Recovery of energy from the waste

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The cycle of the materials

The waste management is part of what is known as cycle of the materials, which includes all the activities of extraction of raw materials, manipulation or manufacture of products, use or consumption and waste management of this consumption. A block diagram of the cycle of materials is shown in Figure 2.1.1.
The first part of the cycle consisting of the extraction of raw materials, production and use stage and consumption stage, corresponds to the activities that generate the wastes. The second part of the cycle, formed by the collection, the treatment and the spill steps, corresponds to the management of this generated waste.
The cycle of the materials can be open or closed, depending whether the waste is re-introduced in the cycle as secondary raw materials source or if it is dumped into the environment.
Closing the cycle, that is, using the waste as secondary raw materials, it is more sustainable because it reduces the disposition of waste and avoids the extraction of new materials. Therefore, both factors, reduction of waste and reduction of new materials extraction, contribute to the preservation of the environment.
For this reason, it is very important that the management of the waste favours its exploitation, which can be made through a good system of segregated collection and the correct treatment of each fraction of waste.
However, the total closing of the cycle is not technically possible, since there is always waste that can not be recovered materially. For this reason, it is still fundamental to have systems of recovery of energy from the waste and of controlled deposition.

Management of waste

The management of the wastes includes all the activities destined to gather and to treat them in a most suitable way, preparing the wastes for the recovery of materials and reintroducing them into the cycle of production, to obtain energy from them, and to dump them in conditions of environmental safety.
Nevertheless, it is more evident that the management of wastes has to include also all the possible actions to anticipate and to minimize its generation.
The most favoured option of managing every waste is the most respectful towards the environment. That is to avoid the pollution, or to avoid extracting more resources.
In this way, the hierarchy for established waste management can be seen in next Figure 2.2.1.
The chosen option will depend on technical conditions, like the quality of waste, the technical feasibility of the recovery of materials and the availability of the necessary infrastructures.

Generation of waste

The quantity of waste that is generated in the domestic area is related to the characteristics of the economic and cultural system. This influences on the appraisal of the products of only one use and foster consuming habits that favour the utilization of containers and packages.
In spite of the efforts of the administrations in the prevention of waste generation, it does not seem possible to separate the generation from waste of the economic growth. In order to achieve this, it would be necessary to change the consumption patterns towards a model which consists of the consumption of both material and immaterial goods.
The composition of waste, however, has varied substantially in the last decades: the proportion of organic material has always reduced, while the percentage of containers, wrappers and packages has increased.
In spite of the total increase of waste, thanks to the selective collection and to the suitable treatments of each fraction, the amount of materials that are destined to final treatments diminishes progressively.

Segregated collection

The segregated collection is the system that allows picking up the separate waste fractions in origin and transporting them to the corresponding plants of treatment.
It has two fundamental and complementary goals: on one hand, to sort out and to retrieve the maximum amount of recoverable materials and, on the other hand, to reduce, as much as possible, the amount of waste that is brought to final treatment, like the incineration and the deposit.
It requires the collaboration and the efforts of the citizens, since the environmental benefits are very important to help and support the quality for the life currently and for the future generations.

Treatment

The treatment, unlike the collection of waste, is a part often little known in the cycle of management of waste. It encompasses a series of techniques which, applied to the municipal waste, which allow reusing them, recycling them, retrieving energy or stoning them a controlled deposit.
The treatments can be divided into two general types: those which allow the re-introduce from of the materials in the cycle of production and consumption, either for reuse or for recycling, and the final ones, that intend to eliminate the waste, in the safest possible way.

Recycling

Recycling and reusing glass helps to reduce the volume of the municipal waste that must be treated, but also has other environmental benefits. For instance, the recycling of glass allows sparing the raw materials with which the same amount of new glass would be made: a ton of recycled glass spares approximately 1.200 kg of raw materials and entails a decrease in the extraction of its aggregates.
Additionally, the manufacture of new glass consumes more energy than the recycling, therefore, each tonne of recycled glass allows the saving of 100 kg of oil or 1,129 TEP (tonnes equivalent petroleum), with the consequent generalized pollution reduction.
Paper and cardboard picked up selectively reduce the volume of municipal waste, especially the amount of waste to incinerate, which contributes to reduce the atmospheric pollution. Furthermore, the manufacture of recycled new paper from paper pulp is much more ecological because it allows saving between 275 and 450 m3 of water and from 0, 25 to 0, 45 TEP per ton of paper.
The recycling of two tons of plastic permits saving a ton of oil, while the recycling of a tonne of aluminium allows to avoid the extraction of four tons of bauxite (the mineral from which it is produced) and the production of two tons of which is highly polluting waste and difficult to eliminate.

