bio-oxidation of the organic matter resulting in the generation of heat, carbon dioxide and water vapor, which are released into the atmosphere (Ipek et al., 2002; Epstein, 1997). At the same time, the volume and mass of the organic raw material is reduced significantly transforming it into a stable organic final product which can be used as soil conditioner, improver as well as for land reclamation (Hogg et al., 2009; Epstein, 1997; Engeli et al., 1993; Carry et al., 1990; Toffey, 1990). The rate of the organic matter decomposition depends upon the evolution of the environmental conditions (e.g. temperature, moisture, oxygen) which regulate the growth of aerobic micro-organisms. Therefore, composting is the “controlled” aerobic biodegradation of most organic (biologically derived carbon-containing) solid matter meaning that the environmental conditions are controlled throughout the process. In that way composting differentiates from the decomposition which occurs in nature (Gidarakos, 2007). Nevertheless, the biochemical process in composting and in the natural decomposition of the organic matter is the same.

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2. Biology of composting

The composting process can generally be divided into two major stages. The first stage comprises of the ‘‘active phase’’ of the process which mainly involves the development of bio-oxidation reactions. Therefore, the readily available organic matter is used as energy source by microorganisms for their metabolic activities. The second phase of the composting process, known as ‘‘curing phase’’, involving the production of organic macromolecules humus-like substances for the formation of mature compost (Cooperband, 2000). All reactions are based on numerous biological, thermal and physicochemical phenomena and involve oxygen consumption, as well as heat, water and carbon dioxide production. A schematic representation of the composting process is shown in Figure 1.

 

In the process of composting, presented in Figure 1, microorganisms decompose the biodegradable organic fraction in order to produce carbon dioxide, water, heat, and humus the biological stable organic end product. Considering that the aerobic degradation is carried out under optimal conditions, than the composting process can be classified into through three separate phases (Figure 2) which may have considerable overlap based on temperature gradients and differential temperature effects on microorganisms. The first phase incorporates a mesophilic or moderate-temperature phase (<40-45oC), which has a duration of a couple of days. The second stage, named thermophilic phase (>40-45oC), involves the development of elevated-temperatures due to enhanced decomposition of the organic compounds which can last from a few days to several weeks depending on the availability of organic matter to be exposed to aerobic degradation. The last stage includes the cooling and maturation phase which results to the biological stabilisation of the end product. The factors affecting the composting process include: the physical and chemical properties of the raw material, the level of oxygen, the moisture content, the temperature and the time over which the composting process takes place.

micro_graph_7975

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3. Factors affecting the composting process

The main parameters which regulate the composting process include: the physical and chemical properties of the raw material, the level of oxygen, the moisture content, the temperature and the particle size of the substrate.

3.1. Feedstock and nutrient balance

In the composting process it is essential to determine the nutrient content of the feedstock, since aerobic micro-organisms responsible for the biodegradation of the organic raw material require various nutrients in order to grow. The main nutrients involved are the following in a decreasing order of importance: carbon (C), nitrogen (N), phosphorous (P) and potassium (K) (Gajalakshmi and Abbasi, 2008). Apart from micro-organisms growth, nitrogen, potassium and phosphorous are also primary nutrients for plants and their concentrations influence the quality of the composted organic material. Of the nutrient elements required for microbial decomposition, carbon and nitrogen (C:N ratio) are considered to be the most important, since the majority of organic materials contain ample quantities of nutrients (EA, 2001). Therefore, the amounts of carbon and/or nitrogen are the substances most likely to affect the composting process by their presence in insufficient or excessive quantities. Carbon is both the energy source and the basic microbial building block of microorganisms, whereas nitrogen is a crucial component of proteins, nucleic acids, amino acids, enzymes and co-enzymes needed for microbial cell growth (Gajalakshmi and Abbasi, 2008). Considering the above, the nutritional balance during composting is mainly de?ned by the carbon to nitrogen ration i.e. C/N ratio. According to literature the optimum C/N ratio for composting is in the range 25–35 (Gaur, 2000; Golueke, 1992), because it is considered that the microorganisms require 30 parts of C per unit of N (Alexander 1977; Bishop and Godfrey, 1983). High C/N ratios make the process very slow as there is an excess of carbonated degradable substrate for the microorganisms (Bernal et al., 1998; Verdonck, 1988). On the other hand, in low C/N ratios there is an excess of N in the organic material which in turn provides excess of inorganic N due to the nitrification process leading to ammonia losses through volatilisation or nitrogen losses by leaching from the composting mass (Pagans et al., 2005; Sanchez-Monedero et al., 2001; Reddy et al., 1979). Controlling the C/N ratio, when mixing different organic materials, is crucial for the successful implementation of the composting process. In general, "green" organic materials (e.g., grass clippings, food scraps, manure), contain large amounts of nitrogen, whereas "brown" organic materials (e.g. dry leaves, wood chips, branches), contain large amounts of carbon but little nitrogen. Therefore a proper balance of "green" and "brown" organic waste shall result in appropriate C/N ratios for the initiation of the composting process. Indicative C/N ratios of various organic waste are presented in Table 1.

Table 1: Indicative C/N ratios of various organic waste streams

organic waste C/N
Diaz et al. (2002) Diaz and Savage (2007) Trautmann and Krasny (1998) IWMI (2003)
Cornstalks 60-73 - -
Fruit waste 20-49 - 20-50
Rice hulls 113-1120 - -
Vegetable waste 11-13 - 10-20
Poultry litter (broiler) 12-15 15 -
Cattle manure 11-30 18 -
Horse manure 22-50 25 20-50
Garbage (food waste) 14-16 - 15
Paper (from domestic refuse) 127-178 - 100-200
Newspapers - - 400-900
Refuse 34-80 - -
Sewage sludge 5-16 11 -
Primary sludge - - -
Activated sludge - 6 -
Grass clippings 9-25 - 10-25
Leaves 40-80 - 40-80
Shrub trimmings 53 -
Tree trimmings
Bark
- hardwood
- softwood
16

-
-
- -

100-400
100-1200
170-500
Wood chips or shavings
- hardwood
- softwood

-
-

-
-

450-800
200-1300
Sawdust 200/750 200-500 200-750

 

3.2. Particle size

The particle size of the substrate and more specifically the surface area of the organic material exposed to microorganisms is another factor which affects the rate of the composting process. The raw material which is grinded, chipped and/or shredded acquire reduced particle size or otherwise increased surface area on which the microorganism can feed thus waste is degraded more rapidly (EA, 2001). In addition, smaller particles also produce a more homogeneous mixture and improve substrate insulation which promotes the maintenance of optimum temperatures (O’Leary and Walsh, 1995). However, if the particles are too small, they might prevent air diffusion through the substrate (Gidarakos, 2007). According to Diaz et al. (2002), an average particle size of 10mm to 50mm generally produces the best results. However, certain composting methods that do not include a turning process require a more robust physical structure to resist settling (e.g. due to gravity and biodegradation process), so larger particles are necessary (greater than 50mm) (Diaz and Savage, 2007). GTZ (2000) recommends chopping all materials to be composted to the length of about 50-100mm, whereas Obeng and Wright (1987) reported that typical particle sizes should be approximately 10mm for forced aeration composting and 50mm for passive aeration and windrow composting.

