Garbage or, formally, municipal solid waste (MSW) is the material eliminated from homes or public roads when it no longer fulfills the function for which it was created. Traditionally, basic and higher education institutions have tried to raise awareness among the student community and society in general about the problems involved in the generation and poor disposal of these wastes.

The activities that have been proposed as alternatives to minimize this problem are recycling and reuse campaigns for some materials such as plastic and paper. Besides promoting a culture of order with respect to the disposal and reduction of the amount of waste to be eliminated. On the other hand, little has been reported about waste energy recovery technologies. These are technological processes whose end product has an energy value as methane (CH4 ) or hydrogen (H2 ), which are of interest from a social, economic, and ecological point of view.

What is the problem of municipal solid waste?

Generally speaking, municipal solid waste can be divided into two main groups: a) organic waste such as fruit and vegetable waste, food and garden waste; b) inorganic waste, which includes plastic, glass, metal, and other materials. Here it is important to mention that, as a consequence of improper disposal, municipal solid waste causes ecological and social problems. When improperly disposed of, MSW pollutes soil, water, and air. During the decomposition process of organic municipal solid waste, gases such as carbon dioxide and monoxide (CO2 and CO, respectively), methane (CH4 ), hydrogen sulfide (H2S), and volatile organic compounds (such as benzene and acetone) are generated.

These are toxic, generate bad odors, and contribute to climate change. Furthermore, rainwater passing through a municipal solid waste landfill, as well as water released from the waste itself, carries with it high amounts of organic and inorganic substances that end up in soils and water bodies. A fraction of this leachate is degraded by microorganisms that produce acidic or basic substances, such as organic acids and ammonium (NH4+), which cause an imbalance in the pH of the medium. Other fractions of leachates are also toxic and therefore directly affect soil ecology.

First, open dumps attract and facilitate the proliferation of insects, birds, and mammals that can transmit diseases such as cholera, salmonellosis, dengue fever, and amebiasis, among others; the other problem is that large tracts of land are required for the final disposal of waste, the production of which is growing and increasingly close to population settlements.

Generation and management of municipal solid waste

In Mexico, 70% of the waste generated is urban waste, in mass, this is equivalent to 42,106 million tons per year or 115,359 tons per day, each inhabitant of the country contributes generating an average of 0.99 kg of waste per day. The above, considering an approximate value to the average per capita production for Latin American and Caribbean countries, which is 1.1 kg/person/day. However, for some countries, production is only 0.11 kg/person/day, as in the case of Uruguay, and in others, such as Trinidad and Tobago, production is up to 14 kg/person/day.

This variation in municipal solid waste production is associated with the level of development of each country, in which the predominant economic activities not only dictate the amount of municipal solid waste produced, but also its composition and the management mechanisms and disposal or treatment technologies. In Mexico, in 1950, when the country's economic transition (industrialization) began, the percentage of organic municipal solid waste was 70%, while by 2012 this fraction was already at 52% (Inegi, 2015).

In all cases, management activities should focus on depositing as little waste as possible in final confinement sites such as sanitary landfills. In industrialized or tourist regions, most of the waste generated is of inorganic nature; therefore, an effective mechanism for reducing the volume of waste is the reuse and recycling of materials such as PET, glass, cardboard, and metals. In places where agricultural and livestock activities predominate, the largest fraction of waste is organic, so the area for its use is biological treatment processes to generate energy and obtain stabilized materials. In other words, materials that can no longer be biologically degraded and therefore no longer have an adverse effect on the environment.

In developing countries, there are three types of disposal systems: landfills, controlled sites, and open dumps. The last two are only confinement sites in which the flow of leachate and the emission of gases into the atmosphere are not controlled. Landfills are engineering works in which an expanse of the earth is impermeabilized and on which municipal solid waste is deposited in layers, each layer of waste is covered with a layer of earth, and so on. In addition, there are systems of channels and flares to control the leachate and gases generated, preventing their flow into the soil and the atmosphere, respectively.

