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Introduction

Composting has been practised by people more or less deliberately and more or less intensively for centuries. Since the middle of the 20th century, composting of waste materials has been employed in a noticeably more targeted manner and methods have been optimized with regard to speed and product quality.

Since the 1970s large-scale composting plants have been built, with Austriaand the Federal Republic of Germany assuming a leading role. For this reason, many terms and also quality parameters, standards, laws and analysis methods have been developed in these countries. Therefore reference is often made to these national documents by way of example.

In the first plants for composting waste, an organic-rich fraction was obtained from mixed household waste. Owing to the high level of pollutants in the product obtained in this manner, separate collection of biological waste was introduced in many places, and the corresponding treatment facilities also created. In this manner a purer, versatile product can be obtained. The spectrum of plants ranges from small-scale plants with processing capacities of 300 t/year to large-scale plants of over 100,000 t/a.

Closely linked with the development of closed plants with higher capacities is the development of adequate exhaust air capture and treatment systems, which are partly introduced in this section.

In a further step, treatment facilities were created for the arising residual waste in plants for pre-sorting and treating the biological fraction, what are known as MBT plants. This takes a great deal of pressure off landfill sites, both in terms of quantity and reactivity (gas formation).

Today rotting plants are usually characterized according to the following criteria:

  • Industrial – agricultural
  • Small capacity (< 1,000 t/year) – large capacity (> 300,000 t/year)
  • Open – closed
  • According to input material: green waste – water treatment sludge – biological waste – digestate – organic-rich fraction from residual waste

Taking this situation as a starting

g point, I will attempt to give an overview of the biological, chemical and thermodynamic principles as well as the methods used, with the emphasis being on the technically more "interesting"

intensive rotting methods.

composting plant overview

Definitions

Composting is the umbrella term under which all the steps are collected which are necessary for producing compost. These include the delivery and preparation of the materials as well as rotting, a maturation process which is present anyway, and packaging the product.

The term composting plant is usually used for plants for treating biogenic wastes, that is, green waste and biological waste. Plants for mechanical and biological treatment of wastes generally contain a composting process as a biological treatment step, but are almost exclusively called MBT plants. The aerobic biological treatment of soil-like materials is likewise closely related to composting, but is generally referred to as bioremediation.

Compost is a soil improver which arises during rotting of organic wastes and with which nutrients and organic substances are fed back into the natural cycle. Compost consists to a large extent of "cell mass", that is, living and dead microorganisms. Compost is rich in humic substances and has a high water retention capacity.

Simply put: Compost = microorganisms in spades.

Rotting is the process by which compost is produced under aerobic conditions by the activity of microorganisms and unicellular organisms, occasionally with the help of worms and others.

Simply put: The little creatures eat the rubbish and make soil out of it.

The terms intensive rotting or main rotting are customary for the first rotting stage in which mainly easily degradable substances are decomposed at great speed and consequently with high levels of oxygen consumption and heat production. This takes 10 - 40 days, depending on the method.

Maturation is the second rotting stage, which usually proceeds in the mesophilic range. Decomposition and conversion processes take place. There is no clear boundary with the intensive rotting and so there is no exact definition.

The terms compost bed and compost heaps have become established for the piles of rotting material.

Reject in connection with composting refers to the coarse fraction arising when screening the product. This has a lot of structure, is less reactive and interspersed with aerobic microorganisms, and is usually fed back into the system.

Aerobic describes a process or state which is characterised by the presence of oxygen – in sufficient quantities. Consequently, the term aerobic decomposition includes microbial metabolic processes which only take place in the presence of oxygen (dissolved in water). In contrast to this, processes or states which are characterised by the absence of oxygen – in sufficient quantities – are referred to as anaerobic.

Humic substances are macromolecular organic compounds, usually dark in colour, which are formed postmortally from rotting material of various organisms (plants, animals and microorganisms). As particles of small size, they have a large specific surface area and can reversibly store water and other molecules and ions, sometimes also heavy metals as a complex. Humic acids are humic substances which form acids in solution. Humic substances make an essential contribution to the particular properties of compost and are an important measure of quality.

Actinomycetes are Gram-positive bacteria which tend to form branched filaments and colonies with a similar appearance to fungal colonies. Actinomycetes are an essential part of soil microflora, and some of them cause the typical "musty" smell of soil.

