Solid Waste & Recycling

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Anaerobic Digestion

Typical estimates show that organ- ic matter represents anywhere between 50 to 80 per cent of all Canadian municipal solid waste. Municipalities are therefore beginning to seriously consider anaerobic...


Typical estimates show that organ- ic matter represents anywhere between 50 to 80 per cent of all Canadian municipal solid waste. Municipalities are therefore beginning to seriously consider anaerobic digestion of organic matter as a viable option to manage waste and divert it from landfill.

The new anaerobic module for the computer-based Integrated Waste Management (IWM) Model will further broaden the capabilities of municipalities to evaluate their waste management practices. The new module is a joint project of the Environment and Plastics Industry Council (EPIC), CSR: Corporations Supporting Recycling, and Environment Canada. Completion is scheduled for late 2001. (See “Calgary’s Waste Impact” in the June/July edition.)

The benefits of anaerobic waste digestion include reductions in greenhouse gas emissions (in comparison to some other waste management technologies) and energy generation (e.g., from methane gas). As a result, the three partners have a keen desire to incorporate this increasingly popular waste management practice into the IWM model and to add even more value to the free computer-based resource. The new module is expected to assist users to evaluate the environmental burdens associated with anaerobic digestion of organic waste, including paper, yard waste and food wastes. It will provide estimates of the quantity of electricity produced and on the solids that (after maturation) may be used in a number of constructive ways.

Controlled digestion

Controlled anaerobic digestion involves placing organic waste in a closed tank (usually called a reactor or digester) that maintains the waste at a given temperature. The process is also referred to as “biogasification,” “biomethanogenesis” or “fermentation,” and involves a consortium of microorganisms.

Recent developments have improved the reliability, speed and energy efficiency of anaerobic digestion treatment methods. In a controlled environment, anaerobic digestion can produce methane, carbon dioxide, inorganic nutrients, and humus without noxious odours or pollution.

The recovered gas can be converted to thermal energy via combustion and used to heat the digester. Any surplus methane can also be marketed as a gas or converted into heat and electricity. In addition to the gases produced by anaerobic digestion, the process also produces a residue that is mechanically removed or pumped out of the digester. This residue is then de-watered and cured through aerobic composting (maturation).

Benefits

There are many benefits inherent in adding anaerobic digestion to municipal waste management practices. In addition to the previously mentioned fuel generation, the process also provides for the stabilization and use of organic waste. It produces solid residue that can be used for compost and soil enhancement.

Anaerobic digestion yields far fewer odours than aerobic treatment methods. Costly aeration is avoided and less sludge is produced.

New module

The new IWM module will accommodate information pertaining to the waste collection and haulage of organic waste to a transfer station and/or directly to the anaerobic digestion facility. As with the other waste management options already included in the model, the new module will extend from the collection of organics at the curb through to the production of products — including heat and electrical energy produced by combusting the biogas and, after curing, the production of a usable compost.

The module will estimate the amount of energy consumed or produced, and emissions to air, water and land associated with anaerobic digestion in relation to the alternatives of recycling, aerobic composting, energy from waste, and landfill. The parameters selected will be the same as those for the current IWM Model.

The module will calculate the net amount of energy produced by using biogas as a source of fuel. The material inputs will include the biosolids processed that may consist of paper, food and yard waste. The user will input the composition of waste in the user’s municipality in a manner similar to how the data are currently entered for aerobic composting.

The energy consumption component will take into consideration the collection and transportation of wastes (either directly to the facility or following “bulking-up” at a transfer station), the energy required to pre-sort and pre-process incoming materials, as well as the energy used in the operation of the digester. It will also incorporate the production and delivery of fuels and electricity used in the aforementioned processes.

The energy production component (when biogas is collected, treated and combusted to produce electricity and heat) will be estimated in a fashion that parallels the existing method used by the IWM model to estimate energy recovery with landfill gas.

Emissions of selected pollutants that arise directly from the anaerobic digestion process will also be calculated within the new module. The emissions produced from delivery and use of any fuels are calculated in the existing transportation module.

More information on the new module and on the IWM Model may be accessed from the EPIC web site, linked to www.solidwastemag.com.

Written by Cathy Cirko, director general of the Environment and Plastics Industry Council (EPIC), a council of the Canadian Plastics Industry Association, in Mississauga, Ontario.


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