Evolution in waste production

The total amount of waste generated per year in Spain experienced an increase of 95.9% in the period between 1990 and 2007, placing the total amount of waste produced in this last year at 25,584,000 tons of waste. Therefore, the generation of urban waste considered from all perspectives is experiencing extremely high growth in the Spanish country. In part, this is due to the non-compliance with certain parameters covered in the national waste plans (as an example is the production of eight million more tons than those foreseen in the National Urban Waste Plan 2000-2006 in 2006), the progressive growth of the Spanish population in recent decades and the fact that Spain is the first tourist destination in the European Union.

Only in 2006 did waste production suffer a small decrease compared to the previous year, although in 2007 waste generation would increase again.

Spain and Europe

For four years, the trend towards stabilization of urban waste production has been generalized in most countries that are part of the European continent, with some exceptions such as Spain, where there is still a trend towards growth in residual production. In fact, in 2006 the residual production per capita in Spain slightly exceeded the average of the EU-27 countries (537 kg/inhabitant/year compared to 517 kg/inhabitant/year), although it is true that there are countries with much higher rates (for example, Ireland produced about 804 kg/inhabitant/year in 2006).

In 2012, the Spanish average in waste production per capita was 484.4 kg per inhabitant and year.

The composition of all urban waste varies depending on three factors, which are the population's standard of living, the activity carried out by it and the region's own climate. Depending on these factors, certain products will be consumed and used, which will ultimately produce the corresponding waste. According to the National Urban Waste Plan (PNRU) 2000-2006, the average production in Spain of the different components of urban waste is shown below:

Es fundamental conocer algunas de las propiedades físicas de los residuos sólidos urbanos para prever y organizar los sistemas de recogida de basura, los tratamientos finales de reciclado o eliminación, así como para decidir cuales son los sistemas de segregación más apropiados en el caso de los residuos que generen riesgos especiales para el medio ambiente.

Degree of humidity

Moisture is present in urban waste at an approximate percentage of 40% by weight; although it is true that it can range between 25% and 60%. The maximum contribution of moisture is provided by those offal that contain organic matter, while the minimum is provided by products of a synthetic nature. This characteristic must be taken into account due to its importance in waste compression processes, leachate production, transportation, transformation processes, incineration or energy recovery treatments, and waste separation processes in the corresponding recycling plant.[4]​.

In municipal solid waste, moisture tends to homogenize, since some products transfer it to others. This is one of the main causes of degradation of certain products such as paper, which absorbs moisture from organic waste, losing characteristics and also value in mechanical recycling processes over recycling at source (which is carried out by citizens in their own homes), which avoids this contact.[4]​.

Specific weight

The density of urban waste is of vital importance to calculate the dimensions of pre-collection containers, both in private homes and public roads (streets, avenues, squares, parks...), as well as a basic factor that determines the volumes of collection and transport equipment, reception hoppers, belts or landfill capacity. The density may vary depending on the degree of compaction to which the waste is subjected. Volume reduction is present in all phases of waste management and is used to optimize the operation, since the large space they occupy is one of the biggest problems faced by the plants in charge of processing it.[4]​.

The specific unit weight of each product does not indicate that the set has an overall value proportional to that of its components. In fact, in homes, these values ​​are usually significantly higher due to the unused spaces in the container that contains the garbage. However, as they are grouped more homogeneously, they will approach the mathematical calculation that gives theoretical average values ​​for uncompacted waste of 80 kg/m³, with considerable variations linked to the composition of the waste in each location.[4]​.

Granulometry

The degree of segregation of the materials and the physical size of the elemental components of urban waste represent an essential value for the calculation of dimensions in mechanical separation processes and, especially, for choosing screens, trommels and similar devices that base their work, consisting of separation, on size. These same values ​​must be taken with great care, since in collection operations the dimensions are affected as a consequence of compression or crushing mechanisms.[4]​.