Biological treatment

The biological treatment can include two types of processes: the composting and the anaerobic digestion. These two processes can be combined; for example, subjecting first the organic material to an anaerobic digestion and making a process of secondary composting. Thanks to these two processes or to their combination of polluting load of waste, the production of leachates and the emission of gases with greenhouse effect are reduced.
This is also a benefit because the organic material can be very pollutant if it is moved to a controlled deposit. On one hand, the action of the rain and of the own physicochemical degradation -so called leaching- can produce a pollutant liquid. On the other hand, methane is generated, which is a very pollutant gas due to its capacity of greenhouse effect.
Moreover, the selective collection of organic material has also other environmental benefits. For example, it allows to generate an organic manure of high quality and to be used in agriculture and gardening. This fact already means a considerable saving by itself, but additionally, it is reducing the use of chemical fertilizers, which can be an important source of pollution for aquatic and for other ecosystems.

Anaerobic digestion

Anaerobic digestion consists of the degradation of the organic material through anaerobic microorganisms. A series of processes gives the production of biogas and the stabilization of the biodegradable waste as a result, so that they are converted in material useful as organic amendments of the ground. The biogas is a renewable source of energy.
The process is carried out in the digesters through controlled changing conditions in about 20 days.
First a process of hydrolysis is applied to the organic material. The goal is to break the cellular membranes and to break down the organic macromolecules, polysaccharides, lipids and proteins, into simpler molecules that can be degraded by microorganisms.
The following step is the acetogenesis, which consists of using these simplest molecules for producing acetic acid, hydrogen and CO2. In this phase, the microorganisms that participate in it produce exhaust oxygen of the environment, so that the process pioneered in an aerobic way turns to anaerobic.
Finally, in the methane genesis the involved microorganisms use the acetic acid as a source of energy and breathe CO2 and hydrogen, which gives the production of methane as a result.
The result of all these processes is the production of biogas, a mixture of dioxide of carbon (CO2) and methane (CH4) in equal parts, as well as of other side-products in minor amounts. The biogas constitutes a source of renewable fuel that can be useful for generating electricity and heat.
Apart from this product, the remaining organic material has become more stabilized and can be destined for the production of compost, to dump it in a safer way or even to use it as an organic improvement of the ground.

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Composting

Composting consists of the degradation of the organic material through aerobics microorganisms. The result of this process is the production of compost or bio stabilized material and of carbon dioxide.
For the process of composting the organic material can be mixed with other materials; for example, with gardening waste or the organic waste of large producers, like markets. In case that waste of pruning and gardening is incorporated, it must be grinded before mixing for facilitating the later decomposition.
The process of composting consists of leaving this mixture of waste to break down initiating the degradation that occurs in the nature, but in an accelerated way. It consists of a controlled bio oxidation process produced by microorganisms that act under determined environmental conditions (temperature, humidity, etc). The process is carried out in impervious compartments, which facilitating its acceleration and, at the same time, avoiding the emanation of smell.
The result is an organic product that, based on its quality, can be used as a fertilizer in gardening and agriculture or for improving soil, infilling, as structuring, etc.

Recovery of energy from the waste

The most implanted systems to recover energy of the wastes are the waste incineration, the fuels derived from waste, anaerobic digestion in plants of biological treatment and catchment of biogas in controlled storage.
Besides the former ones, there are other emerging processes, such as the pyrolysis, the gasification or the plasma technology that also allows retrieving energy from the waste. The systems of energy recovery have to be reserved for the waste that has not been able to be reused nor have been able to retrieve through processes of material recovery.