3.3. Moisture content

Moisture supports the metabolic and biodegradation processes of the micro-organisms, since water is the medium for biochemical reactions, transportation of nutrients and allows the microorganisms to move about (Gajalakshmi and Abbasi, 2008). Generally, the ideal moisture content is considered to be between 40% and 65% for the optimal biodegradation of the raw material. However, the optimal moisture level is depended upon the composted material and more specifically on its porosity (Diaz and Savage, 2007). Organic mix with a low porosity requires higher moisture content than a substrate with a higher porosity level (Diaz and Savage, 2007). Moisture content which is lower or higher than the optimum range results in the inhibition of the microbial activity due to early dehydration and the formation of anaerobic conditions respectively (Gajalakshmi and Abbasi, 2008; de Bertoldi et al., 1983). When the moisture content exceeds 70%, O2 movement is inhibited and the process tends to become anaerobic because the air spaces of the substrate are filled with water obstructing the sufficient oxygen diffusion within the organic mass (Tiquia, et al., 2002, 1996). On the other hand, if the moisture content is lower than required, the microorganisms’ growth and subsequently the decomposition rate of organic matter are significantly reduced creating a final product that is physical but not biologically stabilized (Diaz and Savage, 2007a; de Bertoldi et al., 1983).

3.4. Oxygen flow

The oxygen that is required for the composting process is essential for the aerobic metabolism and respiration by the microorganisms, but also for the bio-oxidation of the organic molecules present in the substrate. Oxygen consumption during composting is directly proportional to the microbial activity providing a direct relationship between oxygen consumption, temperature, moisture and aeration (EA, 2001). Therefore, aeration is a key factor for composting, since proper aeration controls the temperature, removes excess moisture and CO2 and provides O2 for the biological processes. According to Miller, (1992) the optimum O2 concentration is between 15% and 20%. If there is insufficient oxygen, the process can become anaerobic involving a different set of micro-organisms and different biochemical reactions which result in the production of methane gas and malodorous compounds, such as hydrogen sulfide gas and ammonia. Aeration of the organic substrate is achieved through agitation, active aeration (air blowing) and/or passive aeration (natural diffusion of air though negative pressure) (IWMI, 2003).

3.5. Temperature

While high composting temperatures may inhibit or slow down the biodegradation of organic waste, elevated temperatures are desirable for the destruction of pathogens and weed seeds that may be contained in the substrate. According to Hogg et al. (2002) the key parameter for the sanitization of the substrate is the temperature-time regime. Indicative relationships between temperature and time duration during composting for the sanitization of the final product is given in Table 2.
Microorganisms require a certain temperature range for optimal activity. de Bertoldi et al. (1983) state that the optimum temperature range for the maximization of the decomposition rate is 40–65°C. According to Epstein (1997) and Miller (1992), thermophilic microorganisms become less active at elevated temperatures between 60-70°C and thus the microbial activity is reduced. At even higher levels (>70°C) Mena et al. (2003), Fermor et al. (1989) and Finstein et al. (1986) indicate that the microorganisms suffer the effects of high temperatures (inactivation or elimination) and the process slows down. At these temperatures many micro-organisms die or become dormant and the process effectively stops until the micro-organisms can recover.Temperature is one of the main control parameters of the composting process and constitutes a by-product of the microbial activity during organic matter biodegradation. The importance of temperature monitoring lies on the fact that it reflects the activity of microorganisms in the substrate and it represents an indicator of the proper evolution and occurrence of the composting process (Diaz and Savage, 2007a). According to Hassen et al. (2001) substrate temperature determines the rate at which biological processes take place and plays an important role in the evolution and succession of the micro-organisms population.

Table 2: Indicative relationships between temperature and time duration during composting for the sanitization of the final product

Country Composting System Temperature - time regime
Canada
(CCME, 2005)
Windrows ?55°C for 15 days
Static piles ?55°C for 3 days
In-vessel ?55°C for 3 days
USA
(USEPA, 2003)
Α Class compost B Class compost
Windrows ?55°C for 15 days ?40°C for 5 days. For 4 hours during the 5 day period, the temperature must exceed 55°C
Static piles ?55°C for 3 days ?40°C for 5 days. For 4 hours during the 5 day period, the temperature must exceed 55°C
In-vessel ?55°C for 3 days ?40°C for 5 days. For 4 hours during the 5 day period, the temperature must exceed 55°C
UK
(DoE, 1996)
Windrows or Aerated piled The compost must be maintained at 40°C for at least 5 days and for 4 hours during this period at a minimum of 55°C within the body of the pile followed by a period of maturation adequate to ensure that the compost reaction process is substantially complete

 

 

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4. Composting Systems Classification

The currently available compost systems can be generally classified into two broad categories the “windrow” and the “in-vessel” composting systems. The main feature of windrow technology is the accumulation and formation of the organic substrate into piles. Typically, the piles are usually shaped into more or less elongated windrows with specified width and height. With respect to the in-vessel composting systems, the aerobic decomposition of the organic matter takes place in a bioreactor. It should be noted that many of the current in-vessel systems involve the parallel use of windrow systems for the curing and maturation phase of the end product (Dziejowski and Kazanowska, 2002).

4.1. Windrow Systems

Windrow systems are further subdivided on the basis of the aeration method of the substrate into “turned windrow” and “forced air windrow or static pile”. The windrows may or may not be sheltered from the elements. In the windrow composting process, the mixture to be composted is stacked in long parallel rows or windrows. The cross section of the windrows is usually trapezoidal or triangular, mainly depending on the characteristics of the equipment used for the agitation or aeration of the piles. A variety of factors combine to determine the dimensions of the area requirement. Among them are total volume of material to be accommodated during all stages of the compost process, i.e., from the construction of the windrows through disposal of the stored product, the configuration of the windrows, space required for the associated materials handling equipment and the maneuvering thereof, and the aeration system (forced or turning).