Conocer con exactitud las propiedades químicas de los residuos urbanos es un hecho de suma importancia en el tratamiento de estos, puesto que estas propiedades son determinantes para los procesos de recuperación y tratamiento final. Probablemente sean el poder calorífico y el porcentaje de cenizas producidas las características químicas que mayor importancia revisten, ya que son esenciales en los procesos de recuperación energética. Aunque tampoco se deben de subestimar propiedades como la eventual presencia de productos tóxicos, metales pesados o contenido de materiales inertes, debido a la importancia que tienen respecto al diseño de soluciones adecuadas en los procesos de recuperación y para la toma de precauciones hiegiénicas y sanitarias.[5]​.

Chemical composition

To determine the characteristics of energy recovery and the potential for producing fertilizers with the appropriate carbon/nitrogen ratio, it is necessary to study the composition of each waste. It is also necessary to determine the presence and concentration of toxic and dangerous waste to evaluate the risk that its handling may entail.

As a consequence of the enormous variability experienced in the composition of urban solid waste, the resulting chemical composition of the whole is also highly variable.

It is necessary to know the composition of a specific waste to determine its energy recovery characteristics and the potential to produce fertilizers with the appropriate carbon/nitrogen ratio. It is also advisable to know the presence and concentration of toxic and dangerous waste to evaluate the risk that its handling, treatment, reprocessing and reuse may pose to human health and the environment. Arsenic, cadmium, mercury "Mercury (element)"), antimony, chlorinated solvents, elements with characteristics of flammability, corrosivity, reactivity, ecotoxicity, toxicity or carcinogenic, mutagenic or teratological qualities, are usually present in urban waste, normally from industrial and hospital activities.

heating value

The calorimetric characteristics of urban waste determine the design of the facilities that must be installed and the energy recovery. The assessment, which is the result of the variability of the composition of the waste, is predefined by the calorific value of each product.[5]​.

Broadly speaking, it can be stipulated that the calorific value of all urban waste is around 1500 and 2200 kcal/kg. Another value of great interest is the melting and solidification temperature of the ashes from the combustion of these materials, melting at a temperature of 1,200 °C.[5]​.

De acuerdo con la normativa vigente, la gestión de residuos sólidos urbanos comprende la recogida, almacenamiento, transporte, valorización y eliminación (o transformación) de los mismos, siendo también considerada como parte de la gestión la vigilancia de las actividades citadas, además de la vigilancia ejercida sobre los lugares de alojamiento de residuos tras su clausura. Además, esta se halla dividida en cuatro fases diferentes:.

Pre-pick

The term pre-collection includes waste manipulation, separation, storage and processing at source, with the intention of concentrating urban waste, and may modify some of its physical characteristics with the aim of facilitating its collection. The separation for recycling of paper, cardboard or any type of packaging (aluminum cans, glass bottles or plastic jars, for example) in this phase, which is the closest to its generation, is of great importance to achieve the highest possible purity of these fractions. Respect for the schedule established by municipal ordinances for transporting waste from production sites to pre-collection points (in terms of non-buried containers) is also a relevant factor, to avoid bad odors and visual pollution.

Pickup

There are two different types of collection, the collection that is carried out by means of adapted vehicles and the pneumatic:.

This type of collection has a series of advantages over the traditional method, since its installation improves the quality of life of citizens, makes it easier to reduce noise and bad odors.

Transport

It is at this stage when the waste is transferred to transfer stations (facilities in which waste is temporarily stored and compacted to be subsequently transported to the areas where it will be treated by vehicles with large capacity), recycling, classification or energy recovery plants and landfills.

In most Spanish municipalities, compressor collection trucks with a capacity ranging between 10 and 20 m³ are responsible for transporting waste. Although on some occasions and due to the characteristics of the areas in which they operate (narrow public roads, for example), it is necessary to resort to smaller vehicles that lack compression capacity.

Transformation, energy recovery or elimination

The final stage of waste management has three possible variants depending on the nature of the waste components and the possibilities of the region in which they are treated. Therefore, waste components may be transformed in order to obtain new products with other applications (composting and biomethanation"), energetically recovered with the sole purpose of converting them into fuel with which to generate energy (gasification, hydrogenation, pyrolysis, oxidation and, in some cases, incineration) or eliminated.

