Solid Waste & Recycling

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The Bioreactor Strategy

Most jurisdictions in Ontario currently manage their solid waste with the goal of diverting as much from landfill as possible. In some cases this approach is driven by targets such as a certain percentage by a set date. For instance, Ontario...


Most jurisdictions in Ontario currently manage their solid waste with the goal of diverting as much from landfill as possible. In some cases this approach is driven by targets such as a certain percentage by a set date. For instance, Ontario set a target of 60 per cent diversion by the end of 2008, yet a year later had only achieved 22 per cent. Although waste diversion from a conventional landfill has notable benefits, much recent research points to the advantages of waste disposal in bioreactor landfills.

Bioreactor landfills differ from conventional landfills primarily through the addition of liquid to enhance the microbial process. Such a process allows for more efficient degradation and stabilization of organic waste, thus increasing the amount of free space in the landfill and subsequently extending its useful life. The anaerobic environment of a bioreactor landfill, in conjunction with the availability of organic waste, also allows for the deliberate production of landfill gas (i.e., primarily methane) at an earlier stage and at a higher rate than a conventional landfill, which in turn allows for improved energy generation.

A bioreactor landfill also has the potential to reduce transportation demands since a large portion of the organic waste could be combined and collected with the remaining municipal solid waste at the curb, rather than be sent to a separate organics processing facility. A combined collection program with fewer trucks on the road would likely translate into reduced greenhouse gas emissions and other atmospheric pollutants, as well as reduced collection costs and potentially greater social benefits including the reduction of traffic, traffic noise and dust.

So let’s compare the outcomes of a bioreactor landfill strategy against conventional landfill strategies (based on higher waste diversion targets). The question we must answer is this: “Could a bioreactor strategy without organics diversion potentially be more economically efficient, produce less greenhouse gas emissions and consume less energy than the current waste diversion approach?” Though at first it may seem counterintuitive, since the virtues of waste diversion have been widely extolled, the answer may be “yes.”

Study site and strategies investigated

To investigate this, a project was undertaken in which comparisons were performed using the U.S. EPA’s Municipal Solid Waste Decision Support Tool (MSW-DST). The MSW-DST is a lifecycle assessment (LCA) tool that calculates the net total cost and environmental releases associated with each waste process (i.e., curbside collection, composting, landfill disposal and remanufacturing). This research draws into question key elements of provincial waste management policy and suggests that superior policy directions may be worth pursuing.

The research is based on the author’s master’s research under the supervision of Van Geel. Wilkinson selected the City of Ottawa as the study site, where the existing waste diversion strategy included blue box recycling (glass, metal and plastic), “black box” recycling (paper and cardboard) and leaf and yard waste composting at a windrow composting facility. Five integrated solid waste management (ISWM) strategies were compared. The primary differences between the strategies are described as follows:

1) Landfill flare: Post-recycling residual waste (including organics) is disposed of in a traditional landfill.

2) Landfill energy recovery: Captured landfill gas is converted to energy with an internal combustion engine.

3) Organics program: This strategy introduces a green bin program where organics are collected and diverted from the residual waste stream and treated in an aerobic in-vessel composting facility. (Ottawa introduced this type of program in January 2010.) Residual waste is sent to a traditional landfill and captured landfill gas is flared.

4) Bioreactor: Leaf and yard waste and remaining residual waste (including all organic waste and black box materials) are placed in a bioreactor landfill. Captured landfill gas is combusted for energy.

5) Bioreactor blue box: In addition to the Bioreactor strategy, only blue box materials are recycled.

Conclusions and recommendations

The main findings drawn from this study were as follows:

• Cost: Both bioreactor strategies are more cost-effective, primarily because of their reduced collection and treatment costs in contrast to the other methods.

• GHE Emissions: The bioreactor strategy has the greatest greenhouse gas emission offset and is primarily influenced by the offsets associated with remanufacturing and high landfill gas-capture rates.

• Energy Consumption: The bioreactor strategy has the greatest energy offset, again due primarily to the offsets associated with remanufacturing and high landfill gas capture rate (with energy recovery).

The results of this project confirm that, for the City of Ottawa at least, a bioreactor-based ISWM strategy without organics diversion is likely to be more economically efficient, produce less greenhouse gas emissions and consume less energy in contrast to a traditional landfill strategy with organics diversion. In other words, diverting the organic waste of low-market value (i.e. leaf and yard waste and kitchen scraps) away from a landfill with energy recovery is not optimal from a cost or environmental-burdens perspective. As such, the research suggests that the green bin program may not, in fact, be the greenest strategy available.

Bioreactor landfills offer additional benefits over conventional landfills. These include that landfill gas production is available at an earlier stage and higher rate than in a traditional landfill. Enhanced waste stabilization and compaction results in additional landfill space. Also, post-closure care and costs are reduced (Waste Management; Warith, 2002).

In conclusion, although this research project has demonstrated the significant economic and environmental benefits of a bioreactor-based strategy for Ottawa, the benefits must be balanced against social and political elements. The Ottawa program operates in the context of Ontario’s waste diversion policy, which puts some constraints on municipalities. Furthermore, the city has signed a 20-year composting contract with Orgaworld Canada Ltd. for the processing of organic waste from residents’ green bins.

Nevertheless, there’s compelling evidence in favor of greater use of bioreactor landfill technology and the use of LCA in integrated solid waste management planning studies. All municipalities ought to conduct a comprehensive ISWM analysis in order to properly inform citizens, policy-makers and politicians of the pros and cons of various waste management strategies and facilities.

Shelley Wilkinson is a waste management specialist with Golder Associates in Whitby, Ontario. Contact Shelley at shelley_wilkinson@golder.com.


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