Turning old banana peels and last night's leftovers into biogas sounds like a win-win situation for the environment: It eases the guilt about cooking too much food, and the use of biogas reduces carbon dioxide emissions when it replaces fossil fuels.

However, a new study from the Norwegian University of Science & Technology (NTNU) shows that it's not that simple.

In fact, encouraging people to work harder to cut food waste instead of collecting food waste and turning it into biogas reduces energy impacts more than biogas production and use, the researchers found.

Of equal importance, cutting food waste also helps reduce the use of phosphorus, an increasingly scarce but essential plant nutrient that is a key component of fertilizer. This matters because fully one-third of all food produced globally ends up as waste.

"Our work shows that policy and incentives should prioritize food waste prevention and that most savings can be had through a combination of prevention and recycling," Helen Hamilton, a doctoral candidate with the NTNU Industrial Ecology Program, said.

Label confusion

Hamilton and her colleagues used Norway as a case study to evaluate the costs and benefits of recycling food waste versus preventing it. The group looked at what they called "avoidable food waste," or food that should have been eaten but, for various reasons, ended up as waste. The term does not include unavoidable food waste like bones, shells, peels and residues such as coffee grounds.

When they looked across the board at different segments of the food production and consumption sector, they found that 17% of all food that had been sold was wasted. Most of that waste was at the consumer level, partly because of the confusion caused by labeling, the researchers wrote in an article in Environmental Science & Technology.

The problem is the difference between labels that describe a product's "best before" date compared to its "use by date."

"Consumers often mistake 'use by dates' — which refer to highly perishable goods that pose a risk to human health if consumed after a certain period — with 'best before dates,' which merely indicate a food's reduction in quality but not safety," Hamilton and her colleagues explained. "This results in a substantial amount of food waste at the household level."

Phosphorus

The NTNU researchers also looked at how food waste affected phosphorus use in the agriculture sector.

A 2010 doctoral dissertation from Linkoping University in Sweden found that the growth in the global population, combined with increased food demand, will result in a 50-100% increase in phosphorus demand by 2050.

When Hamilton and her colleagues compared what happens to phosphorus demands if avoidable food waste is prevented versus recycled, they found that Norway's need to import mineral phosphorus declined by 14%. The need to import phosphorus decreased 6% under the food waste recycling scenario compared to the baseline demands, but that is a theoretical maximum and would be true if only the leftovers from biogas processing could be perfectly returned to agricultural soils as fertilizer, which is currently not the practice today, Hamilton said.

"This assumption in no way reflects a probable future, as only minimal amounts of residuals produced today are returned to agricultural soils due to many factors, one of which is that farmers are not that eager to accept biogas residuals as a suitable substitute for mineral phosphorus," she said.

Biogas

Some of Norway's major cities — Oslo, in particular, but also smaller cities like Tromsoe — collect food waste in separate green bags that can be sorted from the waste stream using optical sorting. While Tromsoe currently composts its waste, Oslo has its own biogas facility that relies, in part, on food waste collected in the city.

Some of the biogas that is generated by Oslo is used in 36 buses outfitted to burn biogas, which led the Oslo bus company to proclaim in October 2013 that the "buses are fueled by your banana peels."

While that sounds like a good thing — it does, in fact, reduce the need for fossil fuels — in total, it takes more energy to collect and process the food waste than it would if people didn't throw away so much potentially edible food unnecessarily, Hamilton and her colleagues found.

Reducing the demand for animal and plant products by wasting less results in "both reduced upstream production impacts and downstream waste treatment impacts," the researchers said.

"If one only analyzes methods for handling wastes (end of pipe) without regard to upstream impacts, results will often reflect the benefit of producing secondary value-added goods, such as biofuels," the researchers wrote. "With narrow system boundaries, even policies meant to increase sustainability get skewed."

Another risk of prioritizing recycling is that there is a risk of getting locked into "needing" waste to run the biogas facilities, Hamilton said.

"It is important that we address these issues now, because there's a risk," she said. "If we prioritize food waste recycling and build facilities for producing biogas, we risk locking ourselves into needing waste. That is clearly not part of a sustainable future."