The continually growing industrial production and trade in many countries worldwide in the last decennia has accompanied a rapid increase of the production of municipal and industrial waste [1
]. In the second half of the 1990s, the annual volume of waste production ranged between (300–800) kg per capita in developed countries and less than 200 kg per capita in other countries [5
]. The global waste will nearly double to 2.2 billion annual tones by 2025 and almost all the cities in the world are struggling to meet their waste reduction targets [7
]. Landfills and/or open dumpsites were the common practice for municipal solid waste (MSW) disposal all over the world for example, in the U.S.A., 52.6% of MSW was discarded in landfills [8
], in Brazil 59.1% [9
], in The Kingdom of Saudi Arabia (KSA) 85% [10
], in Malaysia 94.5% [11
], in China 79% [12
], in Venezuela: sanitary landfills 32%, controlled disposal 43% and non-controlled disposals or open dumps 24% [13
], in Mexico: sanitary landfills 65%, in uncontrolled and open dumps 30% [14
] and in Thailand 27% [15
The method of landfilling for final disposal of MSW remains a generally accepted and used method thanks to its economic advantages [16
]. Landfilling is defined as the disposal, compression and embankment fill of waste at appropriate sites Landfill for the moment is easy, adjustable with lower cost than the rest of disposal methods and stands alone as the only all waste material disposal method. Although the disposal of MSW in landfills has decreased, landfills are likely to remain an important part of integrated solid waste management systems all over the word. Comparative studies of various waste management (WM) methods (landfilling, incineration, composting etc.) show that among the MSW treatment and disposal technological options, sanitary landfilling or open dumping is popular in most countries because of the relative low cost and low-technical requirement [16
The European Union (EU) Directive on waste landfills has introduced specific goals for reducing the volume of disposed waste and very strict requirements for landfilling and landfilling sites [19
]. Regardless of this directive, the situation in Europe is not homogeneous. For example, Switzerland, Germany, the Netherlands, Sweden, Austria, Denmark and Belgium report landfilling below 5% of waste produced [21
]. In new member countries, candidate countries and on islands, landfilling is still the prevailing technology of WM. For example, the representation of landfilled waste in Poland is 37%, 38% in Slovakia and 64% in Bulgaria, while in the Czech Republic (CR) it reaches 50% [22
The hypothesis is that the landfills can be a source of environmental pollution and risk. The main aims of this review were to: (i) summarize the most recent scientific information on waste disposal options, (ii) present the direct and indirect impact of WM activities on the environment based on literature research.
The principal sources of emissions from landfill sites are as follows: the waste materials as they are brought onto site; emissions from transport; waste blown by the wind; dust generated from the landfill surface; landfill gas generated; leachate produced. The literature search was carried out online databases and included studies and reviews of impacts of WM methods in particular MSW landfills on the environment. In total, 116 references were selected for inclusion in this study; these satisfied the principal criterion for the study that the reported data must come from clearly defined sources and must be accurate. Results of this research may have an important impact on landfill management and the disposal of waste.
2. Waste Production in the European Union
The mass of waste produced in the world as well as in EU countries has been growing considerably for many decades. As mentioned above, the most practical solution for WM in a majority of EU countries remains landfilling due to technical, economic and legal reasons (Table 1
Disposal in landfills are considered an effective method for WM [27
]; however, there are many reasons why landfilling appears as the least rational method of WM. Waste can be landfilled only for limited time and the period of landfill reclamation may last up to hundreds of years [28
]. Biogas and leachates can seriously impact the environment [21
]. In addition, MSW is disposed on landfills without any sorting in numerous countries. Moreover, for the majority of landfills worldwide, unfavorable odor and air pollution bring about serious sanitary problems and are the main causes for the “not in my backyard (NIMBY)” syndrome for the nearby communities [31
]. WM has recorded significant development and its regulation imposed by legislation (e.g., setting of goals for recycling and permitted amount of landfilled biologically degradable waste) curbs the rate of landfill extension now. Evaluation of the impact of landfills on the environment (Figure 1
) is a crucial topic in the literature and has received increased attention recently, given growing environmental concerns.