Fresh mass (FM). The total mass of a fresh sample. Consists of DM and water. Given in % by mass or as a mass fraction.

DM: dry matter, also called dry residue. The residue remaining after drying to constant weight, given as a mass fraction or per cent by mass.

Loss on ignition LOI, also referred to as ODM (= organic dry matter). The mass fraction of a dry sample which is lost on ignition. In materials of biological origin, this corresponds to the fraction of organic, that is, non-mineral material.

C/N ratio: Element ratio between carbon and nitrogen.

Principles

The basic principle of composting can be explained as follows: Organic material is decomposed by microorganisms (bacteria and fungi) and cell mass is formed from it. The process can proceed as far as complete mineralisation. This process takes many months.

For industrial composting however only the first part of the process is relevant, as it is during this process that most of the biomass is converted into CO2, water and microbial biomass. This process, which takes many weeks in nature, is accelerated by technical measures such as aeration and irrigation. In industrial composting a distinction is drawn between 2 stages, namely main or intensive rotting and maturation. Typical rotting duration in industrial composting plants is 4 to 8 weeks. Of this usually 2 to 3 weeks are considered intensive rotting and the rest as maturation.
In accordance with the activity of the material, the temperature of the rotting material is low at the start, then rises rapidly owing to rapid decomposition of easily available substances and decreases gradually over the course of the rotting. In the event of complete rotting as far as substantial mineralisation of the material, ambient temperature is reached at the end.

A large part of the available organic matter is decomposed or converted. Woody fractions are considered unavailable. The mass loss is approx. 50% based on the loss on ignition or approx. 70% if the water loss is included.

During composting bacteria and actinomycetes, including thermophilic species, carry out a large part of the conversion processes. They belong to the decomposers of many organic compounds, for example cellulose, lignin and chitin.

Correspondingly, the most important prerequisites for compostability of organic materials can be summarised as follows.

Prerequisites for compostability:

  • Substances must be biodegradable and free of inhibitors or toxins
  • C/N ratio between 10 and 50
  • Sufficient structure and suitable water content for aeration
  • Favourable C/N ratio: 15-35

Microbiology

The biological processes in industrial composting are characterised by the activity of different microorganisms. During intensive rotting bacteria and thermophilic fungi predominate, whereas during the later rotting and maturation phases the activity shifts increasingly towards actinomycetes, fungi and later also unicellular organisms.

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M+E balance

An essential factor for the design of industrial composting plants is the realistic prediction of the mass and energy balance of the system. This task is characterised in most cases by uncertainties and variations in the composition and quantity of the waste. With regard to the large quantities of material and the great inhomogeneity of the material, many large amounts of material must be analysed in order to obtain accurate data.

Added to this is the fact that it is generally not allowed for waste to be temporarily stored, that is, it must be processed every working day, for hygiene reasons and in order to avoid odour emissions. This means that all systems, mechanical as well as biological, must be designed for the peak load in each case.

Uncertainties usually exist about the water content of the waste. Analyses by material class are very rarely available and are insupportable for plant operation for organisational and cost reasons. A widespread approach for describing the processes proceeds from a pseudo-reaction equation:

Old biomass (DM) + O2 -> CO2 + H2O + new biomass (DM)

Although neither the processes on the microbiological level nor on the material level are described in detail with this equation, the plant balance can be described with sufficient accuracy in this manner. Furthermore, m~∆DM~ = mOld biomass (DM) – mNew biomass (DM)

How much biomass is decomposed to CO2 and H2O and how much biomass is converted into new biomass depends greatly on plant procedure. The more complete the decomposition, the closer the value of the heat effect of the pseudo-reaction comes to the calorific value of the biomass used. Anaerobic decomposition processes have a likewise great effect on the apparent heat effect.

Correspondingly, the specific heat effect also varies in relation to the decomposed dry matter. In the technical literature values vary between 8 and 20 MJ/kg ∆DM. Values between 12 and 16 MJ/kg ∆DM have often been verified in large-scale plants, depending on the rotting phase and the type of plant.

A further important parameter for the design of plants is the air or oxygen requirement. Air has 2 essential tasks: Firstly, the oxygen supply to the microorganisms is to be ensured, and secondly air serves to remove excess heat from the rotting material. The air requirement for aeration can be derived from the stoichiometric oxygen requirement. This is approx. 1.1 g O2 per g ∆DM.