The use of clandestine landfills, which are places where waste accumulates without any type of control, was the first method adopted by humans to eliminate waste, because it was simple and cheap. However, it causes serious environmental problems (landscape alteration, bad odors, soil, water and air pollution) and health problems (diseases transmitted by rodents or insects), lacking adequate controls.

Currently, landfills classified as uncontrolled are sealed and closed, carrying out the corresponding sanitation measures, subsequently using them for various uses, or simply for their landscape integration.

In 2000, the Council of Ministers approved the National Urban Waste Plan 2000-2006, which allows compliance with the Waste Framework Directive of the European Union, as well as developing the Waste Law of 1998. Among some of its objectives is the closure and sealing of uncontrolled landfills that exist in our country.

They are disposal facilities intended for the deposit of waste, located in appropriate locations, where the waste is placed in an orderly manner and under safe and supervised conditions, which aim to avoid problems of water, air and soil contamination. These types of landfills are increasingly being used today.

The characteristics and factors that are taken into account for the installation and good maintenance of the landfill are as follows:

After a certain time has passed and the activities in the landfill have been completed to reach its maximum capacity, it is closed and sealed, to later reforest and restore the area so that it can be used for other uses.

This method of waste disposal, which began to be used in the last decades of the century in England, consists of a controlled thermal combustion process that triggers an oxidation of the carbon and hydrogen present in the organic matter that constitutes the waste, obtaining as products ash, carbon dioxide and water, in addition to dioxins and furans (highly toxic air pollutants) in the event that plastics containing PVC are found among the waste. With incineration it is possible to reduce the volume by 90% and the weight of garbage by 30%, in a short time.

The products resulting from incineration must have their treatment, management and final disposal planned. During the combustion process carried out in incineration plants, large amounts of energy are released, which can be used to generate electrical energy.

It consists of an aerobic process based on the biochemical degradation of the organic matter that is part of the waste. The degradation of matter is carried out by bacteria and fungi present in the waste products themselves, and its main objective is to obtain a biochemically stable compound called compost.

The composting process consists of three stages, throughout which organic matter and carbohydrates are consumed, and carbon dioxide and carbon are released, thus favoring the generation of humic materials:

On some occasions, sludge from water treatment plants (WWTP) is mixed with the organic fraction of urban waste, since the former has a high content of microorganisms that degrade organic matter, and consequently, the composting process is accelerated.

In Spain there are 38 composting plants, most of them located in the Levant and the south of the country. In fact, in the Region of Murcia there is one of the largest composting plants in all of Europe.

Biomethanation is an anaerobic fermentation process of the organic fraction present in waste, through which biogas is obtained. But in addition to this gas, during the anaerobic fermentation process a slightly basic (pH = 7.5) and unstabilized product is also produced, to which a series of fertilizing properties are attributed, as it acts to improve part of the physical characteristics of the soil (increases moisture retention and the amount of water infiltration).

The composition of this product will undergo significant variations depending on the type of organic matter that is degraded, although on average it is made up of 85% organic matter, 2.6% nitrogen and has a percentage of less than 2% phosphorus and potassium.

Biomethanization has a series of advantages over other waste treatment methods, since during its development no pestilence is produced and, furthermore, the fact that it is an anaerobic process favors the minimization of microorganisms that are harmful to human and animal health in a short period of time.

For biomethanization to occur, sealed structures are required that allow certain parameters such as pH, pressure or temperature to be kept under control during fermentation, called biodigesters. In addition, these constructions are also required due to the need to create an oxygen-free environment for the development of anaerobic bacteria.

The characteristics of urban solid waste mean that they cause a series of problems, which can be more or less serious depending on the situation, when they are not treated appropriately. Below are the most common effects caused by MSW:

Heavy metals are also present in leachates that have a high toxicity index, and are therefore harmful to health. Below are some of these materials found in leachates:

— Lead (Pb): this chemical element comes from "Battery (electricity)" batteries in vehicles and a wide variety of electronic devices (in recent years, lead batteries have been replaced by lithium ones, especially in technological objects), as well as from plastics, glass, ceramics and pigments.[7]​.

