July 25, 2007 | General

Toward A Sustainable Solid Waste Management System

BioCycle July 2007, Vol. 48, No. 7, p. 22
A San Francisco Bay area waste management company calculates greenhouse gas emissions from current recycling, composting and disposal practices – and then determines offsets at a 78 percent recycling rate.
Chris Choate and Jim Lord

DETERMINING the carbon footprint on how solid waste is handled is one method that will guide a sustainable approach to solid waste management. The carbon footprint and how that plays out sustainably will play a major role in driving the City and County of San Francisco’s march toward “Zero Waste” citywide by 2020.
Carbon footprint is a representation of the effect specific activities have on the climate in terms of the total amount of greenhouse gases produced. It is measured in units of carbon dioxide. SF Recycling & Disposal, Inc. (SFR&D), a subsidiary of Norcal Waste Systems, Inc., utilizing the U.S. Environmental Protection Agency’s WAste Reduction Model (WARM) has completed the carbon footprint evaluation of three scenarios for managing San Francisco’s municipal solid waste (MSW). The first scenario evaluates all MSW material being taken to the landfill for burial. The second scenario evaluates the 2005 SFR&D recycling rate, which was 39 percent. SFR&D owns and operates a single-stream recycling facility and a construction and demolition recycling line, and delivers organic material to compost sites. Operations are located near the edge of the Bay Area. The last scenario evaluates the future condition and visionary goal of recycling 78 percent of MSW. This goal is commonly referred to as “Zero Waste,” where 100 percent of the waste stream will be processed in one of the three facilities and the only material landfilled will be residuals from the processing.
Sustainability is conducting business in a way that supports and enhances existing operations, helps to protect the environment, and maximizes opportunities going forward. Sustainability requires companies to develop baseline data on their current operations. (You can’t measure what you can’t track.) This baseline data will measure the company’s progress in social, environmental and economic benchmarks. The baseline data should support and highlight the continuous effort to recycle and compost, implement renewable energy programs, reduce overall fleet emissions, meet regulatory compliance standards, and reduce greenhouse gas (GHG) emissions.
USEPA created WARM to help solid waste planners and organizations estimate GHG emissions for baseline and alternative waste management practices, including source reduction, recycling, combustion, composting, and landfilling. The model calculates emissions in metric tons of carbon equivalent (MTCE) and metric tons of carbon dioxide equivalent (MTCO2E) across a wide range of material types commonly found in MSW. In addition, the model calculates energy use for each of the options. The user constructs various scenarios by entering data on the amount of waste handled by material type and by management practice. WARM then automatically applies material-specific emission factors calculated for each management practice to determine the GHG emissions and energy savings of each scenario. Inputs, such as landfill gas recovery practices and transportation distances to MSW facilities, can be modified by the user.
The WARM program was run to determine what level of greenhouse gas emission reductions have already been accomplished with current recycling efforts in the City and County of San Francisco, and then to determine the additional reduction that could occur if all of the primary recyclable elements in the waste stream were recycled. Numerous assumptions had to be made in this evaluation to simplify the process and fit existing waste stream characterization with the WARM model parameters.
Existing recycling programs recover 350,000 tons of material. This equates to a GHG emission reduction of 346,000 MTCO2E. Another 350,000 tons/year can potentially be recycled, which would result in GHG reduction of an additional 470,000 metric tons of carbon dioxide equivalent gasses (MTCO2E).
Inputs into the WARM model include Material Type Parameters and Waste Management Practices. The WARM model used in this evaluation allows the user to classify waste and recyclables into 34 parameters (the model is periodically updated to include new material types as new information becomes available). Thirteen of these parameters were selected and used in the evaluation to identify current and potentially recyclable materials.
Once these materials are selected, a decision could be made regarding the materials baseline and alternative waste management practice. Options are included in the categories of reduction, recycle, landfill, combustion and compost. These waste management practices are further tailored to reflect hauling distances to the proposed use.
The landfill was modeled at 65-mile distance with a gas recovery system that had a default recovery of 75 percent; the gas was used for electrical generation. Composting was assumed to be 75 miles away, and recycling was 20 miles away.