Input substances

Industrial composting is used to treat a variety of starting substances. The most important are listed below.

Main areas of use for composting:

  • Green waste / prunings
  • Biological and food waste
  • Water treatment sludge
  • Organic fraction of household and residual waste
  • Residue from biogas plants

Green waste

Green waste generally means various plant remains. The spectrum ranges from prunings with a relatively high wood fraction and strong structure to straw, hay and foliage with very low porosity. It is very important for builders and operators of composting plants to know the distribution of the fractions over the course of the year.

A wide C/N ratio, a low water content and a low salt content is typical at least for the woodier fractions.

Green waste can be reused for composting in pure form or as a mixture together with biological waste or other materials with poor structure. Green waste is often equated with structuring material, which only however applies to some of the materials.

Biological waste

Biological waste means separately collected, solid, organic waste from households or household-like organisations. It contains a wide variety of compostable substances from households, such as meal leftovers, food preparation leftovers, but also garden waste to a small extent. Generally a higher or lower content of impurities must also be reckoned with. Impurities include all substances which are undesirable for the composting process, regardless of their origin or quality.

The term biological waste is likewise used for restaurant waste. This differs from biological waste from households in particular by its on average higher water and salt content and also the lack of garden waste. The impurity content is usually comparatively low.

Included in biological waste is however waste from food-producing factories and from food retail: expired food, non-saleable rejected batches, production leftovers etc. with often high quality and purity.

Pure biological waste is characterised by high water content and a narrow C/N ratio. (C/N < 15:1)

Water treatment sludge

Water treatment sludge, that is, the excess biomass from aerobic waste water treatment plants, consists almost exclusively of microorganisms and contains the nutrients nitrogen and phosphorus in relatively large concentrations as well as numerous other substances which act as plant nutrients. It is therefore natural to want to use this material which occurs in large amounts as fertiliser.

Dehydrated water treatment sludge can be added to a composting process, as a result of which for example nitrogen is better incorporated, the water retention capacity is increased and other properties which are typical of compost are obtained.

Water treatment sludge can be dehydrated in a purely mechanical manner, that is, without adding lime, to DM contents of 25 to 40% and thus requires the addition of further substances with a lower water content, a wider C/N ratio and higher porosity in order to be efficiently compostable.

Residues from digestion plants

As in water treatment sludge, a large percentage of the dehydrated residue from methane digestion plants consists of bacterial biomass. This anaerobic sludge differs markedly in some respects from water treatment sludge, for example pH value and dehydration capacity. Furthermore, it contains unconverted input substances. A characteristic feature is a high ammonia content, depending on input substances and reactor temperature.

Residual waste

The composition of residual waste can vary greatly locally, depending on the habitation pattern and local living conditions, but in particular on the collecting logistics. Seasonal variations are also considerable.

The fraction of biologically convertible material depends particularly on the availability and the quality of a collecting system for biological waste.

Residual waste these days consists to a large extent of plastics, as well as paper and cardboard, but also contains metals, sand, ashes and glass as well as other biologically inert substances.

A sorting and classification step usually takes place which is targeted at obtaining reusable residual waste fractions, while the rest is to be made biologically inert so that the criteria of the Landfill Ordinance are met and it may be landfilled permanently.

Some important properties of input substances are summarised in the following table:


Water content [%] C/N ratio Structure / specif. weight
Prunings 40 ... 55 > 50 large / 100 ... 300 kg/m³
Biological waste 50 ... 70 15 ... 25 small / 500 ... 800 kg/m³
Restaurant waste 55 ... 70 15 ... 20 very small / 700 ... 1,000 kg/m³
Water treatment sludge 60 ... 75 approx. 10 small / 600 ... 800 kg/m³
Residues from digestion 60 ... 75 5 ... 10 small / 600 ... 800 kg/m³

Plants in Austria

In Austria, plants for composting biological waste, water treatment sludge and digestion residues are operated, as well as mechanical and biological waste treatment plants (MBT plants). MBT plants mean plants for mechanically preparing waste combined with biological treatment of the residual fraction which is to be sent to landfill.