When this compound penetrates our body, a series of anomalies occur in the nervous system, which manifest themselves in the form of cognitive losses and weakness in different parts of our body, especially in the fingers, wrists and ankles. In addition, pregnant women are more likely to suffer miscarriages and sperm production in men can be profoundly reduced. Another effect produced by lead is the radical impoverishment of the blood, which in medical terminology is called anemia.[10]​.

Although it is not scientifically proven, some doctors think that lead is potentially carcinogenic, since people with high exposure to this compound have developed kidney tumors and even brain tumors.[10]​.

— Mercury "Mercury (element)") (Hg): this chemical element comes mainly from "Battery (electricity)") batteries (especially pagers, mobile equipment...), compact fluorescent lamps or alkaline "Battery (electricity)") batteries, although mercury is generated in large quantities in the health sector, where products such as thermometers, vaccines with thimerosal (a product that helps to preserve them), gastric tubes "Probe (biology)") or dental amalgams "Amalgam (dentistry)"), which are largely made up of this heavy metal.[11]​.

If for various reasons mercury penetrates the human body, it must be taken into account that it is an extremely powerful neurotoxin, which will attack the central nervous system.[12]​ In the absence of treatment, the half-life of mercury in the human brain is close to 27.4 years.[13]​ In addition, it can cause irreversible damage to the kidneys and lungs, as well as having the ability to cross the placenta and the blood-brain barrier. When mercury enters the body of a pregnant woman, there is a risk that the unborn child will suffer from deafness, blindness, cerebral palsy, speech difficulties or strokes in the future.[11]​.

When leachates containing mercury reach the sea, this compound disperses and aquatic living beings ingest it in such a way that it accumulates in their bodies. In this way, when these species are captured and marketed, consumers, without realizing it, are introducing considerable amounts of mercury "Mercury (element)") by eating the fish they had acquired due to the presence of mercury in fish, inducing mercury poisoning.

— Cadmium (Cd): this element is present in some low-melting alloys, in solders, in antioxidants (mainly in those that cover iron or brass), in certain pigments and in pieces of jewelry, as well as some cadmium compounds are used as stabilizers for plastics (as is the case of PVC).[14]​.

When a person inhales cadmium they are at risk of suffering from a wide variety of lung diseases, occasionally leading to death. Furthermore, this chemical element damages other organs of the human body such as the liver and kidneys (alteration in the filtration mechanism, with the consequent loss of vital proteins and sugars). Other harmful effects that occur on human health as a consequence of exposure to cadmium are the increase in the fragility of the body's bone structure, infertility, alterations in the central nervous system and the immune system and the appearance of gastrointestinal ailments.[14]​.

Prevention in the production of urban waste is the first of the strategies contemplated. It is not part of management in itself because it is a prior step, but it will be considered here because it is closely related to it.

According to the National Urban Waste Plan, prevention and minimization is understood as the set of measures aimed at achieving a reduction in the production of urban waste as well as the amount of dangerous substances and pollutants present in them.

To do this, it is necessary to act in the following stages of the process:

1. Manufacturing. Here its danger, volume and weight can be reduced. The product must be designed in a way that facilitates its reuse and recycling.

2. Transportation. Reducing unnecessary containers and packaging as much as possible.

3. Consumption. Favoring reuse, lower waste generation through changes in consumption habits and ease of separation.

It should be noted that all those measures leading to prolonging the useful life of items and their ease of repair and reuse help reduce the production of urban waste.

To achieve the objective of reducing the production of urban waste, the aforementioned Urban Waste Plan establishes a series of measures that exemplify very well what must be done in this field:

1. Agreements between the Administration and the productive sectors involved.

2. Promotion of recovery and reuse at source through business prevention plans.

3. Regulations that prioritize the minimization of certain hazardous waste.•

4. Adoption of systems that tax excess waste production and impact the cost of correct waste management on garbage rates.

5. Promotion of R&D aimed at improving the recyclability of articles and reducing the danger of the waste generated.

6. Development of educational and training campaigns aimed at promoting minimization.

The National Urban Waste Plan aimed to stabilize MSW production at 1996 levels by the end of 2002. The aim is to counteract the foreseeable increase due to demographic growth and changes in consumption habits. The aim is to achieve a 6% reduction in the amount of MSW per capita.[17]​.