In Mexico, there are 260 landfills, where around 28 million tons of municipal solid waste are disposed of (Semarnat, 2017). In these landfills, waste can be trapped in anaerobic environments (absence of oxygen) when covered with soil, in this condition, microorganisms are reproduced that when consuming the organic fraction of MSW produce biogas, so that a defective operation of the landfills results in the emission of 10% methane of the global total and bad odors due to hydrogen sulfide H2S and other volatile organic compounds.

The metropolitan area of Monterrey, Nuevo León, is a pioneer in Latin America in energy recovery of the methane produced in its sanitary landfill. In 2003, the public body in charge of the management of municipal solid waste (Integrated System for Ecological Management and Waste Processing -Simeprode-) through the company Bioenergía de Nuevo León, S.A. (Benlesa) builds an electric power generation plant with a capacity of 7.42 MW.h that increased to 20.8 MW.h in 2015. The co-generated energy is destined for the consumption of government offices, public lighting, and the operation of the metro from Monterrey.

On the contrary, in regions where the only purpose of sanitary landfills is to confine waste, it is necessary to extend their life span and control liquid and gaseous emissions. This can be achieved by reducing the amount of waste to be disposed of through the practice of activities such as separation at source and recycling of materials.

In some regions, these activities have not been successful due to several factors. The first of these is the lack of adequate infrastructure; for example, in some cities, containers are placed to deposit garbage separated as organic, metals, plastics, etc., but since there are no adequate means of transportation for each of them, the waste is mixed again; the second refers to the fact that the participation of the population in this classification depends on obtaining some kind of remuneration. In Mexico, 39% of the people who dispose of their waste do so for monetary gain.

When municipal solid waste separation is successfully carried out, the need arises to implement treatment technologies for the non-recoverable fractions, the most widespread worldwide being thermal and biological treatment. Both options have the characteristic of having a positive energy balance, i.e., the process generates the energy necessary for its operation and an extra that can be used, for example, for public lighting.

Heat treatment technologies

After recycling, the remaining waste can be sent for final disposal or disposed of by some method of treatment. Thermal treatments are physicochemical processes that use heat to degrade the waste, thereby recovering metals and releasing heat energy that can be used in industry to generate process steam or electricity. Combustion, pyrolysis, and gasification are thermal technologies collectively referred to as WTE (Waste to Energy).

Combustion reduces the amount of waste by 95%. The waste is burned, whereby temperatures of 1200°C are reached, the oxygen required for combustion is taken from the air, and ash, water, and gases (mainly carbon dioxide) are generated. For each ton of municipal solid waste treated in this way, 685 kW.h are released, of which 339 kW.h are consumed by the process itself.

By applying pressures higher than atmospheric pressure and temperatures of 700°C in oxygen-free conditions, the waste is transformed into gases with high calorific value such as hydrogen (H2 ), methane (CH4 ), and others such as carbon monoxide (CO). By using the fuel gases generated, 544 kW.h of energy can be obtained, of which only 78 kW.h are consumed in the treatment process.

The gasification process seeks to produce combustible gases, such as H2 and CH4, from municipal solid waste. This is achieved by partially incinerating the waste, requiring less oxygen than total combustion. The temperatures reached are around 1100°C, and the energy balance is similar to the combustion process.

All three options reduce municipal solid waste to ash, from which metals can be recovered, and in the case of the ash from the gasification process, it can also be used for construction due to its vitreous characteristics. So, after applying a treatment process of this type, if there was a need to confine one ton of municipal solid waste in a sanitary landfill, now only 50 kg of ashes will have to be disposed of, which are not subject to the action of microorganisms, eliminating the production of polluting biogas and bad odors at the disposal site.

Pyrolysis has the best energy balance, with a net energy gain of 466 kW.h per ton of waste treated, although with the other technologies there are also positive gains of 345 kW.h. The downside of thermal treatments is that their gaseous by-products are toxic, such as hydrochloric acid (HCl), hydrogen sulfide (H2S), hydrogen cyanide (HCN), ammonia (NH3), and nitrogen oxides (NOx). It is therefore important to analyze the technical and economic aspects, energy yields, and environmental impact in order to choose the most appropriate treatment process for each situation.