Current development of mechanical and biological waste treatment from domestic and commercial waste in Austria indicates its increasing importance. In 2006 there were 16 MBT plants in operation, one being built and two in planning, with which Austria has made the step in the direction of increased decentralised waste pre-treatment before being sent to landfill. In the process mechanical and biological waste treatment has established itself as an alternative and complementary pre-treatment method to thermal treatment.

The rapid development in Austria necessitates standardisation of the operation of mechanical and biological waste treatment plants which goes beyond the Landfill Ordinance in order in particular to restrict and control emissions into the environmental media. A first step was taken towards this in Austria in 2002 with the publication of regulations for the mechanical and biological treatment of waste (MBT Directive, BMLFUW [Austrian Federal Ministry for Land and Forestry, Environment and Water Management] 2002). This directive, also notified to the European Commission, specifies a uniform state of the art and is intended to be used to orientate of all those involved (in particular planners and plant promoters) as well as to support the authorities in processes to authorise MBT plants.

In order to be able to assess further requirements in the direction of binding regulations on the state of the art for mechanical and biological pre-treatment of waste, the Austrian Federal Environmental Agency, in co-operation with the Austrian Federal Ministry for Land and Forestry, Environment and Water Management, surveyed and described the current status of MBT plants in Austria. The following statements and conclusions can be drawn from the work:

Plant capacities:

The maximum capacity of all 16 MBT plants increased considerably in the time under consideration 2003-2005 from 441,350 to 669,350 tonnes per year.

The utilisation of the MBT plants operated in 2005 was approximately 91%, with it being possible to count on a further increase in utilisation owing to the increasing full-load operation of MBT plants which have only recently started operating.

Use of compost

The possibilities for using compost are varied and range from for example the use of fresh compost in agriculture to special products such as vermicompost in households. The fertilisation effect, due in particular to the nitrogen contained, always plays a role in addition to other aspects.

Main properties of compost

The most important properties of compost can be summarised as follows:

  • Long-lasting fertilisation effect
  • Water retention capacity
  • Increases the porosity of the soil
  • Increases the content of organic material in the soil
  • Promotes soil biology – soil bacteria

These primary effects produce indirect positive effects on energy input by substituting artificial fertilisers, on the CO2 balance by binding carbon in the soil and on plant health and thus also pesticide use.

Agriculture

Attention is focused mainly on the fertilisation effect. Fresh composts are frequently used. The essential quality parameters are nutrient content, pollutant concentrations, physical parameters (handling) and epidemiological acceptability. Compost use is an essential element of organic farming.

Nurseries and ornamental plants

The main focus lies on fertilisation effect and plant tolerability. Mature composts are mostly used. The essential quality parameters are physical parameters (water retention capacity), foreign substance and germ content and epidemiological acceptability. In soil production, peat can to some extent be substituted by compost, as a result of which a contribution is made to conserving this natural resource.

Domestic gardens

Attention is focused mainly on the fertilisation effect. Mature composts are frequently used. The essential quality parameters are nutrient content, pollutant concentrations, foreign substance content and epidemiological acceptability.

Landscape gardening

Attention is focused mainly on the fertilisation effect. Fresh composts are frequently used. The essential quality parameters are nutrient content, pollutant concentrations, physical parameters (handling) and epidemiological acceptability. Compost use is an essential element of organic farming.

Costs

Financing of the biological treatment of waste comes largely from disposal contributions for the delivered waste. Depending on the plant and quality of the material, a notable additional income can occasionally be made from the sale of compost.

The costs per tonne of input vary greatly both over time and locally. Amounts between 20 and 70 euro per tonne can be assumed in Central Europe. Disposal in composting plants is cheaper in any case compared with alternative methods such as incineration or digestion.

Rotting products

Different products are made in industrial composting plants. The most important are listed below.

Composting products:

  • Compost
  • Structuring material
  • Residual fractions
  • Ammonium sulphate

Quality depends very much on the input. Some quality aspects, like heavy metal or PCB content, phosphorus-content or salt concentration   can not or hardly be changed by the rotting process. Thus the possible uses widely depend on the input composition. Due to the use of the produced material different final product properties are important. e.g. For the application as a fertilizer in agriculture the contents of nitrogen, phosphorus, potassium are important quality parameters, water content is important for the handling and application and it is very importatant to produce clean material free of impurities like glass and stone and stone and free of weed seeds. For material produced in MBT plants it is most important to reduce the content of biodegradable organics as far as possible. 