4.1.1 Forced air windrow – Aerated static system

In a forced air windrow or aerated static composting system, air is either forced upwards through the composting mass or is pulled downwards and through it (Shammas and Wang, 2009). In both instances, the composting mass is not disturbed. The forced aeration composting systems usually involves a combination of drawing air into and through the pile, followed by air forcing upward through the pile. The air that leaves the substrate is either discharged directly into the environment, or is forced through a cone-shaped biofilter (e.g. finished compost or other “stable” organic matter). The use of biofilter serves as a mean for the deodorization of the effluent air stream. According to Bidlingmaier (1996) and Schlegelmilch et al. (2005) finished compost and other organic materials can effectively serve as an odor filter. The basic arrangements of an aerated static pile are shown in Figures 3 and 4. The system includes the following six steps:

1. the mixing of the raw material with of a bulking agent

2. the construction of the windrow (width and height),

3. the decomposition process of the substrate (composting),

4. the screening of the end product and the removal of the bulking agent,

5. the curing phase and

6. the storage of the final product.

The construction of the windrow involves the longitude and parallel placement of a series of perforated longitudinally orientated air pipes (e.g. 10.2–15.2 cm diameter) along each compost pile (Diaz and Savage, 2007).

Figure 3: Schematic diagram of aerated static pile composting (Diaz et al., 2002)

In order to avoid short circuiting of air during air suction or forced aeration, the perforated pipes must be placed in an appropriate distance from the edges of the windrow. According to Diaz and Savage (2007) this distance is between 1.5–2.7m. The pipes are connected to a blower through a length of non-perforated pipe. The piping network is covered with a layer of bulking agent or finished compost that extends over the area of the pile in which the raw material will be composted. The formation of a bulking agent layer is used to facilitate the movement and uniform distribution of air throughout the organic mass during composting. In addition, the formed layer enables the absorption of excess moisture resulting from the composted raw material and thereby minimizing leachate runoff. The organic waste is stacked on the network piping and to bulking agent layer in order to form the windrow pile, as shown in Figure 3. According to the specifications provide by Diaz and Savage (2007), the finished pile should be about 20–30m long, about 3–6m wide, and about 1.5–2.5m high. Additionally, on top of the formulated pile a layer of matured compost or synthetic materials (about 15-20 cm thick) is placed in order to absorb emitted odors from the composting mass and to ensure the homogeneous distribution of temperature throughout the organic matter. The aforementioned arrangement aims at achieving the desired temperature level to optimize the decomposition rate of the organic material and to obtain the required temperature-time regime for the sanitization of the organic mass throughout the pile. Leachate control is provided by sloped and sealed or impervious composting pads with a surrounding drainage system (Diaz and Savage, 2007).

4.1.2. Extended Aerated Pile

In case where large amounts of organic material are to be composted, the extended aerated pile method can be adopted. The extended aerated pile has the following arrangement: On the first day, a pile is constructed in the same way as described in for the conventional aerated static pile, with the exception that only one side and the two ends of the pile are covered with the a finished compost layer. The exposed side of the pile is lightly covered with compost in order to prevent the escape of odors. On the second day, a second piping network and bulking layer is laid parallel to the exposed side of the pile erected on the first day, and the pile is erected in the same manner as was the first pile. This procedure is repeated for 28 days. The first pile is removed after 21 days; the second pile on the day after, and so on. An important advantage of this approach is a substantial reduction in spatial requirements. The land area requirement for systems that use a single pile is about 7–11 tonnes d.w. of organic waste treated per hectare. The estimate of about 7 tonne/ha allows for sufficient land area to accommodate leachate collection, administration, and storage (e.g. raw material, end product).

4.1.3. Economics

The aerated static pile method is probably the least expensive method of all of the various types of composting technologies that are currently available. Aerated static pile method is a low technology which requires minimum capital investment in terms of equipment (the amount of material handling is limited) as well as in the operation and maintenance cost (Shammas and Wang, 2009). It is difficult to present a generally applicable capital cost for static pile composting technology since the treatment process and the developed compost markets are usually site specific. With respect to material and operational costs, Diaz et al. (2007) state that the cost for composting a mixture of sludge and woodchips is about $50 per tonne (2005 US dollars), of which about $10 per tonne is for woodchips whereas according to Mavropoulos et al. (2008) the windrow composting cost of green waste ranges between 20 to 35 € per tonne.

4.1.4. Limitations

The aerated static pile method is not the most suitable for all types of raw materials and under all conditions. Since aerated static pile method does not acquire a mechanism for the agitation of the substrate during the composting process, the material used requires having relatively uniform particle size not exceeding 3.5–5 cm in any dimension (Diaz and Savage, 2007). Granular materials such as sludge are the most appropriate. In case of organic mixtures of large particle size that exhibit a wide spectrum of dimensions the composting process shall probably result in uneven distribution and movement of air through the pile. This uneven distribution of air through the pile promotes air short-circuiting and the development of anaerobic pockets of decomposing material.

4.2. Turned Windrow System

The turned windrow method is the one that traditionally and conventionally has been associated with composting. The term “turned” applies to the method used for aeration. Aeration of the windrow is achieved by agitation of the substrate using tractors with front end loaders or any other appropriate machinery which tears down the piles and reconstructs them. Turning not only promotes aeration, but it also ensures uniformity of decomposition by exposing at one time or another all of the composting material to the particularly active interior zone of a pile. In addition, the mechanical agitation of the substrate reduces to some extend the particle size of the organic material, whereas water loss due to evaporation (elevated temperatures) is accelerated (Cornell University, 2010). The water loss can be considered as a benefit of the composting method in cases where the moisture content of the substrate is too high. However, in low moisture content the turning of the substrate can be potentially disadvantageous and water addition is required during the turning process.

4.2.1. Construction of Piles

In operations in which the turning is carried out mechanically, the pile configuration that results will obviously be the one imparted by the machine. Ideally, the windrow should be about 1.5–2.0m high (Diaz et al., 2002). In situations in which it is practical to perform the turning manually, the height should be roughly that of the average laborer. At most, it should not be higher than that easily reached with the normal pitch of the equipment used in turning. Another factor that impacts the maximum height is the tendency of stacked material to compact. The height for mechanical turning depends on the design of the turning equipment — generally, it is between 1.5 and 3.0 m (Diaz et al., 2002). The pile’s width is a function of convenience and expediency, since it has little effect to the diffusion of oxygen into a pile and, therefore, it does not contribute significantly to meeting the oxygen requires within the composting mass. With manual turning, a width of about 2.4-2.7m is considered to be suitable, whereas the width of the pile with mechanical turning depends upon the design of the mechanical equipment (usually 3.0 to 4.0 m) (Diaz et al., 2002). In theory, the length of the windrow is indeterminate. For example, the length of a 180 tonne conical shape windrow of material at a height of 1.8m and width of 2.5m would be about 46.0 m. A nearly continuous system can be set up by successively adding each day’s input of raw waste to one end of the windrow. The continuous windrow system is employed by adding fresh material to one end of the pile and removing material from the other as it reaches stability.