Evolution in waste production

The total amount of waste generated per year in Spain experienced an increase of 95.9% in the period between 1990 and 2007, placing the total amount of waste produced in this last year at 25,584,000 tons of waste. Therefore, the generation of urban waste considered from all perspectives is experiencing extremely high growth in the Spanish country. In part, this is due to the non-compliance with certain parameters covered in the national waste plans (as an example is the production of eight million more tons than those foreseen in the National Urban Waste Plan 2000-2006 in 2006), the progressive growth of the Spanish population in recent decades and the fact that Spain is the first tourist destination in the European Union.

Only in 2006 did waste production suffer a small decrease compared to the previous year, although in 2007 waste generation would increase again.

Spain and Europe

For four years, the trend towards stabilization of urban waste production has been generalized in most countries that are part of the European continent, with some exceptions such as Spain, where there is still a trend towards growth in residual production. In fact, in 2006 the residual production per capita in Spain slightly exceeded the average of the EU-27 countries (537 kg/inhabitant/year compared to 517 kg/inhabitant/year), although it is true that there are countries with much higher rates (for example, Ireland produced about 804 kg/inhabitant/year in 2006).

In 2012, the Spanish average in waste production per capita was 484.4 kg per inhabitant and year.

The composition of all urban waste varies depending on three factors, which are the population's standard of living, the activity carried out by it and the region's own climate. Depending on these factors, certain products will be consumed and used, which will ultimately produce the corresponding waste. According to the National Urban Waste Plan (PNRU) 2000-2006, the average production in Spain of the different components of urban waste is shown below:

Es fundamental conocer algunas de las propiedades físicas de los residuos sólidos urbanos para prever y organizar los sistemas de recogida de basura, los tratamientos finales de reciclado o eliminación, así como para decidir cuales son los sistemas de segregación más apropiados en el caso de los residuos que generen riesgos especiales para el medio ambiente.

Degree of humidity

Moisture is present in urban waste at an approximate percentage of 40% by weight; although it is true that it can range between 25% and 60%. The maximum contribution of moisture is provided by those offal that contain organic matter, while the minimum is provided by products of a synthetic nature. This characteristic must be taken into account due to its importance in waste compression processes, leachate production, transportation, transformation processes, incineration or energy recovery treatments, and waste separation processes in the corresponding recycling plant.[4]​.

In municipal solid waste, moisture tends to homogenize, since some products transfer it to others. This is one of the main causes of degradation of certain products such as paper, which absorbs moisture from organic waste, losing characteristics and also value in mechanical recycling processes over recycling at source (which is carried out by citizens in their own homes), which avoids this contact.[4]​.

Specific weight

The density of urban waste is of vital importance to calculate the dimensions of pre-collection containers, both in private homes and public roads (streets, avenues, squares, parks...), as well as a basic factor that determines the volumes of collection and transport equipment, reception hoppers, belts or landfill capacity. The density may vary depending on the degree of compaction to which the waste is subjected. Volume reduction is present in all phases of waste management and is used to optimize the operation, since the large space they occupy is one of the biggest problems faced by the plants in charge of processing it.[4]​.

The specific unit weight of each product does not indicate that the set has an overall value proportional to that of its components. In fact, in homes, these values ​​are usually significantly higher due to the unused spaces in the container that contains the garbage. However, as they are grouped more homogeneously, they will approach the mathematical calculation that gives theoretical average values ​​for uncompacted waste of 80 kg/m³, with considerable variations linked to the composition of the waste in each location.[4]​.

Granulometry

The degree of segregation of the materials and the physical size of the elemental components of urban waste represent an essential value for the calculation of dimensions in mechanical separation processes and, especially, for choosing screens, trommels and similar devices that base their work, consisting of separation, on size. These same values ​​must be taken with great care, since in collection operations the dimensions are affected as a consequence of compression or crushing mechanisms.[4]​.