Water content

Degree of rotting

ODM content

Heavy metal content

Hygienisation

Impurity content

Impurities mainly come in with input material. Impurties are plastics, glass, stones, pieces of metal foils and similar materials. The content of impurities is important for the application of the end product. Impurities give the impression of being dirty and may be a relevant source of heavy metals.

Impurities have to be sorted out by hand and weighed. The impurites content is defined as dry weight of impurities divided by the total dry weight of the sample.

Plant technology

Plant sizes and construction types:

The plants are differentiated according to the processing capacity, input materials, operator, degree of automation and other criteria.

In Austria a distinction is usually drawn between agricultural composting and large-scale or industrial composting, for organisational reasons.

A distinction is also made between open and closed composting plants.

Open means on fixed, dried areas outdoors. Closed means in a hall or in a reactor so that the exhaust air can be completely captured.

The heap shape can be triangular, trapezoidal or tabular.

For aeration, a distinction is made between

  • suction and pressure aeration,
  • circulation and through-air operation and between
  • continuous and discontinuous aeration.

Plants with a wide variety of combinations of the above-mentioned basic properties have been and are being built, for which reason a complete overview cannot be given.

In the meantime the most common construction is the tunnel composting plant, that is, horizontal, closed rotting reactors in which part of the rotting exhaust air is usually recirculated.

Plant dimensions

The following figures should allow an estimation of the dimensions of composting plants.

An essential factor is the load per unit surface area in the composting process. For open heap composting, the gross surface area incl. driveways and interspaces can be estimated at 0.5 - 1 t/m².

The most important criterion for selecting heap height is the porosity and the resulting capacity of the heap to be aerated. If the pressure is too high the heap structure collapses, free water escapes and the affected areas can no longer be sufficiently aerated. The tolerable values also always depend on the structure, water content and water retention capacity of the material. The loads per unit surface area in large-scale plants are between 0.1 and 1.8 t/m². For aerated heaps in reactors or as tabular heaps, 1.3 t/m² is a more favourable value.

Aeration and heat removal are closely connected to each other and form the core of any plant dimensioning process. Depending on the rotting phase, either the air requirement or the heat removal by means of the air is decisive for the amount of air required. The air requirement for aeration can be derived from the stoichiometric air requirement. Furthermore, the boundary condition applies that increased occurrence of anaerobic zones is to be expected at oxygen concentrations below 10 to 15%, depending on the rotting system. The minimum air requirement is thus approximately twice the stoichiometric air requirement. The air requirement for heat removal results from the heat balance and the reaction heat. The heat effect is usually between 12 and 14 MJ/kg DM decomposed.

Design criteria for irrigation are irrigation intensity, that is, the maximum flow rate of the irrigation apparatus, and the irrigation amount per rotting stage. The irrigation amount depends greatly on the rotting system, material and type of introduction into the heap, so that no general values can be given.

The energy consumption of composting plants varies considerably depending on the material and technology used. The greatest consumers of energy are mechanical preparation of the material, in particular the comminutors, aeration units and machinery for handling and working the composting material. Some examples of this are:

  • Biological waste and water treatment sludge composting plant of 20,000 t/a

Energy consumption: Electricity 260,000 kWh/a, diesel 60,000 l/a
In total: 323,000 kWh/a, thus specifically: 16 kWh/t input

  • Mech. and biol. waste treatment (MBT) plant of 85,000 t/a

Energy consumption: Electricity 3,000,000 kWh/a, diesel 86,000 l/a
In total: 3,220,000 kWh/a, thus specifically: 32 kWh/t input

  • Mech. and biol. waste treatment (MBT) plant of 42,000 t/a

Energy consumption: Electricity specifically: 30...40 kWh/t input,
Diesel consumption not known, therefore assumed to be 5 kWh/t (see above)
In total specifically (approx.): 35...45 kWh/t input

Plant operation:

The operation of composting plants in the technical sense consists in constantly optimising the living conditions of the aerobic microorganisms which carry out the decomposition and conversion of the biomass. Although this sounds simple, in practice it demands a great deal of knowledge and effort on the part of the operator and often also plain experience and instinct, as it all has to be done with complex systems with a low degree of freedom, and often compromises have to be made between contradictory requirements for optimising different goals.

