Solid Waste & Recycling

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Anaerobic Digestion of Municipal Solid Waste

Canada sends over 75 per cent of its annual 33 million tonnes of municipal solid waste to landfill. This contrasts with the European Economic Community (EU) and the USA, which bury less than 60 per ce...


Canada sends over 75 per cent of its annual 33 million tonnes of municipal solid waste to landfill. This contrasts with the European Economic Community (EU) and the USA, which bury less than 60 per cent of their municipal waste. Moves are afoot to overcome Canada’s dubious distinction as landfill king. Quebec and Ontario have mandated targets of 60 per cent diversion from landfill or incineration, and waste diversion is a theme across the country.

Anaerobic digestion (AD) could play an important role in achieving aggressive diversion targets. AD is a proven approach, already achieving up to 76 per cent waste diversion in certain European situations. It has significant advantages over other options like aerobic composting. AD takes the organic waste of from the municipal waste stream and converts it into renewable energy at the same time as it reduces greenhouse gas emissions.

Yet for all this, AD has made little penetration of the North American market. There are historical reasons. The systems require important capital investment. AD’s main product is methane that must compete with natural gas. This fossil fuel remains relatively low in price because it’s a resource formed in immense quantities via natural anaerobic digestion over millions of years. However, at the rate we’re using up natural gas, the demand/supply equation will eventually favour higher prices.

Landfill area has also been far more available in North America than in Europe. Only recently have some North American municipalities begun to integrate the full lifecycle costs into landfill charges. These include costs for leachate control, greenhouse gas emissions and the opportunity costs of losing valuable land. How many landfills, initially neglected because of their remoteness, have become the centre of intense debate because of environmental impacts on the surrounding area?

Finally, the AD process has been considered temperamental and unreliable in North America. It is true that the lessons learned from initial experiences must be applied diligently to avoid problems, but examples in Europe now strongly suggest that the process can be reliable. AD can be exploited in an economically and environmentally responsible manner to achieve Canada’s important diversion and Kyoto objectives. And there are practical means of integrating it with various collection modes to ease municipal infrastructure and handling costs. AD’s day in the sun may finally have come for Canada.

Harnessing nature

AD originated soon after the first life appeared on earth. Bacteria growing in the absence of oxygen derived their energy from the breakdown of organic carbon compounds. The bacteria ended up recycling compounds into methane and carbon dioxide.

When such anaerobic digestion takes place unfettered in nature, it’s a significant source of greenhouse gas (GHG) emissions. Methane has a global warming potential 21 times more potent than carbon dioxide. However, if the methane can be captured it becomes a valuable renewable energy source displacing the need for fossil fuels. From a climate change perspective there is clearly much to be gained in harnessing AD and putting it to use.

AD was harnessed decades ago to digest sewage and other wastes. It was first adapted to process municipal solid waste over 15 years ago in the EU using in-vessel techniques operating at elevated temperatures. This approach accelerates digestion of the readily-available organic components, but any lignin present digests very slowly. Lignin, the binding agent in wood, is the most common organic compound resistant to AD. Therefore, operations in the EU typically complete the AD process with an aerobic composting step, to reduce in-plant treatment times.

Proven diversion in Europe

As the EU moves away from landfill, AD is becoming a proven, competitive option at a cost of some US $50 to $80 per tonne. Annually, some 87 AD plants are fed 2.6 million tonnes of municipal solid waste, ranging from mixed to source-separated wastes. Plants of up to 250,000 tonnes/year capacity now provide desirable large-scale economies.

The Brecht facility in Belgium (see photo) illustrates the compact footprint of a modern industrial AD plant. This kind of plant with mechanical sorting/recovery steps has led to 76 per cent diversion. (See process diagram.)

The Freiberg facility in Germany demonstrates another AD advantage. This facility is situated right next to a Burger King restaurant because the former is odour free. These factors mean that an AD facility can process municipal waste in urban areas, thereby minimizing transport costs for the waste itself and the recycled material derived from it. The latter advantage can impact the degree of diversion. Many recycled materials, such as commingled plastics, command low prices. Any significant transport cost leads to an economic equation that favours landfill rather than recycling.

It’s very difficult for a large totally enclosed composting system, suitable for an urban setting, to match this performance. Recent lifecycle analyses of such systems show that their environmental advantages are seriously offset by the infrastructure and operational requirements.

The EU has fostered AD by introducing a landfill ban on biodegradable waste and incentives for renewable energy. This pro-activity contrasts sharply with Canadian actions. For example, the Ontario government recently excluded municipal solid waste from its definition of biomass for renewable energy. Yet it simultaneously recognized new landfill gas energy projects as renewable. Thus, Ontario waste is currently seen as a renewable energy source only if buried in landfill, a position at odds with the Federal Government’s Innovation Roadmap on Bio-based Feedstocks, Fuels and Industrial Products, the International Energy Association and established EU successes, all advocating non-landfill utilization of the municipal solid waste biomass resource.

At a recent international AD conference held in Montreal, a vigorous debate arose over the extent to which an engineered landfill can be converted into an efficient anaerobic digester. For all this debate there is no doubt that a significant portion of the methane gas emitted by landfills cannot be captured. The GHG emissions continue over decades and it is simply uneconomic to retrieve these fully, given their diluted state. In contrast, the AD process actually achieves GHG emission reductions through the full use of the captured methane as renewable energy, plus the other benefits just described.

