The answer depends on what type of compostable bottle you’re asking about.
Oxodegradable plastic bottles were not specifically evaluated in DEQ’s report. They are typically comprised primarily of PET, so the impacts of producing and transporting the bottles are very similar to the PET bottles profiled in DEQ’s study. The major difference occurs at end-of-life. If recycled, the oxodegradable plastics will blend with traditional plastics, potentially causing new products made from these recycled plastics to fail.
In the landfill, the oxodegradable plastics may eventually decompose, producing carbon dioxide and methane in the process. While some of the methane will be captured and used to produce energy, the remainder will escape to the atmosphere. Because methane is such a potent greenhouse gas, the oxodegradable PET bottles will likely have higher greenhouse gas impacts than traditional PET bottles if they degrade in the landfill.
PLA is difficult to evaluate, in part because it is a relatively new resin. Most PLA used in the United States is produced at a single plant in Blair, Nebraska. The manufacturer claims to have made significant process improvements in mid-2009 that significantly reduce the environmental impacts of production. Data reflecting these improvements were not available at the time DEQ was completing its study, so the PLA water bottles included in its study uses production characteristics from 2005. As such, these results may overstate some impacts over the life cycle of PLA bottles.
Comparing a PLA bottle composted 100% of the time against a PET bottle recycled 100% of the time, the PLA bottle consumes slightly less energy over its life cycle, and contributes slightly more to greenhouse gas emissions. The ecotoxicity impact of the PLA bottle is about half that of the PET bottle, and smog impacts are about 20% lower, but respiratory potential impacts are 50% higher and eutrophication potential is about 4 times higher than the PET bottle.
If the PLA compostable bottle is not composted or recycled (since there is no current recycling system established for PLA in Oregon) then the bottle will likely be disposed in the landfill. Once landfilled, the bottle could potentially decompose. Since not much is known about PLA’s decomposition in landfills, DEQ modeled both 100% and 0% decomposition rates. If the PLA bottle decomposes in the landfill, it contributes to the production of landfill methane, a potent greenhouse gas. In the 100% decomposition scenario, the PLA bottle’s life cycle global warming impact is approximately 60% greater than the typical PET bottle (recycled at a rate of 62%, an estimate of the current recycling rate for PET bottles under Oregon’s bottle bill). If the bottle doesn’t decompose, it has the potential to reduce global warming impacts by 23% compared to the average PET bottle.