Conocer con exactitud las propiedades químicas de los residuos urbanos es un hecho de suma importancia en el tratamiento de estos, puesto que estas propiedades son determinantes para los procesos de recuperación y tratamiento final. Probablemente sean el poder calorífico y el porcentaje de cenizas producidas las características químicas que mayor importancia revisten, ya que son esenciales en los procesos de recuperación energética. Aunque tampoco se deben de subestimar propiedades como la eventual presencia de productos tóxicos, metales pesados o contenido de materiales inertes, debido a la importancia que tienen respecto al diseño de soluciones adecuadas en los procesos de recuperación y para la toma de precauciones hiegiénicas y sanitarias.[5]​.

Chemical composition

To determine the characteristics of energy recovery and the potential for producing fertilizers with the appropriate carbon/nitrogen ratio, it is necessary to study the composition of each waste. It is also necessary to determine the presence and concentration of toxic and dangerous waste to evaluate the risk that its handling may entail.

As a consequence of the enormous variability experienced in the composition of urban solid waste, the resulting chemical composition of the whole is also highly variable.

It is necessary to know the composition of a specific waste to determine its energy recovery characteristics and the potential to produce fertilizers with the appropriate carbon/nitrogen ratio. It is also advisable to know the presence and concentration of toxic and dangerous waste to evaluate the risk that its handling, treatment, reprocessing and reuse may pose to human health and the environment. Arsenic, cadmium, mercury "Mercury (element)"), antimony, chlorinated solvents, elements with characteristics of flammability, corrosivity, reactivity, ecotoxicity, toxicity or carcinogenic, mutagenic or teratological qualities, are usually present in urban waste, normally from industrial and hospital activities.

heating value

The calorimetric characteristics of urban waste determine the design of the facilities that must be installed and the energy recovery. The assessment, which is the result of the variability of the composition of the waste, is predefined by the calorific value of each product.[5]​.

Broadly speaking, it can be stipulated that the calorific value of all urban waste is around 1500 and 2200 kcal/kg. Another value of great interest is the melting and solidification temperature of the ashes from the combustion of these materials, melting at a temperature of 1,200 °C.[5]​.

De acuerdo con la normativa vigente, la gestión de residuos sólidos urbanos comprende la recogida, almacenamiento, transporte, valorización y eliminación (o transformación) de los mismos, siendo también considerada como parte de la gestión la vigilancia de las actividades citadas, además de la vigilancia ejercida sobre los lugares de alojamiento de residuos tras su clausura. Además, esta se halla dividida en cuatro fases diferentes:.

Pre-pick

The term pre-collection includes waste manipulation, separation, storage and processing at source, with the intention of concentrating urban waste, and may modify some of its physical characteristics with the aim of facilitating its collection. The separation for recycling of paper, cardboard or any type of packaging (aluminum cans, glass bottles or plastic jars, for example) in this phase, which is the closest to its generation, is of great importance to achieve the highest possible purity of these fractions. Respect for the schedule established by municipal ordinances for transporting waste from production sites to pre-collection points (in terms of non-buried containers) is also a relevant factor, to avoid bad odors and visual pollution.

Pickup

There are two different types of collection, the collection that is carried out by means of adapted vehicles and the pneumatic:.

This type of collection has a series of advantages over the traditional method, since its installation improves the quality of life of citizens, makes it easier to reduce noise and bad odors.

Transport

It is at this stage when the waste is transferred to transfer stations (facilities in which waste is temporarily stored and compacted to be subsequently transported to the areas where it will be treated by vehicles with large capacity), recycling, classification or energy recovery plants and landfills.

In most Spanish municipalities, compressor collection trucks with a capacity ranging between 10 and 20 m³ are responsible for transporting waste. Although on some occasions and due to the characteristics of the areas in which they operate (narrow public roads, for example), it is necessary to resort to smaller vehicles that lack compression capacity.

Transformation, energy recovery or elimination

The final stage of waste management has three possible variants depending on the nature of the waste components and the possibilities of the region in which they are treated. Therefore, waste components may be transformed in order to obtain new products with other applications (composting and biomethanation"), energetically recovered with the sole purpose of converting them into fuel with which to generate energy (gasification, hydrogenation, pyrolysis, oxidation and, in some cases, incineration) or eliminated.

