Adding air and moisture to a landfill to form an aerobic rather than an anaerobic environment speeds up waste degradation and paves the way for the recovery of valuable resources through landfill mining.
by Mark Hudgins, James Law, David Ross & Jun Su
Bioreactor landfills – also called ‘wet landfills’ – are an emerging trend in waste management worldwide. Adding moisture to the waste in a suitably designed and operated landfill should increase its degradation, leading to less risk and a move towards sustainability. Adding air along with moisture in a bioreactor system holds further promise as various laboratory, pilot- and field-scale projects have demonstrated. Aerobic conditions can lead to lower leachate treatment costs, reduced methane gas and less odour.
A landfill cell can be viewed as a ‘treatment system’ rather than just a long-term waste containment structure. When the ‘treatment’ is complete, the conditioned waste, soil cover and sacrificial plastic wells (used to inject air and water) can be mined and excavated so that ‘new’ cell airspace can be created. From a life-cycle perspective, the bioreactor landfill could be the basis for a cost-effective sustainable solution to solid waste.International aerobic bioreactor projects
Depending on site-specific parameters, landfill bioreactor systems can be anaerobic, aerobic or semi-aerobic. There are also hybrid designs that first use an aerobic process and then switch to an anaerobic process to create early onset of landfill gas (LFG) production.
The aerobic landfill process involves the growth and control of aerobic and facultative bacteria within the waste instead of anaerobic micro-organisms; an ‘underground cousin’ to composting, the rate of decay in aerobic landfills matches closely rates observed in composting.
Proper aeration, moisture addition and gas extraction are needed to control the environment required for aerobic processes to thrive with optimum efficiency. Air is typically injected via vertical injection wells installed through the landfill cover to maximize air delivery and to help control the heat generated. Because over two-thirds of the original water content in the waste mass may be lost during decomposition, water or other liquids are often added to maintain aerobic activity.
Carbon dioxide, trace amounts of non-methane organic compounds (NMOCs), water and salts (typically the by-products of aeration) are removed via a second set of vertical wells connected to a gas vacuum and header system. This helps transfer heat from the waste to minimize fires and is used for LFG ‘polishing’ (if needed). Atmospheric oxygen is made available to microbes in sufficient quantities to promote vitality of the aerobic types and to minimize odours, but not in concentrations that promote explosions (methane is present during the aerobic system start-up at 45–50% v/v and trace amounts can remain during operation).
There are now over a dozen aerobic projects worldwide,1 with many different versions reporting similar performance results.2 For example, the Fukuoka method, an anaerobic approach practised in Japan is simple to construct and operate, while the ‘Bio-Puster Method’ (a patented landfill aeration system) has been used in Austria since 1991. In 2000, a Dutch study of selected methane gas control technologies ranked landfill aeration of waste as the highest and most economical.3 In the USA, aerobic projects have been conducted in over 10 states.
These projects showed that using the aerobic process as a remediation tool lowered the hazards and risks typically found in anaerobic waste environments (high pathogen mortality, reduced methane). Thus where waste excavation may have been dangerous in the past, aerobic landfill projects can be conducted to allow waste mining after the waste has been degraded.
For example, at the 6.5 ha Baker Place Road landfill in Columbia County, Georgia (USA), air and leachate were injected into the waste via vertical wells for 18 months. The biodegradation rate reportedly increased by 50%, leachate BOD fell by 65%, methane production decreased by 90% and NMOC levels declined by 75%. A 1 ha aerobic test cell was operated at a lined facility in Atlanta, Georgia, for nine months. Both tests demonstrated that solid waste degrades at a significantly faster rate under aerobic than anaerobic conditions, the volume and strength of the leachate are reduced and the amount of methane generated falls.4 In another study, post-aeration samples collected from waste excavations indicated that the largest fraction (over 50%) appeared ‘as a suitable soil/compost material with sufficient moisture content’ (30%).5 The composted material was biologically stable, with little odour. Plastic products, metals and glass made up over 30% of the remaining materials, with inert materials as the balance. Lignin-containing materials (e.g. wood and paper) degraded slightly. Laboratory analysis showed that soluble salts, metals and pH were within safe ranges, and no pathogens were detected in the materials.6
Based on projects such as these, other aerobic projects are moving forward. For example, the two-year project at the closed 16 ha Heishitou landfill near Beijing began in 2007 to stabilize the waste mass and lower risks as part of a possible site redevelopment. Tsinghua Unisplendour Taihetong EnviroTech Ltd (THUNIST), a subsidiary Jun Su of Tsinghua Group, was contracted to design, construct and operate this first aerobic bioreactor remediation system in China.New mining projects advance landfill sustainability
With a rapid, in-place waste decay process available, operators can now recover airspace, reduce risks, lower post-closure care and realize many other benefits in less time than nature can achieve. For example, the Perdido MSW landfill in Escambia County, Florida, recently undertook an innovative on-site mining project. As a result of waste mining and materials recovery, the landfill will be able to operate for an additional 26 years. The expected benefits at these type of sites include:Enabling previously non-compliant sites to meet waste management regulations. ‘Recovery’ of landfill airspace. Removing a source of leachate and landfill gas production. Reclaiming soils from excavated areas for on-site reuse and recycling materials previously discarded. Figure 1. The sustainable landfill (courtesy LG Aerobic Solutions)
The aerobic landfill bioreactor process moves from being a remediation tool to a waste treatment method that is part of an integrated landfill management strategy. Referred to as the ‘sustainable landfill’ (Figure 1), the aerobic bioreactor could be the basis for a revolutionary new approach to solid waste management and generally consists of the following steps:
The high Btu materials are tested for use as refuse-derived fuel (RDF). The metals and glass are characterized. The composted materials and soils (generally over 50% by volume) are tested for use as either daily cover material or as a marketable agricultural product. The high Btu materials are sent off-site to an RDF facility or a waste-to-energy (WTE) plant. The metals and glass are added to similar recycling streams. C&D and other landfilled materials that cannot be recycled are shipped to another facility. The cell floor is rehabilitated and the cell refilled with incoming MSW. Alternatively the mined cell can be closed and the land redeveloped.