The following example may illustrate this: One of the tasks of intensive rotting is hygienising the material, that is, substantially killing germs which are pathogenic to plants and humans and weed seeds on the one hand and the optimisation of decomposition and of formation of humic substances on the other. Whereas attempts are made to achieve the first task by maintaining high temperatures, precisely these high temperatures have a negative effect on the decomposition and on the formation of humic substances and also lead to increased emissions and nitrogen losses.

Several of the following options are usually available to the operator:

Preparing and mixing the input material: A mechanical preparation stage consisting of transporting, comminution, mixing, screening and sifting processes usually precedes the actual composting process. The objective is to remove impurities (batteries, cardboard, plastics, glass and many others), to open up coarse material (whole potatoes, oranges, aubergines etc.), to adjust the water and C/N content for composting (mixing wet material with reject from packaging, adding fresh green waste) and to homogenise the material (green waste from the outskirts of towns, moist waste from urban areas etc.)

Aeration: The rotting material can be aerated passively by convection based on heat development and moisture content or actively by forced aeration using fans or by conversion processes. As the supply of air is associated with the removal of heat and water, a sensible path must be trodden between temperature control and oxygen supply. Excessive aeration leads to drying out or cooling of the material, inequalities and excessive energy consumption. See also rotting control

Irrigation: Microorganisms, especially bacteria, depend on high water contents. If the water content is too high, put simply if free water occurs in the rotting material, uniform supply with oxygen and removal of heat from the heap is no longer guaranteed and anaerobic zones occur. The "ideal water content" is therefore that with which sufficient moisture is available as a habitat for bacteria and fungi and the pore volume is also large enough for uniform aeration of the material. This condition can be best expressed in figures with the rule M(H2O)=2xM(LOI). The amount of water in the material should be twice the amount of organic material expressed as loss on ignition.

Example: A compost sample has a water content of 60% of fresh mass and a loss on ignition (LOI) of 70% of DM. Therefore 40 g DM and (40x0.7=) 28 g LOI are present per 100 g compost (moist mass). The ratio of water to loss on ignition is therefore: 60/28= 2.14 and therefore too high, but in the favourable range.

The rule for the practitioner consists in the "fist test". Here, a handful of material is taken and squeezed in one's fist. The material should be moist enough to form a stable ball but no liquid should escape from the material even when squeezed hard.

A problem for the operator is the very restricted options for obtaining representative samples. Meaningful analyses are usually obtained in the course of conversion processes. Material moisture control is usually based on a combination of analysis, calculation and balancing, and experience. The optimisation of the operation requires data recording and evaluation over a long period.

Conversion:

During conversion, the material is generally homogenised, loosened and aerated, and can in many cases also be irrigated in the course of the conversion process. This means that the aeration capacity for further rotting can be improved for a few days. Attempts are usually made to treat the material statically for no longer than 14 days.

Rotting control:

The rotting temperature in industrial composting plants can be controlled by irrigation, conversion, but in particular aeration. The rotting control can be carried out in a "manually" controlled or regulated manner. Corresponding measurement data is an essential prerequisite for process control. Whereas temperature and pressure measurements and concentration measurements in the exhaust or circulated air can take place online in composting plants too, only laboratory measurements are available for measuring material moisture, loss on ignition and other rotting material parameters. The effort involved in obtaining samples and measuring restricts the sensible measurement frequency considerably.

Preparation:

  • Delivery
  • Handover
  • Screening
  • Comminution
  • Mixing / homogenisation

Intensive rotting

Input into the rotting system

Output from the rotting system

Maturation rotting

Sometimes designed in the same way as the intensive rotting stage, but usually simpler technology, for example open, aerated heaps.

Complete maturation:

Usually not carried out on an industrial scale as the time required (months) cannot be shortened.

Post-treatment

Post-treatment, often also referred to as fine preparation or packaging, comprises the steps after biological treatment, which serve to improve the usability or marketability of the product.

Depending on the starting material and the degree of biological maturation, different steps are used for preparation. The essential steps are classification of the material, that is, separation into different screened fractions, removal of light foreign material such as plastic parts and removal of heavy foreign material such as glass and stones.

