Municipal Solid Waste Landfilling Approaches: Comparison
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Finding a sustainable approach for municipal solid waste (MSW) management is becoming paramount. However, as with many urban areas in developing countries, the approach applied to MSW management in Karachi is neither environmentally sustainable nor suitable for public health. Due to adoption of an inefficient waste management system, society is paying intangible costs such as damage to public health and environment quality. In order to minimize the environmental impacts and health issues associated with waste management practices, a sustainable waste management and disposal strategy is required. 

  • municipal solid waste
  • sanitary landfill
  • open dumps

1. Open Dumps

The open dump method is an elementary level of solid waste disposal, and is identified with the uncontrolled deposition of waste with only limited or without any control measures [20][1]. Overall, 33% of waste is openly disposed of at dumpsites globally, and in lower income countries (where dumpsites are the leading waste disposal facilities) more than 90% of waste is openly disposed of [5,21][2][3]. In Pakistan, 70% of waste generated ends up in dumpsites [17][4].
The operation of open dumps poses serious threats to the environment and human health [22][5]. The environmental and public health damage caused by open disposal of waste includes ground and surface water contamination through the generation of leachate, contamination of soil by solid waste or leachate, air pollution due to gaseous emissions, provision of breeding grounds to disease vectors such as mosquitos, flies, and rodents, odour problems, and uncontrolled methane emissions [23,24][6][7]. Furthermore, open burning of MSW, commonly practiced in developing countries, leads to the release of harmful contaminants including fine particulates (PM2.5), and damages the air quality in urban areas [25][8].

2. Anaerobic Landfills

Anaerobic sanitary landfills are known as well-designed waste disposal facilities which do not require any processes to influence waste degradation [26,27][9][10]. However, control measures to minimize environmental and public health effects are incorporated at the site, including a bottom liner and surface top cover as well as leachate and gas treatment (heat/power generation or flaring) facilities [26,27][9][10].
The sanitary landfill approach is the most popular waste treatment method due to its high volume handling capacity, low investment, and minimal technical requirements [28][11]. It has been reported [29][12] that the biodegradation processes of the organic fraction of municipal solid waste are slower under anaerobic conditions than under aerobic conditions in a landfill. Investigation results from old landfills in Germany and other European countries showed noticeable emission potential from landfills operated under anaerobic conditions, and it is estimated that gaseous emissions can last at least for thirty years, and that leachate emissions can last for many decades or even centuries depending on site-specific conditions [30][13].

3. Semi-Aerobic Landfills

The semi-aerobic is the oldest approach regarding landfill aeration; this method was developed in the early 1970s in Japan and is known as the “Fukuoka method” [9,31][14][15]. The semi-aerobic landfill process is driven by a natural air ventilation mechanism which provides a speedy waste stabilization solution through the availability of oxygen in the waste mass without demanding high resources and technology [31][15]. The semi-aerobic landfill system can be a suitable method for meeting the sustainability requirements cost-effectively and with low technical input, especially in developing countries which are lacking in sustainable waste disposal due to funding issues and technical limitations [32][16].
A semi-aerobic landfill system consists of a horizontally-installed perforated pipe network with an adequate slope at the bottom of landfill for leachate collection, with perforated pipes erected vertically at intersections and at the end of each branch for air ventilation [9,31][14][15]. Furthermore, in a semi aerobic landfill system, air flows through the pipe network by means of a natural advection process due to temperature differences between the landfill body and the ambient environment [9,31][14][15]. The temperature difference is a result of exothermic biodegradation of the organic fraction of the waste mass; the release of this heat can raise the temperature in the waste body by 50–70 °C [31][15].
This temperature difference leads to density differences in the gas inside the landfill, creating a buoyance force which allows the gas to flow up through the waste mass and vent out the vertical gas extraction pipes, developing negative pressure as a result that allows more air to be drawn inside the landfill body through the leachate collection pipes [31,33][15][17]. In an aerobic environment, organic matter degrades more effectively than in anaerobic conditions; thus, air circulation through the waste mass results in enhanced waste stabilization and improved emission quality and quantity [31][15].
A study on full-scale aeration in semi-aerobic landfills by [34][18] has shown that the relationship between airflow rate and ambient temperature is negatively proportional, as in winter a large flow rate was noticed, while no flow of air was observed in summer. In a semi-aerobic landfill system, anaerobic conditions prevail inside the waste mass due to insufficient air distribution, which promotes methane formation. However, the CO2 and CH4 emission ratio of a semi-aerobic landfill (4:1) is much lower than an anaerobic landfill system (1:1) [31][15].