AD for Canadian diversion targets

Current Canadian plans to achieve 60 per cent waste diversion from landfill appear expensive. Typically the tactics include broader source separation, expansion to three-stream collection and large scale composting of organics. According to Andr Giroux of Laval’s solid waste division, that city’s provisional plans to go from the current 15 per cent diversion to the 60 per cent targeted for Quebec municipalities anticipate that municipal waste costs will double. Even these costs are dependent on some rather optimistic provisions for the use of large quantities of low quality compost. Part of the problems in Laval and elsewhere are the varying needs and difficulties for introducing three-stream collection and processing for single family vs. multi-tenant housing.

The combination of a Materials Recovery Facility (MRF) with an AD processing plant has the robustness and flexibility to receive various waste streams from source-separated organics through to mixed waste. (See diagram.) The Canadian waste is similar to EU waste (and, if anything, is richer in organics). EU modes for AD diversion of various waste streams can be readily applied here at the same competitive costs.

Thus, for a city like Laval, segregated organics from single family dwellings could be routed directly to an AD facility while the residual fractions from single stream recyclables processing (see

article, “Single Stream Recycling,” Oct/Nov 2004 edition) or separate garbage streams would be processed for mechanical recovery of any remaining recyclables prior to AD processing of the organic-rich residues. Mixed waste from high-rise units where separation is not practical could route through recovery in the MRF followed by AD. The EU approach of operating an AD facility in concert with a MRF appears to be just as attractive in Canada as it has proven to be in Europe.

A key to achieving cost competitive performance for AD (US $50/tonne) is to organize the full municipal waste recovery and transformation industry so that each AD facility receives some 150,000 to 250,000 tonnes per year. (Important economies of scale are associated with both the MRF and the AD plant.)

According to Standard and Poor’s (2003), the national US average price for large waste landfill operators working on the same scale is $US 45/tonne. Even when profit margins are factored into the cost of AD (to yield a price of about US $60 to 70/tonne), it’s clear that more complete cost accounting of landfill operations (including long-term value of the land, GHG emissions, etc.) can make combined MRF and AD attractive both from an economic and environmental standpoint.

What will it take?

In Canada AD is caught in a conundrum. Canadian municipalities cannot individually take on any substantial risks associated with new technologies. Whatever the successful track record in the EU there is a perceived risk associated with AD here. We need Canadian demonstrations of the fully operational technology, but these initial demonstrations will have to be at a size where the economies of scale cannot be achieved. Operations at such demonstration sites will need some financial support through some combination of private and public-sector funding.

The kind of problem that can arise if there is not a good mutual understanding of the issues to be addressed is exemplified by the current dispute (as of January 2005) between SUBBOR and the City of Guelph. They are in litigation over contractual matters related to the supply of waste and services for an AD demonstration facility in which some $30 million has been invested to date by the company. (See article “City of Guelph vs. SUBBOR,” April/May 2004.)

This Guelph facility, as well as a Newmarket plant previously run by CCI-BTA, still constitutes the most logical and economic base for good Canadian demonstrations of AD technology. The problem at Guelph has not been the innovative SUBBOR technology itself. It remains promising with three distinct advantages.

* It has up to 50 per cent higher methane production;

* It produces a lignin-rich soil amendment, free of residual plastics, with good water retention and slow-release carbon attributes; and,

* The latter is produced without the need for an aerobic polishing stage, thus making the facility footprint smaller and augmenting the facility’s suitability for an urban setting.

All of these advantages accrue from a simple intermediate step of steam explosion that liberates the lignin and makes any compounds like cellulose closely associated with it much more digestible.

As the previous sections point out, there are also fundamental long-term issues beyond successful demonstration that must be resolved if AD is to find its place in Canada. Even if a private company were to step forward to apply AD to municipal waste operations under contract, appropriate tipping fees must be integrated into the contract so there’s a level playing field between AD and landfill (with full cost accounting for the latter). A reasonable price must be offered for the renewable energy produced at the site. The initial contract would have to be over a sufficiently long time frame to allow amortization of the initial investment of several tens of millions of dollars (for a plant large enough to realize economies of scale). Financial recognition must also be provided for the substantial GHG emission reductions due to the AD operations. These reductions would constitute a major contribution to Canada’s Kyoto requirements. (See sidebar.) Discussion with representatives from the Federation of Canadian Municipalities suggest that Canadian towns and cities are willing to take on their share of responsibilities once some clear opportunities for action emerge in the demonstration of AD technologies.

James P. Rollefson, Ph.D., (jim.rollefson@nrc-cnrc.gc.ca) is with NRC’s Industrial Research Assistance Program (IRAP) in Ottawa, Ontario. Bruce E. Holbein, Ph.D. (beholbein@sympatico.ca) of Guelph, Ontario has 23 years experience in industrial research and implementation of biotechnologies in Canada and Europe. As an independent expert, he has also guided several bio-energy technology initiatives, including the SUBBOR AD technology and two recent NRCan/ BIOCAP initiatives on biodiesel and pyrolysis.

Anaerobic digestion delivers Kyoto dividend

Canada, as a signatory to the Kyoto Accord, is committed to reducing its annual GHG emissions by over 200 million tonnes of CO2 by 2010. This is a daunting challenge. But with current municipal solid waste landfill emissions estimated at more than 10 per cent of these targeted reductions, the proper processing of waste represents a major opportunity in helping achieve the Kyoto target. Where applied, anaerobic digestion can not only eliminate these landfill emissions, it offsets other GHG emissions from its production of renewable energy. This has led Professor Murray Haight of the University of Waterloo to conclude that, of all the options available, AD has the highest potential for lowering GHG emissions related to waste. Composting consumes rather than offsets energy. Engineered landfills with gas collection still release substantial amounts of their overall GHG emissions to the atmosphere. In contrast, AD treatment of Canada’s municipal waste is an attractive option that could contribute a large Kyoto dividend.


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