Solid Waste Management: An Overview Of Bioreactor Landfills
Anusha John#, Ajay kumar K#, Abdul Shiyas M A#, Sreenath H#

Department of Chemical Engineering, National Institute of Technology Calicut
NIT Campus P O, PIN 673601, Kerala
1manjalyjohn@gmail.com
2ajaykumar@nitc.ac.in
3shiyas2003@yahoo.co.in
4srinad@gmail.com

Abstract--- A landfill is a site for the disposal of waste materials by burial and is one of the most common methods of solid waste management in various parts of the world. Significant environmental and economic benefits can be gained by making small changes in the way the landfills are operated. Bioreactor landfill is a promising biotechnological option for faster stabilization of Municipal Solid Waste (MSW). It changes the goal of landfilling from the storage of waste to the treatment of waste.
In a bioreactor landfill environment, the solid waste actively decomposes rather than being simply buried in a dry tomb. This active decomposition is possible because over half the MSW is comprised of organic material (food, paper, etc.), which will decompose fairly rapidly under the right conditions. Rather than being kept dry, the solid waste is actively moistened by injecting leachate into the landfilled solid waste to accelerate decomposition. The anaerobic conditions in a conventional landfill can be replaced with aerobic conditions by introducing air into the solid waste and thus enhancing the biological process. Studies show that a bioreactor landfill transforms and stabilizes the readily and moderately decomposable organic waste constituents within five to ten years. A bioreactor increases the potential for waste to energy conversion, stores and/or treats leachate, recovers air space, and ensures sustainability. Sustainability has the greatest potential for economic benefit due to reduced costs associated with avoided long-term monitoring and maintenance and delayed siting of a new landfill
This paper presents an overview of the bioreactor landfill concept, benefits to be derived, design and operational issues. It also gives a general comparison between a dry tomb landfill and a bioreactor landfill from various studies undergone and thus finds bioreactor landfill an effective and feasible method of MSW management. I. INTRODUCTION
The generation of solid waste has become an increasingly important global issue over the last decade due to the escalating growth in world population and large increase in waste production. This increase in solid waste generation poses numerous questions concerning the adequacy of conventional waste management systems and their environmental effects. Landfill disposal is the most common waste management method worldwide. Landfills have served as ultimate waste receptors for municipal refuse, industrial residues, recycle discards and wastewater sludge. The method of Municipal Solid Wastes (MSW) disposal in most of the urban and rural areas of developing countries is open dumping. This practice causes public health and environmental problems.

Wastes in landfill experience physical and biological changes resulting in solubilization or suspension of high concentrations of organic matter in the landfill’s leachate. Source reduction and waste minimization, resource recovery and recycling, waste processing and treatment, combustion and land filling have all significantly affected the sufficiency of waste management systems. Of all available management options for solid waste management, landfill disposal is the most commonly employed waste management worldwide. Landfilling is the final and ultimate disposal of solid waste.
The problem with open dumpsites is that when it rains, the leachate seeps through the soil and contaminates the neighbourhood. In a bio-reactor landfill, the leachate is trapped and put back into the dump. The micro-organisms in the leachate help faster degradation of the waste (in about five years). If this degradation happens in the absence of oxygen (anaerobic) then landfill gas (mainly methane) will come out which could be piped away for use. The degraded waste could be scooped out, making room for fresh wastes. And out of the scoops, all that is not degraded could be separated. [1]
Engineered bioreactor landfills are usually designed to minimize the infiltration of rainwater and/or snowmelt into the solid waste. The design objectives of these landfills are to minimize leachate migration into the subsurface environment and maximize landfill gas generation rates under controlled conditions. [2] A bioreactor landfill is a sanitary landfill that uses enhanced microbiological processes to transform and stabilize the readily and moderately decomposable organic waste constituents within 5 to 10 years of bioreactor process implementation. The process requires significant liquid addition through leachate recirculation to reach and maintain optimal conditions. Strategies including waste shredding, pH adjustment, nutrient addition, waste pre-disposal and post-disposal conditioning, and temperature management, may also serve to optimize the bioreactor process [3]. Engineered bioreactor landfill sites can provide a more controlled means by which society can reduce the emission of global warming landfill gas (LFG), additionally they can provide immediate improvements to the surrounding local environment [4]. Bioreactor landfills provide accelerated waste biodegradation, a means for recovery of capacity or air space and a means to enhance LFG generation rates [5].
As compared to many developed countries, the concept of bioreactor landfills is still relatively very new to India [6]. There are bio-reactors in many parts of the world, but India is yet to have its first. Such engineered landfills are the crying need of the hour, not only because it is increasingly becoming difficult to find adequate dumpsites near cities, but also in view of the health hazards the open landfills pose.