As in the case of static pile method, forced windrow piles are constructed by stacking the prepared feedstock in the form of an elongated pile. The cross section of the windrows is trapezoidal or triangular depending on the conditions of the area in which the system is cited (e.g. geared to climatic conditions and efficient use of the composting working area.). According to Diaz and Savage (2007) in dry and windy areas the piles usually acquire a trapezoidal shape because the ratio of exposed surface area to volume is lower with such a configuration. In addition, the volume of the overall hot zone is greater in a trapezoidal shaped pile in relation to a triangular or conical cross section, since heat loss is less and windrow volume per unit pad area is greatest. On the other hand, during wet weather the flattened top is a disadvantage because water is absorbed into the composting mass changing the moisture conditions within the organic mass. Although the climate condition might influence windrow geometry, in practice, the determinant is mainly the turning equipment (e.g. manual, mechanical type).

4.2.2. Arrangement of the Windrows

The specific arrangement of the windrows at a composting facility depends upon the availability of land and the accessibility of the equipment used. Whatever the arrangement, the windrows should be positioned such that each day’s input can be followed until the material is completely composted. An important requirement is the space that is required to perform the turning of each day’s input material, whether the turning is done manually or mechanically. With manual turning, the total area requirement is at least two times that of the original windrow depending upon the type of machinery used, since turning varies with type of machine (Diaz and Savage, 2007). It is worth mentioning that some machines accomplish turning in such a manner that as the original windrow is torn down, the new windrow is reconstructed directly behind the machine. This type of machine requires little more area than that of the original windrow, whereas other types of machines rebuild the windrow adjacent to its original position the area requirement of which is comparable to that described for manual turning. According to Diaz et al. (2002) a considerable degree of advancement has been made in the design of mechanical turning machines. Emphasis has been placed on the comfort of the operator, on the size of the windrow, and on the overall space requirements. Indicative types of turners are the auger turner, the elevating face conveyor, and the rotary drum with flails.

4.2.3. Methods of Turning

With respect to manual turning of the piles, the most common and convenient equipment is the pitchfork with four or five tines. In the manual turning of the pile, when reconstructing building the pile, material from the outside layers of the original windrow should be placed in the interior of the rebuilt windrow. In this way during the compost cycle every particle of the material is at one time or another in the active interior zone of the pile. If this ideal situation is not practical to attain, the deficiency can be compensated by increasing the frequency of turning. Finally, it is important not to compact the raw material when constructing the original windrow, and when rebuilding the pile.

4.2.4. Frequency of Turning

The turning frequency of the pile is strongly related to the rate of oxygen uptake by the active microbial population. Practically, there is a compromise between required turning frequency and technical and economic feasibility of meeting that the required frequency. Structural strength and moisture content of the substrate are the most important physical characteristics when it comes to the determination of the turning frequency. Other parameters involved with the effectiveness of the turning procedure are the pathogen elimination and the uniformity of substrate decomposition. Another variable factor is the decomposition duration desired by the operator. High-rate composting requires very frequent turning, since the rate of degradation is directly proportional to the turning frequency. The lower the moisture content of the organic matter and the firmer the structure of the particles, the less frequent will be the required turning. For instance, when straw, rice hulls, dry grass, dry leaves, woodchips, or sawdust are used as bulking material and the moisture content of the mixture is about 60% or less, turning on the third day after constructing the original pile and every other day thereafter, for a total of about four turnings, is sufficient to accomplish “high rate” composting. After the fourth turning, the frequency can be reduced to once each 4 or 5 days. The same program is applicable if paper is the bulking material, provided that the moisture content does not exceed 50% (Golueke and McGauhey, 1953). If the composting mass gives off fouling odors, it means that the composting process has become anoxic, which in most cases it is due to the presence of excessive moisture. To overcome the anoxic condition additional turning, at least once each day, is required to foster evaporation until odors are disappeared.

4.2.5. Equipment Used for Turning

There are several types of windrow turners available on the market with higher capacity than rototillers which are quite satisfactory for relatively small composting operations (Diaz and Savage, 2007). Windrow turners can be generally categorized into three main groups divided according to the design of the turner mechanism. The three groups include the auger turner, the elevating face conveyor, and the rotary drum with flails. Some types of turners are designed to be towed and others are self-propelled (Figure 5). The self-propelled types are more expensive than the towed types. An advantage of the towed type is the fact that the tractor can be used for other purposes between turnings. In addition, the self-propelled type requires much less space for maneuvering and, therefore, reducing the required area, since windrows can be closer to each other. The turning capacity of the machines ranges from a few tonnes per hour to as much as 3,000 tonnes/h depending on model, whereas the costs (2005 US dollars) of the self-powered machines range from about $200,000 to 300,000 (Diaz et al., 2002).

Figure 5: Examples of a self-propelled and a towed windrow turners (Diaz et al., 2002; Recycle & Composting Equipment Pty Ltd, 2010)

4.2.6. Site Preparation

The composting piles should be placed on a hard paved surface. The pad should be sufficiently rugged to support the combined weight of the composting mass and associated materials handling equipment, as well as the maneuvering of the machines. The main reasons for the paving are: (1) to facilitate materials handling, (2) to control any leachate that may be formed, and (3) to prevent fly larvae from escaping the area. In summary, preservation of sanitation and materials handling are the two key factors. In operations processing less than about 10 tonnes/day, the paving may consist simply of well-compacted clay as a base with a layer of packed gravel or crushed stone on the surface. In the event that crushed stone and gravel are not available, a layer of soil can be used. The soil should be firmly packed on top of the clay. Of course, when soil is used as the top layer, a problem arises during the rainy season. Paving is especially essential if mechanical turners are utilized. The machines are fairly heavy and, accordingly, can operate properly only on a firm footing. Paving materials in addition to gravel and crushed stone are asphalt and concrete. Special provisions should be made for collecting the leachate that might be generated. The fresh leachate has an extremely objectionable odor and unless controlled, it can lead to the development of problems. In desert regions, the windrows should be protected from the wind so as to reduce moisture loss through evaporation. In regions of moderate to heavy rainfall, the windrows should be sheltered from the rain. If shelters are not available, the possibility of the windrows taking in an excessive amount of moisture would be particularly high.

