The first sheet of each Excel file provides an explanation of what is in the other sheets. The second sheet gives the composition of the jurisdiction as a whole, and subsequent sheets give the composition of specific waste sources in that jurisdiction, such as “residential route trucks,” “commercial route trucks,” and “loose drop boxes.”
The methodology used in the 2016/17 study is very close to the methodology described in the 2002 study. A description of each of the material categories is linked below:
This study, like previous DEQ waste composition studies, gives two values for the composition of each materials in the wastestream, “Field Results” (columns B and C of each sheet) and “Contamination Correction Results” (columns D and E of each sheet). “Field Results” are based on the weight of each material as sorted and weighed at the disposal site. However, many materials in the garbage are dirty and wet. For example, cardboard may have become wet from rain or from absorbing moisture from food in the garbage, and there may be materials inside of cardboard boxes or that adhere to the cardboard when sorted at the disposal site. This study cleaned and dried samples of the materials in the field to estimate “contamination correction factors” that can be used to estimate how much “clean, dry” material is present in the dirty, wet materials as sorted at the disposal site. Columns D and E show these “contamination-corrected” results. The methodology for this part of the study is explained in Appendix C of the 2002 study. For estimating the tons of each material disposed each year, it is best to use the contamination-corrected results. For comparing this study to studies in other jurisdictions though, it is best to use the field results as most waste composition studies do not attempt to estimate the cross-contamination of materials being sorted.
Each Excel file was updated in July 2018 to provide estimates and make adjustments to the overall waste composition for:
- Shredded tires, gypsum wallboard, and medical waste that was disposed as a single material or stream that was not sampled as part of the waste composition study
- Scrap metal recovered from ash from the Marion County Energy Recovery Facility in Brooks, Oregon, and
- Material recovered from mixed waste from drop boxes and self-haul vehicles at three Metro-area transfer stations at a point after we had collected and sorted the solid waste samples at those facilities.
These estimates and adjustments were made only for the waste disposed overall in the jurisdictions included in the Excel file, and not for any of the individual waste substreams, as explained in the first tab of each Excel file.