The bioreactor landfill is increasingly being touted as an environmentally sound MSW disposal method, although numerous practical questions need to be answered before its widespread adoption.

By John Pacey

The Bioreactor Landfill: In-Landfill Composting
Leachate Recirculation: How It Relates to the Bioreactor Concept
Anaerobic vs. Aerobic Operations
Limit Environmental Impacts to the Landfill’s Operational Life
Bioreactor Landfills Offer Future Environmental and Cost Benefits

The US Environmental Protection Agency (EPA) is currently collecting information and holding hearings on the bioreactor landfill concept to determine whether Subtitle D should be changed to facilitate the concept’s implementation. As part of the reexamination of Subtitle D to determine what is best for our future landfill disposal protocol, this overview attempts to provide some clarity on the meaning and applicability of the bioreactor landfill. Key issues include:

• definition of a bioreactor and process conditions,
• the difference between a true bioreactor landfill and leachate recirculation,
• an examination of benefits of anaerobic vs. aerobic operations,
• reduction in the environmental impact period.

The Bioreactor Landfill: In-Landfill Composting

The bioreactor landfill is nothing more than an in-landfill composting activity at a Subtitle D sanitary landfill in which liquid, temperature, and air - in the case of the aerobic process - are managed in a controlled manner to achieve rapid stabilization of the food, greenwaste, and paper-waste constituents. To optimize the rapid waste stabilization of these elements, moisture conditioning of the waste must be established and maintained at a relatively uniform level, and gas composition, flow, and temperature must be carefully maintained and monitored.

In most cases cited in recent literature and papers, the bioreactor is examined based on the anaerobic process condition (no oxygen present) normally associated with a sanitary landfill. The bioreactor landfill, however, can also be operated in an aerobic condition (with oxygen) and even in a semiaerobic condition (a combination of aerobic and anaerobic conditions). During site operations, managers could switch among these conditions; specially cultured microorganisms could also be used to enhance the process. The options for process variation are in their demonstration-phase infancy. Interest in the aerobic process system is rapidly growing. We can expect to see significant headway over the next decade in establishing the most environmental and cost-effective in-place composting methods in waste management.

Leachate Recirculation: How It Relates to the Bioreactor Concept

In both anaerobic and aerobic conditions, leachate is recirculated to achieve a sufficient and uniform moisture distribution throughout the waste, provide nutrient transfer for all of the microorganisms’ needs, provide a mechanism for heat transfer, and treat the leachate as a trickling bioreactor filter process. The high moisture content mitigates fire potential. Both anaerobic and aerobic conditions require that liquid be kept at slightly above the FC moisture content (above which leachate will be produced) and that a relatively high temperature from 100º to 140ºF (anaerobic) and 130º to 150ºF (aerobic) be maintained. To achieve FC, between 25 and 50 gal. of liquid must be added per ton of waste. The anaerobic mechanisms produce gas at a ratio of 60% methane and 40% carbon dioxide. Methane is the same natural gas that fuels our gas stoves; carbon dioxide is the same gas we exhale as we breathe.

It should be stressed that a bioreactor landfill is more than one that simply recirculates leachate. Subtitle D already allows the recirculation of leachate and condensate, but it does not allow the introduction of other free liquids. In most cases, existing landfills do not produce sufficient leachate and condensate to optimize the composting benefit and therefore they attain only a portion of potential stabilization benefits. Leachate recirculation is usually done to defray annual offsite leachate and condensate transport and disposal costs, pretreat leachate before onsite disposal, or achieve a higher effective waste density (less annual use of landfill airspace). It is also useful in accelerating waste degradation, resulting in increased landfill gas (LFG) generation, which in turn could increase energy sales and revenues. It has been reported that there were more than 130 landfill leachate recirculation projects in the United States by 1997. Most of these did not have the Subtitle D composite lining, and yet the environmental results have generally been satisfactory.

Existing regulations do not facilitate current leachate recirculation projects’ achievement of FC and thereby attain the optimum waste-stabilization performance of a true bioreactor landfill. Some landfill recirculation projects come close to achieving FC, but without sufficient leachate, this goal is difficult to meet. Thus, they have - or will - achieve only partial degradation at the time of closure. Nonetheless, compared to the conventional Subtitle D landfill, leachate recirculation projects generally reduce long-term environmental risk because they significantly reduce LFG generation and produce less contaminant loading in the leachate.

