Dry phase anaerobic digestion of organic waste: Resource recovery

waste processing facility rendering
Typical dry source segregated organic waste processing installation. Click to enlarge.

By Christian Cabral

There is currently a strong movement in North America to prevent landfill disposal of organic waste. As a result, many municipalities and private companies are seeking diversion solutions to comply with bans and avoid financial penalties. In each case, the solution must address specific social, economic and environmental aspects. To add to the challenge, alternatives must be projected 30 to 50 years in the future. This requires consideration of future environmental policy, economic context, energy values, carbon footprints and nutrient management.

If energy recovery is part of the organics recycling strategy, two well-proven routes are available: thermal processes and anaerobic digestion (AD). This article will explore the benefits of continuous, plug-flow, dry or high solids AD as an appropriate solution for many Canadian cities.

To digest or not to digest

There are good reasons for the municipal and private sector to look at AD of municipal organic waste:

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1. Power companies are increasingly looking to diversify their offerings, accommodating clean energy as part of their portfolio.

2. Anaerobic digestion can significantly help downstream composting platforms by minimizing maturation time, increasing capacity, controlling odour and wildlife nuisances and preventing pathogen vectors.

3. Provinces are looking at biogas to help offset fossil fuel demands.

4. Organics landfill bans prevent raw organic waste from disposal in landfill cells. However, stabilized residual organics, in certain cases, may be used as landfill cover after AD and/or composting.

Depending on feedstock quality, processing organic waste can be challenging. Starting at waste containers, there are two basic avenues to recycle organic waste:

•Collection of source segregated organic waste (SSO), followed by treatment at a composting site or energy producing AD plant or combination of the two.

• Collection of mixed municipal solid waste including organic wastes, followed by mechanical separation of any recyclables at a mechanicalbiological treatment (MBT) plant. The organic fraction of municipal solid waste is then either composted, or anaerobically digested for energy production.

Both organic recycling avenues have their strengths and weaknesses. While source segregated collection requires high participation rates and separate collection and transport logistics, the resulting organic waste stream is typically “clean.” This means it contains a limited amount of impurities such as plastics, glass, metals and debris. Therefore, it is possible to produce higher quality fertilizer products with greater reuse potential.

For MBT plants, waste producers have it easy and no additional collection and transport logistics are required. However, substantial technical efforts are required at the sorting plant. This leads to far more impurities that “spoil” the subsequent treatment process and typically require additional post-treatment to satisfy regulators and users. The optimum route depends on the waste source and ease of separation.

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