3. Landfill as a Potential Source of Pollution
The need for the collection and sanitary disposal of MSW was not recognized until recently. Fifty years ago, throughout the world, most MSW was disposed of in open dumps or tips. In 1959, the American Society of Civil Engineers (ASCE) defined “sanitary landfilling” as a controlled operation in which MSW is deposited in defined layers, each layer being compacted and covered with soil before depositing the next layer [32
The fundamental difference between a dump and a landfill is that in a dump there is no attempt to separate the waste from the underlying soil or rock strata and where the hole extends to below the groundwater level, waste is dumped directly into the groundwater [32
]. In contrast, a sanitary landfill is an engineered structure consisting of bottom liners, leachate collection and removal systems, and final covers (Figure 2
). Landfills are designed both to store and to treat wastes. Much of the potential risk from MSW landfill results from the migration of contaminated leachate and landfill gas therefore the environmental impacts of the many landfills existing throughout the world cannot be ignored. Major emissions (leachates and biogas) are considerably affected by biological processes occurring in them. If the MSW is disposed on the landfill with no pre-treatment, emissions develop during the landfill operation period, which are produced even after the landfill will have been closed [33
]. According to Białowiec (2011) these emissions produce on average circa 150 (range of 70–300) m3
of biogas per 103 kg of municipal waste (related to dry matter weight) and about 5 m3
of severely contaminated leachates, depending on waste composition, climatic conditions etc. [28
]. The values approximately correspond to heavily compacted landfills in central Europe with annual total precipitation amounts ranging from (550−750) mm. The produced biogas has to be collected and incinerated or it can be used as a source of energy. The produced leachates have to be collected and treated [28
3.1. Landfill Gas
One of the products of biological processes occurring in MSW landfills is biogas or landfill gas (LFG) [35
]. Biogas is a product of the decomposition of biologically decomposable organic matter [36
]. The rate of biogas production and its composition change during the landfill lifetime. Landfill gas production (Figure 3
) is based on the composite model of bacterial waste decomposition and on the model of long-term behavior of landfill gas from old waste repositories [37
LFG obtained from the MSW develops during anaerobic decomposition of organic matter. MSW contains about (150–250) kg of organic carbon per ton of waste, microorganisms in which transform it into landfill gas during anaerobic processes. The generated LFG with (40–60%) methane has an average heating value of 17,765 kJ/N m3
. With the energy conversion efficiency being 34%, the produced electric energy is approximately 2.5 kW h/N m3
]. Considering the presence of methane and carbon dioxide within the LFG, the total amount of greenhouse gases (GHE) in the landfill life can be estimated at about 1.37 E+09 kg of equivalent carbon dioxide [39
Main factors affecting the amount of LFG are as follows: waste composition, moisture content, temperature, landfill age etc. LFG generation starts one to two years after waste disposal into the landfill and continues for (15–25) years [40
]. Moreover, according to Directive 31/1999/CE: “LFG shall be collected from all landfills receiving biodegradable waste and the LFG must be treated and used. If the collected LFG cannot be used to produce energy, it must be incinerated”.
3.2. Landfill Leachates
Rain has a major influence in the formation of leachates. Precipitation percolates through the deposited waste and binds to dissolved and non-dissolved waste constituents by means of several physical and chemical reactions. Groundwater tributaries, surface runoff and biological decomposition also participate in the formation of leachates. Liquid fractions in the waste combined with soil cover moisture content take part in the formation of leachates as well.
Moisture can be eliminated from the landfill by the consumption of water in LFG formation by means of water vapor evaporated in LFG or by means of leachate removed through drainage system.