Biological treatment technologies

In a natural way, the transformation of matter by various microorganisms takes place in the environment. An example of this is when the peel of a fruit is deposited on the soil of a garden, after some time it is transformed into a material with characteristics similar to those of the soil. That is, microorganisms growing on the surface of the peel consume it, producing CO2, water, and mineralizing the material. In swamps, the water is saturated with solids, so there is no gaseous oxygen (O2) dissolved in the medium, in these anaerobic conditions, the metabolism of the microorganisms present is different, so the material they consume is transformed into methane and carbon dioxide, mainly. Biological waste treatment technologies are the application of the biological processes that occur in the environment, reproducing these phenomena in systems (engineering works) where the physicochemical characteristics maximize the activity of the microorganisms responsible for the biotransformation of the materials are controlled.

On the other hand, composting is the aerobic technology for the treatment of organic waste. In this technology, a mixture of organic waste, soil, and support material, which can be wood chips, is deposited in devices or on impermeable surfaces; these materials must be periodically mixed with the purpose of supplying them with air. The microorganisms present in the soil carry out the degradation of the organic municipal solid waste. The support material increases the porosity of the mixture, allowing air to diffuse through it, avoiding the generation of anaerobic zones where gases such as methane and aromatic compounds would be produced. The end product of the process is a stabilized material (30% of the initial volume of waste subjected to treatment) with fertilizing characteristics due to its high nitrogen and phosphorous content. The main weakness of this technology is the long process time (more than 90 days), which implicitly means the need for large extensions of land.

Anaerobic digestion processes are the other biological alternative. It is carried out in bioreactors, closed devices to ensure anaerobic environments inside them, which are controlled at a neutral pH (value of 7) and operating temperature (usually set at a value of 36 or 45°C). Bioreactors can be tanks equipped with impellers that promote mixing and homogenization of the materials throughout the working volume, or tubular devices with mechanisms that force the materials to pass like a plunger through their entire length. The organic municipal solid waste is fed to the bioreactors together with wastewater (water that has already been used in another activity) to adjust the solids concentration to a value of less than 35%. A group of anaerobic microorganisms is in charge of disintegrating and producing organic acids and finally producing biogas with high methane content (>70%). At the end of the process, we have a material rich in nutrients such as nitrogen and phosphorus that can be used as a soil improver and biogas with a volumetric calorific value of 10.49 kW.h.m3 that can be used as fuel directly or to generate electricity.

An energy balance of an anaerobic process can be made by taking as a basis for calculating the generation of municipal solid waste, estimated for a polygon of 100,000 inhabitants (99 tons per day), of which 52% is organic in nature. If this municipal solid waste is treated by an optimally operated system, 3434.75 m3 /d of methane and 1.10 t/d of compost would be generated, and 102.75 m3 of wastewater would be treated. The compost generated, as well as the treated water, are valuable products for agricultural activities. The methane resulting from the process would yield 36,030.53 kW.h/d of energy, considering a volumetric calorific value of 10.49 kW.h./m3 for methane, which is enough to supply electricity to 1242 typical Mexican homes. In another scenario, if the energy generated is used for other purposes such as public lighting, a municipality would obtain savings of between 2.6 and 10 million pesos per year (considering prices of between 0.85 and 2 pesos per kW.h).

Conclusions

Currently, the generation of municipal solid waste is a social, economic, and ecological problem that is most prevalent in underdeveloped and developing countries. All human activities generate waste that must be disposed of or confined in a way that minimizes the negative effect on the environment.

Thanks to scientific research and technological development in this field, thermal and biological treatment technologies have been proposed that make it possible to reduce the amount of waste to be disposed of in landfills by up to 95%, thus prolonging its useful life and preventing large areas of land from being used for storage. In addition, the products of these technologies are mainly combustible gases, thus changing the social-economic focus from a problem situation to a situation of energy opportunity. In countries such as Mexico, the proportion of organic MSW opens the possibility of implementing these technologies. However, it is necessary for the government at different levels to provide the necessary infrastructure to encourage the culture of waste separation and recycling, key aspects for the success of sustainable municipal solid waste management.

By José Vian-Pérez, Alejandra Velasco-Pérez, and Tania García-Herrera, Source: CIENCIA UANL / No.97