In this evaluation, the 2005 Norcal generated tonnage breakdown was used to determine current GHG emissions reductions. A 2004 waste characterization study was used to determine potential reductions in material currently being landfilled.
The 13 different material types used to define current recycling practices included corrugated cardboard, food scraps, mixed MSW, mixed recyclables, personal computers, yard trimmings, concrete, dimensional lumber, glass, mixed organics, mixed metals, mixed paper, mixed plastics, and tires. In terms of potential recycling, the waste characterization broke the material into five categories accounting for about 85 percent of the waste stream being landfilled – paper, plastic, glass, metals and organics. These did not match up with the WARM model categories so an aggregate lumping of materials was undertaken. This resulted in compostable/ soiled paper and waxed OCC/kraft to be mixed organics. All other paper became mixed paper (broad), all plastics became mixed plastics, all glass became mixed glass, all metal became mixed metal, and all organics became food scraps.
In this classification, numerous subcategories of materials were identified but for the purposes of this evaluation, all were considered recyclable. This resulted in a potential number that is never expected to be reached since both 100 percent recovery is unrealistic, as is the ability to market or use all of the identified categories. To reach a high percentage of this potential, a wet-dry collection system will need to be employed to help force generators to separate the wet organics from the other recyclables and prevent contamination of the paper fraction.
A summary of the recycling tonnages is presented in the Table 1. The Zero Waste reflected in this breakdown accounts for recovery of all organics and mixed recyclables that have been identified in the waste characterization of material currently being landfilled.
Based on the parameters or inputs selected for both model inputs and material identification, the net greenhouse gas emissions were calculated and are presented in Table 2. To understand exactly where this emission tonnage reduction originates, a breakdown was prepared that equates MTCO2E per ton of recycled material. This was done under both recycling and disposal scenarios and is presented in Figure 1.
From this data, it can be seen that a premium emission reduction is given for recycling while composting of organics can easily result in emission increases. Part of this is due to the model’s gas recovery feature at landfills. Additional research is needed to better verify this feature. The suggested default of 75 percent gas capture seems somewhat arbitrary based on how landfills are constructed and how gas is extracted.
While the WARM model presents an easy to use tool for solid waste planners to track greenhouse gas emissions reductions, the accuracy of the model can potentially be improved. Several issues came to light in this application of the model that should be evaluated for potential future upgrades.
Landfill Gas Extraction: After waste is placed in a landfill, the organics will undergo continued decomposition long before any gas extraction occurs. Typically no methane gas is generated the first year a material is in place and methane generation continues to increase over the next five or so years when it peaks and begins a general decline. However, gas extraction is not typically put in until areas are completed or there are no plans for additional fill placement for several years. Also gas wells are designed to extract gas mostly from the deeper area of the fill where methane quality is the best. Only minimal extraction occurs at the upper levels to minimize air intrusion and potential poisoning of the field.
Organics Recycling: Composting and incineration are the only potential options for organics recycling in WARM. Technology advancements and air emissions constraints (odor and VOCs) are leading to changes in the field that need to be reflected. Flexible coverings and enclosed operations have changed the emissions in composting, and digestion and other conversion technologies are recovering energy that both reduces emissions and creates offsets that need to be reflected in evaluations.
MSW Characterization: As material is reduced, reused and recycled, the characteristics of the residual waste material changes. For example, in this evaluation, essentially all of the organics are removed from the waste but the residual is still treated as if it is typical MSW that can easily have an organics content greater than 60 percent. A method for accounting for material characteristic changes would improve the model.
Mixed Recyclables: Accounting for dry recyclables in the model presents a problem, especially on the West Coast. One of the key inputs in the model relates to how far the material is hauled. A considerable amount of bulk recyclables may be shipped overseas. There is no way to account for transport or reflect how the material will be recycled.
Chris Choate is with Norcal Waste Systems, Inc. in San Francisco, California. Chris can be contacted at: or (415) 875-1000. Jim Lord is with LandFirst Consultants in Los Gatos, California. He can be reached at or (408) 314-1785.

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