Thus, the discharge of leachates is closely related to rainfalls, surface runoff and infiltration of groundwater soaking up the landfill. The method of landfilling (water-proof covers, requirements for insulation layers such as clay (mineral cohesive soil), geotextiles (GCL) and/or plastic materials) is crucial for checking the amount of water reaching the upper layers of the landfill, and hence, for mitigation of pollution risk. The production of leachates is also greatly affected by climatic conditions since they influence the input of precipitation into the landfill and losses due to evaporation. Moreover, the production of leachates depends on the character of disposed waste, namely on water content and on the degree of compaction of upper landfill layers [43
Technology applied as routine on landfills established in Europe in the 20th century was technology of leachate treatment based on the "dilute and disperse" system. Landfills were usually not properly sealed and the leachates produced were leaking into the wider surroundings where they mixed with groundwater in which they were dispersed. Such facilities for dilution and dispersion are no more built nowadays; however, the existing ones have left old environmental burdens all over Europe in the form of landfills, which may represent a risk of contamination, particularly in areas with high groundwater table [44
3.2.1. Composition of Leachates
Many studies have proven that landfill leachate is a significant source of pollutants as a consequence of the leaching of hazardous substances [21
]. Leachates contain four main components: nutrients (namely nitrogen), volatile organic compounds, heavy metals (HM) and toxic organic compounds [48
]. Nitrogen in the form of NH3
was identified as one of priority substances to be eliminated for the mitigation of leachate toxicity [51
There are many factors affecting the quality of leachates, e.g., landfill age, seasonal weather fluctuations, total precipitation amount, waste type and its composition [54
]. Composition of leachates significantly changes especially in dependence on landfill age. There are three types of leachates defined according to landfill age (Table 2
). Concentration of organic compounds (COD) in leachates decreases with the increasing age of the landfill, while the concentration of NH3
3.2.2. Toxicity of Landfill Leachates
Leachates represent complex mixtures of substances including dissolved organic matter, inorganic macro-components, HM and a wide range of xenobiotic organic compounds. A great amount of these substances occurring in landfill leachates is hazardous and toxic to human health and the environment. Moreover, chemicals can bioaccumulate in organisms and be passed along the food chain, eventually reaching humans [48
The assessment of risks from landfill leachates is traditionally based on the evaluation of individual chemical substances by means of chemical analysis. Although the approach is important, it features some limitations. First, some chemical contaminants may occur in the leachates that are below the detection limits of chemical analysis and their detection can be difficult due to the limitations of analytical techniques. For the regime of continual chemical sampling, changes caused by regular refluxes may be necessary; however, this system is rather costly and labor force intensive.
Furthermore, these chemical techniques do not allow prediction of the effects of pollutants on recipients (ecosystems). Eco-toxicological studies of the responses of model biological organisms exposed to the effects of toxic substances can provide important information complementing the standard chemical analyses [46
]. Unlike the chemical analysis, the biological test of toxicity can include biological effects of all present compounds as well as their biological availability [46
]. The chemical analysis by itself provides only limited information about the environmental fate of complex refluxes of leachates [46
]. Thus, eco-toxicological tests can provide the necessary link between the traditional chemical analysis and comprehensive field studies with using biological organisms in controlled (defined) environmental conditions [57
] published one of the first studies on the toxicity of leachates. In this study, they highlighted shortcomings of traditional chemical testing and used a recently developed test with the salmoniform fish of rainbow trout (Oncorhynchus mykiss
), setting up standard methodology for the evaluation of biological toxicity of leachates. The authors identified main constituents of leachate toxicity (non-ionized ammonia, tannins and copper) on the model fish species. NH3
can have negative toxic effect on the environment. It was demonstrated that NH3
is highly toxic for aquatic organisms both acutely and chronically. This statement was confirmed in previous studies demonstrating that NH3
is a primary cause to the toxicity of leachates from MSW landfills [59
]. In addition, NH3
is released from leachates by means of volatilization (Henry’s constant is 32.2 Pa.m-3
at 20/25 °C) and increased concentrations of NH3
in the atmosphere may have adverse impacts on vegetation [59
]. Comparative toxicological studies also revealed mutagenic properties of these leachates. Helma et al. [62
] demonstrated that the genotoxic potential of leachates MSW landfills is higher than that of wastewater from the manufacture of cellulose, industrial wastewater, contaminated surface and ground waters or even of drinking water and bathing water samples.
