Composting Plant Open Windrow

concentration temperature, microbial _ activity

Fig. 13.3 Factors influencing the composting process.

temperature [°C]

temperature [°C]


postcomposting, mature phase

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 pre- and 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:

• Pre- and 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 pre- and 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.

Pre- and Main Composting

Postcomposting, Mature Phase

Degradation of easily degradable com

Degradation of difficult-to-decay degradable

pounds: sugar, starch, pectin, protein

compounds: hemicellulose, wax, fat, oil, cellulose,


Inactivation of pathogenic micro

Composition of high molecular weight compounds

organisms and weed seeds


High oxygen demand

Low oxygen demand

Emissions of odor and drainage water

Low emissions

Time: 1-6 weeks

Time: 3 weeks to 1 year

• 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


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 microbi-al 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

Windrow Aeration


- disintegration

- screening

- dehydration

- drying

- wetting

- mixing

- inpurity separation

- aeration

- mixing

- wetting

- drying

- impurity separation

- disintegration

- mixing

- screening

- impurity separation groundsteps

- disintegration

- screening

- dehydration

- drying

- wetting

- mixing

- inpurity separation

- aeration

- mixing

- wetting

- drying

- impurity separation

- disintegration

- mixing

- screening

- impurity separation

• alternative

Fig. 13.5 Basic flow sheet of compost production.

• 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-preparation 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, drum- and plain-screens (with hole plates, wire 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].


Composting Systems

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

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, composting systems

Organigram Mkb
Fig. 13.6 Classification of composting systems.
Dobsonian Telescope Mount Plans
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-

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 homogen-ization (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,

(?) bulking material

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

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

© bulking material as layer at the bottom airr

@ aeration tunnel or pipe

(4) perforated floor

airr 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-driven turning machine (or a self-driven machine). The finished compost is screened with a mobile screening device. The oversize fractions from the screening are

Fig. 13.9 Self-driven windrow turning machine (drawing: Backhus GmbH, Edewecht).
Composting Plant Open Windrow
Fig. 13.10 Open windrow composting plant with turning machine [3].

reused in the compost plant, and the finished compost is stored under a roof until it is sold.

Reactor Composting

Every method of composting in an enclosed space (e.g., container, box, bin, tunnel, shed) with forced exchange of respiration gases is a type of reactor composting. Composting reactors (Fig. 13.12) can be classified according to the manner of material flow as biowaste water greenwaste separation impurities

mobile shredder i

store i

windrows by wheel loader


turning machine

mobile screen


Fig. 13.11 Flow sheet of an open windrow composting plant.

impurities waste air compost leakage water

• horizontal-flow reactors having

- a static solids bed or

- an agitated solids bed

• vertical-flow reactors

• rotating-drum reactors

With few exceptions, all reactors have a means of controlled forced aeration and enable the waste air and the drainage water to be collected and treated. Addition of water or other additives is possible only when the waste material is mixed in the reactor. If possible, composting processes are used only for precomposting, because of the high costs of reactor composting compared to windrow composting.

Horizontal-flow reactors with static solids bed: This is a batch system in which the waste material is loaded with a wheel loader or transport devices into a horizontal reactor or is covered by a foil or textile material. The forced aeration, in positive or neg-

348 | 13 Composting of Organic Waste (l) Horizontal reactor with static solid bed

(3) Vertical reactor waste [

_l_iv leakage


Static Bed Reactor

waste air

(2) Horizontal reactor with agitated solid bed

(4) Rotating drum




Fig. 13.12 Reactor composting systems.

—C^ waste air air leakage


—fc waste air leakage water O compost

—E^ waste air leakage water compost

Fig. 13.12 Reactor composting systems.

ative mode or alternating, issues from pipes at the bottom of the material or from holes in the floor. The waste air, with its odor components and water, can be treated in a biofilter or biowasher. In some systems some of the waste air is recycled. The air flow rate can be controlled according to the temperature in the material or the oxygen/carbon dioxide concentration in the air. The retention time in the reactor is between several days (precomposting) and several weeks (mature compost). The end product can be inhomogeneous (partially too dry) and not biologically stabilized, because there is no turning/mixing and no water addition to the waste material and is forced aeration from only one direction.

Horizontal-flow reactors with agitated solids bed: In this reactor type the waste material can be turned mechanically and water can be added, in contrast to a horizontal-flow reactor with a static solids bed. The devices for mixing the wastes can be horizontally or vertically operating rotors or screws, scraper conveyors, or shovel wheels. Fully automated functioning of the whole process is possible.

Vertical-flow reactors: In this reactor type the waste material flows vertically, with or without stages, with mass flow from the top to the bottom as a plug flow system or, if the material from the outlet at the bottom is loaded back in at the top, as a mixed system. Forced aeration occurs from the bottom or from vertical pipes in the material. This process can also be run in a fully automated way.

Composting Factory Designs Drawings
Fig. 13.13 Composting plant with rotting drum in closed shed [3].

Rotating-drum reactor: The waste material is loaded into a horizontal slowly rotating drum with forced aeration. The filling capacity is approximately 50%. The material is transported (plug flow) in the helical pathway from one end of the drum to the other and is mixed intensively. Self-heating starts after a short time. Water addition is possible. Rotating-drum reactors can also be used as mixing equipment.

An example of a composting plant with a rotating-drum reactor is shown in Figures 13.13 and 13.14. This plant is characterized by an enclosed building for receiving the biowaste, preparing it for the rotting process (disintegration, separation of impurities), pre-rotting in a drum, and main-rotting in turned windrows (composting I, Figure 13.14). All highly contaminated waste air can be collected and treated with a biofilter. Only the maturing phase of the compost (composting II, Figure 13.14) occurs under natural climatic conditions in windrows under a roof. In a fully enclosed composting plant, even the maturing phase takes place in a closed shed.


Compost Quality

To be used as a fertilizer and soil conditioner, compost must meet certain quality requirements, such as (1) optimal maturity, (2) favorable contents of nutrients and organic matter, (3) favorable C/N ratio, (4) neutral or alkaline pH, (5) low contents of heavy metals and organic contaminants, (6) no components that interfere with plant growth, (7) mostly free from impurities, (8) mostly free from germinatable seeds and living plant parts, (9) low content of rocks, (10) typical smell of forest soil, and (11) dark brown to black.

'Maturity' and 'stability' are different properties of compost. Stability is defined in terms of the bioavailability of organic matter, which relates to the rate of decompo-

air biowaste water greenwaste closed building mill


> 80 mm


screen 80 mm screen 80 mm magnet mobile shredder store t rotting drum composting I in windows; turning equipment composting II in windows; turning equipment waste air biofilter


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