The unique nature of every landfill can make development of a sustainable landfill strategy a challenge. Successful strategies also require a multi-discipline approach that addresses the many environmental, political and social issues surrounding landfills.Aerobic landfills, economics and carbon offsets
There are many financial incentives in developed countries to consider aerobic landfills. For example, about $2 million has been invested since 2000 in the development and operation of an aerobic landfill in Williamson County, Tennessee, to process 68,000 tons of MSW at a unit cost of about $29 per ton (including electricity costs). Yet the landfill owner has saved over $2 million in leachate treatment costs, does not require LFG or odour control or a flaring system, stands to save over $1 million in closure capping and post-closure monitoring (due to reduced risk), and potentially will avoid millions more that might otherwise be spent on groundwater remediation.
Overall the costs, savings and additional revenues could together be worth well over $10 million, making the unit ‘benefit per ton’ of waste handled approximately $150 (a five-to-one ratio). Furthermore, these benefits could be realized much sooner than with other approaches. If desired, landfill mining performed after degradation could increase the landfill’s capacity to receive more waste. If the landfill is redeveloped, this could not only generate additional revenues from the sale of the remediated property but also increase property tax revenues.
In developing countries, however, it may be difficult to finance such projects. Yet, new financial drivers in some countries could help provide funding from the sale of emission reduction credits. Provided a project follows the Clean Development Mechanism Executive Board’s approved Baseline and Monitoring Methodology AM0083, ‘Avoidance of landfill gas emissions by in situ aeration of landfills’, MSW landfill owners who treat or remediate their landfills aerobically by means of air venting or low pressure aeration can generate carbon offsets, or credits.7
Now recognized as a technology that ‘avoids’ methane generation and release to the atmosphere, governments have enacted new legislation and protocols based on aerobic landfill technology. For example, the ‘Quantification Protocol for Aerobic Landfill Bioreactor Projects’ published by the Alberta Offset System in Canada recognizes the opportunity to generate carbon offsets by directly avoiding methane emissions from materials anaerobically decomposing in landfills; instead, wells are drilled to allow aeration and the addition and/or recirculation of leachate.
The Alberta protocol also offers a unique economic driver. Although GHG offsets can be realized using methodologies such as flaring or LFG-to-energy, the protocol allows all the estimated Voluntary Carbon Units (VCUs) to be obtained in less than five years rather than say 30 years. It recognizes that, since aerobic systems can ‘avoid’ the conversion of organic matter to methane, such actions should be credited based on their ability to reduce the potential for methane generation over the timeframe of this reduction.
This is significant in terms of net present value (NPV) as it improves the economics of a GHG reduction project and, through ‘forward trading’ of VCUs, aerobic landfill/sustainable landfill projects can obtain upfront capital generation. Taking into account the reduction of LFG collection and destruction efficiencies over time, the aerobic approach compares even more favourably since it converts much of the organic mass to carbon dioxide, water and salts in a relatively few years.
For example, an economic analysis of various closure/LFG control approaches was conducted in 2009 for a 6.3 ha unlined landfill in northern Alberta that accepts MSW and varying amounts of C&D waste. Its projected capacity on closure in 2010 is approximately 1.4 million tons. It has no leachate or LFG collection system. The options evaluated were: capping-only, LFG flaring, LFG-to-energy (LFGTE), aerobic degradation, and methane oxidization.
Under the flaring and LFGTE options, modelling suggested approximately 113,000 carbon dioxide equivalents (CO2e) per year, or VCUs, could be captured each year. A LFG recovery potential of approximately 160 standard cubic feet per minute (scfm) was estimated assuming 70% system coverage, beginning in 2010 and declining thereafter following closure. Capital and operating costs were assumed for 13 years, and environmental monitoring for 25 years thereafter. The value of a VCU was estimated at $8.
Figure 2. Estimated methane offsets due to aerobic treatment
The VCUs for the GHG reductions at this site were estimated over three reporting periods of eight years each (24 years total) (Figure 2). VCUs could be obtained annually for the flaring and LFGTE options. However, the aerobic degradation option would allow most to be captured in less than five years as it would degrade the organic waste in much less time. Under these conditions, capital costs were estimated to be within 25% of each other with the exception of aerobic degradation. However, this option offers the prospect of significantly more revenue from VCU sales. Furthermore, its operating costs were lower due to less operating time (four versus 25 years) and a less frequent monitoring schedule (seven versus 25 years). Table 1 presents a summary of the economic comparisons.Conclusion
Whether as a remediation tool or as the basis for improved landfill operations, the aerobic landfill bioreactor approach can be valuable. Research and projects to date support the use of this approach as a promising strategy that could be applied to many landfills worldwide. Such an approach would help to:Address many public concerns. Lower post-closure care costs. Recover landfill airspace. Serve as the foundation for developing landfills into more useful real estate in land-short countries such as China.
Mark Hudgins is a Project Manager, James Law is Project Director and David Ross is Senior Vice-President at SCS Engineers.
Jun Su is Assistant General Manager at Tsinghua Unisplendour Taihetong EnviroTech Ltd (THUNIST), Beijing, China.