The above-described properties of leachates from landfills clearly point out the necessity of introducing high-quality strategy in the management of landfill leachates (collection, re-circulation and final treatment prior to discharge into the environment) as a final strategy for the management of pollutants [63
3.2.3. Landfill Leachate Treatment Technologies
There is considerable scientific literature on the collection, storage and suitable treatment of landfill leachates. A range of technologies are available for the treatment of landfill leachate, aimed at achieving the standards established by legislation. Typically, the leachate treatment can be carried out through: (1) biological processes (activated sludge, aerobic and anaerobic stabilization lagoons and biological filters); (2) physical-chemical processes (flotation, coagulation/flocculation, adsorption, chemical precipitation, air stripping, pH adjustment, chemical oxidation, ion exchange, electrochemical treatment); (3) membrane filtration (microfiltration, ultrafiltration, nanofiltration and reverse osmosis), (4) advanced oxidative treatments (ozonization) and (4) natural systems (wetlands) [9
3.3. Landfill Fires—A Source of Pollution
Landfill fires are unexpectedly common [67
]; however, the environmental aspects of landfill fires have been given little attention within the research community. In 2004–2010, there were 840 fire events recorded in U.S., which resulted in the subsequent loss of material and machinery as well as of human health damages [69
]. More than 60 landfill fires were recently recorded in Poland and the authorities announced that many of them are likely to have had been set up on purpose in order to devour illegal waste imported from other countries. These incidents show that landfill fires are controllable only with difficulties and have a great impact on the environment. Published reports on the impact of landfill fires on the environment due to the emission of toxic substances are few (Table 3
Landfill fires can significantly harm the environment due to emissions of toxins into the atmosphere, soil and water. Risk factors depend on the type of burning waste, on the geographical location of the landfill and on the type of fire [69
]. In general, these fires occur at low temperatures and under anoxic conditions. Hydrocarbons, chlorinated materials and pesticides produce a variety of toxic gases in such conditions, which may contain dioxins/furans [79
], polynuclear aromatic hydrocarbons, respirable particulates (PM) [80
] and HM [81
] as well as other harmful compounds [78
]. Smoke produced during the landfill fire may contain hazardous toxic gases such as CO, H2
etc. and carcinogenic substances such as dioxins.
Emitted bad odors and smoke bother the neighborhood and may put into danger even human health, especially among vulnerable populations such as the elderly, children, pregnant women and/or people with pre-existing chronic respiratory conditions [81
4. The Future of Landfills
In the hierarchy of WM, Europe increasingly moves from landfilling towards recycling and reuse [87
]. The directive in force since 1999 comprises requirements for the reduction of waste amounts disposed in landfills. Since 2016, member countries are not allowed to landfill more than 35% of biodegradable MSW landfilled in 1995. Some countries managed to achieve these goals four years later. The main objective of the WM strategy is to prevent waste formation. If prevention is not possible, then the waste should be reused or recycled. If even this is impossible, the waste should be used to generate energy thereof (thermo-valorization). Waste should be disposed in landfills only if no other possibilities of its management exist.
Regardless of what preventive measures, reuse or recycling can be realized by the society, landfills will always play a role in the WM system. Furthermore, globally, and even in EU countries, landfill rates are still high; meanwhile, waste prevention and recycling rates are too low [89
]. Many authors agreed that waste reducing, reusing and recycling (3R) behaviors have been a widely accepted WM strategy [89
]. However, sufficient capacity for recycling and reuse of all types of waste will not be economically acceptable under all conditions. In the pursuit of greater prevention, investment into recycling and reuse of wastes that to disappear in the future would be economically disadvantageous. Moreover, the amount of waste fluctuates throughout the year. Sometimes, the amount of waste determined for recycling, reuse or incineration exceeds the capacity. Not all types of waste can be recycled or incinerated. For some of them, landfill is the only choice and in case that the recycling facility or incinerator are out of operation due to maintenance, repair or breakdown, the waste should not remain in residential areas. This shows that some types of waste have to be disposed in landfills even when recycling and reuse are in place [95
]. In the sound system of WM, these landfills serve as ’security networks’. Landfills should be established by using sustainable methods so that they do not represent an environmental burden for future generations.