II. OPERATION OF A BIOREACTOR LANDFILL A. Essential Components Of A Bioreactor Landfill
The essential components of a landfill are:
(a) A liner system at the base and sides of the landfill
(b) A leachate collection and control facility
© A gas collection and control facility (optional for small landfills)
(d) A final cover system at the top of the landfill.
(e) A surface water drainage system
(f) An environmental monitoring system
(g) A closure and post-closure plan

B. Bioreactor Mechanism

Bioreactor landfill is an emerging technology for solid waste management. The basic concept of bioreactor landfill is to use specific design and operation practices to accelerate the decomposition of food waste, green-waste, paper and other organic wastes in a landfill by promoting optimum moisture content and sufficient nutrients for the microorganisms to degrade the waste. The method not only enhances the degradation processes, but also stabilizes the landfill as quickly as possible. Landfill stabilization means that the environmental performance parameters (landfill gas composition, generation rate and leachate concentration) remain at a steady level. Biological processes are known to reduce the fraction of solid waste. Leachate recirculation system in a bioreactor landfill is one of the techniques that can be used to enhance solid waste biodegradation. Landfill’s leachate may contain high concentration of organic and inorganic materials including toxic compounds and heavy metals [7]. The recirculation of landfill’s leachate accelerates the rate at which the waste is broken down, thus decreasing the time required to stabilize the landfill’s site. This is important because the longer a landfill remains un- stabilized, the longer it remains as a source for potential environmental problems (e.g., groundwater contamination and methane gas migration). Bioreactor landfill avoids the problem of future liner failure because the wastes decompose within (5-10) years.