4.2.7. Economics

The cost of windrow composting depends on factors such as the type and particle size distribution of the organic feedstock (whether preprocessing is required for size reduction), the contamination level of the raw material, the requirements for permits, the labor costs and the use and value of the produced compost. Generally windrow composting facilities are relatively low-cost processes.  If the composted raw material is yard waste, then the cost of the process using turned windrow technology is in the range of $15–30 per input tonne inclusive of capital and operation and maintenance expenses (Diaz et al., 2007). According to recent data provided by ARCADIS & EUNOMIA (2010) the windrow composting costs ranges marginally higher than 20 to 40 €/tonne (net of compost sales) depending upon the specifications of the composting facilities. Additionally the operating costs is estimated at 6.5 €/tonne whereas the annual maintenance costs is considered at 3.15 €/tonne.

4.2.8. Limitations

Improper and/or insufficient turning might lead to the generation of fouling odors. Even with a suitable protocol, some odors are certain to be generated. This situation is typical for all composting system that involves handling and processing of organic waste, regardless the method employed (e.g. static, turned windrow, or mechanized composting). The generation of objectionable odors is mainly occurring, in nuisance proportions, during the preparation and the active process of composting. Therefore, appropriate preventing measures can be taken only during that time. A slower then optimum rate of organic matter decomposition and the subsequent larger land requirement often have been alleged against the use of the turned windrow as contrasted to the “highspeed” composting which is claimed for mechanical composting. With respect to the composting rate, it should be emphasized that enhanced composting is achieved only when high-priced land area and costly machinery usage are involved. If machinery means are not involved and land area is not critical, rapid composting loses its advantage. Furthermore, under those conditions, the intensity and frequency of turning can be reduced. The reason is that very little odor is emitted from a pile of composting material that is not disturbed. It is mainly during the turning process that foul odors, if present, are released from the pile. However, it must be emphasized that this relaxation in terms of turning frequency of the pile is safe only when no human habitations are nearby (i.e. distance >150m).The major limitation of turned windrow systems probably is related to public health issues. The limitations are particularly applicable to operations that involve the processing of sewage sludge or animal waste which incorporate pathogenic microorganisms. This limitation stems from two basic features when operating turned windrows. The first feature is related to the fact that elevated temperatures, which favor pathogens elimination, do not generally prevail throughout a windrow, since in its outer layers the temperatures are lower than in the active interior zone of a pile. The second feature involves the recontamination of the sanitized material when turning the pile. In case when outer layers of the pile do not acquire the desired temperature level there is a risk of pathogen exposure of sanitized organic material (Bustamante et al., 2008). However, repeated turning eventually reduces the pathogen populations to concentrations that are less than infective. This latter condition is reached by the time the material is ready for final processing and use.

4.3. In-Vessel Systems

The retention time, during which the active phase of composting takes place within the bioreactors, generally lasts from 7 to 15 days and largely depends upon the type of substrate used. Since the detention time is rather short, upon completion of the rapid degradation phase, the material that exits the bioreactor most of the times is placed in windrows for further maturation. A brief description of each type of bioreactor is presented to the following paragraphs.
In general, bioreactors can be divided into two main types (1) vertical and (2) horizontal (Haug, 1993). Horizontal bioreactors are further categorized into (1) channels, (2) cells (3) containers (4) tunnels and (5) “inclined” reactors or rotating drums (Crowe et al., 2002). In-vessel bioreactors can also be classified as a function of the movement of the material. Consequently, the reactors can be denoted as static and dynamic.
In-vessel composting occurs within a contained vessel, enabling the operator to maintain closer control over the process in comparison with other composting methods. The in-vessel systems are designed to minimize odors (e.g. biofilter) and process time by controlling environmental conditions such as air ?ow, temperature, and oxygen concentration. In this section the term “in-vessel” or “reactor” is applied to the unit or set of units in which the “active” stage of composting takes place. These units are also called bioreactors, since composting essentially is a biological process. There are several in-vessel systems on the market. The growth in new designs is partly related to the regulatory requirements enacted by some European member states and by the EU. The primary objective of the in-vessel design is to provide the best environmental conditions, particularly aeration, temperature, and moisture. Nearly all in-vessel systems use forced aeration in combination with stirring, tumbling, or both.

4.3.1. Vertical Reactors

Generally in vertical in-vessel bioreactors the organic material is introduced through the top of the system and removed from the bottom of the unit as shown in Figure 6. The end product is usually is discharged out the bottom of the bioreactor by a horizontally rotating screw auger. As such, the bioreactors acquire the configurations to operate in a continuous basis. Air is introduced in these systems by forced aeration either from the bottom, by means of aeration pipes, traveling up through the composting mass where it is collected for treatment or through air lances hanging from the top of the bioreactor. The emitted gas is removed from the reactors is transported to a gas treatment system. Typically, vertical in-vessel bioreactors involve some type of cylindrical container or tank and they are manufactured from steel and concrete, whereas they are thermally insulated. The capacity of these systems ranges from a few cubic meters to more than 1500m3 (Diaz et al., 2007). It must be stated that most vertical bioreactors are used for composting solid waste and sewage sludge and they have been plagued by a number of operational difficulties (Diaz et al., 2002).

4.3.2. Horizontal Reactors

Horizontal reactors are units that, as their classifications suggest, operate in the horizontal position. Horizontal reactors can be further classified into: channels, cells, containers, tunnels and rotating drums.

Channels or Trenches

These designs are similar to windrow composting facilities, since the organic material is placed in piles. The main difference between the channels and windrows is that in channel composting, the material to be treated is placed between walls, whereas in most cases the facility is housed inside a building. The walls vary in height from 1-3m with a distance of approximately 6m apart from each wall, whereas the piles are about 50m long. The air is introduced within the organic mass through forced aeration or air suction, while at the same time the substrate is agitated with mechanical turning. In order to manage the potentially negative impacts of the emissions from the composting mass, the processing building is kept under negative pressure. The intake emitted gas is removed from the building to a biofilter (Misra et al., 2003) or other air pollution control device (e.g. bioscrubbers) (Shammas and Wang, 2009). Generally, the raw material is loaded into the trenches by means of a conveyor belt or with automated units that use Archimedean screws or front-end loaders, whereas in similar manner the end product is unloaded. Channels can be operated either on a batch basis or on a continuous basis. On a batch basis, the incoming material is loaded into the channel as soon as the first phase of the composting process is finished and treated material has been removed. In channels which operate in a continuous mode, the incoming material is loaded on a daily basis. Channels operating on a continuous basis can be further classified into longitudinal and lateral channels according to the movement direction of the organic material treated.