Insulation of landfills by means of impermeable membranes becomes a European standard. The insulation stops all processes in the landfill. These membranes have a lifetime of up to 50 years and can even sustain up to 500 years. However, they inevitably fail at a certain time moment, and if they are disrupted, the emission generating processes will start again. Hence, the potential emissions are postponed for future generations (Figure 1
). In many countries, after-care landfill management is governed by legislation and it must be ensured in general for at least 30–60 years after landfill closure. Some countries require the after-care as long as the relevant authority of state administration considers necessary. After-case is apparently required for a time-period longer than one generation. A safe solution would be more sustainable. It follows that a society that strives for long-term sustainable development needs long-term sustainable landfills.
There are no internationally recognized definitions of sustainable landfilling (Figure 4
). When it comes to landfills, discussions about sustainability frequently use terms such as stability, completion, end technologies and threat to the environment [96
]. Scharff et al. [98
] presented the following selection of definitions:
The Solid Waste Association of North America (SWANA) sub-committee for stability: landfill is "functionally stable" as long as the waste matter does not represent threat to human health and the environment after its closure. This condition must be assessed with respect to the quality and amount of leachates, gas production and composition, cover, slope gradient, slope and design of insulation, site geology and hydrogeology, climate, potential recipients, exposure of ecosystems and humans to landfill impacts, and with respect to other factors considered as relevant for specific localities.
Anglo-Welsh agency for environment protection: completion is defined as a condition when the landfill is physically, chemically and biologically stabilized to such extent that its undisturbed contents are likely not presenting any risk of environment pollution to landfill surroundings. After the landfill closure, neither active pollution controls are required within after-care (e.g., leachate and gas management) nor any monitoring systems.
Technical University Hamburg (TUH): the stage of after-care can be terminated in the moment when the potential of the development of emissions is so low that actual emissions do not harm the environment.
Although various definitions have slightly different reading, it appears that there is a general agreement that sustainable landfill or landfill in which after-care termination is considered safe is such a landfill that will reach a condition within a limited time-period when its undisturbed content will no more represent any threat to human health or the environment. This is the moment (often referred to as completion) in which the after-care of landfill can be terminated. It is important to realize that this condition is in basic agreement with the intention of EU waste legislation. Annex II to the Landfill Directive requires neither insulation nor after-care in landfills of inert waste and defines inert waste in a similar way as not representing and threat to human health or the environment [98
]. A question remains to be asked whether inert waste actually does not represent a risk for human health and the environment because research in this field is scarce.
Advantages of Waste Landfilling
The most frequent threats following out from the operation of landfills have been described above. Now, the time has come to take into account arguments advocating advantages of limited disposal of waste in landfills. Especially in the long-term time horizon, landfilling can transform the troublesome share of waste flow into the short-term gas generation and provide long-term carbon supply. Moreover, the after-care of landfills can also promote renovation of brownfields, which may finally lead to new opportunities for betterment of landscape condition and of the environment.
Options of WM are not interchangeable. Diverse strategies available for solid WM are generally considered as the hierarchy of opportunities for sustainability with the reduction of waste amount at the source being the best option and landfilling the worst. However, alternative removal of waste is currently subject of criticism by both environmentalists and landfill operators and suppliers.
Although the possibility of ’zero waste’ is discussed frequently [92
], according to Song [100
] transforming currently over-consuming activities into zero waste is still challenging.