C. Bioreactor Landfill Types 1) Aerobic-Anaerobic: The aerobic-anaerobic bioreactor is designed to accelerate waste degradation by combining attributes of the aerobic and anaerobic bioreactors. The objective is to cause a rapid biodegradation of food and other easily degradable waste in the aerobic stage to reduce organic acids in the anaerobic stage resulting in the earlier methanogenesis. In this system, the uppermost lift of waste is aerated while the lifts below receive liquids. Horizontal wells, installed in each lift during construction, are used to transport landfill gases, liquids and air. 2) Anaerobic: The anaerobic bioreactor seeks to accelerate the degradation of waste by optimizing conditions for anaerobic bacteria. Anaerobic bacteria are responsible for converting organic wastes into organic acids and ultimately into methane and carbon dioxide. The moisture condition that is required to optimize anaerobic degradation is about field capacity or (35-40) % moisture. The moisture content in typical landfills is around (10-20) % water. Moisture is typically added in a form of leachate through a variety of delivery systems. Sewage sludge can be added too as a source of moisture. This leads to an increase in the amount of landfill gas production that is then collected by the gas collection system in the landfill. 3) Aerobic: The Aerobic Bioreactor accelerates waste degradation by optimizing conditions for aerobes that are organisms that require oxygen for cellular respiration. In aerobic respiration, energy is delivered from organic molecules in a process that consumes oxygen and produces carbon dioxide. Aerobes require sufficient water to function as anaerobes. Aerobic organisms can grow more quickly than anaerobes because aerobic respiration is more efficient at generating energy than anaerobic respiration. In landfills, aerobic activity is promoted through injection of air into the waste mass. The aerobic process does not generate methane. The aerobic process is much accelerated and typically requires less than two years for full biodegradation. 4) Facultative Bioreactor: The facultative bioreactor combines conventional anaerobic degradation with a mechanism for controlling the high ammonia concentration that may be developed when liquids are added to the landfill. In this system, leachate containing high levels of ammonia is treated using the biological process of nitrification. The nitrifying bacteria convert the ammonia to nitrate that can be used by facultative bacteria in the absence of oxygen for respiration. A Facultative bioreactor requires adequate moisture to function optimally. Leachate and other liquids (like surface water) are used to increase the moisture content of the waste. D. Generation Of Landfill Gases
The concept of bioreactor landfill considers water and solid waste, as major inputs while leachate and gas are the principal outputs. The primary gases that can be found in landfills include ammonia, carbon dioxide, methane, nitrogen and oxygen. They are produced from the decomposition of the organic fraction of municipal solid waste. Methane and carbon dioxide are considered the main landfill gases that are produced from biodegradable organic waste components in landfill. The generation of landfill gases can be divided into five-phases: 1) Initial Adjustment Stage: The organic biodegradable components in municipal solid waste undergo a microbial decomposition with the presence of oxygen soon after the waste is placed in the landfill. In this phase, biological composition occurs under aerobic reaction in which oxygen is consumed by aerobic microorganisms. Digested wastewater treatment plant sludge and recycled leachate can be considered as other sources of organisms. The aerobic decomposition generates heat, and temperature (10-20_C) higher than the refuse placement temperature [8]. 2 ) Transition Phase: In this phase, oxygen is exhausted and the anaerobic conditions begin to develop. Nitrate and sulfate, which serve as electron acceptors, are converted to nitrogen gas and hydrogen sulfide during biological conversion reactions. The onset of anaerobic conditions can be observed by measuring the oxidation/reduction potential of the waste. The generation of methane occurs when the oxidation/reduction potential values are in the range of 150 to 300 millivolts [9]. During the transition phase, the Ph of the generated leachate starts to drop due to the high concentrations of carbon dioxide and the presence of organic acids. 3) Acid Phase: Microbial activities which are originally initiated in second phase accelerate significant amount of organic acid and reduce the hydrogen gas. There are two major reactions that occur in this phase. The first reaction is the enzyme-mediated transformation (hydrolysis) of higher molecular mass components (e.g., lipids, polysaccharides, proteins, and nucleic acids) into compounds that can be used by microorganisms as sources of energy. Acidogenesis is the second reaction of this phase, which involves the conversion of microbial compounds resulting from the first reaction into lower molecular mass, compounds such acetic acid (CH3COOH). The microorganisms involved in this conversion are often known as non-methanogenic or acidogens [8]. Carbon dioxide is the primary gas generated during this phase with a small amount of hydrogen gas. The Ph of generated leachate drops to 5 or lower due to the elevated concentration of CO2 inside the landfill. The biochemical oxygen demand (BOD5), chemical oxygen demand (COD) and conductivity of the leachate will increase due to the dissolution of the organic acids in the leachate. Many inorganic constituents in particular heavy metals will be solubilized during this phase due to the low Ph value of landfill’s leachate. 4) Methane Fermentation Phase: A group of microorganisms convert the acetic acids and hydrogen gas to methane and carbon dioxide. The microbes responsible for this reaction are called methanogenic or methanogens. The Ph values, due to methanogenic reaction, will rise to more natural values in the range of (6.8-8). The BOD5 and COD values of the landfill’s leachate will be reduced. Also, the concentration of heavy metals in the landfill’s leachate will drop [9]. 5) Maturation Phase: This phase starts after the biodegradable organic material is converted to methane and carbon dioxide. The rate of landfill gas generation is reduced significantly in this phase as most of the available nutrients are removed with leachate during the methanogenic phase and the remaining substrates are slowly biodegradable. CO2 and CH4 are the primary landfill gases in this phase. Small amounts of nitrogen may be also found in the landfill gas. During maturation phase, leachate will contain humic and fulvic acids which are difficult to process biologically. The production of CH4 declines as waste organics get depleted. However, the slowly biodegradable organics generate methane for decades (e.g., cellulosic organics such as wood and paper).

E. Solid Waste Decomposition Process
Decomposition of refuse in a landfill occurs through a complex series of microbial reactions. Initial aerobic decomposition occurs producing mainly carbon dioxide and water vapor. Anaerobic decomposition begins somewhere between 2 months and 2 years after waste is placed in the landfill. Hydrolytic and fermentative microorganisms convert complex organic materials (such as food and garden waste, and paper products), to intermediate carboxylic acids, particularly acetic acid, alcohols, hydrogen, and carbon dioxide. Acetogenic bacteria produce acetate, hydrogen, and carbon dioxide from longer-chain carboxylic acids and alcohols. The end reaction is the production by methanogenic to bacteria of methane [10].

F. Landfill Leachate
Leachate is a liquid that has percolated through solid waste and has extracted, dissolved, and suspended materials that may include potentially harmful materials. The type of solid waste, physical, chemical, and biological activities that occur in the solid waste determines the quality of leachate. The quantity of leachate seeping from the landfill is proportional to the buildup of leachate within the landfill, alternatively called leachate mound. Leachate can cause serious problems since it able to transport contaminating materials that may cause a contamination of soil, groundwater and surface water.