Longitudinal Channels: In this type of channel the substrate is gradually moved from one end to another by the turning machine. Therefore, the processing time is related to the design of the turner which in turn specifies the movement rate of the composting mass. Indicatively, during the intensive phase of decomposition (the first phase of the process), the processing time is about 4 weeks (Diaz et al., 2007). Longitudinal channels incorporate different channel shapes the most common of which are the straight, elliptical and U-shaped.

Lateral Movement Channels: In this type of channel the substrate is transferred by the turning machine laterally to the next row. The loading operations are usually performed through the use of conveyors. Loading is carried out approximately every 2–3 days, depending upon the volumetric capacity of the channel. Most of the designs include forced aeration and rely on the use of conveyors to remove the composted material and transport it to the maturation area (Diaz et al., 2007).

Cells

Cells, or biocells, are hermetically enclosed units, generally rectangular in shape, in which the composting takes place. In that way the environmental conditions of the composting process can be fully controlled and optimized, whereas the outside surfaces are thermally insulated to minimize thermal losses during composting. Biocells operate on batch basis and they can be built onsite or can be prefabricated. In a typical operational sequence, the substrate is introduced into the cell by means of front-end loaders or conveyors. Once the unit has been filled, the biocell is closed and the composting process begins. Typically, the period of intensive composting lasts for approximately 14 days depending on the type of treated material. Air supply is provided to the organic matter by means of a forced aeration system (pipes or channels) through the bottom layer of the cell forcing the air to move upwards through the organic mass. The gas emitted during the bio-oxidation phase is removed at the top of the biocell and usually directed to a biofilter or partially recirculated. Some biocell systems incorporate a heat exchanger to pre-heat the air prior to introduction into the composting mass. Water addition to the substrate is performed by means of a hydration system (nozzles and pipes) which is typically installed at the top of the biocell, whereas the generated leachate (excess moisture) is collected and recirculated to regulate the moisture content. Furthermore, some biocell models incorporate screw conveyors and moving floors aiming to agitate the substrate, while being in the container. After the end of the intensive composting process, the organic material is removed from the biocell with a front-end loader. The biocells capacity ranges between 100 and 1000m3, whereas typical dimensions are: 6m wide, 4m high and more than 50m long. The height of the material inside the container must be carefully chosen in order to limit substrate’s compaction and enhance proper air diffusion throughout the composting mass.

Containers

Typically the top of the container is opened or removed and the raw material is loaded into the container by means of a conveyor belt or a front-end loader. Containers usually are rectangular in shape, with volumetric capacities ranging from 20 to 40m3. Air is supplied to the substrate via force aeration from the bottom of the system (e.g. nozzles), while the exhaust gas is removed from the container and directed to an air control system (i.e biofilter). The containers are usually equipped with a hydration system for the moisture content control of the substrate, whereas generated leachate is gravitationally removed through perforated pipes located at the bottom of the system. The processing time is about 8–15 days, at the end of which the organic material is discharged from an opening located at the one end of the container by means of a roll-off collection truck. The containers are usually installed in parallel system/modules with which a nearly continuous system can be set up. Each module includes approximately 6 to 8 containers with a total capacity which ranges from 3,000 to 5,000 tonnes/year of organic matter.

Tunnels

Tunnels or biotunnels are insulated, rectangular shaped structures which are made out of metal, concrete or brick and their typical measurements are: 4–5m wide, 3-4m high, and up to 30m long. The tunnels operate on a continuous basis and the substrate is introduced at the one end of the tunnel on a daily basis. The organic mass is forced forward toward the opposite end of the tunnel by means of a hydraulic piston or through the reciprocating motion of moving floors. Moisture and oxygen levels are monitored at all times, whereas water and air are introduced whenever deemed necessary. Aeration is provided to the organic mass via compressors which deliver the air through the floor while in some tunnel systems air is provided by the use of centrifugal fans in order to reduce the noise level that is generated. Pipes placed on the roof of the unit remove the discharge air through negative pressure. Some of the units recirculate the process air through reversing aeration systems which can reach up to 80%. The entire process is automated and controlled by means of a computer. The retention time of the organic material is approximately 14 days whereas the overall treatment process lasts about 2 weeks.

Inclined Bioreactor or Rotating Drum

Another type of in-vessel composting system is the inclined bioreactor or rotating drum consisting of a rotating cylinder. The cylinder is built at a slight inclination so that the substrate is moved from one end to another. The rotating drums usually incorporate internal vanes which, combined with the rotating action of the drum, contribute to the size reduction as well as to the agitation of the organic matter. This type of reactor normally is used for the active phase of composting and by carefully controlling the oxygen and moisture contents and maintaining at optimum or near-optimum levels, the composting process can be accelerated. The cylinder is approximately 45m long and 2–4m in diameter, whereas the rotational speed is about 0.2–2 rpm (Diaz et al., 2002). Under normal operating conditions, the bioreactor is filled to about two-thirds, while the retention time of the substrate is about 1 week. After the active phase of composting within the rotating drum, the organic material is cured for a few weeks in windrows. (Diaz et al., 2002).

4.3.4. Economic Limitations

As has been mentioned in-vessel composting systems reserve the bioreactor for the active stage of the composting process and rely upon windrow systems for the curing and maturation phase of the organic matter. The rationale of these systems is to maintain conditions at optimum levels during the active stage of the process and thus accelerating the microbial activity rate and consequently shortening the active phase. The economic gain of in-vessel systems in comparison to windrow composting is the reduction of residence time and the increase of its processing capacity as well as the better quality of the end product, since the conditions during the process are usually optimized and controlled at all times. On the contrary the capital and O&M costs of in-vessel composting systems are significantly than those of windrow systems. According to Diaz et al. (2007), in the early 1970s, capital costs for compost plants in the USA were of the order of $15,000–20,000 per tonne of daily capacity. The operational costs were about $10–15 per tonne processed. In 2005 the costs range from about $25,000 to about $80,000 per tonne of daily capacity weeks (Diaz et al., 2007). According to recent data the financial cost of in-vessel biowaste treatment in EU member states ranges from 30 to 41 € per tonne treated. The unit cost includes annualized capital, O&M expenditure as well as other specific costs or revenues (e.g. revenues from sale of the energy) without considering taxes or any form of subsidization of the end products (EC, 2010).

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5. Post-Processing

Post-processing practices involve the various stages that are employed to in order to refine the produced compost and to meet market and regulatory standards. Among the main processing units that are included in compost post-processing practices are: size reduction, screening, air classification, and de-stoning. It must be stated that the post processing techniques achieve adequate separation in cases where the moisture content of the orgaic end product is lower than 30%.