Efficient introduction of recycling into practice at a reasonable price is difficult since it would require effective separation of individual waste fractions both at the source and after collection. Separation of organic matter from waste, either biologically degradable or combustible is considered a significant source of various pathogens. Composting, as the most valuable method for recycling of biologically degradable waste, inevitably also generates bioaerosols impairs the health of workers and inhabitants near large composting facilities [101
]. Incineration is the main alternative of landfilling but brings a number of related problems. It represents a very costly technology needed for the solution of all possible risks, namely emissions into the air [104
]. It is also a subject of criticism in relation to possible hazardous emissions [107
], non-removal of pathogenic substances, non-provision of HM immobilization and also in relation to yet other shortcomings of WM economy. In addition, the perception of risks by the public considerably exceeds actual risks. Ash material from municipal waste incinerators can be disposed in landfills after or without pre-treatment but the risk of inhaling ash from the source in landfills has been assessed as insignificant.
Landfilling brings the lowest own costs. Its secondary impacts (hygienic, social and environmental—calculated on an economic basis) are traditionally referred to as significant. However, their expression in numbers is questionable and any considerations again do not take into account the new available technologies [28
It should be emphasized that the current enormous volumes of dumped waste in landfills could be regarded as potential resource reservoirs for metals, high quality recycled aggregates and waste-derived fuels by landfill mining. 150,000–500,000 old and still active landfills exist throughout EU representing an estimated total volume of 30–50 Gm3
of waste. Landfills should be seen as ‘urban stocks’ and be considered as resource reservoirs for future recovery, ‘a bank account’ for coming generations [108
5. Landfill Mining
Landfill mining (LFM) has become very current topic. LFM has been proposed as an innovative strategy to mitigate environmental risks associated with landfills, to recover secondary raw materials and energy from the deposited waste and to enable high-valued land uses at the site [109
]. Landfills are mined to obtain materials for use as raw materials (e.g., metals) or as energy resources (e.g., plastics) [110
]. Moreover, the concept of LFM is designed to close the material loops towards a circular economy (CE), recovering landfill waste [111
]. However, according to the latest literature, there are many problems associated with LFM. According to Hölzle [111
], LFM operations required on average 103 MJ diesel (≙2.4 kg) and 1.9 MJ electricity per ton excavated waste, producing 12 kg of CO2
equivalent. Transportation proved to be the sub-process with the largest energy consumption by far, producing 58% of total emissions, followed by processing (27%). Laner at al., [109
] has pointed out that our understanding about the climate impact of LFM is limited to a few case-specific assessments [112
]. While some of them conclude that LFM would lead to reduced climate impacts compared to business-as-usual [112
], others have found that such projects would instead result in net contributions to global warming (GW) [113
]. Laner at al. [116
] pointed out that the economy of LFM is very important. Several case study assessments on the economy of LFM exist, a broader understanding of the driving factors is still lacking. It has been suggested [116
] that 80% of the generated LFM scenarios show negative results. Therefore, the development of a treatment plant that enables maximum resource recovery and environmentally and economically reliable remains one of the technological challenges for further development of LFM. For LFM to reach its full potential, strategic policy decisions and tailored support systems, including combined incentives for material recycling, energy utilization and nature restoration are required.
On the basis of the above discussion, it can be concluded that in many countries, landfilling remains a dominant method of municipal Waste Management. Landfills continue to be one of the main methods of waste disposal despite their relatively high potential to pollute the environment. Therefore, regular landfill monitoring is required to identify and define landfill hazards for the environment. This literature research shows the necessity of identifying knowledge gaps and establishing bases for developing a more holistic framework of landfill risk analysis.
Thus far, however, with respect to the current situation and rich style of living adopted in industrially developed countries, the idea of waste management systems functioning without landfilling—at least in the foreseeable future within one generation—seems to be somewhat utopian.
From the literature review, it is evident that even if high levels of waste avoidance, reuse and recycling are achieved, some waste materials will always need to be forwarded for disposal. Therefore, the concept of sustainable landfill should be implemented. A truly sustainable landfill is one in which the waste materials are safely assimilated into the surrounding environment.
The framework of this study that obtain from a comprehensive literature review can be used as a systematic approach for evaluation of landfill impacts on the environment for all stakeholders involved in the landfill industry.