The use of clandestine landfills, which are places where waste accumulates without any type of control, was the first method adopted by humans to eliminate waste, because it was simple and cheap. However, it causes serious environmental problems (landscape alteration, bad odors, soil, water and air pollution) and health problems (diseases transmitted by rodents or insects), lacking adequate controls.

Currently, landfills classified as uncontrolled are sealed and closed, carrying out the corresponding sanitation measures, subsequently using them for various uses, or simply for their landscape integration.

In 2000, the Council of Ministers approved the National Urban Waste Plan 2000-2006, which allows compliance with the Waste Framework Directive of the European Union, as well as developing the Waste Law of 1998. Among some of its objectives is the closure and sealing of uncontrolled landfills that exist in our country.

They are disposal facilities intended for the deposit of waste, located in appropriate locations, where the waste is placed in an orderly manner and under safe and supervised conditions, which aim to avoid problems of water, air and soil contamination. These types of landfills are increasingly being used today.

The characteristics and factors that are taken into account for the installation and good maintenance of the landfill are as follows:

After a certain time has passed and the activities in the landfill have been completed to reach its maximum capacity, it is closed and sealed, to later reforest and restore the area so that it can be used for other uses.

This method of waste disposal, which began to be used in the last decades of the century in England, consists of a controlled thermal combustion process that triggers an oxidation of the carbon and hydrogen present in the organic matter that constitutes the waste, obtaining as products ash, carbon dioxide and water, in addition to dioxins and furans (highly toxic air pollutants) in the event that plastics containing PVC are found among the waste. With incineration it is possible to reduce the volume by 90% and the weight of garbage by 30%, in a short time.

The products resulting from incineration must have their treatment, management and final disposal planned. During the combustion process carried out in incineration plants, large amounts of energy are released, which can be used to generate electrical energy.

It consists of an aerobic process based on the biochemical degradation of the organic matter that is part of the waste. The degradation of matter is carried out by bacteria and fungi present in the waste products themselves, and its main objective is to obtain a biochemically stable compound called compost.

The composting process consists of three stages, throughout which organic matter and carbohydrates are consumed, and carbon dioxide and carbon are released, thus favoring the generation of humic materials:

On some occasions, sludge from water treatment plants (WWTP) is mixed with the organic fraction of urban waste, since the former has a high content of microorganisms that degrade organic matter, and consequently, the composting process is accelerated.

In Spain there are 38 composting plants, most of them located in the Levant and the south of the country. In fact, in the Region of Murcia there is one of the largest composting plants in all of Europe.

Biomethanation is an anaerobic fermentation process of the organic fraction present in waste, through which biogas is obtained. But in addition to this gas, during the anaerobic fermentation process a slightly basic (pH = 7.5) and unstabilized product is also produced, to which a series of fertilizing properties are attributed, as it acts to improve part of the physical characteristics of the soil (increases moisture retention and the amount of water infiltration).

The composition of this product will undergo significant variations depending on the type of organic matter that is degraded, although on average it is made up of 85% organic matter, 2.6% nitrogen and has a percentage of less than 2% phosphorus and potassium.

Biomethanization has a series of advantages over other waste treatment methods, since during its development no pestilence is produced and, furthermore, the fact that it is an anaerobic process favors the minimization of microorganisms that are harmful to human and animal health in a short period of time.

For biomethanization to occur, sealed structures are required that allow certain parameters such as pH, pressure or temperature to be kept under control during fermentation, called biodigesters. In addition, these constructions are also required due to the need to create an oxygen-free environment for the development of anaerobic bacteria.

The characteristics of urban solid waste mean that they cause a series of problems, which can be more or less serious depending on the situation, when they are not treated appropriately. Below are the most common effects caused by MSW:

Heavy metals are also present in leachates that have a high toxicity index, and are therefore harmful to health. Below are some of these materials found in leachates:

— Lead (Pb): this chemical element comes from "Battery (electricity)" batteries in vehicles and a wide variety of electronic devices (in recent years, lead batteries have been replaced by lithium ones, especially in technological objects), as well as from plastics, glass, ceramics and pigments.[7]​.