G. Leachate Management Systems
The management of leachate is the key to eliminating potential landfill problems. A number of specific practices have been used to manage the leachate collected from landfill sites. These techniques include: 1)Leachate recirculation: An effective method for the treatment of the leachate is to collect and recirculate the leachate through the landfill. During the early stages of landfill operations, the leachate will contain significant amounts of TDS, BOD, COD, and heavy metals. When leachate is recirculated, biological, physical and chemical reactions will occur and provide treatment for BOD. The recirculation of leachate provides the landfill with various nutrients, which are required for the growth of bacteria that participate in anaerobic degradation of the waste. Leachate recirculation will increase a landfill’s moisture content. Moisture is essential for the activities of all microorganisms because it serves as a medium for transporting nutrients and bacteria. During leachate recirculation, the leachate is returned to a lined landfill for reinfiltration into the municipal solid waste. This is considered a method of leachate control because as the leachate continues to flow through the landfill it is treated through biological processes, precipitation, and sorption. This process also benefits the landfill by increasing the moisture content, which in turn increases the rate of biological degradation in the landfill, the biological stability of the landfill, and the rate of methane recovery from the landfill [11]. 2) Leachate evaporation: One of the simplest techniques that can be used as a leachate management system is leachate evaporation. It involves the use of lined leachate evaporation ponds. Leachate that is not evaporated is sprayed on the completed portions of the landfill. In some locations with high rainfall, the lined leachate storage facility is covered with geomembrane during the winter season to exclude rainfall. Odorous gases that may accumulate under the surface cover are vented to a compost or soil filter. 3 ) Leachate treatment: A number of options have been used for the treatment of leachate. The treatment process selected will depend on the contaminants to be removed. The principal biological, physical and chemical treatment options used for the treatment of leachate include: * Activated sludge, responsible for the removal of organics * Nitrifications, responsible for the removal of nitrogen * Neutralization, responsible for Ph control * Precipitations, responsible for the removal of metals and some anions * Ion exchange, responsible for the removal of dissolved inorganics 4 ) Leachate characteristics: * Young Leachate: Landfill’s leachate contains a biodegradable organic matter for the first few years [12]. Therefore, this young leachate tends to be acidic due to the presence of volatile fatty acids. The Ph is generally in the range of 6 to 7 and may be lower in a dry-stressed landfills. The young leachate is derived from processes such as the complex biodegradation of organics (e.g., cellulose) and simple dissolved organics (e.g., organic acids). Gradually, leachate becomes less strong and simply dissolved organics (e.g., gases CH4, CO2, H2, H2O and biomass)[9]. * Old Leachate: The Ph of landfill’s leachate will increase to the range of 7 to 8 due to the depletion of the biodegradable organics and the production of gases. The changes occur after 4 to 5 years of waste deposited in the landfill. The Nitrogen level will indicate the age of the leachate. Ammonia nitrogen (NH3-N) and organic nitrogen (Org-N) are produced by the decomposition of organic and are stable in the anaerobic environment. Nitrate nitrogen (NO3-N) will be consumed during the anaerobic reaction [9].

H. Methods Of Leachate Recirculation
Leachate recirculation is one of the emerging techniques that can be used to manage leachate from landfill. Leachate should always be controlled, treated, or contained before it is released to the environment. During leachate recirculation, the leachate is returned to a lined landfill for reinfiltration into the municipal solid waste. This is considered a method of leachate control because as the leachate continues to flow through the landfill, it is treated through biological processes, precipitations and sorption [13]. This process also benefits the landfill by increasing the moisture content which in turn increases the rate of biological degradation in the landfill, the biological stability of the landfill, and the rate of methane recovery from the landfill. There are several methods of leachate recirculation. These include: 1) Direct application to the waste during disposal:
In this process, the leachate is added to the incoming solid waste while its being unloaded, deposited, and compacted. The problems with this method include odor problems, health risks due to exposure, and off-site migration due to drift. This method also requires a leachate storage facility for periods such as high winds, rainfall, and landfill shutdown when the leachate cannot be applied. 2) Spray Irrigation of landfill surface: The leachate is applied to the landfill surface in the same method that irrigation water is applied to the crops. This method is beneficial because it allows the leachate to be applied to a larger portion of the landfill. Also, the leachate volume is reduced due to evaporation. However, the disadvantages associated with direct application are associated with this method as well. 3) Surface application:This is achieved through ponding or spreading the leachate. The ponds are generally formed in landfill areas that have been isolated with soil berms or within excavated sites in the solid wastes. The disadvantages associate with this method include an increase in the amount of required land area, monitoring of the ponds to detect seepage, leaks, and breaks that would make it possible for the leachate to escape directly or with storm water runoff. 4) Subsurface application: This method achieved throughout placing either vertical recharge wells or horizontal drains fields within the solid waste. There is a large amount of excavation and construction required with this method, but the risk of atmosphere exposure is radically reduced. Another method of leachate management is to pump out the leachate from the bottom of the landfill and store it in a large basin. The leachate is often shipped to a municipal wastewater treatment facility where it can be effectively treated. Since the concentration of leachate’s contamination is not high as that of commercial wastewater, the leachate serves as a dilutor and is consequently helpful.