Step Objective Implementation, comments
Classification Production of different screened fractions. Fine fraction for agricultural use, soil production and household purposes.
Medium fraction for green areas, as a biofilter material etc.
Coarse fraction is generally added to the input material or discarded. Use as biofilter material is possible.
Screening using a drum screen, star screen or flip flow screen
Separation of hard material Removal of glass and stones, but also fruit pits and pieces of bone to improve usability of the product. Usually impact separator or hard and light material separation combined as an oven separator
Removal of light material Removal of plastic parts, plant fibres, pieces of aluminium foil and similar materials. Air separator, oven separator
Air separator often in combination with screening
Bagging up Packing into bags. Bag sizes from 10 to 50 l are customary. Only suitable for stable mature compost, A low water content is necessary.
Labelling Meeting obligations with regard to declaration, but also facilitating use and prevention of misuse Declaration of quality classes and analyses. Inclusion of recommendations for use.

See also section Use

Collection

Collection forms an essential part of the composting waste recycling system. Depending on the collection system and area, the proportions of impurities in the separately collected biological waste are usually between 0.1% and 10%. In some cases they are even far above this, which is however no longer considered biological waste.

Collection logistics and the motivation of the population have a decisive effect on the quality of the starting product. This is in turn a considerable cost factor and ultimately has an effect on the quality of the end product.

In the most favourable cases sorting of the delivered waste is not necessary, but if there is a high level of contamination more expenses are incurred for transporting, sorting and disposal of residual waste and ultimately more contamination is to be found in the end product.

Informing people

An important measure in connection with the operation of composting plants is providing the waste producers, that is, the population, with information. Educational information can have a considerable influence on the collection quality, which is not just a cost factor, but also contributes to lower energy consumption and lower emissions and allows production of a higher quality product.

Educational work must begin in primary schools and end in retirement homes. It is furthermore necessary to repeat information campaigns to the public on a regular basis, provide information on new developments and communicate the value of the product. Education work can considerably reduce the impurity content in the delivered material.

Providing containers

The organisation of waste collection at the waste producer is the first important way of influencing waste quality. Positioning and container size have a significant influence on the composition of the collected waste.

The introduction of separate collection of biological waste is virtually indispensable to achieve high quality.

Furthermore, the rule applies that small collecting containers produce higher purities than larger ones.

Central bring systems generally produce purer waste but smaller quantities than convenient pick-up systems. In this case it needs to be judged how much weight should or must be placed on the quality of the waste. The organisation of waste collection depends on the plant technology used and should in any case be planned for the long term and in consultation with disposal companies.

Transporting

Quality

Pre-treating input material

Delivery

Input

Transporting material

Classification

Comminution

Energy consumption

Quality

Rotting

Input

Rotting / reactors

Inputting material

Aerating material

Moistening material

Maintaining structure

Process control

Cooling by evaporation (2,500 kJ/kg) instead of air heating (16 kJ/kgK)

The most important parameters for process control and monitoring:

Temperature T

pH

Water content and dry matter DM

Loss on ignition (LOI or ODM)

C/N

Oxygen content in exhaust air c[O2]

Carbon dioxide content in air c[CO2]

Output

Bagging

Energy consumption

Quality

Preparing product

Producing fractions

Removing hard material

Mixing

Bagging

Energy consumption

Quality

Special aspects

Standards and laws

At EU level:

Waste Framework Directive

Composting Directive

Landfill Directive

Waste Directive

In Austria:

Waste Management Act

Composting Ordinance

The requirements with regard to pre-treatment before landfilling according to the European Directive on the Landfill of Waste (Landfill Directive 99/31/EC) have been legally implemented in Austria by the Landfill Ordinance (Austrian Official Journal No. 164/1996 in the version of Austrian Official Journal II No.49/2004) on the 1st January 2004 (in exceptional cases from the 1st January 2009). This national reform means that the process of mechanical and biological waste treatment has now been approved as legally equal to the process of thermal waste treatment.

Quality parameters

Different quality criteria are to be used depending on the starting material. The most important are:

  • Degree of rotting
  • Breathing activity
  • LOI or DM decomposition
  • Water content

Dirt

Mechanical treatment but also just handling and transporting material causes dust to form. This is naturally rich in organic material and microorganisms.