Many of the leachate recirculation projects with a leachate shortfall recirculate the leachate in a uniform manner, as the waste is placed, and/or as the leachate is available. This might only raise the waste moisture content a modest amount with only moderately aggressive degradation impact. An optional management scenario may be to designate separate cells where each cell might be operated optimally by raising its moisture content to slightly above FC and continuing with leachate recirculation to achieve the best results. Once a bioreactor cell was completed with an adequate liquid supply, the next waste placement would continue in the conventional manner, without additional liquid. A bioreactor cell could be implemented later as the needed liquid became available. In this manner, part of the landfill would be the conventional, dry landfill and a portion would use the bioreactor process to achieve the goal of stabilized waste.

Anaerobic vs. Aerobic Operations

Should the bioreactor landfill be managed using the anaerobic or aerobic process? For both the aerobic and anaerobic processes, degradation will be aggressive compared to the conventional Subtitle D landfill. The very aggressiveness of either aerobic or anaerobic in-landfill composting under managed and monitored programs offers the opportunity of nearly complete stabilization of food, greenwaste, and paper within short periods compared to today’s conventional landfill. The aerobic process is more aggressive than the anaerobic; waste should be completely stabilized in only a few years following process initiation, as compared with a five- to 10-year term for the anaerobic process. This must be compared to a conventional landfill, which achieves only partial stabilization in 15-80 years.

A major factor in comparing the performance of two processes is that the anaerobic process will significantly increase the quantity and generation rate of methane generated during its shortened stabilization term. This will provide more opportunity for LFG-to-energy projects and significant expansion for projects already in place. The aerobic process differs in that it almost eliminates the generation of the methane component of LFG. This could greatly affect EPA’s goal of reducing methane emissions from landfills.

Limit Environmental Impacts to the Landfill’s Operational Life

Rapid stabilization offers a major long-term environmental benefit in terms of reducing risk: Waste and leachate will have been exposed to all potential detrimental environmental impacts during the operational life of the landfill, rather than during a long postclosure period. Recirculated leachate is more aggressive than would occur in the event of future flow-through of surface waters. Therefore, postclosure liquid flow through the waste should not increase gas generation nor result in further release of organic or metal constituents into the leachate. Most external environments should be able to naturally manage long-term waste-related emission or leakage from a well-managed bioreactor landfill.

Waste stabilization is a relatively gray term in the literature. Life cycle consideration for the bioreactor landfill is for 20, 100, or 500 years. For the purpose of the bioreactor landfill, food, greenwaste, and paper products can be biodegraded to a stabilized status within a few years of landfill closure. The level to which these items are degraded in the bioreactor landfill extends well beyond what would otherwise occur in the conventional Subtitle D landfill, even in the event of total failure of its environmental containment system. Other organic constituents, such as wood, rubber, plastic, leather, and textiles, are slowly degradable and should not pose much of a long-term environmental threat from either a greenhouse gas or groundwater standpoint.

Bioreactor Landfills Offer Future Environmental and Cost Benefits

The bioreactor landfill offers several well-known and proven processes to achieve rapid degradation, and thus stabilization, of the relatively rapidly degradable organic waste materials within a relatively short term. Although it requires increased management and more environmental controls, the bioreactor landfill can result in enhanced performance, fewer long-term environmental risks, and higher potential revenue to help defray operational costs. Over the long term this should result in considerable environmental and cost savings.

The operational issues for the bioreactor landfill are the same as have been permitted in the past 130 leachate recirculation projects. The main difference is that bioreactor landfills will be subject to very tight management plans with increased focus on performance, operational details, and expanded instrumentation and monitoring.

Recognition of the potential environmental and economic benefits of the bioreactor has brought a new focus on the use of anaerobic and aerobic bioreactor processes. With the advent of Subtitle D landfills, it is now a real possibility to rapidly stabilize our waste so as to minimize postclosure environmental risk and gain near-term environmental and economic benefit. The bioreactor process is not complicated. Although the degree of management and monitoring is more sophisticated and challenging than with the conventional landfill, the benefits can be outstanding.

John Pacey is the founder of Emcon, an engineering consulting firm with offices throughout the US.

MSW
Elements 2001