Supporting Information for How to Manage Residual Leftovers on your Way to Zero Waste
Major assumptions in the report
Landfill gas capture rates are a contentious issue and a large factor in determining the environmental impacts of burying leftovers, particularly when the leftovers contain high quantities of organic materials. We chose two landfill gas capture rates to reflect the uncertainty in this area. We also chose to model no landfill gas capture for post-MRBT leftovers because the material will not be biologically active and will produce little to no landfill gas. There was not estimated to be enough gas production from the MRBT leftovers to warrant a LFGTE system.
Our decision to run the model at 40% and 80% gas capture rates was to offer some sensitivity analysis around how the rates affected the environmental performance of landfills. Our lifecycle analysis measures the impacts over a 100-year period, and there is no empirical data on landfill gas capture rates over that time period since the technology and practice are relatively new. The data shows that a landfill with a higher gas capture rate would have a lower environmental impact, but the report does not imply that 80% gas capture is feasible. The report also clearly shows MRBT outperfoms landfill gas to energy systems, even at the assumed high gas capture rates.
Residue from MRBT process
The residual material after MRBT processing is not considered to be marketable and is sent to a landfill for disposal. Some MRBT facilities may produce a stabilized inert residual that can be used for specific applications such as land reclamation of old mines and landfills, or landscaping along railways and highways, but this was not considered in our study. Using the MRBT residual instead of landfilling could increase the environmental benefits of using MRBT by displacing the use of a substitute material with a higher pollution profile.
Recovery rates from MRBT
Recovery rates at MRBT facilities vary with the composition of the incoming feedstocks. For example, it may be more difficult to recover market-quality recyclable materials if there are a lot of food scraps in the residuals. This study assumed two different sets of capture rates for recyclables in the MRBT process based on data and conversations with operating facilities. Here's a comparison of our high/low recovery rates for the recyclable materials:
The energy generated by WTE and LFGTE facilities was assumed to displace electricity that would have otherwise been produced by natural gas. Natural gas is replacing coal as the dominant source of new energy in the US.
Frequently asked questions
What are "leftovers?"
Leftovers is our term for the materials that remain after recycling and composting. We use "leftovers" instead of "waste" for several reasons:
- These are valuable resources, not "trash."
- Many of these materials could be recycled or composted but are not sorted properly.
- The amount of leftovers decreases in a community with high recovery goals and strong programs for recycling, composting and reuse. What's left over today may not be in a few years or may change in composition as more materials are recovered. This underscores the need to design disposal facilities for fewer and fewer tons, instead of investing in technologies that rely upon a steady stream of tons with high energy content.
Why did you choose Seattle for your model?
We wanted to model a community on the cutting edge of resource recovery and we also needed a communtiy with good data on what was left in their trash bin. Seattle has one of the best recycling rates in the US and had a recent waste composition on its leftovers so it became our model community.
Does that mean 70% recycling is the best achievable recycling rate?
Absolutely not. Nearly 90% of a community's discards can be recovered and marketed. Much of Seattle's remaining waste could be recycled or composted instead of ending up in the trash. For example, nearly 29% of Seattle's leftovers was food waste that could have been composted. Almost 19% was paper that could have been recycled or composted. Seattle households could certain reach higher recovery rates and will continue to do so. Their current recycling rate of 70% is a great accomplishment but is not the final goal.
What do the environmental categories, such as ecotoxicity and eutrophication, represent, and how do you place a monetary value on pollution and health risks?
For more information on the types of pollution assessed and their associated monetary values, please read the background documentation on the MEBCalcTM model.
Where is MRBT happening?
European countries are leading the movement to pre-process leftovers for additional recycling and biological stabilization. This is driven by European Union policy, the Landfill Directive, which reduces the amount of biologically active materials that can be landfilled. Germany has gone one step further in banning the direct landfilling of any materials, requiring all leftovers go through a pre-treatment process. There are more than 330 plants in Europe processing 33 million tons per year, with more than 450 plants expected by 2016.
Can we use MRBT instead of having people separate materials for recycling and composting?
While MRBT facilities are able to recover some recyclables from the leftovers, these facilities are not able to sort and market many of the materials because they are too dirty and contaminated from the other leftovers. For example, the compost from MRBT facilities is often not marketable as a soil amendment and is commonly buried in the landfill. MRBT is a solution for treating leftovers after source separation efforts and is not a substitution for having residents and businesses separate their discards. Source separating materials for recycling and composting, such as cardboard, plastic bottle and food scraps, guarantees the highest economic value and greatest environmental benefits from recovering these materials.
Why is managing residuals important--shouldn't communities focus on increasing recovery efforts first?
Communities should absolutely give top priority to increasing the availability of and participation in recycling and composting programs to recover materials from their discards. Infrastructure investments are a big part of this effort and it is important for a community to consider how its discards stream will change as recovery rates increase. By looking ahead and considering the best options for managing its leftovers in the present and the future, a community can avoid investing in disposal technologies that are not compatible with a decreasing amount of leftovers or cause excess pollution and environmental health impacts. Pre-processing leftovers through MRBT facilities may also help communities reach their other environmental goals such as minimizing air and water pollution and reducing greenhouse gas emissions.