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6. Mass and energy balances

A typical mass balance diagram of the composting process is shown in Figure 8. According to the figure, for 100 tn of processing MSW, 95tn is assumed to be the input for the composting process after the mechanical pretreatment. The degradation process of the organic matter causes the loss of 63tn DS and water, thus producing a final end compost which accounts to 30tn and 2 tn of residues.


In composting, energy is generated during the biodegradable organic solids oxidation in the form of heat release (enthalpy of reaction). Also, energy enters the composting system through the ambient air (sensible and latent heat), and exits the system through the flue gas (sensible and latent heat and the heat of reaction associated to the presence reduced gaseous species, namely ammonia (NH3) and hydrogen sulphide (H2S)). The enthalpy accumulated (EAc) in the composting materials along the processing time can be evaluated through the following equation (Neves et al., 2007):

where NI is input rate of energy associated to the inflow rate of ambient air:

NE is the output rate of energy associated to the composting flue gas rate:

with , and

is the molar saturation ratio of water vapor at the atmospheric pressure p1 (= 1,013?105 Pa); pvs is the water saturation vapor pressure calculated from the Clausius–Clapeyron equation at the actual temperature of the composting materials (TR), considering that flue gas is moisture saturated.

NG is the rate of energy released by the biological oxidation of the biodegradable materials,

and NU is the rate energy transfer between the composting materials at temperature TR and the environment, at temperature TS (considered constant and equal to 298 K),

where AR is the estimated superficial area of each load of waste (?0,24 m2) and U is the global heat transfer coefficient. It was considered that heat is transferred though conduction, based in a compost thermal conductivity of 0,1 W?m-1?K-1.

The temperature of the composting materials can be calculated at any moment by:

(Neves, D.S.F, 2007)

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7. Market potential for products

As mentioned above the end product of the composting process is the compost. Compost has the potential to be used as a soil amendment in various applications. Compost can substantially improve the fertility, texture, aeration, nutrient content and water retention capacity of the soil. Due to its beneficial characteristics, compost has a variety of potential applications and can be used by several market segments. Some of the markets include:

agriculture (small- and large-scale);

landscaping;

gardening (residential, community);

nurseries;

top dressing (e.g., golf courses, parks, median strips);

land reclamation or rehabilitation (landfills, surface mines, and others); and

erosion control

The markets or uses listed in the previous paragraph are constrained by: (1) the characteristics of the compost, (2) the limitations applicable to its use, and (3) pertinent laws and regulations (Alexander, 2000; Harrison et al., 2003).

Results of marketing studies and surveys conducted in several countries have demonstrated that some of the most critical elements in the use and marketability of the compost products are: (1) quality, and (2) consistency. The quality of a specific type of compost is a function of its chemical, biological, and physical characteristics. Assuming that a composting process is properly carried out, the quality of the finished product is determined by: (1) the composition and characteristics of the input material used in the production of the compost, and (2) the type and thoroughness of the process used to remove impurities. Some of the physical characteristics that are normally desired for a particular compost product are color, uniform particle size, earthy odor, absence of contaminants, adequate moisture, concentration of nutrients, and amount of organic matter (Eggerth et al., 1989). The size of a particular market for compost depends, to a large extent, on the quality of the compost and on the types of uses for the material. Composts from different types of substrates (e.g., yard waste, source-separated MSW) have different characteristics and consequently have different potential markets (Franklin Associates et al., 1990).

7.1. Limitations

The use of compost and therefore the limitation of its use on land is depended on  the potential adverse effects on human health and safety, animal & livestock health and safety, crop production and the quality of the air, water, and land resources. The significance of the aforementioned limitation is related to whom or what is affected and the extent to which they are affected. The limitations associated with the use of the compost with respect to the health and safety of humans are related to the harmful substances that may be present in the product. Among the main substances that are being regulated in orde to prevent potential adverse effects include the pathogenic organisms, heavy metals, persistent organic pollutants (POPs) and level of contaminants (e.g. plastic, glass). The level of their appearance is mainly associated of to the feedstock material to the composting process. Although the output from processing mixed MSW is not considered compost in several countries that are members of the EU, this practice is still being conducted and considered in other countries.

EU initiatives with respect to the heavy metals maximum consentration in compost are also seen in Table 3. The second draft EU Working Document on the Biological Treatment of Biowaste lays down heavy metal limit values for two classes of compost both towards the high quality end product, while a third class of material, stabilised biowastes, which is still considered as ‘waste’. In addition according to the commission decision 2001/688/EC (EC Eco-label) on “establishing ecological criteria for the award of the Community eco-label to soil improvers and growing media” environmental performance in regard to the heavy metal concentration of soil improvers  and growing media  have been set. For the utilization of compost as fertilizer or soil conditioner within organic farming (Eco-agric), specific compost quality standards for heavy metal concentration are also provided. Within the Eco-agric (EC 2092/91 - EC 1488/97) only composted source separated household waste containing only vegetable and animal waste is accepted. Limitations on the use of the finished compost are related to the possible adverse effects on: (1) human health and safety; (2) animal (livestock) health and safety; (3) crop production; and (4) the quality of the air, water, and land resources. The importance of each limitation depends upon whom or what is affected and the extent to which they are affected. The limitations associated with the use of the compost with respect to the health and safety of humans are related to the harmful substances that may be present in the product. Examples of such substances are pathogenic organisms, heavy metals, persistent organic pollutants (e.g. PCBs) and impurities (e.g. plastic, glass). The waste used as feedstock for the compost process is the source of harmful substances that may appear in the product. Although the output from processing mixed MSW is not considered compost in several countries that are members of the EU, this practice is still being conducted and considered in other countries.

3: Heavy metal limits for European compost standards (mg/kg dm)

Policy measures

Cd

Crtot

CrVI

Cu

Hg

Ni

Pb

Zn

As

EC

Draft W.D. Biological Treatment of Biowaste (class 1)

0.7

100

 

100

0.5

50

100

200

 

Draft W.D. Biological Treatment of Biowaste (class 2)

1.5

150

 

150

1

75

150

400

 

Stabilised Biowaste**

5

600


600

5

150

500

1500


EC/'eco-label'

2001/688/EC

1

100

 

100

1

50

100

300

10

EC/'eco-agric'

2092/91 EC-1488/97 EC

0.7

70

0

70

0.4

25

45

200

 

Figure 9 gives a comparative survey on heavy metal limit and guide values for composts in European countries expressed as relative mean limits as compared to the maximum concentration of the EC Eco-Label for soil improver (= 100 %).