When this compound penetrates our body, a series of anomalies occur in the nervous system, which manifest themselves in the form of cognitive losses and weakness in different parts of our body, especially in the fingers, wrists and ankles. In addition, pregnant women are more likely to suffer miscarriages and sperm production in men can be profoundly reduced. Another effect produced by lead is the radical impoverishment of the blood, which in medical terminology is called anemia.[10]​.

Although it is not scientifically proven, some doctors think that lead is potentially carcinogenic, since people with high exposure to this compound have developed kidney tumors and even brain tumors.[10]​.

— Mercury "Mercury (element)") (Hg): this chemical element comes mainly from "Battery (electricity)") batteries (especially pagers, mobile equipment...), compact fluorescent lamps or alkaline "Battery (electricity)") batteries, although mercury is generated in large quantities in the health sector, where products such as thermometers, vaccines with thimerosal (a product that helps to preserve them), gastric tubes "Probe (biology)") or dental amalgams "Amalgam (dentistry)"), which are largely made up of this heavy metal.[11]​.

If for various reasons mercury penetrates the human body, it must be taken into account that it is an extremely powerful neurotoxin, which will attack the central nervous system.[12]​ In the absence of treatment, the half-life of mercury in the human brain is close to 27.4 years.[13]​ In addition, it can cause irreversible damage to the kidneys and lungs, as well as having the ability to cross the placenta and the blood-brain barrier. When mercury enters the body of a pregnant woman, there is a risk that the unborn child will suffer from deafness, blindness, cerebral palsy, speech difficulties or strokes in the future.[11]​.

When leachates containing mercury reach the sea, this compound disperses and aquatic living beings ingest it in such a way that it accumulates in their bodies. In this way, when these species are captured and marketed, consumers, without realizing it, are introducing considerable amounts of mercury "Mercury (element)") by eating the fish they had acquired due to the presence of mercury in fish, inducing mercury poisoning.

— Cadmium (Cd): this element is present in some low-melting alloys, in solders, in antioxidants (mainly in those that cover iron or brass), in certain pigments and in pieces of jewelry, as well as some cadmium compounds are used as stabilizers for plastics (as is the case of PVC).[14]​.

When a person inhales cadmium they are at risk of suffering from a wide variety of lung diseases, occasionally leading to death. Furthermore, this chemical element damages other organs of the human body such as the liver and kidneys (alteration in the filtration mechanism, with the consequent loss of vital proteins and sugars). Other harmful effects that occur on human health as a consequence of exposure to cadmium are the increase in the fragility of the body's bone structure, infertility, alterations in the central nervous system and the immune system and the appearance of gastrointestinal ailments.[14]​.

Prevention in the production of urban waste is the first of the strategies contemplated. It is not part of management in itself because it is a prior step, but it will be considered here because it is closely related to it.

According to the National Urban Waste Plan, prevention and minimization is understood as the set of measures aimed at achieving a reduction in the production of urban waste as well as the amount of dangerous substances and pollutants present in them.

To do this, it is necessary to act in the following stages of the process:

1. Manufacturing. Here its danger, volume and weight can be reduced. The product must be designed in a way that facilitates its reuse and recycling.

2. Transportation. Reducing unnecessary containers and packaging as much as possible.

3. Consumption. Favoring reuse, lower waste generation through changes in consumption habits and ease of separation.

It should be noted that all those measures leading to prolonging the useful life of items and their ease of repair and reuse help reduce the production of urban waste.

To achieve the objective of reducing the production of urban waste, the aforementioned Urban Waste Plan establishes a series of measures that exemplify very well what must be done in this field:

1. Agreements between the Administration and the productive sectors involved.

2. Promotion of recovery and reuse at source through business prevention plans.

3. Regulations that prioritize the minimization of certain hazardous waste.•

4. Adoption of systems that tax excess waste production and impact the cost of correct waste management on garbage rates.

5. Promotion of R&D aimed at improving the recyclability of articles and reducing the danger of the waste generated.

6. Development of educational and training campaigns aimed at promoting minimization.

The National Urban Waste Plan aimed to stabilize MSW production at 1996 levels by the end of 2002. The aim is to counteract the foreseeable increase due to demographic growth and changes in consumption habits. The aim is to achieve a 6% reduction in the amount of MSW per capita.[17]​.