Incineration

The incineration is the process in which a controlled combustion is produced with minimum temperatures of 850º C, duration of minimum 2 seconds and turbulence of oxygen superior to 6%. The fumes that come out of the oven pass through the boiler, where they heat water which, once transformed into steam, it is useful for obtaining electrical energy through a turbine or using it for other uses energy, like the air conditioning (heat or cold) distributed in an urban sector.
As a result of the treatment, the volume of the incinerated waste is reduced by 75% while 20% of inert slag and 5% of ashes are produced.
The slag goes for a vibratory sieve in order to sort out the metals that they can contain. Afterwards they can be used for tasks of infilling of the ground or deposit in a controlled way.
The ashes are accumulated in closed silos until, depending on the harmfulness; they are brought to centres of special waste treatment or poured into controlled deposits for hazardous waste.
Product of combustion and gases are also generated like the carbon dioxide, steam, nitrogen, oxygen (the leftover of the combustion), chlorine, chloride of hydrogen, oxides of sulphur and organic compounds, among other. Before being expelled by the chimney the gases have to fulfil a series of conditions marked by the European valid legislation. For this reason, they pass through an electro filter and a system of washing [11].
The incineration of waste is a treatment regulated in order to limit the environmental impact. The valid legislation establishes very strict parameters regarding the emissions of gases. These parameters are controlled in a permanent and discontinuous way at the exit of gases (chimney), and in periodic way in the case of the ashes.

Fuels derived from waste

In the case of the municipal waste, reference to fuels derived from waste is made when the recovery of energy is made through the combustion of a material prepared for selecting and treating waste that a fossil fuel substitutes.

Anaerobic digestion in plants of biological treatment

The anaerobic digestion consists of the degradation of the organic material through anaerobic microorganisms and gives as a result the production of biogas a renewable source of energy.

Catchment of biogas in controlled storage

The fermentation of the organic material deposited in the dumps generates methane and carbon dioxide. These two gases can be picked up with a system of wells and be profitable as a biogas.

Other treatments

There are other methods of thermal treatment, like the pyrolysis or the gasification. The pyrolysis consists of transforming chemically the waste in absence of oxygen. In the case of the gasification the waste does not burn directly, but it transforms into a combustible gas mixture through a partial oxidation with application of heat.
The difference between the incineration and these processes consists of the presence of oxygen. The incineration implicates combustion in presence of oxygen, while the gasification is made in absence or low concentration of oxygen.

Benefits of the recovery of energy

In general, all the processes of energy recovery, especially the ones of combustion, allow reducing the volume and the weight of the waste that can not be material recovery. Nevertheless, it must be noticed that in the environmental topics, it is always better to prevent, to reuse or to retrieve the waste. Therefore, the energetic recovery has to be the last step in the stages of waste recovery.

Landfill

The controlled deposition is a final treatment that consists of dumping the waste in conditions of environmental security, that is, in a way that they can not be a source of pollution for the environment.
When the waste arrives to the controlled deposit it is positioned on a terrain, previously waterproofed, spreading it on layers of thin and compacting it to reduce the volume. Afterwards, it is covered with a layer of land, which avoids sanitary problems and allows confining itself the waste correctly.
The dumped waste contains part of organic material. This material is fermented and produces biogas (a mixture of methane, CO2 and other volatile compounds) and leached (liquids resulting from the process of degradation). They both can mean a source of pollution. For this reason, the controlled deposits are structured for minimizing the impacts.
The terrain is waterproofed and a system of catchment and treatment of leached is incorporated into it in order to assure that the leached are not filtered to the ground and not pollute possible aquifers in the zone.
To guarantee that the rain is not filtered inside the deposit and mixed with the leached, there is a system of pluvial waters pipeline.
To avoid the emission of methane at the atmosphere, a system of catchment and treatment of biogas is placed in the deposit.
As last, the waste is poured into cells isolated by layers of compacted land, so that they can come off suitably confined.
It must also be noticed that the controlled deposition is a treatment regulated in a very strict way by the European legislation, and that the waste that is poured has to be previously stabilized, based on to several parameters.

Table of contents :

1 Introduction
1.1 Agenda
1.2 Local Agenda of Barcelona
1.3 Local Agenda of Gävle
1.4 Aim and method
2 Theory
2.1 The cycle of the materials
2.2 Management of waste
2.3 Generation of waste
2.4 Segregated collection
2.5 Treatment
2.5.1 Recycling
2.5.2 Biological treatment
2.5.3 Recovery of energy from the waste
2.5.4 Landfill
3 Process and results
3.1 Generation of waste
3.1.1 Generation of waste in Barcelona
3.1.2 Generation of waste in Gävle
3.2 Segregated collection
3.2.1 Segregated collection in Barcelona
3.2.2 Segregated collection in Gävle
3.3 Types of treatment
3.3.1 Types of treatment in Barcelona
3.3.2 Types of treatment in Gävle
4 Discussion
4.1 Generation of waste
4.2 Segregated collection
4.3 Treatments
5 Conclusions
6 Appendices
6.1 Tables
6.2 Information about Gävle
6.3 Information about Barcelona
7 References.

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