I. Advantages And Disadvantages Of Leachate Recirculation

The advantages of leachate recirculation are: * Landfills that use leachate recirculation experience a decrease in the concentration of the leachate compared to landfills without recycle treatment. This reduced the amount of leachate treatment that is needed it and therefore costs are also reduced. * The increased moisture content within the solid waste enhances the system conditions for improved biological decomposition of organic matter in the landfill. * The organic matter in the leachate receives further treatment each time it recycled through the landfill. This reduced treatment costs of the landfill. * The reducing environment within the landfill removes inorganics in the leachate through precipitation and sorption. * Leachate recirculation stabilizes the biological system in the landfill and this reduced the environmental threats of the landfill, and reduces the amount of post closure mentioning that is required. It also provides the opportunity for landfill mining and space reclamation. * Leachate recirculation increases the rate at which the waste decomposes and this increases the rate of methane production. * Leachate recirculation is a leachate management method that is relatively simple and inexpensive.
The disadvantages of leachate recirculation are: * Since landfills are heterogeneous, the leachate may find discrete channels to travel through. This makes it difficult to insure that the leachate is reacting with all of the waste and is thoroughly treated. * The risk of environmental exposure when leachate is applied to the surface of a landfill.

III. COMPARISON BETWEEN A DRY TOMB AND A BIOREACTOR LANDFILL

A conventional landfill site is an engineered waste disposal facility where garbage is deposited in the ground compacted into a cell and covered with earth fill materials. Environmental controls are incorporated into the engineering design of the facility to protect both the human and natural environments.
A Bioreactor landfill uses enhanced microbiological processes to transform and stabilize the readily and moderately decomposable organic waste constituents within 5 to 10 years of bioreactor process implementation [14]. The bioreactor landfill significantly increases the extent of organic waste decomposition, conversion rates, and process effectiveness over that would otherwise occur within the landfill. Landfill stabilization means that the environmental performance measurement parameters (i.e. landfill gas composition and generation rate and leachate constituent concentrations) remain at steady levels, and should not increase in the event of any partial containment system failures beyond 5 to 10 years of bioreactor process implementation. The volume of bioreactor is smaller than the conventional landfill even after 10 years. In a conventional (dry) landfill, the wastes are delivered to the landfill, spread out, compacted and covered at the end of the day with a thin layer of soil, until a planned depth is reached, then the waste is covered with clay as a final layer. The problem with conventional-dry landfills is that it takes several decades (30-200 years) for the waste to fully decompose. Also, liner failure could happen in conventional-dry landfill sometime in future, which can cause serious groundwater contamination [15].
Studies show that Bioreactors bring about extensive reduction in organic loads. Pollution indicators like COD, BOD and DOC are reduced by 80-90% in this kind of operation. Reduction in solids content of about 50-60% is also achieved[16].

IV. SUMMARY AND CONCLUSION

Advantages of Bioreactor Landfill can be summarized as follows: * Enhance the LFG Generation Rates * Reduce Environmental Impact * Production of End Product that does not Need Land filling * Overall Reduction of Land filling Cost * Reduction of leachate Treatment Capital and Operation Cost * Reduction in Post-Closure care, maintenance and * Overall reduction of contaminating life span of the landfill due to a decrease in contaminant concentrations during the operating period of the bioreactor landfills

It is now time to seriously consider acceptance and adoption of the bioreactor landfill as a key strategy for deriving short and long-term environmental, regulatory, monetary and societal benefits. The bioreactor option is a direct result of engineering and building a new generation of environmentally sound landfills; it provides environmental security while permitting and encouraging rapid stabilization of the readily and moderately decomposable organic waste components[17]. It is hoped that the emerging bioreactor landfill technology will point our solid waste industry towards taking a new look at a very effective option to manage our waste disposal.

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