Corrosion

In the field of composting there are numerous influences which lead to material damage. Some of the important themes are listed here.

In composting plants it is warm, moist and dirty in many areas. Organic acids, ammonia, sometimes H2S and other chemical substances with high corrosion potential also occur. Furthermore, the possible effects of the microorganisms, which occur in high concentrations, in combination with the above-mentioned conditions are to be noted.

The moisture or wetness which is present in a wide variety of areas in composting plants has adverse effects on many materials both alone and in combination with the substances mentioned below. Liquid which escapes from delivered and stored materials should be noted as well as condensation from water vapour and in pipelines.

Organic acids are formed in various stages of the process but also outside the rotting area in all dusty and moist zones.

The formation of ammonia (NH3) cannot be completely prevented, even with the best rotting control. Ammonia itself dissociates in water and has an alkaline effect. More important however is the damage to materials caused by NO3-, which is formed from NH3 by microbiological activity.

Hydrogen sulphide (H2S) often occurs in freshly delivered and not yet treated material. Both the sulphides and the sulphates formed by biological oxidation have considerable corrosion potential.

Microorganisms contribute in many ways to material damage. A specific effect consists in that many organic materials can be degraded by microorganisms. For instance not only wood is easily attacked but also a wide variety of adhesives. This is to be noted particularly in connection with supporting parts such as laminated wood beams.

A wide variety of materials are attacked by the above-mentioned corrosive substances. Some materials are only partially suitable. Particular care should therefore be taken when choosing materials.

In addition to suitability in principle the concentrations to be expected in the various parts of the plant are to be taken into account as well as additional influences such as abrasion, changes in chemical milieu (pH, moisture etc.)

Material Danger Assessment
Aluminium Corrosion at pH > 7 Fit for limited use – not suitable if there is also abrasion
Polyethylene Temperature >50°C Fit for limited use – drop in strength above 50°C
Polypropylene UV radiation Well suited, with restrictions in outdoor areas
PVC Temperature < 0°C Well suited, with restrictions in outdoor areas
Steel Corrosion!! Low-alloy fit for limited use, in some areas only high-alloyed steel types can be used
Galvanised steel Corrosion Fit for limited use
Concrete Corrosion Fit for limited use – protection by additives, coating, lining
Wood Biological attack on wood and adhesive Fit for limited use, caution with composite materials (laminated wood beams)

Table: Suitability of materials

Risks

Various dangers can arise in industrial composting plants. Firstly, typical dangers in relation to lorry and wheel loader traffic are to be noted, which are often exacerbated by driving surfaces which are often dirty and wet. Secondly, specific dangers owing to uncontrolled biological activity occur in solids and process water as well as in the rotting stage air.

The formation of H2S and CO2 is to be reckoned with in various input materials. Corresponding procedures for prevention and monitoring should be defined, and extraction of low points should be provided. There is generally a lack of oxygen or CO2 is present in harmful concentrations in the rotting stage air. Just the high temperature and humidity of the rotting stage air can lead to it no longer being breathable and circulatory failure occurring after staying in it for a relatively long time. Fine dust, germs and spores are always present. Dangers caused by these substances are to be investigated case by case. In general it can be said that dust and spores only result in dangers to human health (for example farmer's lung) in combination with relatively high temperatures and dryness. Immunodeficient persons should take particular care.

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Teaching material / multimedia

Test questions and exercises

Q: Write out the equation for aerobic decomposition of biomass in general and transfer it to glucose.

Facts in brief

DESIGN FIGURES:

  • Specific reaction enthalpy in relation to decomposed DM: 12 - 14 MJ/kg
  • Ideal water content: M(H2O)=2xM(LOI)
  • Stoichiometric O2 requirement: approx. 1.1 g O2 per g DM decomposed
  • Load per unit surface area in composting:
    Typical: 0.5- 2 t/m²
    Favourable: up to 1.3 t/m²

Name Size Creator Creation Date Comment  
JPEG File composting plant overview.jpg 152 kB gerhard.hampejs@fh-wels.at Mar 31, 2010 00:37 NE composting plant overview  
JPEG File Composting process balance general.... 743 kB gerhard.hampejs@fh-wels.at Aug 27, 2010 16:51    
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