4. Aerated Landfills

In situ aeration is a quite new technology for intensified removal of biodegradable organic material left in old landfills [35][19]. For aeration of landfills, two approaches are applied; one is forced aeration, while in the second air is supplied in natural conditions. Forced aeration is realized by injection of air into the landfilled waste mass through means of different types of blowers [36][20]. The major objective of the aerobic in situ aeration is to stabilize and change the emission behavior of organic matter deposited in the landfill [37][21].
Aerobic degradation processes in landfills enable the significantly faster decomposition of organics (e.g., hydrocarbons) compared with anaerobic processes, resulting in increased carbon discharge in the gas phase and decreased leachate concentration [38,39][22][23].
A study by [35][19] reported that when landfill gas production is decreased to such a level that energy generation is not economically feasible and even flaring of extracted gas is not practical, there will be up to 10–20% residual gas production potential remaining of the total production potential. Moreover, it may take decades to stabilize the remaining organic material in the anaerobic environment; by providing aerobic conditions, the residual organic matter can be degraded in a limited time (<10 years under a conducive environment) [35][19].
The in situ aeration approach goes beyond the concept of injecting air into the landfill, including a well design and spacing options for the suitable volume and pressure of air, air distribution, temperature, and moisture control as well as pollution discharge in the leachate and gas phases [9][14]. The major objective of aerobic in situ aeration is to oxidize and change the emission behavior of organic material deposited in landfill, and in the end to significantly reduce the emission potential in a more appropriate way [37][21].
Aerobic degradation processes in landfills enable the significantly faster decomposition of organics (e.g., hydrocarbon) compared with anaerobic processes; as result, carbon discharge in the gas phase increases and leachate concentration decreases [38,39][22][23]. In all, nitrogen elimination is the most significant advantage that can be obtained from aeration technology [40,41][24][25]. Several authors [9,42][14][26] mention that the aeration of waste material in the landfill body is an essential and unavoidable pretreatment step in the landfill mining process to prevent uncontrolled gaseous emissions from waste during excavation activity. Presently, various approaches and concepts are applied in the aeration of landfills, such as semi-aerobic landfills, high pressure aeration, low pressure aeration (including active aeration with and without off-gas extraction), passive aeration via air venting, and energy self-sufficient landfills [9][14].

5. Bioreactor Landfills

A bioreactor landfill is an engineered and modern shape of a conventional anaerobic/aerobic landfill where moisturization of the waste takes place by injecting water (fresh or wastewater) and recirculating the leachate to optimize waste degradation processes [43,44,45][27][28][29]. The recirculation of leachate facilitates cycling of microbes and nutrients into the waste mass and maintains an optimal moisture content in the landfilled waste [46][30]. The cycling of microbes and nutrients is intended to enhance microbial processes for transformation and stabilisation of easily and moderately degradable organic waste fractions, within the timeframe of 5–10 years for bioreactor process execution [47][31].
Various studies [48,49,50,51][32][33][34][35] have reported the positive effects of moisturization of the waste and leachate recycling during landfill operation, which includes speedy waste biodegradation and stabilization, increasing LFG (methane) production, rapid settlement, reduced leachate quantity, and leachate treatment cost savings. Furthermore, bioreactor landfills and their variations represent a sustainable alternative approach to conventional sanitary (dry tomb) landfills [52][36]. However, bioreactors can have drawbacks, e.g., odours and physical instability of the waste material due to increased moisture [53][37].
Moreover, establishment of infrastructure for leachate recirculation and/or aeration may cause increased capital and operational costs [53][37]. Studies have suggested that the high upfront costs involved in operation and construction of bioreactor landfills can be balanced by future economic benefits, including an increase in the active life of the landfill (waste disposal period), more efficient use of airspace [54][38], lower minimum leachate treatment/disposal costs, delay in the need to construct a new cell and cap, savings in the post-closure care period thanks to less need for monitoring and lower financial guarantee obligations, and higher efficiency in landfill gas collection, resulting in larger revenues generated from production [55][39].
According to [53][37], the bioreactor approach can be applied when the waste to be deposited possesses a high quantity of biodegradable organics. Bioreactor landfills can be designed as anaerobic, aerobic, semi-aerobic, and hybrid landfills [36,56][20][40]. The basic differences between these designs of bioreactor landfills are linked with their operations, layouts, and arrangements for leachate recirculation, landfill gas collection, and (optional) air injection system [45][29]. Bioreactor landfills are mostly operated under anaerobic conditions [57,58][41][42]. In a hybrid bioreactor landfill, a series of aerobic and anaerobic conditions are observed [53,59][37][43]. The aeration of the bioreactor landfill is realized through injection of air/oxygen to establish an environment for aerobic biodegradation of the landfilled waste in order to control methane emissions and accelerate waste stabilization [60][44].
However, hindrances in oxygen distribution in the waste mass due to high moisture content and leachate recirculation have been reported by various research studies [61,62,63][45][46][47]. Moreover, other studies [64,65][48][49] have stated that degradation of waste is significantly influenced by the rate of oxygen distribution. 

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