[mean percentage relative to threshold values of the EC Ecolabel for soil improver]. Countries with more than one compost category or quality class referring to PTE thresholds are indicated with ‘I / II /III’]

There are thousands of chemically synthesised compounds that are used in products and materials commonly used in our everyday life. Many of them are potential contaminants of biowaste, although, due to their low concentration or easiness to be broken down by micro-organisms, as to the buffering capacity of soils, they do not cause a threat to the environment. However, there are some organic compounds that are not easily broken down during waste treatment and tend to accumulate and be the source of concern due to their eco-toxicity, the eco-toxicity of the products resulting from their degradation or to their potential for bio-accumulation. There are usually three main reasons why an organic compound may be subject to preventive action:

(a) the break down by soil micro-organisms of the compound concerned is slow (from some months to many years) and therefore there is an actual risk of build-up in the soil;

(b) the organic compound can bio-accumulate in animals and therefore it poses a serious threat to man;

(c) the degradation products of the organic compound are more toxic than the initial compound.

Therefore, there is likelihood a very high number of organic contaminants to be found in compost made from collected and treated biodegradable organic waste. Each year, the use of new compounds increases by a few thousand. Some of these compounds break down or undergo a transformation during the composting operations, while others remain stable. The presence of organic contaminants in compost used on soils could represent a potential risk to the environment and to the quality of crops intended for human or animal consumption.

Limits for organic contaminants were proposed only within the second draft of the Working Document on biological treatment of biowaste, concerning the polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) only (Table 4), and their concentration was set to be in consistence with the Sewage Sludge Directive (86/278/EEC). In general, organic contaminants are expected to be at low levels in composts derived form source separated materials and, therefore, in most European countries there are no set limit values for organic contaminant in composts.

Table 4: Organic pollutants standards for compost and stabilised biowaste

Parameter

(mg/kg dm)

Compost Class 1

Compost Class 2

Stabilised Biowaste

PCBs (mg/kg dm)

-

-

0.4

PAHs (mg.kg dm)

-

-

3

Threshold values for these organic pollutants to be set in consistence with the Sewage Sludge Directive.

Impurities or any inert non organic contraries may be found in composts from biodegradable municipal waste. The better the performance of separate collection from households or small enterprises the higher the purity. When developing an industry standard for compost quality, the presence of foreign matter in compost should be taken into consideration, since it has a negative impact on consumers and on the composting industry in general. The consumers look for compost free of visible foreign matter or otherwise harmful foreign matter. Table 5 presents the classification of compost according to the level of impurities in compost as has been laid down by the second working document on Biological Treatment of Biowaste in EU.

Table 5: Impurities standards for compost and stabilised biowaste

Parameter

Compost Class 1

Compost Class 2

Stabilised Biowaste

Impurities >2mm

<0.5%

<0.5%

<3%

Gravel and stones >5mm

<5%

<5%

-

From the very beginning of the implementation of compost standards hygienic aspects have been addressed in order to “guarantee a safe product” and to prevent the spreading of human, animal and plant diseases. Provisions for the exclusion of potential pathogenic microorganisms within process and quality requirements are established on two levels:

• direct methods by setting minimum requirements for pathogenic indicator organisms in the final product

• indirect methods by documentation and recording of the process showing compliance with required process parameters (HACCP concepts, temperature regime, black and white zone separation, hygienisation/sanitisation in closed reactors etc.).

Table 6 shows the requirements of the Decision of EC ECO-label.

Table 6: EU requirements on pathogens/weeds in compost based on the EC eco-label

Pathogens /Weeds

Approval of technology (AT)

Salmonella sp.

absent in 25 g

E. coli

< 1000 MPN (most probable number)/g

Helminth Ova

absent in 1.5 g

weeds/propagules

 

germinated plants: ? 2 plants /l

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8. Environmental impacts

Significant environmental benefits can be obtained by the use of compost as a soil conditioner, a fertilizer, or a growth medium. Those benefits are related to the nutrients recycling back to the soil which enables the reduction of synthetic fertilizers. When it is used as daily cover at landfills, it replaces other materials that would otherwise be used for that purpose.

However, there are also negative impacts on the environment associated with the production and utilization of compost. These impacts depend both on the operational composting process and to a large extent to the waste composition of the input organic streams. Mixed MSW and sewage sludge composting pose greater risks because these materials typically contain higher concentrations of heavy metals, POPs and pathogenic microorganisms than do source separated organic waste.

Negative impacts of composting on the environment can also be caused from gases that are released due to the improper operation and maintainance of the compost piles. More specifically, in cases when composting piles are not properly operated in that they do not deliver the required oxygen within the organic mass, anaerobic bacteria are being developed which result in the production of methane.. The release of methane significantly contributes to the problem of greenhouse gases in the atmosphere. Additionally, poorly operated composting facilities are always associated with unpleasant odors and the creation of nuisance. Other air emissions are generated by the combustion engines used to power windrow turning machines and grinders.

During composting leachate production is another matter of concern especially in open composting systems. Leachates are produced from water runoff and condensation at the compost facilities due to the increased moisture content of the feedstock material as well as due to the need for water addition to the organic mass in order to maintain the moisture content at an acceptable level during the biodegradation process. Leachates occasionally acquire increased levels of biological oxygen demand (BOD) that may exceed the acceptable discharge limits. Leachates runoff to surface water can reduce significantly the amount of dissolved oxygen in the aquatic ecosystem resulting in eutrophic conditions. Sound practice here is to avoid discharge to water and to capture or direct all leachate to absorption in sand or soil.

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9. Economic data

The capital costs of the in-vessel composting varies significantly according to the scale of the facility, the input material that is being treated, characteristics of the exhaust gases that are being treated and the retention time of the organic matter.

According to the World Bank, the capital cost for the development of an in vessel composting system is approximatetely of the order of 240€ per tonne of capacity (35-55 million € for a capacity of 500 tonnes/day) while the operation and maintenance cost is 20-40 €/tonne. According to recent data ASCARDIS & EUNOMIA (2010) typical capital costs are of the order of 190€ per tonne of capacity and suggest operating and maintenance cost of 12.5 and 10.25€ per tonne respectively.

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10 Applicability

Composting is considered of significant applicability for the case of Romania and Bulgaria, since both countries are characterized by very high content of biodegradable waste within municipal waste, as confirmed by the waste analysis in both countries in the framework of the implementation of the BALKWASTE project. It should be noted that the separate collection of organic waste constitutes a necessary precondition for the production of good quality compost that can find useful uses and relevant market. Otherwise, it can be expected that the produced compost will not have the required properties for being used, there will be a lot of skepticism about its use and, in fact, its relevant market value could be zero.

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